Abstract:
A head assembly for an extrusion-based 3D printer includes: a fluid-dispensing head having a manifold and at least two fluid-dispensing nozzles, of different sizes, which are mounted in communication with a melt chamber in a manifold. Outlets of each nozzle are closed by respective valve members. A rocker serves both to pivot the nozzles to their lowermost nozzle-operating position and to actuate the valve members, for ready switching between the valves, such that the smaller nozzle can be used for high resolution work, and the larger nozzle can be used for bulk infill.

Description:
TECHNICAL FIELD 
     The present invention generally relates to additive manufacturing systems for building three-dimensional (3D) parts in a layer-by-layer manner, also known as 3D printers. The invention particularly relates to printing heads used in 3D printers for extruding a flowable part material. 
     BACKGROUND OF THE INVENTION 
     In an extrusion-based additive manufacturing system, a 3D part is printed from a digital representation by extruding a flowable part material to build up the part in a layer-by-layer manner. A filament is fed to an extrusion nozzle in the printer head where it is melted, and then ejected onto a substrate in fluid form while the printer head moves in a horizontal plane to trace out a layer. The extruded material fuses to previously deposited material, and solidifies quickly. 
     The printer head on such a tool should be compact, so as to maximise the size of the object that can be produced, and lightweight, so as to enable it to be rapidly moved and accurately positioned without adverse inertia effects. Providing multiple extrusion nozzles on the printer head allows for improved flexibility, such as the ability to readily change extrusion materials or colours, without nozzle removal and replacement. However, prior designs have not been optimal, and there is a need for a printer head with improved flexibility, which is not compromised by excessive weight, size or structural complexity. 
     One of the advantages of extrusion-based additive manufacturing is that the capital and operating costs of the tools are lower than for competing technologies, such as stereolithography. However, it has previously been considered that high resolution must be sacrificed in using this technology in order to obtain reasonable operating speeds. There is therefore an unmet need for an extrusion-based additive manufacturing tool that is able to offer both higher resolution and faster operating speed. It is an object of the present invention to address these needs or, more generally, to provide an improved 3D printer. 
     DISCLOSURE OF THE INVENTION 
     According to one aspect of the present invention there is provided a printer head assembly for a 3D printer comprising: 
     a fluid-dispensing head including a manifold; 
     first and second nozzles mounted to the manifold, each of the first and second nozzles having an outlet closed by a respective valve member; 
     a coupling member coupling the fluid-dispensing head to a support so as to allow movement of the fluid-dispensing head by which each of the first and second nozzles may be moved to and from a respective nozzle-operating position, wherein with one of the first and second nozzles in its nozzle-operating position the outlet of the other of the first and second nozzles extends below the outlet of the one of the first and second nozzles; 
     a rocker mounted to oscillate about a rocker axis, the rocker having at least one cam surface engaging a respective fixture for driving the movement of the fluid-dispensing head, the rocker further including first and second abutments engaging respective ones of the valve members for opening or closing the nozzle outlet of each nozzle in its nozzle-operating position, and 
     an actuator controlling the angular position of the rocker. 
     Preferably the coupling member comprises a pivot and the fluid-dispensing head pivots to move the first and second nozzles to their nozzle-operating positions. Alternatively, for instance, the coupling member may include arcuate or linear rails along which the fluid-dispensing head may be moved, by the rocker, relative to the support so as to place the fluid-dispensing nozzles in their nozzle-operating positions. Preferably the first and second nozzles are of different sizes and are in fluid communication with one another. Alternatively, each nozzle may be fed from a separate supply, so that no communication is provided between the nozzles. 
     Preferably the at least one cam surface comprises first and second cam surfaces engaging respective first and second fixtures, such that engagement of the first cam surface and first fixture pivots the fluid-dispensing head in a first direction and engagement of the second cam surface and second fixture pivots the fluid-dispensing head in a second direction opposite the first direction. 
     Preferably the rocker is mounted to the fluid-dispensing head, and the first and second fixtures are disposed on the support. Preferably first and second cam surfaces are external surfaces of the rocker. Preferably first and second abutments include either surfaces: a) disposed at the end of circumferentially elongated slots in the rocker, or b) on the periphery of lobes on the rocker. 
