Patent Publication Number: US-2021162664-A1

Title: 3d printer

Description:
TECHNICAL FIELD 
     The present invention relates to a 3D printer, and to a filament drive mechanism for a 3D printer. 
     BACKGROUND 
     3D printers enable simple products to be manufactured simply and cheaply. However, existing 3D printers are unable to make complex finished products, because they print only a single family of materials and cannot decorate the 3D printed product. It would be desirable to be able to 3D print commercial products which are more than simply inanimate plastic or metal but are instead a combination of electronics (for example) and plastic. 
     The present invention has been devised to address some of the limitations of existing 3D printers, and seeks to provide a 3D printer which can print finished products or, or at least to print a product in kit form requiring minimal assembly. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber; and   a liquid print head, movably mounted within the housing, for dispensing a liquid; and   a print bed at the base of the printing chamber for receiving an object to be fully or partially filled by the liquid print head; and   one or more apertures for drawing air from the object using a vacuum pump.       

     By drawing air from the object (either by disposing the aperture(s) within, at or proximate an opening into the object, or by disposing the aperture(s) elsewhere within the 3D printer and relying on a global vacuum within the printing chamber to draw air from the object), the filling of an object with printed liquid can be achieved more effectively than relying on gravity alone, and a reduction in the number of air bubbles in the printed liquid can be achieved. 
     The apertures may be disposed in or at one or both of the housing and the print head. If disposed in the housing, the apertures may be provided in the walls, ceiling, or at any other component within the 3D printer. If disposed at the print head (for example on a platform bearing the print head and optionally one or more other print heads and/or other tools), a flexible tube may extend from the aperture to a vacuum pump provided within or externally of the 3D printer. Either alternatively, or in addition, the apertures may be disposed in the print bed. 
     The printing chamber may be a sealed chamber, and the vacuum pump may be operable to draw air out of the printing chamber via the one or more apertures. 
     A further print head may be provided for printing the object onto the print bed prior to it being filled using the liquid print head. The further print head may be a fused deposition modelling (FDM) print head. 
     The liquid print head may be operable to dispense liquid into a first opening into the object while the one or more apertures draw air from the interior of the object via a second opening into the object. In this case, the further print head may be operable to complete the mould by printing over the first and/or second opening once a process of dispensing liquid into the mould has been completed. 
     The 3D printer may comprise a hopper containing the liquid, and the hopper may be pressurised by the pumping action of the vacuum pump. 
     The 3D printer may comprise the vacuum pump. 
     The further print head may be operable to pause printing, and the housing may be opened to permit the manual insertion of an external object into the mould. The manual insertion of the external object may occur prior to, or part-way through, liquid being dispensed into the mould. 
     The 3D printer may comprise a filament drive mechanism supplying filaments of printing material to the further print head, the drive mechanism comprising:
         one or more drive surfaces for driving respective filaments along one or more respective filament paths while the filaments are pressed against respective drive surfaces, each drive surface being unable to drive its respective filament along its filament path if the filament is not pressed against the drive surface;   a plurality of pressure surfaces, each being associated with one of the drive surfaces, each pressure surface being arranged to press against a respective filament to enable it to be driven by the driving surface; and   a plurality of cams, each cam being associated with one of the pressure surfaces, wherein the rotational position of the cams controls which of the plurality of pressure surfaces are pressed against their respective filaments.       

     According to another aspect of the present invention, there is provided filament drive mechanism for a 3D printer, the drive mechanism comprising:
         one or more drive surfaces for driving respective filaments along one or more respective filament paths while the filaments are pressed against respective drive surfaces, each drive surface being unable to drive its respective filament along its filament path if the filament is not pressed against the drive surface;   a plurality of pressure surfaces, each being associated with one of the drive surfaces, each pressure surface being arranged to press against a respective filament to enable it to be driven by the driving surface; and   a plurality of cams, each cam being associated with one of the pressure surfaces, wherein the rotational position of the cams controls which of the plurality of pressure surfaces are pressed against their respective filaments.       

     In this way, cam position can be used to control filament engagement, with the cams potentially being driven by a single actuator (stepper motor or servo for example), rather than requiring a separate actuator to control each filament. This leads to a reduction is weight, and a reduction in a number of parts (potentially improving reliability). 
     The plurality of drive surfaces may be arranged on a single drive shaft. 
     The plurality of cams may be arranged on a single cam shaft, and the rotational position of the cam shaft may control the rotational position of the cams. Alternatively, the cams may be on separate cam shafts, but coupled together via gears such that a single stepper motor or servo and be used to drive them. 
     The plurality of pressure surfaces may form part of respective cam following units, each cam following unit being movable within a channel under the action of the respective cam to engage and disengage the pressure surface from the filament. The plurality of pressure surfaces may be wheels mounted within the cam following units. Each cam following unit may comprise first and second parts, the first part being coupled to the cam and the second part bearing the pressure surface, the first and second parts being biased away from each other via biasing elements, wherein when the pressure surface is pressed against the filament the force applied from the cam to the pressure surface via the biasing elements overcomes the bias to move the first and second parts closer to each other. The biasing elements may each comprise a pair of opposed magnets, the action of the cams overcoming a magnetic repulsion between the pair of magnets. 
     In another embodiment, each cam following unit may comprise first and second parts, again with the first part being coupled to the cam and the second part bearing the pressure surface. The second part may be movably disposed within the first part. The first part may be biased towards the cam by a first biasing element, with the action of the cam overcoming the bias provided by the first biasing element to drive the first part (and with it the second part) towards the drive surface. The second part may be biased towards the drive surface (or away from the first part) by a second biasing element. When the first part is urged towards the filament and the drive surface by the cam, the second part moves with it because of the bias provided by the second biasing element. As a result, the pressure surface comes into contact with the filament, and the second biasing element prevents too much pressure being applied to the filament. Accordingly, and similarly to the previous embodiment, when the pressure surface is pressed against the filament the force applied from the cam to the pressure surface via the biasing elements overcomes the bias to move the first and second parts closer to each other. The first and/or second biasing elements may be springs. A single spring having two parts may be used, one part defining the first biasing element and the other part defining the second biasing element. 
     The filament drive mechanism may comprise at least 3 filaments, at least 3 drive surfaces, at least 3 pressure surfaces and at least 3 cams. It will be appreciated that a smaller number (zero or one) may be provided, or a larger number may be provided, depending on specific requirements. 
     The cam following unit may be coupled to its respective cam by a pin which engages with a groove on the cam, the groove defining a path on the cam which causes the pin, and therefore the cam following unit, to move towards or away from the filament as the cam is rotated and the pin follows the path of the groove. The groove may be similar in shape to, and proximate, a circumference of the cam. The cam following unit may comprise a cut-out portion or slot which the rotational axis of the cam extends through. 
     Alternatively, the cam following unit may have a cam follower which contacts its respective cam, and causes the cam following unit to be deflected as the cam rotates to move the cam following unit (and thus the pressure surface) closer to or further from the filament and the drive surface. 
     The drive surfaces may each comprise a drive wheel coupled to a motor. The drive wheel may be toothed. 
     The filament path may enter the filament drive mechanism at an inlet and exits the filament drive mechanism at an outlet, the filament path being surrounded by a first guide formation proximate the inlet and a second guide formation proximate the outlet and being exposed between the first and second guide formations in the vicinity of the respective drive surface. The first guide formation and the second guide formation may taper towards the position of the drive surface to permit the filament to be in contact with the drive surface over a short length. 
     The rotational position of the cams may be controlled by a servo or stepper motor. 
     The paths of the grooves on the cams and/or the rotational positions of the cams on the cam shaft relative to each other and/or the shape of the cams may be such that by varying the rotational position of the cam shaft a selected two or more of the pressure surfaces can be brought into contact with the respective filaments at the same time. The paths of the grooves on the cams and/or the rotational positions on the cams on the cam shaft relative to each other and/or the shape of the cams may be such that at one or more rotational positions of the cam shaft all of the pressure surfaces can be brought into contact with the respective filaments at the same time. A first of the two or more pressure surfaces in contact with the filaments may apply sufficient pressure to fully engage the filament with the drive surface while a second of the two or more pressure surfaces in contact with the filaments applies sufficient pressure to only partially engage the filament with the drive surface. 
     Any individual pressure surface or combination of pressure surfaces can be brought into contact with the respective filaments at the same time by setting an appropriate rotational position of the cam shaft. An amount of engagement of a filament with the respective drive surface may be controlled by setting an appropriate rotational position of the cam shaft. 
     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber; and   a print head, movably mounted within the housing, for dispensing a printing material; and   a filament drive mechanism according to the above, the filament drive mechanism being operable to supply a selected filament to the print head.       

     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber;   a print bed at the base of the printing chamber; and   a pump device;   wherein the print bed comprises a heating element, and one or more conduits disposed proximate the heating element, and wherein the pump is operable to drive or draw air through the conduits and into the printing chamber.       

     In this way, heated air can be introduced into the printing chamber by using heat from the heating element in the print bed. This results in the introduced air being ready-heated upon its introduction into the printing chamber, without requiring a separate external heating element to be provided. 
     The heating element may be substantially planar and extend across the print bed. The one or more conduits may extend from an inlet to an outlet at or near the periphery of the print bed. The inlet may be disposed beneath the heating element. The inlet may be disposed substantially centrally of the heating element, and a plurality of conduits may be provided which extend from the inlet to a plurality of outlets about the periphery of the print bed. 
     The plurality of conduits may extend radially from the inlet to their respective outlets. The one or more conduits may extend horizontally beneath the heating element. 
     The pump device may be operable in a vacuum mode to draw air from the printing chamber and through the one or more conduits to evacuate the printing chamber. 
     An exhaust may be provided through which the evacuated air is expelled to the atmosphere. A filter may be provided for filtering the evacuated air. 
     A temperature sensor may be provided for measuring a temperature within the printing chamber, and a controller for controlling the operation of the pump device in dependence on the measured temperature to achieve and/or maintain a desired temperature within the printing chamber. 
     The pump device may comprise a valve and a pumping motor, and the controller may be operable to control one or both of the valve and the motor to regulate an air flow rate through the one or more conduits. The pump device may comprise an air pump for driving air through the one or more conduits and into the printing chamber, and a vacuum pump for drawing air out of the printing chamber and through the one or more conduits, and a valve for selecting which one of the air pump and the vacuum pump is in fluid communication with the one or more conduits. 
     The pump device may be located beneath the printing bed. 
     A print head may be provided, movably mounted within the printing chamber, for dispensing a printing material onto the printing bed. The print head may be operable to print over and seal one or more of the apertures to control the flow of air into and/or out of the printing chamber. 
     The 3D printer may comprise a liquid print head, movably mounted within the housing, for dispensing a liquid; wherein the print bed is for receiving an object to be fully or partially filled by the liquid print head; and the 3D printer comprises one or more apertures for drawing air from the object using a vacuum pump. 
     The 3D printer may comprise a filament drive mechanism for supplying filaments of printing material to the print head, the drive mechanism comprising:
         one or more drive surfaces for driving respective filaments along one or more respective filament paths while the filaments are pressed against respective drive surfaces, each drive surface being unable to drive its respective filament along its filament path if the filament is not pressed against the drive surface;   a plurality of pressure surfaces, each being associated with one of the drive surfaces, each pressure surface being arranged to press against a respective filament to enable it to be driven by the driving surface; and   a plurality of cams, each cam being associated with one of the pressure surfaces, wherein the rotational position of the cams controls which of the plurality of pressure surfaces are pressed against their respective filaments.       

