Patent Publication Number: US-2023150196-A1

Title: Liquefier for an extrusion-based additive manufacturing system and method for its manufacturing

Description:
This invention relates generally to a liquefier for use in an extrusion-based additive manufacturing (EAM) system. More specifically, although not exclusively, this invention relates to a liquefier tube, a liquefier assembly, an extrusion head and a method of manufacturing a liquefier tube for use in an EAM system . 
     Extrusion-based additive manufacturing systems, e.g. Fusion Filament Fabrication (FFF) systems, are used to build a physical object from a digital representation in a layer-by-layer manner by extruding, through a nozzle, a melted feedstock material. 
     The feedstock normally takes the form of a continuous filament of thermoplastic material. Commonly, the filament is fed from a spool and advanced through a movable extrusion head containing a liquefier assembly. The liquefier assembly is configured to melt the filament such that it can be extruded through a nozzle and deposited on a print bed. 
     The liquefier assembly includes a liquefier section, which can be in the form of a tube, along which the filament is advanced, and a heater or heater cartridge external of the liquefier tube. The heater heats the liquefier section and, by virtue of thermal transfer, the filament. The temperature of the filament is raised above its melting point, causing it to liquify and allowing the material to be extruded from the nozzle. Extrusion pressure is applied by virtue of the continual advancement of the filament upstream of the nozzle. 
     Traditionally, both the filament and the channel or passage within or extending along the liquefier section are of substantially circular cross-section. It has been observed that because of the low thermal conductivity of thermoplastic material, heat from the heater is not efficiently and quickly transferred to the centre of a circular filament. This, in turn, can have a negative impact on both the rate and quality of extrusion. 
     It is therefore a first non-exclusive object of the invention to provide a liquefier that overcomes, or at least mitigates the drawbacks of the prior art. 
     It is a further non-exclusive object of the invention to provide an improved liquefier, liquefier tube, liquefier assembly, extrusion head and method manufacturing a liquefier. 
     Accordingly, a first aspect of the invention provides a method of manufacturing a liquefier or liquefier tube, e.g. for use in an extrusion-based additive manufacturing system, the method comprising providing a tubular body having a substantially circular cross-section, deforming or crushing the tubular body at a location spaced from one of its ends, so as to provide an inlet portion having a substantially circular cross-section for receiving a filament of material, and a non-circular portion having a non-circular cross-section downstream of the inlet. 
     The method may comprise providing a tubular body having a substantially constant wall thickness. 
     The method may comprise providing, e.g. forming, attaching or connecting, an extrusion tip at the other end of the tubular body, e.g. opposite the inlet portion, to form an outlet for dispensing material in a molten state. Providing an extrusion tip may comprise forming, attaching or connecting the extrusion tip to the other end of the tubular body, e.g. opposite the inlet portion. The extrusion tip may form or describe an outlet, e.g. for dispensing material in a molten state. 
     The method may comprise deforming or crushing the tubular body at a location spaced from one of its ends, such that the tubular body transitions from the substantially circular inlet portion to the non-circular portion downstream of the inlet. 
     The method may comprise deforming the tubular body by hydroforming. Deforming the tubular body by hydroforming may comprise placing the tubular body, or a portion thereof, in a forming tool or between a pair of dies. Deforming the tubular body by hydroforming may comprise injecting fluid under pressure into a passageway described by the tubular body so as to deform the tubular body, or a portion thereof, e.g. such that it conforms to a profile described by the forming tool or pair of dies. 
     According to another aspect of the invention there is provided a method of manufacturing a liquefier or liquefier tube, e.g. for use in an extrusion-based additive manufacturing system, the method comprising deforming a first sheet of material to form a first part of a liquefier or liquefier tube, deforming a second sheet of material to form a second part of a liquefier or liquefier tube, and attaching the first part to the second part to form a tubular body of the liquefier or liquefier tube. 