     Preferably the pivot includes at least one through-extending feed passage in communication with the manifold. Optionally, the pivot includes a plurality of through-extending passages. Preferably the manifold includes a heating element, and the feed passage is adapted to transmit a consumable filament. Preferably the printer head assembly further comprises a mounting member to which the rotary actuator is mounted, and a neck is provided in the pivot adjacent the manifold, providing a thermal bridge separating the manifold from the mounting member. 
     Preferably the valve members comprise needles that are retractable from the nozzle openings. Preferably the first and second abutments engage respective ones of the needles for opening the nozzle outlets, and a respective spring cooperates with each needle for closing the nozzle outlets. 
     Preferably a pivot axis of the pivot lies in a central plane substantially symmetrically bisecting the manifold, and the nozzles are disposed on opposing sides of the central plane. Preferably a rocker axis of the rocker is substantially parallel to the pivot axis and lies in the central plane. 
     Preferably the printer head assembly further comprises a fan mounted to the support for directing a cooling air flow adjacent to the nozzle to increase the speed of solidification. 
     Preferably the fan and manifold are generally disposed on opposite sides of the printer head assembly. 
     Preferably the support is adapted to be reciprocated linearly along a rail defining one of three orthogonal axes of the 3D printer, and most preferably the support supports linear bearings for engaging the rail. Preferably the support is reciprocated linearly along the rail by an endless belt, and the support includes at least one jaw for gripping the endless belt. In another aspect the invention provides a 3D printer having the printer head assembly of any of the preceding claims, wherein one of three orthogonal axes of the 3D printer is defined by a rail, the support includes an aperture, and the rail is received in the aperture such that the printer head is reciprocated linearly along the rail. 
     A fluid-dispensing head according to the invention provides for two nozzles that can be readily switched into operation for improved flexibility, without excessive weight, size or structural complexity. Providing large and small nozzles can readily be swapped, allows the nozzles to be selected as required for maximum speed and resolution, for instance, by using the small nozzle to accurately produce exposed finished surfaces and the large nozzle for bulk infill. By proving a single rocker that controls both nozzle position and nozzle actuation, a simple, reliable, compact and lightweight mechanism is also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: 
         FIG. 1  is an isometric view of a first embodiment of a head assembly according to the invention; 
         FIG. 2  is a schematic illustrating the operation of the printer head assembly of the invention; 
         FIG. 3  is a section through the printer head assembly of  FIG. 1  in an upright central plane through the pivot axis of the fluid-dispensing head; 
         FIG. 4  is section  4 - 4  of  FIG. 3 ; 
         FIG. 5  an isometric view of a second embodiment of a head assembly according to the invention; 
         FIGS. 6 and 7  are section through the printer head assembly of  FIG. 5  in parallel upright planes orthogonal to the pivot axis of the fluid-dispensing head; 
         FIGS. 8 and 9  are side and end views respectively, of the rotor of the printer head assembly of  FIG. 5 ; 
         FIG. 10  is a side view of the printer head assembly of  FIG. 5  that includes a fragmentary section  10 - 10  of  FIG. 11 , and 
         FIG. 11  is a section through the printer head assembly of  FIG. 5  in a horizontal plane through the pivot axis of the fluid-dispensing head. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present disclosure is directed to a print head assembly for use in an extrusion-based 3D printer and in  FIGS. 1 to 4  a first embodiment is shown. Referring to  FIGS. 1 and 2  of the drawings, the print head assembly generally comprises a structural support assembly or support  10  to which a fluid-dispensing head  11  may be coupled by a pivot  12 , allowing the fluid-dispensing head  11  to rotate about a pivot axis  13 , relative to the support  10 . The fluid-dispensing head  11  includes a manifold  26  which may be fixed to the pivot  12  and to which are mounted first and second nozzles  14 ,  15 . 