     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber;   a print head platform, movably mounted within the housing, having a print head for dispensing a printing material, the print head platform being movable vertically and horizontally; and   a filament elevator, movably mounted within the housing, for carrying one or more filaments of printing material for use by the print head, the filament elevator being movable vertically to stay within a first predetermined distance from the print head platform.       

     In this way, the separation between the print head platform and the filaments can be kept relatively uniform (within a range), reducing the distance the filament is required to travel to the print head, and reducing flexing and kinking of the filaments. 
     The filament elevator may be movable vertically to stay outside of a further predetermined distance from the print head platform. 
     The 3D printer may comprise a plurality of pillars, each pillar having first and second carriages arranged to move vertically on the pillar, the second carriage being disposed above the first carriage;
         wherein the print head platform is movably mounted within the housing via the first carriages of the plurality of pillars, and is movable vertically and horizontally by moving the first carriages independently of each other;   wherein the filament elevator is movably mounted within the housing via the second carriages, the filament elevator being movable vertically by moving the second carriages together; and   wherein the second carriages are constrained to move together vertically on the pillars, and wherein the vertical position of the second carriages is dictated by the vertical position of the highest one or more of the first carriages       

     A drive mechanism may be provided for controlling the vertical position of each of the first carriages independently, wherein if the drive mechanism causes the highest one or more of the first carriages to move upwardly, the second carriages are carried upwardly with the highest one or more of the first carriages, and wherein if the drive mechanism causes the highest one or more of the first carriages to move downwardly, the second carriages are permitted to move downwardly. 
     The filament elevator may carry a filament drive mechanism for extruding the filaments towards the printing head. The extruded filament may be conveyed to the printing head via a tube. 
     The first carriages may be coupled to the printing head platform by connecting rods, each connecting rod being pivotally mounted both to the printing head platform and to its first carriage, and wherein the second carriages are coupled to the filament elevator by fixed connecting rods. 
     The filament elevator may be connected to an upper part of the housing by a support mechanism, the support mechanism bearing a substantial portion of the weight of the filament elevator, the remainder of the weight of the filament elevator being sufficient to drive the second carriages downwardly to follow the highest one or more of the first carriages when the highest one or more of the first carriages is caused to descend. The support mechanism may comprise one of a pulley or an elasticated support. 
     The print head platform may have a liquid print head, for dispensing a liquid; wherein a print bed is disposed at or near the base of the printing chamber for receiving an object to be fully or partially filled by the liquid print head; and the 3D printer comprises one or more apertures for drawing air from the object using a vacuum pump. 
     The drive mechanism may comprise:
         one or more drive surfaces for driving respective filaments along one or more respective filament paths while the filaments are pressed against respective drive surfaces, each drive surface being unable to drive its respective filament along its filament path if the filament is not pressed against the drive surface;   a plurality of pressure surfaces, each being associated with one of the drive surfaces, each pressure surface being arranged to press against a respective filament to enable it to be driven by the driving surface; and   a plurality of cams, each cam being associated with one of the pressure surfaces, wherein the rotational position of the cams controls which of the plurality of pressure surfaces are pressed against their respective filaments.       

     A print bed may be provided at the base of the printing chamber, and a pump device; wherein the print bed comprises a heating element, and one or more conduits disposed proximate the heating element, and wherein the pump is operable to drive or draw air through the conduits and into the printing chamber. 
     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber; and   a print head, movably mounted within the housing, for dispensing a printing material; and   a utility head, movably mounted within the housing, bearing one or more of a laser, ink pen or inkjet print head, for adding surface decoration onto the dispensed printing material.       

     Preferably, the print head and utility head are both mounted onto a common platform. 
     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber;   a print bed at the base of the printing chamber, the print bed comprising one or more apertures;       

     a pump device, for drawing air through the apertures, to generate a vacuum beneath an object located on the print bed to retain it in position; and
         a utility head, movably mounted within the housing, bearing a cutting tool for cutting the object on the print bed while it is retained in place by the vacuum generated beneath it.       

     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber;   a print bed at the base of the printing chamber, the print bed being tiltable; and   a print head, movably mounted within the housing, for dispensing a printing material while the print bed is horizontal or tilted with respect to the horizontal.       

     The 3D printer may comprise a plurality of pillars, each pillar having a bed carriage arranged to move vertically on the pillar;
         wherein the print bed is movably mounted within the housing via the bed carriages of the plurality of pillars, and is tiltable by moving the bed carriages independently of each other.       

     According to another aspect of the present invention, there is provided a 3D printer, comprising:
         a housing providing a printing chamber; and   a dual liquid print head, movably mounted within the housing, for dispensing a liquid;   wherein the dual liquid print head comprises a first liquid print head and a second liquid print head each comprising a pump for drawing liquid from a hopper and dispensing it from a respective outlet; and   wherein the dual liquid print head is configured such that as the pump of the first liquid print head draws liquid from the hopper (and dispenses it), the pump of the second liquid print head draws fluid into its respective outlet (from outside the print head). It will be appreciated also that the dual print head configuration may permit the pump of the second liquid print head to draw liquid from the hopper (and dispenses it), while the pump of the first liquid print head draws fluid into its respective outlet (from outside the print head). In other words, either liquid print head can be used both to dispense liquid, or create a vacuum (suck).       

     The hopper may be pressurised, for example using an air pump. 
     The 3D printer may comprise a print bed at the base of the printing chamber for receiving a mould to be filled by the dual liquid print head, the mould having a first opening and a second opening at a predetermined relative position with respect to each other, wherein the dual liquid print head is operable to be positioned with respect to the mould such that the first liquid print head dispenses liquid into the mould via the first opening while the second liquid print head creates suction through the second opening. 
     The pumps of the first and second liquid print heads may be driven by a common motor. In this case, the pumps of the first and second liquid print heads may be cavity pumps comprising a rotor and a stator, and the common motor may be configured to drive the rotors of the respective pumps in opposite directions simultaneously. 
     Each of the first and second liquid print heads may draw liquid from a different hopper. In this case, the respective hoppers of the first and second liquid print heads may contain different liquids. 
     In some cases, at least part of the first liquid print head and the second liquid print head are interchangeable. 
     In some cases, as the pump of the second liquid print head draws fluid into its respective outlet, this drives air into the hopper thereby pressurising it. 
     It will be appreciated that various individual features and combinations thereof provide an enhanced capability 3D printer, and address certain disadvantages of the prior art. For example, the combination of FDM printing, liquid printing and UV curing enables a solid, hollow object to be printed and filled with liquid, and then the liquid cured to form a composite object. It will be appreciated that the liquid may be cure not only with uv light, but instead by heat, solvent evaporation or chemical reaction. The further introduction of a vacuum (either in the printing chamber as a whole, or specifically within the FDM printed object) enables such a composite object to be formed more effectively (more reliable filling of the object with liquid, with fewer air bubbles). A combination of a twin molyneux vacuum pump print head, FDM, print head, vacuum bed and laser, ink pen and enclosed sealed enclosure enables complex composite products to be formed with a single 3D printer with minimal human interaction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings where like parts are provided with corresponding reference numerals and in which: 
         FIGS. 1A to 1F  schematically illustrate a 3D printer comprising various features of the present invention; 
         FIGS. 2A and 2B  schematically illustrate a twin molyneux vacuum pump; 
         FIGS. 3A and 3B  schematically illustrate vacuum pump arrangements&#39;  FIGS. 4A to 4C  schematically illustrate a vented hot bed; 
         FIGS. 5A to 5H  schematically illustrate a filament drive mechanism; 
         FIGS. 6A to 6D  schematically illustrate a filament elevator; 
         FIGS. 7A to 7D  schematically illustrate a magnetic carriage; and 
         FIGS. 8A and 8B  schematically illustrate a vented print head. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     The present technique provides a novel combination of technologies to allow (for example) 3D printing of encapsulated electronics and rubber mouldings in combination with plastic printed parts. Current 3D printers operate using either FDM (fused deposition modelling), SLS (selective laser sintering), SLA (stereolithography) or metal printing to print items that are limited to the material range of the printer. These printed parts may contain moving mechanisms, but generally not to the extent necessary to manufacture finished items without significant further processing once the 3D printing process has been completed. Most products desirable for printing contain solid parts, electronics and mechanisms. Some products combine solid and rubbers for example to create rubber over-moulding such as a grip on a tooth brush. Current 3D printers do not offer the printing of liquids, although printers have been built that can print curing materials. As an example of the limitations of existing 3D printing techniques, no existing 3D printer combines the necessary methods to print finished products such as a WiFi operated toy car. 
     By combining fluid injection printing with fused deposition modelling (FDM), or other solid printing method, in the manner described below, moulded products housing encapsulated electronics can be printed (note that the 3D printer does not print the electronics, but may “print around” the electronics, with the electronics being manually or automatically inserted before or during the printing process). Certain of the techniques described herein allow for waterproof products to be manufactured, for example by printing rubber materials around printed solid plastic parts (the printed rubber materials may for example be deposited in liquid or gel form and cured to form an elastic/rubber material). The use of liquid deposition while simultaneously drawing air from the mould (either generally evacuating the printing chamber, or preferably more directly applying a vacuum to, at or in, the mould) makes the injection of liquids/resins more effective than simply relying on gravity. In particular, using a vacuum, areas of the mould above the gravity limit can be filled by sucking out the air from the mould, and thus drawing resin into these areas of the mould. 
     By providing one or more of an ink pen, ink jet, laser and vinyl cutter, it is possible to implement additional processes to bring the printed part(s) to a finished product state. These additional tools could (for example) be mounted onto the print head platform, such that the horizontal and vertical position of these tools can be controlled in an analogous manner to the print head. A CNC router could be added to allow for the removal of material, although care would be required to avoid clogging the vacuum inlets and air outlets which will be described below. Items can be placed on the print bed and filled with liquid, and/or graphics/artwork added by laser, or FDM printed on the items. This could entail printing an outline perimeter or other markings onto the print bed and placing the item within the printed perimeter (either manually, or with a robotic arm), to achieve accurate alignment and ensure that the artwork is printed on the item at a desired position. If the perimeter or markings are 3D printed rather than simply defining a visual guide, the resulting raised outline would assist with alignment, since the object would be retained in place. The same principles could apply for locating vinyl at a desired position to be cut. By way of example, the outline of a phone may be printed and raised letters added or text laser engraved thereon. 
     Having a light weight print head means less likelihood for backlash, overshoot, and frame wobble, all of which may cause inconsistent printing. 
     Overall Structure and Vacuum Chamber 
       FIGS. 1A and 1B  show the 3D printer as a whole, in an assembled view and exploded view respectively.  FIG. 1C  shows a close-up view of the circular portion ‘C’ marked in  FIG. 1A .  FIG. 1D  shows a close-up view of the circular portion CA′ marked in  FIG. 1B .  FIG. 1E  shows a cross sectional view of a vertical pillar (described below), both in situ (top drawing), and separately (bottom drawing).  FIG. 1F  is a schematic side view of the printer as a whole, emphasising different features, and demonstrating several variations on the design. Generally, in  FIGS. 1A to 1F , a 3D printer  1  is shown arranged in a delta configuration. The 3D printer  1  carries both an FDM (fused deposition modelling) print head which melts filament material at a diamond hot end to be deposited in the printing process, and also a liquid print head (nozzle) which dispenses liquid from one or more hoppers. 
     In  FIGS. 1A to 1E , a dual liquid print head  13   a  is provided, including two nozzles  15  which dispense liquid from two separate hoppers  12 . In contrast, the 3D printer of  FIG. 1F  comprises a single liquid print head  13   b  which dispenses liquid from a single hopper  12   a . The liquid print head  13   b  draws liquid from the hopper  12   a  through a tube  12   b . It will be understood that many of the techniques described within the present application may be applicable to 3D printing techniques other than liquid printing and FDM. In particular, some of the printing techniques described herein may be applied to a 3D printer having a first and second print heads of different types. Preferably, the first print head prints a material which immediately, or following a short period of cooling, forms a solid, while the second print head dispenses a liquid, which may be solidified by way of a subsequent treatment such as UV curing. Some of the printing techniques described herein may require only a single print head. 
     Referring to  FIG. 1C , the two print heads of the dual liquid print head  13   a  are more clearly visible, as are the nozzles  15  which are directed downwardly towards the print bed  3 . The print heads can be seen to extend through a vented platform, which will be described inn more detail below. 
     The delta configuration comprises three vertical pillars  2  located around a print bed  3 , the print bed  3  being provided on an upper surface of a base  6 . The vertical pillars  2  mount into the base  6  via mounting slots  6   b . The pillars  2  each comprise a linear rail  2   a  disposed inwardly of the printer  1 , and a trim  2   b  disposed outwardly of the printer. Curved side panels  4  extend between (and slot into the sides of) each pair of the three pillars  3  to form a generally cylindrical enclosure. An upper circumferential panel  5  rests on the tops of the three pillars  3 , and forms an outer, annular, portion of a roof of the printer  1 . The central opening within the annular portion  5  can be covered with a central roof portion (not shown) in order to define a fully enclosed printing chamber between the pillars  3 , side panels  4 , print bed  3 /base  6  and the roof. The printing chamber is preferably both fully enclosed, and sealed (or sealable), such as to permit a vacuum to be formed within the printing chamber if desired. 
     A print head platform  7 , bearing one or more print heads/nozzles is suspended from three printhead carriages  8 , each printhead carriage  8  being mounted to a respective one of the pillars  2  via (two) connecting rods  9 . The carriages  8  are independently movable up and down their respective pillars. This allows for movement of the print head platform  7  (and thus 3D printing) in an x, y (horizontal) and z (vertical) direction. In particular, any vertical and horizontal printing position of the print head platform  7  within the chamber is achievable by way of selecting the appropriate vertical position for each of the carriages  8 . While  FIGS. 1A and 1B  show all three printhead carriage  8  and pillars  2 , for simplicity,  FIG. 1F  is shown with only two pillars  2  and their respective carriages  8 . 
     Referring to  FIG. 1D , each printhead carriage  8  comprises a first part  8   a  and a second part  8   b . The connecting rods  9  are connected to the second part  8   b . The first part  8   a  and second part  8   b  are detachably engageable with each other, as shown. Referring to  FIG. 1E  (bottom) a cross section through one of the pillars  2  is provided, showing the linear rail  2   a  and the trim  2   b . Various longitudinal slots are provided both externally and internally of the rail  2   a . An internal region  50  is provided, which has an opening  51  inwardly towards the printing chamber, opposite to the trim  2   b . Referring to the upper part of  FIG. 1E , it can be seen that the first part  8   a  of the carriage  8  is provided within (and movable vertically within) the internal region  50 . The second part  8   b  of the carriage  8  then engages with the first part  8   a  through the opening  51 , such that both parts  8   a ,  8   b  of the carriage  8  are movable vertically on and in the rail  2   a . The part  8   a  comprises wheels  52  which engage with, and permit movement along, the inside of the linear rail  2   a . Similarly, the part  8   b  comprises wheels  53  which engage with, and permit movement along, the outside of the linear rail  2   a . It can also be seen in the upper part of  FIG. 1E  how the panels  4  fit into external channels/slots in the linear rail  2   a . The carriages  8  may be caused to move vertically (upwardly and downwardly) on the linear rail  2   a  by any suitable means. In  FIGS. 1A, 1B and 1C , a threaded lead screw  60  extends vertically within each of the columns  2 . The threaded lead screw also extends through an internal (partially or completely) threaded bore (not shown) in the part  8   b  of the carriage (see aperture  62 ). In one implementation, a motor is provided within each of the pillars (for example mounted at or within the base of the 3D printer) which causes the screw within that pillar to rotate. As the screw rotates, its rotation within the threaded bore of the carriage  8  causes the carriage to move upwardly or downwardly within the pillar (depending on the direction of rotation of the lead screw  60 ). In an alternative version, the lead screw  60  is fixed (does not rotate), but a motor is provided within the part  8   a  of the carriage  8 , which is able to rotate a threaded sleeve about the fixed lead screw  60 , causing the carriage  8  to move upwardly or downwardly within the pillar. An advantage of the latter arrangement is that if further carriages are provided within the same pillar (for example the tilting print bed carriages described below), these can be operated independently. 