     The fluid-dispensing head  11  generally pivots between a first position indicated by  11   a  and a second position indicated by  11   b  and shown in dashed outline in  FIG. 2 . In position  11   a  the first nozzle  14  is in its nozzle-operating position in which flowable material may be dispensed, and its outlet  70  extends below the outlet  71  of the second nozzle  15  to ensure that the unused second nozzle is kept clear of the horizontal layer being extruded. The tips of the nozzles  14 ,  15  are planar, and in positions  11   a ,  11   b  the tips of the nozzles  14 ,  15  are horizontal to assist in smoothing the top of the extrudate. By turning the fluid-dispensing head  11  between the positions  11   a ,  11   b  either the first nozzle  14  or second nozzle  15  can be accurately moved to its nozzle-operating position. In this embodiment, the first nozzle  14  may be smaller than the second nozzle  15  and may be used for fine detailing and the second nozzle  15  for bulk filling, thus allowing the 3D printer to select nozzles as required, for instance, based upon an algorithm that determines the optimum printing speed and resolution required to produce a given product. 
     The printer head assembly is shown upright in  FIG. 1 . The support  10  provides the main structural and housing elements of the printer head, and comprises a frame  10   a  and a pair of housing shells  10   b ,  10   c  (shell  10   c  being omitted from  FIG. 1  to reveal the fluid-dispensing head  11 ). An inlet opening  18  is provided at the top of the housing shells  10   b ,  10   c  inside which a fan  19  is mounted, to draw air in through the inlet opening  18 , ejecting it from an opposing outlet opening  20  at the bottom of the housing shells  10   b . The housing shells  10   b  generally enclose the pivoting fluid-dispensing head  11  and may be connected to one another and to the frame  10   a  by fasteners (not shown). For use in a Cartesian system, the printer head is adapted to be reciprocated linearly along a straight rail (not shown) defining one of three orthogonal axes of the 3D printer. For this purpose, a rail-receiving aperture  24  may extend horizontally through the frame  10   a . The frame  10   a  may carry linear bearings (not shown) for engaging the rail. At least one jaw (not shown) may be provided on the frame for gripping an endless belt by which the printer head assembly is reciprocated linearly along the rail. A mounting wheel  23  fixed to the frame  10   a  is located on an rear side of the printer head assembly. 
     The fluid-dispensing head  11  is an assembly that includes the manifold  26 , nozzles  14 ,  15 , a mounting member  34 , a rocker  30  and a rotary actuator  21 . The rotary actuator  21 , is shown in  FIG. 1  separated from the bosses  22  on the mounting member  34  to which it is mounted by fasteners (not shown). The rotary actuator  21  is disposed adjacent a front side of the printer head assembly. 
     With particular reference to  FIGS. 3 and 4 , the manifold  26  to which the nozzles  14  and  15  are fixed, serves to direct fluid to the nozzles  14 ,  15 . The pivot  12 , which may be supported in bearings  27  received in the frame  10   a  and between the housing shells  10   b ,  10   c  and may include an axially extending feed passage  28  adapted to transmit a consumable filament (not shown) into a melt chamber  63  within the manifold  26 , the melt chamber  63  communicating with both nozzles  14 ,  15 . The manifold  26  may also include an embedded electrical heating element  29  for melting the filament. The adjacent nozzle outlets  70 ,  71  of the nozzles  14 ,  15  may each be closed by a respective valve needle  32 ,  33  which controls the dispensing of fluid from the printer head. The mounting member  34  may be fixed to the pivot  12 , which thereby fixes it to the manifold  26  so that the mounting member  34  oscillates with the manifold  26  relative to the support  10 . The mounting member  34  extends generally above the pivot  12  and serves to mount the rotary actuator  21 , and the rocker  30  connected thereto and may also support the valve members. 