     A filament elevator  10  is located above the print head platform  7 , and carries (in this example) three different filament spools  17   a ,  17   b ,  17   c . The filament elevator  10  is mounted to the three pillars  2  via three respective elevator carriages  16 , which are rigidly connected to each other, and to the elevator  10 . The filament elevator  10  may be considered as a platform  18  having three arms  19 , each of which attaches to one of the elevator carriages  16 . The three filament spools  17   a ,  17   b ,  17   c  are located on the platform  18 . The filament spools  17   a ,  17   b ,  17   c  each carry a respective filament which can be melted at a hot end  14  to be deposited by the 3D printer  1 . More specifically, each filament spool  17   a ,  17   b ,  17   c  stores a filament of a particular colour or material. 
     A filament drive mechanism  11  is suspended beneath the filament elevator  10  and is operable to pull filaments from each of the spools of the filament elevator  10  and deliver them to the print head platform  7 , and in particular to and through the diamond hot-end  14  (see  FIG. 1F ) which is able to melt the filament material and to deliver it to an appropriate position within the printing chamber to print an object. The spools  17   a ,  17   b ,  17   c  are rotatably mounted on a spindle (not shown), and are able to rotate independently from each other, such that as the drive mechanism  11  pulls a filament from the spool  17   a  (for example), the spool  17   a  is causes to rotate to release the filament, while the spools  17   b ,  17   c  may remain stationary. As will be discussed further subsequently, the drive mechanism  11  may draw filaments from two or more of the spools  17   a ,  17   b ,  17   c  simultaneously, at the same or different rates. A tube  20  extends (at least part of the way) between the filament drive mechanism  11  and the diamond hot-end  14 , through which the filament passes. The tube  20  protects the filament and reduces kinking during its delivery. Similarly, a tube  21  extends (at least part of the way) between the spools  17   a ,  17   b ,  17   c  to the drive mechanism  11 , through which the filament passes. 
     Also mounted on the printhead platform  7  is a (twin) moineau pump  13   a  (or single pump  13   b  in the case of  FIG. 1F ) which is fed with fluid from a hopper  12 , the latter being located on shelves formed on the side panels  4 , above the maximum upper position of the print head platform  7 , both to avoid providing an obstruction inside the effective printing area, and so that gravity can assist the provision of fluid from the hopper  12  to the print head platform  7 . The moineau pump  13   a ,  13   b  allows for fluids to deposited directly on the print bed  3  or inside an object located on (and optionally printed on, for example using the FDM print head) the print bed  3 . The print head platform  7  may be vented to control the temperature at the print head(s). The print bed  3  is (or comprises), in some embodiments, a vented hot bed. A UV light source and one or more of a laser, ink printing mechanism, vinyl cutter or router may also be provided. One or more of these could be readily mounted onto the print head platform  7 , or might alternatively be mounted elsewhere (for example on a utility platform). Alternatively, the print head platform  7  might be interchangeable with a utility platform bearing one or more of these tools. A UV light source could be mounted on one (or more) of the shelves of the side panels  4 , along with the hopper  12 . 
     In  FIG. 1F , the printhead platform  7  can be seen to carry the hot-end nozzle  14  in addition to the liquid print head (Moineau) pump  13   b . An inkpen  39   a , laser  39   b  and vinyl cutter  39   c  are also disposed on the printhead platform  7 . Only a single liquid hopper  12   a  is shown in  FIG. 1F . The hopper  12   a  is a sealed container, which can be evacuated (a vacuum formed therein) or pressurised via a pump/vacuum which is in fluid communication with the interior of the hopper  12   a  via an inlet/outlet. A filament elevator counterbalance mechanism  40  is shown in  FIG. 1F . This suspends, or supports, the filament elevator  10  from above, and in particular from the roof of the 3D printer  1 . In this way, some (and preferably a majority) of the weight of the filament elevator  10  is borne by the counterbalance mechanism  40 , making it easier for the carriages  8  to (in some cases singly) move the filament elevator up and down, as will be described in detail below. The counterbalance may comprise springs or elastic which connect between the roof  5  and the elevator at three or more points. A pulley arrangement may be provided, if required. 
     In  FIG. 1F , print bed carriages  41  are provided which are able to move vertically on the vertical columns  2  to reposition and/or tilt the print bed  3 . The print bed carriages  41  are of similar form to the print head carriages  8 , and may be mounted within, and movable within the columns in the same or a similar manner to the carriage  8 , for example in how they are mounted to the lead screw  60 . The print bed carriages  41  are connected to the print bed  3  at three positions around its circumference. It will be appreciated that with three carriages  41  on the three columns  2 , it is possible to achieve any direction of tilt of the print bed  3  (which as discussed later, may be a hotbed). In particular, the vertical height of the print bed  3  at the three positions around its circumference can be controlled to influence the gradient and direction of tilt of the print bed  3 . This allows printing (3D or 2D using an inkjet print head or pen) on surfaces which would not be horizontal for a horizontal print bed, either on an FDM printed object already laid down by the 3D printer, or an item placed manually on the bed. It also makes it possible to use a laser to smooth (bevel or chamfer) layered edges. More generally, a tilting hotbed/print bed makes it possible for a for laser, CNC, ink pen inkjet print head or FDM nozzle to be positioned to print on an item in the printer that would not otherwise be parallel to the print head platform. For example, an item could be printed then tilted to print on the previously angled surface. Since the surface is tilted the layering of the FDM would make for better printed letters. As an example of operation, first a location perimeter matching the shape of an item can be printed onto the print bed, then the item can be placed within the perimeter and printed on (the perimeter prevents the item from sliding down the inclined surface of the print bed once it has been tilted). The tilting of the print bed also allows for graphics to be printed on a curved surface by simultaneously tilting the print bed and moving the print head. It will however be appreciated that the print bed can only be tilted a small amount without interference between the print head and the object to be printed and/or the print bed. 
     From the above explanation, it will be appreciated that, by fixing the print bed to 3 points that can move up and down, it is possible for the print bed to be levelled and tilted. A level print bed is important for consistency whilst FDM printing. If the 3D printed is placed on a surface which is not completely level, the print bed may be adjusted to compensate for with. An auto-levelling function could be provided, or a spirit level or similar integrated into the print bed to enable levelling by eye. As discussed, intentional tilting of the print bed allows for printed parts having an angled upper surface (not horizontal to the printbed) to be tilted so as to make them horizontal. This allows for tools mounted on the printhead platforms to work on a horizontal surface, or allows for further printing on the now-horizontal upper surface of the printed object. It is also possible to control the angle and direction of tilt of the print bed during a printing process, under the control of a software algorithm, to (for example) enable the twisting of the bed in a circular motion in combination with a print head movement to create patterns, art or engraving both on horizontal surfaces or any other surfaces within the limits of tilt to be worked upon. 
     The use of a vacuum chamber and hot bed that blows air into the printing chamber allows for the chamber to be heated whilst printing FDM and the provision of a vacuum environment while using liquid resins. The vacuum function also removes harmful solvents and other unwanted smells or smoke from the chamber, such as those produced by the use of a laser. The 3D printer may use a compressor/vacuum pump and pneumatic valves to control and or change the use of the pump through a bank of pneumatic solenoid valves. 
     Twin Moineau Vacuum Pump 
     As discussed above, benefits can arise from evacuating air from the printing chamber. Further benefits may arise from more directly evacuating air from a mould or other object into which liquid is being printed/dispensed. This may be achieved by way of a liquid print head (for fluid injection printing) in combination with a vacuum inlet in the general vicinity of the liquid print head for drawing air out of the mould or object while liquid is being dispensed into the mould or object. This may for example be achieved by providing two openings into the mould, with filling of the mould with liquid being carried out by aligning the liquid print head with a first of the openings and the vacuum inlet with the second of the openings, and drawing air from the second opening (to create a vacuum, or at least draw some of the air from the mould) at the same time as dispensing liquid into the first opening. The first and second openings, and the liquid print head nozzle and vacuum inlet, may be dimensioned such that the liquid print head nozzle generally or substantially fills or covers the first opening (or even provides a complete seal) while the vacuum inlet generally or substantially fills or covers the second opening (or even provides a complete seal). Referring back to  FIG. 1C , it will be appreciated that this could be achieved by removing one of the liquid print heads and in particular by replacing the associated nozzle  15  with a vacuum inlet. 
     Conventional 3D printing heads such as paste extruders feed paste by loading a syringe, which is prone to trapped air. Some extruders use a container pressurised with air to force the paste out. The presence of air in the paste results in air being expelled instead of the paste, such that no material is printed. The use of Moineau pumps to extrude paste has been considered, and commercial form-in-place printers have a shutting valve. Syringes and bags are messy to fill. In contrast, the twin pump described herein uses a pressurised hopper, meaning that the fluid in the hopper can settle, while the pump draws the fluid from the bottom (where bubbles should be at a minimum once the fluid has settled, due to the tendency for bubbles to rise naturally in thin fluids). With more viscous fluids this advantage would be reduced, but still present. The hopper may have a vacuum applied thereto in order to remove more bubbles. The pump draws fluid as per all moineau pumps. 
     Proposed is a dual operated pump mechanism utilizing two screw positive displacement stators pumps (moineau pumps). An air pressurised hopper feeds fluid into one of the pumps, so as to dispense accurately metered amounts of fluid onto the print bed. In combination with a UV light, the dispensed fluid can be cured in specific locations on the print bed in a similar fashion to FDM. The other pump can be used to print a different colour or material. When used for mould filling, the first pump may be utilized for pressurised injection of the liquid into the mould while the second pump is utilised for vacuuming the injected fluid through the mould and/or to de-gas the injected fluid. The hopper may be primed by evacuating air or gas from it prior to printing. Once the pump and hopper are purged of air (or other gas), little or no gas and bubbles are contained within the fluid leading to better prints and moulding. The pump utilizes “luer” lock tips, which offer the advantage of being available in multiple designs of tip size and angles so that fluids of different viscosities can be printed consistently. The twin Moineau vacuum pump has easily changeable pump chambers and stators. As a result, if a fluid should cure inside the pumping chamber, the chamber and/or stator can be quickly swapped with a replacement to minimise down-time. The pump mechanism prevents fluids from entering the drive mechanism, so that the pump stator can be driven reliably without the drive mechanism being fouled with the liquid. 
     In  FIG. 2A , a twin moineau pump  200  is shown. Two pumps are provided, each of which is a cavity pump comprising a stator  201 ,  202  within which rotates a rotor  203 ,  204 . Cavities are defined between the stator  201  and rotor  203  of the first pump (due to the external shape and dimensions of the rotor  203  and the internal shape and dimensions of the stator  201 ), and these cavities effectively progress longitudinally as the rotor  203  rotates, driving the contents of each cavity towards and out of a nozzle  213 . The nozzle  213  is mounted onto the pump using a luer lock tip thread  217 , permitting ready replacement of the nozzle. In  FIG. 2B , a side view of the twin moineau pump is shown, in which a fluid inlet  215  can be seen to permit fluid to enter the cavities between the stator  201  and rotor  203 . The fluid inlet is in fluid communication with the hopper  12 . As the cavities (and the liquid within them) progresses away from the inlet  215  towards the nozzle  213 , this creates a vacuum in the vicinity of the inlet  215 . This generates a pressure difference between the vicinity of the inlet  215  and the hopper  12  (which is accentuated if the hopper  12  is pressurised), causing the fluid from the hopper  12  to be drawn towards the inlet and into the cavities between the stator  201  and rotor  203 . Similarly, cavities are defined between the stator  202  and rotor  204  of the second pump, and these cavities effectively progress longitudinally as the rotor  204  rotates, driving the contents of each cavity towards and out of a nozzle  214 . The nozzle is mounted at the end of the pump using a luer lock tip thread  218 , permitting the nozzle  214  to be replaced (for example with a nozzle of a different dimension). An inlet (not shown) is provided, in like manner to the inlet  215  of the first pump. 
     Returning to  FIG. 2A , the progressive cavity pumps each operate by rotation of their respective rotors  203 ,  204  under the action of a shared stepper motor  205  driving a shared drive gear  206 . The drive gear  206  in turn drives the first and second rotor drive gears  207 ,  208  of the first and second pumps respectively. The first and second drive gears  207 ,  208  drive respective offset drive plates  209 ,  210 . The offset drive plates  209 ,  210  have a pin which is offset from the centre of the drive plate which engages with rotor drive plates  211 ,  212 . The pins locate in the rotors  203 ,  204  to allow for the offset rotation of the rotors  203 ,  204 . 
     As mentioned above, the rotation of the rotors  203 ,  204  transfers fluid by progressing a sequence of discrete cavities. It will be understood that the flow rate is proportional to the rotation rate of the rotor, enabling the flow rate to be easily controlled. It will also be appreciated that only one rotational direction of a rotor will cause liquid to be dispensed through the nozzle. When the rotor is rotated in the opposite direction, air will be drawn in through the nozzle of the pump, and pushed out through the inlet and towards the hopper  12 . The same motor is used to drive both the rotor  203  and the rotor  204  at the same time (and at the same speed), and the arrangement of gears  206 ,  207 ,  208  causes the rotors  203  and  204  to rotate in the same direction. In this example the two pumps have opposite geometries (or handedness) in terms of the progressive cavity actions provided by their respective stator and rotor, and so the driving of the motor will cause a first of the pumps to dispense liquid from the hopper  12  with which the first pump is in fluid communication, while simultaneously second of the pumps to draw air in through its nozzle and towards the hopper  12  with which the second pump in is fluid communication. This makes it possible to pressurise a hopper  12  using the same drive process as dispensing fluids. In an alternative embodiment, the pumps may have the same handedness, but be caused to contra-rotate by the inclusion of an intermediate gear, thereby achieving the same effect of being able to dispense fluid with one pump while drawing air in with the other. 
     The liquid is therefore pushed from the top of the first pump to the bottom of the first pump and out through the nozzle  213 . When the stepper motor  205  is caused to rotate clockwise (for example), the resultant clockwise rotation of the gear  206  will cause the gear  207  to rotate the pump rotor  203  counter clockwise creating a pumping action. This causes fluid to be drawn in through the pump inlet  215 . Simultaneously the pump rotor  204  will be rotated by its gear  208  and drive plate  210  creating a vacuum action sucking through nozzle  214  and expelling through the inlet  215 , due to the opposite handedness of the cavity structure created between its rotor and stator when compared with the first pump. 
     A single moineau pump using a pressurised hopper is shown in  FIG. 3A . Fluid  314  for printing is contained within a hopper  311 . An air/vacuum pump  309  feeds air through an air inlet  310  into the hopper  311 , thus pressurising the hopper  311 . A moineau pump  315  is provided. When the moineau pump  315  starts pumping, fluid is sucked through a hopper outlet  312  and into an inlet  313  of the moineau pump  315 . The pressurisation of the hopper  311  aids this process, by increasing the pressure differential between the hopper outlet  312  and pump inlet  313 . The fluid  314  is pumped by the progressive cavity action described above. This causes the fluid  314  to exit a nozzle  308  of the pump  315 . By reversing the above action, the air pump  309  draws air (sucks) through the air inlet  310  whilst the pump  315  operates in reverse. This allows air or fluid to be sucked in by nozzle  308  through to the hopper. The air pump  309  may assist this process by reversing its operation to depressurise the hopper  311 , or even to create a vacuum in the hopper  311 . In another variation, the air pump  309  may not be used during the reverse operation, but a release valve (not shown) may be used to depressurise the hopper  311  to ambient pressure prior to operating the pump  315  in reverse. 
       FIG. 3B  shows the pump/motor arrangements of  FIGS. 2A and 2B  in combination with the hopper/pump arrangement of  FIG. 3A . Each of the land hand side and right hand side of  FIG. 3B  corresponds to an instance of the arrangement shown in  FIG. 3A . However, the respective moineau pumps of the two instances are both driven (at the same time) by the same motor. This allows for 2 pumps and hoppers to be utilised in parallel with a single driving motor. The air pumps are both able to draw air from their respective hopper (vacuum mode) and pump air into their respective hopper (pressurise mode). The action of a common stepper motor  340  creates the necessary mechanical movement to drive both rotors, as outlined in  FIG. 2A , for one pump and hopper (for example the left hand pump and hopper  320  in  FIG. 3B ) to act as a pump whilst the other pump and hopper (for example the right hand pump and hopper  330 ) in  FIG. 3B ) acts as a vacuum. Reversing the stepper motor would then cause the right hand pump and hopper  330  to pump while the left hand pump and hopper  320  sucks. 
     One advantage of the two moineau pumps driven by one motor is the reduced weight, which is important given that the pumps will need to be mounted onto the print head platform. Further, because the two pumps are driven together, one is able to suck while the other pumps. Both pumps can therefore operate as either a pump or a vacuum. The air feed line, either between the hopper and the dispensing pump, or between the air pump and the hopper, may be provided with a valve to shut when not printing to stop fluid dribbling. Appropriate selection of lure lock tip in relation to the fluid also aids preventing of dribbling. Because one pump sucks and the other pumps, the filling of a mould by liquid from one of the pumps can be aided by the sucking action of the other pump, as discussed above. In particular, the generation of a vacuum within the mould causes fluid to be more effectively drawn into all parts of the moult than by relying on gravity alone. The air compressor pump may use adjustable valves to set the flow rate depending on the type of fluid being used, and may run at a set level by PWM (pulse width modulation) to control the pressure. Relatively thin (non-viscous) liquid such as water requires lower pressure to pump compared with thicker, more viscous liquids such as silicones. The fluid in the hoppers can be degassed by running the vacuum for a short period after the material is loaded (this would be most effective if just one hopper was to be degassed). 
     Vented Hot Bed 
     A vented hotbed is provided inside the sealed chamber. The hotbed itself serves as the printbed discussed previously (and subsequently). The hotbed provides a planar surface onto which articles can be printed. The hotbed may be a metal or ceramic plate (or other suitable material) which is, or contains, a heating element. The vented hotbed uses a pneumatic operated vacuum to drive air into the chamber and/or draw air out of the (sealed) chamber. Presently available 3D printers use a heated bed to print on, and some of these enclose the printer to reduce heat loss. Heating the printing chamber creates a consistent ambient temperature for the build area and aids consistent printing by reducing shrinkage and warping (which may occur as the dispensed printing material cools on a print bed). Some 3D printers use fans to circulate the air. The proposed 3D printer improves upon prior designs by providing a vented bed having channels that are provided directly below the heating element. Air is fed through the channels, extracting heat from the bed to heat the chamber. The injection of heated air may be switched on and off in dependence on feedback from a temperature sensor or sensors within the printing chamber. By switching the direction of flow (from pumping air into the printing chamber to drawing air out of the printing chamber) it is possible to extract fumes and gases trapped in the liquid printed in a mould or in an open-top-enclosure can be drawn out. This is achieved using a pneumatic valve that directs air from the compressor pump. In vacuum mode the pump runs at its normal level (speed) and an adjustable valve and PWM (Pulse Width Modulation) of the pump motor are used to set the required sucking level. 
     The vented heat bed uses an air compressor to blow air into the heat chamber through the inlet the duct(s) in the bed, utilizing the heat from the heat mat to create a constant temperature in the chamber. The air duct directly below the heat mat allows for the heat exchange. In vacuum mode air is sucked from the chamber to enable degassing of liquids within a mould produced by the 3D printer (as discussed above in relation to a dedicated vacuum inlet on the print head platform, or as part of the dual pump). The vacuum also removes toxic fumes from the chamber. The toxic fumes may either be passed through a filter (and either recirculated to the chamber or exhausted to the outside) or exhausted directly to the outside air. The change from vacuum mode to pump mode may be achieved by any number of combinations of controllable valves. 
       FIGS. 4A to 4C  show a vented hot bed  401 .  FIG. 4A  shows the vented hot bed  401  mounted within a printing chamber  404  of a 3D printer, while  FIG. 4B  shows the vented hot bed  401  when viewed from above, and  FIG. 4C  shows the internal conduit structure of the vented hot bed  401 . The vented hot bed  401  comprises a heat mat  403  directly fixed to its top surface. The heat mat  403  may be metallic, or ceramic, or any other suitable material, and may be electrically heated by an electrical power source (not shown). Items are 3D printed on top of print bed  403   a  mounted on the heat mat  403 . While operating in a chamber heating mode a valve  411  is configured to allow an air compressor  412  to blow air through an inlet  409  into ducts extending past (or through) the heat mat  403  (in this example the ducts are provided below the heat mat  403 ) and thus extracting heat from the heat mat  403  on the way to the printing chamber, which it enters through the inlets/outlets  402 . It will be appreciated that the ducts need only pass sufficiently close to the heat mat  403  to draw heat from it, they are not required to be immediately adjacent. In this way, the chamber is heated using air which is already at a high temperature at its point of entry into the chamber  404 . A sensor  413  in the chamber  404  senses the temperature of the chamber and uses software/electronics to control the air compressor to be on/off as necessary to achieve the required temperature. It will be appreciated that a plurality of temperature sensors could be used, at different positions within the printing chamber  404 . It will also be appreciated that the temperature within the chamber could also be controlled by increasing or decreasing the temperature of the hotbed itself. However, the temperature of the hotbed may be required to be within a predetermined range in order to serve its primary function in relation to maintaining the temperature of the printing material printed thereon, and so control via air flow is generally preferable. 
     To switch from heating mode to vacuum mode (note that the chamber, or at least the base thereof may still be heated using the heat mat  403  during the vacuum mode, but heated air will not be introduced into the printing chamber) the valve  411  is used to switch from connecting an air pump to the inlet  409  to connecting a vacuum pump to the inlet  409 . It will be appreciated that, rather than providing separate air compressor and vacuum pumps, a single pump may be used and operated in two different modes. An item printed on the print bed  403  can be a mould item  407 , enclosed on all sides apart from the top, or could have open vents on the top surface into which liquid could be dispensed. After liquid  406  is dispensed into the mould  407  it may contain unwanted trapped air  405 . Furthermore, fumes  408  could be released (in this case from the liquid  406 , but in other cases, from printed solid materials). The vacuum pump  412  applies a vacuum (negative pressure) at the outlet  409  and therefore sucks air from the chamber  404  through the duct  402  and via the inlets/outlets  402   a . This causes negative pressure in the enclosure and as such air bubbles  405  contained in the liquid  406 , and fumes  408  released into the chamber  404 , to be removed and sucked through the air paths of the vacuum pump to outside or through a filter  410 . 
     Referring to  FIG. 4B , the upper part thereof provides a top view of the vented hot bed  401 , in which the apertures  402   a  can be clearly seen, distributed about, and proximate to, the circumference of the upper surface of the hot bed  401 . A central aperture  415  is also shown, which can be used to draw air from (create a vacuum within) an object having an (partially or fully) open bottom disposed over the aperture  415 . The aperture  415  may be in communication with the pumps  412  in the same manner as the apertures  402   a.    