     The rocker  30  serves to pivot the fluid-dispensing head  11  and is supported upon a shaft  38  that is oscillated by the rotary actuator  21  about a rocker axis  39  which may be parallel to the axis  13  of the pivot  12 . The rocker  30  has first and second cam surfaces  40 ,  41  that engage respective fixtures  35 ,  36  which may be integral with the housing shells  10   b  and  10   c  respectively. Engagement of the first cam surface  40  and first fixture  35  displaces the rocker  30  relative to the support  10  and drives the pivoting movement of the fluid-dispensing head  11  in a first direction and engagement of the second cam surface  41  and second fixture  36  pivots the fluid-dispensing head  11  in a second direction opposite the first direction direction. In this manner, actuation of the rotary actuator  21  may turn each nozzle  14 ,  15  to its nozzle-operating position. The first and second cam surfaces  40 ,  41  may be symmetrically arranged on opposite, external surfaces of the rocker  30 . First and second abutments  42 ,  43  may be formed as internal abutment surfaces disposed at the end of circumferentially elongated slots  44 ,  45  in the rocker  30  and engage respective ones of the valve needles  32 ,  33  for opening or closing the nozzle outlets  70 ,  71 . 
     As best seen in  FIG. 3 , an air gap  46  may separate the mounting member  34  from the manifold  26  and a neck  47  may be formed in the pivot  12  where it passes through the air gap  46 , providing a thermal bridge separating the manifold  26  from the mounting member  34 . 
     Referring to  FIG. 4 , each valve needle  32 ,  33  may be fixed to a respective block  48 , such that a respective spring  49  held between the blocks  45  and a flange  50  on the mounting member  34  serves to bias the valve needles toward the nozzle outlets  70 ,  71  to their closed positions. The slender form of the valve needles  32 ,  33  and the spacing between the blocks  48  and flange  50  minimise the path for conducting heat upwardly from the manifold  26 . The upper ends of the valve needles  32 ,  33  pass through a slot  52  in the lower side of the rocker  30  and cylindrical stoppers  53 ,  54  are fixed proximate their ends, the stoppers  53 ,  54  being sized to slide freely within the slots  44 ,  45  in the rocker  30 . The lower ends of the valve needles  32 ,  33  are supported in the nozzles  14 ,  15  themselves, the nozzles  14 ,  15  in turn being located in recesses in the manifold  26 . A shroud  56  may generally surround the tip of both nozzles  14 ,  15 . This shroud  56  may be formed of polytetrafluoroethylene, or another non-stick material. 
     The fan  19  is mounted is to the housing shells  10   b  of the support  10  generally on the opposite side of the head to the manifold  26 . The openings  18 ,  20  allow a stream of air to pass down from the fan  19  and serves for directing a cooling air flow adjacent to the nozzles  14 ,  15  to increase the speed of solidification of the extrudate, and for moving cold air over the mounting member  34 , in order to keep the cold side of the fluid-dispensing head  11  cold and increase the thermal gradient across the heat bridge between the mounting member  34  and the manifold  26 . The housing shells  10   b  thereby assists in focusing this air stream  18  to flow adjacent to the mounting member  34 , manifold  26  and nozzles  14 ,  15 . 
       FIGS. 5-11  illustrate a second embodiment of the print head assembly, which is of generally like construction to the first embodiment, and like numbers are used to refer to like components where appropriate. As shown in  FIGS. 5 and 6 , as in the first embodiment, a rocker  130  of this second embodiment is used both to pivot the fluid-dispensing head  111  between the between the angular positions  11   a  and  11   b  and also to open and close the valves by displacing the valve needles  32 ,  33 . Two lever arms  60 ,  61  may be provided, each mounted to the mounting member  134  to pivot about respective axes parallel to the axis  39  of the rocker  130 , as on the cylindrical bosses  22  which also engage fasteners to secure the rotary actuator  21 . With their outer ends pivoting on the bosses  22  the inner ends of the arms  60 ,  61  are positioned adjacent one another and overlapping, but spaced apart in the direction of axis  39  so as they can freely pivot independently of one another. The cylindrical stoppers  53 ,  54  fixed on the ends of the valve needles  32 ,  33  are received in complementary openings in the inner ends of the arms  60 ,  61 . In this manner, pivoting of the arms  60 ,  61  displaces the valve needles  32 ,  33 , opening and closing the valves to control the dispensing of extrudate during printing. The lever action of these arms  60 ,  61  multiplies the force produced by the rocker for displacing the valve needles  32 ,  33  compared to the first embodiment in which the abutments  42 ,  43  of the rocker  30  directly engage the cylindrical stoppers  53 ,  54 . The abutments  142 ,  143  of the rocker  130 , on the other hand, indirectly engage the cylindrical stoppers  53 ,  54  via the arms  60 ,  61 . 