     Referring to  FIG. 4C , this is a cross section through the hot bed  401 , in the plane of the ducts  402 . Here, the ducts  402  can be seen to comprise a plurality of ducts which extend radially from a central internal area of the hot bed  401  (from which extends the inlet/outlet  409  to the pump  412 ) to each of the apertures  402   a.    
     It will be appreciated that the vented hotbed provides one way of evacuating the printing chamber (alternative/additional ways are discussed above), as well as of heating the chamber more effectively. The vacuum could be achieved with just an inlet to the chamber, but the use of a vented hot bed gives the advantage that air can be blown into the printing chamber while taking heat from the bed to heat the chamber, and also by sucking through the bed multiple inlets may be used which defines a short distance for bubbles to travel to an inlet, irrespective of where the bubbles are located. 
     Filament Drive Mechanism 
     As discussed above, FDM 3D printers heat and melt filament(s) in the hot-end and extrude it through a nozzle. Filament drive mechanisms are used to drive the filament to the hot end (in the present case from the spools mounted on the filament elevator). Drive mechanisms are either mounted directly before the hot-end or remotely. The remote type drives filament directly to the drive mechanism through a tube. A diamond-hot-end takes (in the present case) 3 filaments and melts them through a single nozzle. Currently there are only single and double drive mechanisms available. Using 3 single drive mechanisms to feed the diamond hot-end makes the assembly large and requires mores space. Together, the diamond hot-end and filament drive mechanism are referred to as a filament extruder. 
     The present technique uses a smaller drive mechanism, enabling it to be mounted on or from the filament elevator. The print head platform  7  carries a diamond hot-end for melting the filaments delivered by the drive mechanism  11 . The filament drive mechanism  11  in this case is a triple filament drive mechanism for delivering three separate filaments to the diamond hot-end. The drive mechanism  11  may deliver one or more of the filaments at a time, such that the diamond hot-end is able to melt either a single one of the filaments to form a printing material for deposition in the printing process, or a combination of two or more (in preferably adjustable proportions) of the filaments. The diamond hot-end takes the 3 filaments and melts them through a single nozzle so as to extrude 3 separate colours or different materials or a combination of these. 
     Referring to  FIG. 5A , a diamond hot-end nozzle  501  is provided. The diamond hot-end nozzle  501  receives filaments  502   a ,  502   b ,  502   c  along three respective filament paths  501   a ,  501   b ,  501   c . The filaments  502   a ,  502   b ,  502   c  are fed through tubes along the filament paths  501   a ,  501   b ,  501   c  from filament spools mounted on the filament elevator, to melt in the diamond hot-end nozzle  501 . Then, melted filament material  503  leaves the nozzle and is deposited on the print bed  505  to form a 3D printed item  504 . In order for the filaments  502   a ,  502   b  and  502   c  to be forced into the diamond hot-end nozzle, they need to be pushed (or fed) by a drive mechanism. 
       FIGS. 5B, 5C, 5D and 5E  show a first suitable drive mechanism, taking the form of a triple drive mechanism which drives multiple (in this case three) filaments by use of a single stepper motor, and in particular enables the driving of 1, 2 or 3, or any combination of 1+2, 1+3, 2+3, 1+2+3 or 1, 2 and 3 by use of a cam system driven by a servo to engage the drive of the filaments. In particular,  FIG. 5B  is an exploded view of the drive mechanism,  FIG. 5C  is a cross section through part of the drive mechanism showing the parts that make up a single drive (of the triple drive),  FIG. 5D  is a cross section (perpendicular to that of  FIG. 5C ) which shows the triple drive in assembled form, and  FIG. 5E  shows a closer view of the channel through which a filament passes through the device.  FIGS. 5F, 5G  an  5 H show a second suitable drive mechanism, having certain differences compared with the first drive mechanism. The example described herein is a triple extruder, having a cam which is driven by a servo, and which moves a cam follower. Referring to  FIG. 5B , a single filament  515  is driven by filament drive teeth  516   a . Pressure from a pressure wheel  512  allows for the filament to be driven. Upper and lower nipples  515   a ,  515   b  prevent the filament from kinking and feed the filament to the filament drive teeth  516   a . The pressure wheel  512  engages and disengages from the filament  515  to apply or remove the pressure that allows the filament drive teeth  516   a  to engage with and move the filament  515 . 
     The pressure wheel  512  may in some embodiments operate in two states—a first state in which it is fully disengaged from the filament  515  (or at least provides insufficient pressure for the drive teeth  516   a  to be able to engage the filament  515  strongly enough to drive it through the mechanism), and a second state in which it is fully engaged from the filament  515  (or at least provides sufficient pressure for the drive teeth  516   a  to be able to engage the filament  515  strongly enough to drive it through the mechanism). In alternative embodiments the degree of pressure exerted by the pressure wheel  512  may be controllably variable, so that the filament  515  is driven at a controllable speed through the mechanism, without needing to vary the speed of the stepper motor. This is beneficial, since the stepper motor can be operated at a constant (and predictable) speed, and different filaments can be driven through the mechanism at different rates by varying the amount of pressure being applied thereto by the respective drive wheel (effectively by controlling slippage). For example, five four states could be used; an “maximum” state providing a filament drive speed of 100%, an “off” state providing a filament drive speed of 0%, a “slow” state providing a filament drive speed of 25% and a “medium” state providing a filament drive speed of 50%. It will be appreciated that a smaller or greater number of speed settings could be provided. 
     The pressure wheel  512  is mounted in a pressure wheel carriage  513 . On the rear of the pressure wheel carriage number  113   a , a magnet  511   b  is provided. The pressure wheel carriage  513  is mechanically linked to a cam follower  510 , such that the two components cannot move beyond a fixed separation from each other. A magnet  511   a  is provided on the cam follower  510  in a position proximate the magnet  511   b . The magnets are magnetically opposed, to repel each other. The cam follower  510  controls the pressure wheel carriage  513  to move forwards and backwards between two positions, which may for example relate to the first and second states (or the “off” and “maximum” states) described above. a cam axle  507  forms a drive shaft which is driven by a stepper motor (not shown in  FIG. 5B ). A set of cams are mounted at different positions on the cam axle  507 . 
     In  FIG. 5B , which is a cross sectional view through the drive mechanism, only a single cam  506   a  (and the other components at the same cross-sectional position) can be seen. The cam  506   a  can be seen to comprise a pin groove  508 , which generally follows the perimeter of the cam  506   a , and in particular is between the cam centre  506   a  and its outer perimeter  506   b . A pin  509  is shown to be located within the pin groove  508 . The pin  509  is fixedly mounted to the cam follower  510 , and both the cam follower  510  and the pin  509  are constrained to move only in a direction which is parallel (or anti-parallel) to the “Forwards” arrow marked in  FIG. 5B . As a result, as the cam  506   a  rotates about the cam axle  507 , the irregular shape of the pin groove  508  will cause the pin  509 , and thus the cam follower  510 , to move towards or away from the cam axle  507 . As the cam follower  510  moves, it pushes or pulls the pressure wheel carriage  513  with it. When the cam follower  510  is driven to move towards the filament, it will cause the magnets  511   a ,  511   b  to move closer together. The magnetic repulsion between the magnets  511   a ,  511   b  will push the pressure wheel carriage  513  towards the filament  515 . When the pressure wheel  512  comes into contact with the filament  515 , part of the force exerted by the cam  506   a  will be taken up in overcoming the magnetic repulsion between the magnets  511   a ,  511   b . When the cam follower  510  is driven to move away from the filament  515 , the cam follower  510  will first move away from the pressure wheel carriage  513  until it reaches the maximum separation extent (during which time the pressure wheel  512  will remain in contact with the filament  515 , but while exerting gradually reducing pressure), and then the mechanical engagement between the pressure wheel carriage  513  and the cam follower  510  will cause the pressure wheel carriage  513  to be pulled away out of engagement with the filament  515 . 
     In  FIG. 5C , a top view of the assembled filament drive mechanism is shown. This shows three cams mounted on a single shaft/axle  507 . A single servo  518  is connected to the cam axle  507 . This servo  508  controls the rotational position of the cam axle  507 , and thus the rotational position of each of the three cams mounted thereon. Three cam followers are connected to respective ones of the cams, and three pressure wheel carriages are each linked to a respective one of the cam followers. The cams (and more specifically the path of the pin groove) are preferably shaped such that one revolution of the cam bar pushes the relevant cam follower  510  such that its respective filament is engaged against the stepper drive shaft  516   b . The position of the cams allows for the differing combinations to be made in one revolution of the cam axle  507 . The stepper motor  519  causes rotation of the stepper drive shaft. 
     The rotational position of the cam axle  507  is controlled (indexed) using a servo or stepper motor. The drive shaft for driving the filament through the drive mechanism is driven by a stepper motor, which could be geared to increase force. The use of cam followers which are coupled to the pressure wheel carriages with magnet springs brings a variety of benefits. In particular, when the force applied by the cam-follower urges the drive wheel to press the filament on to the stepper-drive, the opposing magnets via which the cam follower is connected to the pressure wheel carriage act as a spring to take up differences and apply tension. Slippage of the filament is a common problem, and the magnetic carriages help to provide a consistent force whilst allowing for changes in filament diameter (for example due to the use of a filament with an inconsistent diameter, or to account of different filaments being used and having different diameters). A servo can be used, which accurately maintains its rotational position (thus enabling the cam axle rotational position to be accurately controlled), but equally a stepper motor could be used, but in this case a sensor would be required to know the position and this would be lost with power down. 
     In  FIG. 5E , the drive wheel  516   a , and upper and lower nipples  515   a ,  515   b  can be seen more clearly. The nipples  515   a  and  515   b  assist the filament material in being fed smoothly, and without kinking, through the mechanism. The nipples guide the filament as close to the stepper-drive before and after. This helps prevent buckling of the filament particularly a problem with flexible filaments. The inlet nipple and tapered entry helps smoothly guide the filament from the tube so it easily loads and has no space to buckle. Equally the triple drive mechanism could have 4, 5 6 or more cams so as to feed more than 3 filaments, meaning different colours and materials could be used in many different combinations. 
     It will be appreciated that alternative cam-based mechanisms could be used. For example, and as shown in  FIG. 5F , a cam-based mechanism using the perimeter of a cam  556   a  in combination with a biasing component  570  and a cam follower  575  are certainly envisaged as viable alternatives. In this embodiment, a wheel carriage  563  comprises the cam follower  575  which causes the wheel carriage  563  to move towards and away from the drive wheel as the cam  556   a  rotates. The biasing component  570 , in this case a folded metal spring, is engaged between the casing of the drive mechanism (at points ‘A’) and shoulders CB′ on the wheel carriage  563 , thereby biasing the wheel carriage  563  towards the cam  556   a . A pressure wheel  562  is movably mounted to the wheel carriage  563 , and biased towards a drive wheel  566   a  by an extension  570   b  to the biasing component  570 . In  FIG. 5G , it can be seen that an axle  580  of the pressure wheel  562  is located within a slot  582 , to permit the relative movement between the pressure wheel  562  and the wheel carriage  563 . Accordingly, the pressure wheel  562  is able to move (with respect to the wheel carriage  563 ) over a short distance, to achieve the same effect as the magnetically opposed first and second parts of the previous embodiment. The biasing part  570  can be considered as a metal spring having first and second spring elements. The first spring element biases the wheel carriage towards the cam (and away from the drive wheel), while the second spring element biases the pressure wheel towards the drive wheel. In use, the cam action overcomes the influence of the first spring element, while contact with the filament overcomes the influence of the second spring element, thereby retaining the pressure wheel in close contact with the filament when the filament is to be driven through the mechanism. 
     In this alternative embodiment, the moulded nipples  515   a ,  515   b  are not used. Instead, as can be seen in  FIG. 5H , replaceable inserts  515   c ,  515   b  are provided. These are separate from the main body of the drive mechanism housing, and may be inserted into openings in the housing, as shown. As a result, the inserts  515   c ,  515   d  may be formed of a different material than the main body. It will be appreciated that the inserts may be subject to high levels of wear due to being in a friction relationship with the filament, which may include strengthening fibres. The inserts may therefore be made of a harder material than the body to accommodate wear. The inserts may also be made of a lower friction material to assist with the passage of the filament. Furthermore, as discussed above, it may be desirable to be able to accommodate different filament pitches. This can be readily achieved simply by swapping out an insert having one internal bore diameter for an insert having a different internal bore diameter. Each insert has an open end  590  proximate the drive wheel and pressure wheel, which is chamfered to enable it to extend more closely to the wheels to minimise the length over which the filament is unsupported. 
     A simplified version of the mechanism could simply use a cam to directly engage filaments, rather than being used to urge a separate spring plate against the filaments. 
     Filament Elevator 
     As discussed above, it is desirable to keep the length of the filament path between the filament drive mechanism and the print head relatively constant. A reduced variation in the filament path requires the filaments are stored within the enclosure. Further, filaments may suffer from water absorption, and thus housing them inside would reduce this without needing to contain the filaments within a sealed housing. Moreover, by having the filament and drive mechanism close to the print head rather than at the top or side of the printer the paths are consistent and shorter resulting in less friction. As a result, a consistent force is required to drive the filament to the print head, rather than a force which varies substantially as a function of current distance between filament drive mechanism and print head. Slippage of the filament is a common problem and because FDM requires a high level of accuracy in the filament being driven to the print head, changes in friction within the tubes through which the filament are driven (between the drive mechanism and print head) can change the amount of filament fed to the print head. Further, flexible filaments may suffer from jams when used with long print paths. 
     As discussed above, a delta printer has a fixed print bed with the print head moving in x, y and z directions. Excess weight on the print head platform adds extra momentum to the print head platform and therefore any excess weight decreases its accuracy (or requires it to be moved more slowly, increasing print times). A filament driven through tubing to the printhead is susceptible to friction, because the longer the tubing the greater the friction, and twists in the filament tubing path also increase friction. Greater friction leads to a higher probability of jams and an inconsistent amount of filament being driven to the print head. With filament reels at a fixed position, which is common among delta printers, the filament tubing path must flex and change its position as the printhead moves. This will alter the friction in the tubing and thus variation in the necessary driving force of the filament drive mechanism. 
     Ideally, both the filament reel tubing path and the filament drive mechanism would be mounted directly onto the print head platform to avoid friction related problems. However, the excess weight on the printhead platform would decrease the accuracy of printing. The proposed filament elevator addresses this by “floating” above the print head, which keeps the distance between the filament spools and the print head platform more consistent. The filament elevator bears the filament reels on its upper surface, and has the drive mechanism suspended beneath it, which allows for a consistent filament path from the reels to the drive mechanism. From the drive mechanism to the printhead the filament tubes and therefore paths are held within an elasticated plate which minimizes the tube (path) length and the number of twists within the tube length to minimize pathlength. This creates a more consistent pathlength and therefore reduces changes in the in the friction experienced by the filament increases the accuracy of prints. The filament elevator moves up and down by means of the upper most printhead drive carriage. The weight of the filament elevator is taken up by means of a pulley or elastic mechanism suspended from the top of the printer. Due to the need to drive only a small proportion of the overall weight of the filament elevator (most of the weight being supported by the pulley or elastic mechanism), the driving forces within the printhead drive carriages are little changed by the additional weight of the filament elevator. 
       FIGS. 6A to 6D  shown the filament elevator within the printing chamber. In particular,  FIGS. 6A and 6B  show the filament elevator and associated print head platform at two different positions within the printing chamber.  FIG. 6C  shows the filament elevator and drive mechanism in greater detail.  FIG. 6D  shows the filament elevator, drive mechanism and print head in greater detail. A printhead platform  607  is linked by connecting rods  606  to moving carriages  604 . The rods  606  are able to pivot freely at both ends (that is, at the carriage  604 , and at the printhead platform  607 ). One carriage  604  is provided on each of the 3 pillars. By moving the carriages  604  up and down independently, it is possible for the printhead platform  607  to move about and print material on a print bed. This allows for movement of the printhead in the x, y and z directions, as described above. Generally the carriages  604  are driven up and down their pillars by either a screw drive or belt drive (not shown in the diagrams, and not material to the invention). For simplicity of explanation, the delta printer is represented with 2 pillars. The printhead platform  607  has a printhead  612  mounted on it. Suspended above this and from a filament reel  603 , is the filament drive mechanism. The filament reel  603  is suspended from the pillars  601  in a similar fashion to the printhead platform  607 . In particular, 3 sets consisting of a connecting rod  605  connected to a carriage  602   a  which is constrained to move up and down the pillar  601 . Filament  611  is drawn from the filament reel  603  by the filament drive mechanism  609  into the printhead  612 . 
     In  FIG. 6C , the filament elevator is shown without the connecting rods  605 . The filament reel  603  is mounted on a filament reel platform  603   d . Suspended below this by  3  flexible connecting rods  610  is the drive mechanism platform  610   a . Mounted to the bottom of the drive mechanism platform is the drive mechanism  609 . 
     In  FIG. 6D, 3  filament reels  603   a,b  and  c  are shown to be housed within the filament reel platform  603   d . One of each of the filaments are fed through holes in the filament reel platform  603   d  and holes in the drive mechanism platform to the drive mechanism. The triple drive mechanism  609   a  feeds the 3 filaments to a triple hotend  612   a . The filaments  611   a,b  and  c  run through tubes. The tubes act as guides to prevent the filament from kinking as it is pulled and pushed by the drive mechanism  609   a.    
     Comparing  FIGS. 6A and 6B , it can be seen that as the printhead platform  607  is moved to the left, the carriage  604   a  must rise and carriage  604   b  must fall. Filament carriage  602   a  is raised by the action of carriage  604   a  moving up. Since the filament reel connecting rods  605  are rigid, the carriage  602   b  does not fall with the carriage  604   b , but instead moves with the carriage  602   a . If the printhead platform were to move to the right, then carriage  604   b  would rise and lift carriage  602   b  up with it. As a result, the filament elevator remains at a maximum distance of the length of rod  606  above the printhead platform  607 . Since the printhead platform is on the left hand side the drive mechanism platform  610   a  is pulled and tilts in the direction of the printhead platform. The fact that the rods  610  connecting the filament reel platform  603   d  to the drive mechanism platform  610   a  are flexible allows for this movement. As the filament platform  607  is moved around to print items this mechanism allows the drive mechanism to follow the printhead. 
     It will therefore be appreciated that the filament elevator keeps the drive mechanism and filament paths more consistent and shorter, and permits filaments to be stored in the sealed enclosure of the printing chamber, thus keeping the filaments dry. The filaments can easily be changed because the three spools are separable. When an upper spool of the set of spools is released from a lower spool, it is lifted upwards on the counterbalance mechanism, enabling easy removal from the 3D printer. Once removed from the printer, the spool can then be separated from the counterbalance mechanism, and replaced. In other words, the top spool is releasably attached to the counterbalance mechanism, the middle spool is releasably attached to the top spool, the bottom spool is releasably attached to the middle spool, and the elevator platform may be releasably attached to the bottom spool. 
     In a variation of the above, the elevator could take the form of a disc, mounted between the upper (filament) carriages, with the spools mounted (suspended) beneath it. If provided, the counterbalance may then be connected to the disc shaped elevator platform. 
     Magnetic Tensioners 
     As discussed above, the print head platform is moved using three independent carriages which move vertically on respective vertical pillars. This is the standard setup for a delta printer. Conventional carriages suffer from certain disadvantages, and to address these disadvantages the present 3D printer may utilise magnetic spring wheel carriage tensioners. Linear rails are commonly used for achieving linear motion in various machines, not just for 3D delta printers. Carriages run on these rails, and driving the carriage can move items in a machine (such as, but not limited to, a delta printer). One common rail system is a v-slot, in which some carriages are retained against the linear rail by gravity while other use opposing wheels running either side of (at least a portion of) the rail(s). These commonly have 4 wheels, 2 either side but some may have many more. 
     CNC routers, laser cutters and other similar machines typically utilize extruded aluminium profile with a “V slot” for form the vertical pillars. Carriages that run on this type of extruded profile comprise wheels having chamfered edges. When pairs of these wheels are used on opposite sides of the profile a method of adjusting the distance between the pair of wheels, and thus the clamping pressure on the pillar, and thus the friction which prevents slippage is required. Tension of the wheels against either side of the rail is controlled by providing the wheels on one side on a cammed screw, which can be used to adjust tension so that the carriage is closely retained against the rail but also runs freely. This requires regular maintenance to ensure the tension remains at the correct level. If the wheels are too loose the carriage will wobble, while if the wheels are too tight then slight changes in the profile/thickness of the rail would lock up the carriage and inhibit motion. If twin rails are used with a common carriage travelling between them, the dimension between the pair of rails may change along the length of the twin rails. This would conventionally require the cam-screws to be adjusted to be looser to prevent the wheels jamming, but at the loss of accuracy (increased wobble). By use of magnets the need for adjustment is eliminated, as changes in dimension along the single or double rails are automatically taken up, preventing wobble at all points. 
     The 3D printer described herein uses opposed magnets to address the disadvantages of the prior art. The opposed magnets act within the carriage to urge the wheels against the profile of the pillar. This reduces the need for periodic/continual adjustments which would otherwise be necessary to maintain the correct friction between the carriage wheels and the profile, thereby allowing the carriages to move freely along the profile/linear rails. The present technique is advantageous over the used of springs for the same purpose. In particular, the force applied by a conventional spring would generally decrease over time, while a cam would become looser as the wheel wears. In contrast, by directly replacing the spring with magnets, these problems are alleviated—provided that the magnetic attraction/repulsion between the opposed magnets does not diminish over time. With a cam as the dimension of the machine change along the length of two parallel rails the friction increases and decreases, with a spring or magnet spring this is taken up by the change in position allows for by the spring/magnet but not by the cam. This is particularly emphasized when using rails in the vertical direction. 