     As best seen in  FIGS. 8 and 9 , the abutments  142 ,  143  of the rocker  130  are lobes integrally formed on the rocker  130  with convex surfaces that abut and slide along mating faces of the arms  60 ,  61 . The abutments  142 ,  143  are axially spaced apart from one another, and from the cam surfaces  40 ,  41  which are aligned transversely to axis  39 . The abutment  142  is innermost and abuts the inner end of the arm  60  and the abutment  143  is outermost and abuts the inner end of the arm  61 . 
       FIGS. 10 and 11  illustrate the construction of the pivot  112  of the second embodiment which supports the fluid-dispensing head  111  in bearings  127  engaging a shaft portion of the pivot  112 . The bearings  127  may be fixed between the housing shells  10   b  of the support  10  and allow the pivot  112  to turn about axis  13 . The pivot  112  includes two passages  128 ,  228  along which two consumable filaments (not shown) may be fed to the melt chamber  63  of the manifold  126 . The passages  128 ,  228  may be arcuate and generally symmetrically disposed either side of an plane bisecting the pivot  112 . A neck  47  passing through the air gap  46  may be formed in the pivot  112  by a thin wall section of each of the passages  128 ,  228  providing a thermal bridge separating the manifold  126  from the mounting member  134 . 
     Adjustable stops limiting the angular positions  11   a ,  11   b  are provided by two dog point set screws  65 , one on each of the housing shells  10   b ,  10   c  which engage with magnets  66  fixed at the top of mounting member  134 . 
     In operation, the fluid-dispensing head  11 ,  111  can be pivoted between the angular positions  11   a  and  11   b , via the intermediate position  11   c  illustrated in  FIGS. 4, 6 and 7 , which is equally angularly spaced between positions  11   a  and  11   b . The fluid-dispensing head  11 ,  111  is rotated from position  11   c  anticlockwise to the position  11   a , where the first nozzle  14  is in its nozzle-operating position, by rotation of the rocker  30 ,  130  (in a clockwise direction when viewed from the front as in  FIGS. 4, 6 and 7 ). This rotation of the rocker  30 ,  130  engages the cam surface  41  with the fixture  36  which is thereby deflected, pivoting the head  11 ,  111  and attached nozzles  14 ,  15  about axis  13 . 
     During this turning movement, in the first embodiment ( FIG. 4 ) the stopper  53  slides in the arcuate slot  44  until it abuts the first abutment  42 . Continued rotation beyond this point then requires the stopper  53  to rotate with the rocker  30 , thereby retracting the needle  32  and thus opening the outlet  130  of nozzle  14 . Correspondingly, in the second embodiment ( FIG. 7 ) a circular face of the rocker  130  abuts the arm  60  until it abuts the leading edge of the abutment  142 . Continued rotation beyond this point then starts to pivot the lever  60 , thereby retracting the needle  32  and thus opening the outlet  130  of nozzle  14 . The 3D printer can then, by translating the printer head assembly, lay out one or more layers of extrudate. 
     In the case of the first embodiment where the first nozzle  14  is smaller than the second nozzle  15  the first nozzle  14  may be used first to accurately establish the surfaces of the object being printed. Once the high resolution surface finishing has been completed using the small nozzle  14 , bulk filling behind the perimeter layer is then completed using the large nozzle  15 . For instance, three 0.1 mm perimeter layers may be completed using the small nozzle  14  before a single 0.3 mm fill layer is laid down. This change-over is completed by reversing the rotation of the actuator output and the attached rocker  30 , rotating the rocker  30  anticlockwise, which reverses the previously described actions, first closing the nozzle outlet then, by cooperation between the cam surface  40  and fixture  35 , pivoting the fluid-dispensing head  11  clockwise about pivot  12  through the position  11   c , to the position  11   b.  Likewise, contact between the stopper  54  and the second abutment  43  serves to raise the needle  33  as the rocker  30  continues to pivot, opening the outlet  70  of the smaller nozzle  14 . 
     Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.