       FIGS. 7A to 7C  show the proposed mechanism for engaging a carriage with a linear rail.  FIG. 7A  shows a cross section of a part of a linear rail in the form of a V slot extrusion  701 .  FIG. 7B  shows a carriage positioned with respect to a linear rail.  FIG. 7C  shows the engagement between a cross section of linear rail and a carriage, while  FIG. 7D  shows the arrangement of  FIG. 7C  viewed from below. The proposed mechanism could equally be another type of linear rail. Referring to  FIG. 7A , the extrusion  701  is generally square in cross section, but has a groove or channel  701   a  extending along each face. The wheels of a carriage intended to travel along the linear rail are required to engage with one or more of the channels  701   a , and thus have shaped edges in order to achieve this. Referring to  FIG. 7B , a carriage  702   b  is shown positioned on and movable along the linear rail  701 . In the case of a horizontal rail (as shown), a carriage may simply rest on the rail. However, in the case of a vertical rail, a carriage would be required to engage with two opposing sides of the rail in order to be retained against the rail, such as in the manner of  FIG. 1E , as described above. 
     Referring to  FIG. 7C , a carriage  702  is mounted to the linear rail via four sets of wheels. A first two sets of opposing wheels respectively engage with opposing faces of the linear rail  701 , while a second two sets of wheels engage with the same face of the linear rail  701 . The carriage  702  is “U” shaped, having a first part which extends between the second and third parts. The first two sets of wheels are disposed on the inside of the second and third parts (the legs of the “U”), and the second two sets of wheels are disposed on the inside of the first part of the “U”. As a result, the carriage extends around, and engages with, 3 faces of the linear rail  701 . 
     The carriage  702  comprises a plurality of wheel carriages  704 , each of which bear one of the wheels  703   b . In particular, each of the wheel carriages  704  has a pivot, on which the respective wheel  703   b  is mounted. Not all of the wheels are mounted on wheel carriages (in this embodiment, although in other embodiments they may be). In particular, the wheels  703   a  rotate about pivots  703   c  provided in the carriage  702  itself. The wheel carriages sit in a wheel carriage slot inside of the carriage  702 . A first magnet  705   a  is fixed to the wheel carriage  704 . A second magnet  705   b  is fixed to the rear of the carriage slot. The first and second magnets  705   a  and  705   b  are orientated such to repel one another. The repulsion force of the magnets forces wheels carriages  704  and accordingly the wheels  703   b  against the v-slot extrusion  701 . This creates the necessary force to correctly position the wheels against the v-slot extrusion  701 . Equally the wheels could be replaced by ball bearings and the extrusion replaced by a rail with a location slot for the ball bearings. 
       FIG. 4D  shows the carriage  702  viewed in the direction ‘A’ in  FIG. 4C . Here, it can be seen that a pair of opposed wheels  730   a ,  730   b  comprise a first wheel, on one side of the rail  701 , which is mounted directly to the carriage, and another wheel, on the opposite side of the rail  701 , which is mounted to the carriage  702  via a magnetic wheel carriage. The same applies for the pair of opposed wheels  732   a ,  732   b.    
     Vented Print Head 
     During FDM printing plastic filament is driven by a drive mechanism to the print head and then melted through the hot end nozzle, as described above. Current printers use two fans per hot end—one fan to cool the printed material and another fan to cool the filament path that leads to the hot end. Cooling the filament path is important so that the plastic filament remains unmelted prior to it entering the hot end, as it needs to be solid so that it can be pushed into the hot-end. If the filament has melted, or has softened, this will not be possible. Cooling the printed material may be necessary to aid adhesion to the print bed and solidify the printed material. It has been found that by using channels in the print head platform, it is possible to direct air to both of these points. This also reduces the weight on the print bed due to the absence of fans. Further, the printhead may be able to print closer to some edges due to the absence of a fan preventing movement in that direction. Accordingly, the use of a vented print head removes the need for 2 fans, thereby reducing the weight on the print bed, and enabling the print head to reach closer to the edge of the print bed (where a fan would normally be provided). 
     In particular, the proposed vented print head utilizes an air compressor to pump air through ducts within the print head platform and through holes pointing toward the hot end and the print bed. The holes pointing towards the hot end enable controlled cooling of the filament path to the hot end. For accuracy placement and bonding or print material cooling fan are used by current invention to accurately control the plastic cooling. The vented print bed has holes aimed at the print bed enable controlled cooling of printed material. 
     Referring to  FIG. 8A , a filament  801  enters a hot-end guide  804  and is melted in a nozzle  805  by a heating element  808 . A melted filament  809  is extruded from the nozzle  808  onto a print bed  806  as per the FDM process describe above. Air is directed through a print head duct  803  towards the filament hot-end guide  804 . This air is directed over cooling fins of the guide  804 , to cool it. Air is also directed through a print head duct  802  towards the print bed  806  to cool the extruded filament  807 . Referring to  FIG. 8B  (which shows the underside of the vented print head), a single air inlet  810  may feed multiple outlets  811  through multiple ducts  812 . Similarly, for the material cooling ducts  802 , a single inlet  813  may feed multiple outlets  814  through multiple ducts  815 . 
     Working Example 
     The 3D printer  1  utilizes a sealed chamber to enable a vacuum to be established. By virtue of the combination of the features described above, mouldings can be printed using FDM and then filled with liquid resin (UV, solvent or 2 pack (2 part adhesive, comprising a hardener and resin which are mixed to chemically react and solidify)) and this can be vacuum de-gassed/infused to aid the moulding. Using an existing FDM process a 3D object can be printed, and this object can be designed to form a mould. The mould could for instance be for a watch. The printing process would entail a pause during printing to allow for a pre-manufactured watch mechanism to be placed into the mould. Printing would continue after this pause to complete the mould, with an allowance for the watch glass and for inlet and outlet ports to the mould. Utilizing one of the twin molyneux vacuum pump outlets it is possible to engage the mould whilst simultaneously using the second outlet of the pump (vacuum inlet) to engage the second port on the mould. The vacuum pump would be started and fluid injected through the pump from the first hopper aided by the fact that the hopper is pressurised. This would begin filling the mould. The TMVP second pump, now being utilised as an inlet, sucks the fluid through the mould. 
     After the fluid has completely filled the mould, the watch face may be printed using the TMVP other hopper a clear fluid would be used and surface tension utilised to obtain a concave surface (if desired, or alternatively convex or flat). The vacuum in the vented hot bed operates through these process to ensure bubbles are removed. If the fluid is either 2 pack solvent or UV curing, the fluid would be left to cure in the necessary manner. For instance, in the case of the product outlined here the FDM printed mould would be of a clear plastic and fluid would be a UV cure resin. This would enable the UV lights to be turned on after the mould has been filled to cure the fluid inside the mould. Other materials such as 2 pack or solvent cure or air cure or any either curing process could be injected without the necessity for the clear mould. 
     Equally an opaque mould could be used provided that UV light is transmitted through the material. Due to the fact the printer utilizes a triple filament hotend a combination of necessary materials can be printed using the FDM process to enable the printing of a mould made of dissolvable material, parts for the end product to be printed in a variety of plastics and conductive plastics to be printed in the form of electronic circuits. This combination along with the ability to print liquids by the TMVP which can be cured to soft elastomers enables the printing of electronics encased within sealed designs. If the mould is utilised for this and is of a dissolvable material the mould can be dissolved away to leave the end product. Equally, a non-dissolvable material could be used and the mould broken or machined a way. A pause during this process allows for electronic components to be placed within the circuit board printed using the FDM process. These components could be complete and populated circuit boards or individual components (resistors, diodes relays etc.). Motors, or watch mechanisms or similar parts could be placed as the design dictates. 
     3D Labels may be printed using the FDM process. A flat badge could be printed with a raised perimeter and raised letters or artwork printed within this perimeter using one, two or three different colours or materials. The TMVP can then be used to fill inside this perimeter with a resin having sufficiently viscous fluid allowing it to flow in between and around the artwork or letters. The vented hotbed could be engaged in vacuum mode to aid degassing of the liquid filled FDM printed part, thereby improving the clarity of a clear material by removing air bubbles. For a UV cured clear material, the UV lights of the 3D printer may be turned on thereby curing the resin within and around the FDM printed material. 
     In some cases, forming a vacuum within the printing chamber and/or the mould may not be necessary because the fluid driven through the pressurised hopper and drawn into the pump may naturally reduce the prevalence of air bubbles within the fluid. The more viscous the fluid, the more likely that bubbles would not be fully expelled from the fluid in the hopper and as such the use of a vacuum may be more beneficial with thicker fluids, for example RTV silicon. This, when placed into the hopper, is more likely to contain air bubbles, which may remain trapped within the fluid when the fluid is deposited from the TMVP. In an example in which two different materials are to be deposited by the TMVP (a double mould), the second pump which has been used to vacuum resin through a mould may create bubbles within the hopper of the second pump. The second pump would generally not use a different material without contamination if a single mould with both an inlet and outlet was made since the fluid dispensed from the first hopper may be drawn into the second hopper. In practice, the use of two different materials may be practical only if the inlet of the mould would be engaged and the outlet left open to the enclosure, thereby allowing for the second pump to suck air from the enclosure (rather than the mould) and therefore not contaminate the different fluid in the second hopper. At the same time, to aid fluid entering the mould and fully filling the mould, the vented hotbed would be engaged in vacuum mode. 
     The vented hotbed would enable open top moulds to be degassed. Using one half the TMVP enables fluids to be printed in exact amounts. Using UV cure materials and a UV light mounted in the print head platform (or elsewhere within the printing chamber) means these can be printed and cured in a manner similar to FDM. The UV cure fluid can be printed within a channel with the necessary surface tension to create a concave, flat or convex surface, before being cured. With the use of flexible material, this can act as a sealing surface. Open picture frames created using the FDM print head can be filled with liquid. UV curing can be achieved using a series of UV lights mounted through the printer. This enables the fluid to level before being cured. Certain liquids, such as 2 pack or solvent cure resins would cure naturally (without the need for UV light). Operating the vented hotbed in vacuum mode would aid gasses being drawn from the material. 
     When printing liquid into a mould, the two liquid print heads can be used to engage the mould, such that one of the print heads injects fluid and the other print head vacuums it through the mould, similar to vacuum casting or infusion. The use of laser, cutter and inkpen would allow for engraving art work and vinyl cutting of stickers. The laser can be used to smooth the surfaces and aid bonding of layers by melting between the paths creating a stronger surface. The cutter could cut vinyl placed on the print bed. Tilting of the print bed would enable letters or art to be printed on the tilted surfaces. By combining all of the above processes complete kits of products could be printed. For instance, a supplied kit of parts may be used during the printing process to create electronic or mechanical moving or operating items. These can be combined with stickers or labels cut using the vinyl cutter and or laser/inkpen. The printer process, either created by the user or supplied as part of the kit would be uploaded (to software running on a computer connected to the printer), with pauses in the printing process being define for placement of kit parts, changes of materials, by the user. The kits would then be complete or would require further manual assembly by the user. 
     Items can be placed on the print bed to be printed onto. For example, a phone may be placed and artwork drawn or lasered thereon, or FDM printed letters added. In order to achieve this, first a print outline is printed then the item placed and printed on. In another example, a dice could be manufactured by first printing a cube, which is then taken from the print bed. A perimeter matching the size of the dice is then printed onto the print bed, and the dice is placed back on the printer (within the perimeter) to print the numbers or dots, then rotated repeatedly for the other 5 sides. This enables consistent printing onto each side of the dice.