     The method may comprise deforming the first sheet and/or the second sheet into a channel section. The method may comprise deforming the first sheet and/or second sheet using a press, forming machine or metal stamping machine. The method may comprise deforming the first sheet and/or the second sheet by hydroforming. The method may comprise deforming the first sheet and/or the second sheet by stamping or forming. 
     The first sheet and/or the second sheet may be or comprise metal, for example stainless steel. The first sheet and/or the second sheet may be or comprise brass, copper, tungsten, titanium, molybdenum, beryllium copper or any other suitable metal or alloy. 
     The first sheet and/or the second sheet may comprise a polymeric material. The method may comprise heating the first and/or second sheet prior to deforming, e.g. above its glass transition temperature and/or so as to increase ductility and/or malleability. 
     The first part and/or second part of the liquefier or liquefier tube may each comprise one half of the liquefier, liquefier tube or tubular body of the liquefier tube. 
     The first part may be attached to the second part by welding or brazing. 
     According to another aspect of the invention there is provided a method of manufacturing a liquefier or liquefier tube, e.g. for use in an extrusion-based additive manufacturing system, the method comprising providing a first block of material, machining a first part of a liquefier or liquefier tube into a surface of the first block, providing a second block of material, machining a second part of a liquefier or liquefier tube into a surface of the second block and bringing the first block and second block together to describe a passageway of the liquefier or liquefier tube. 
     The first block and/or the second block may comprise a metal, e.g. stainless steel. The first block and/or the second block may be formed of or comprise brass, copper, tungsten, titanium, molybdenum, beryllium copper or any other suitable metal or alloy. 
     The first block and second block may be connectable, e.g. removably connectable, to one another so as to describe the liquefier, liquefier tube or a passageway thereof. 
     According to an aspect of the invention, there is provided a liquefier, e.g. for use in an extrusion-based additive manufacturing system, the liquefier describing a passageway having an inlet portion with a substantially circular cross-section, e.g. for receipt of a filament of material and an outlet downstream of the inlet, wherein the passageway transitions from the inlet portion to a non-circular portion downstream of the inlet, the non-circular portion having a non-circular cross-section. 
     The liquefier may comprise a tube, or a hollow or tubular body, which may have a substantially circular cross-section. The outlet may comprise an extrusion tip, e.g. for dispensing material in a molten state. 
     According to another aspect of the invention, there is provided a liquefier tube, e.g. for use in an extrusion-based additive manufacturing system, the liquefier tube comprising tubular body, an inlet portion having a substantially circular cross-section, e.g. for receipt of a filament of material, an outlet downstream of the inlet, with an extrusion tip for dispensing material in a molten state, wherein the tubular body transitions from the substantially circular inlet portion to a non-circular portion downstream of the inlet, the non-circular portion having a non-circular cross-section. 
     The liquefier or body may have a substantially constant wall thickness. 
     Advantageously, providing a tubular body having a substantially constant wall thickness allows for easier, and cheaper, manufacture of the liquefier tube. 
     The extrusion tip may be formed of or comprise a nozzle. The method may comprise providing, for example forming, attaching or connecting, a nozzle, e.g. at the other end of the body. 
     The extrusion tip or nozzle may be connected, e.g. removably, to the liquefier or body. The extrusion tip or nozzle may be brazed or welded to the liquefier or body. 
     The method may comprise forming, attaching or connecting the extrusion tip or nozzle, e.g. removably, to the liquefier, liquefier tube or body. The method may comprise brazing or welding the extrusion tip or nozzle to the liquefier, liquefier tube or body, e.g. the other end of the body. 
     The non-circular portion of the liquefier or body may comprise a deformed or crushed portion. 
     In examples, deforming or crushing the liquefier or body or providing the non-circular portion comprises deforming or crushing the liquefier or body in a first region, e.g. to provide a first segment or section, and deforming or crushing the liquefier or body in a second region, e.g. to provide a second segment or section. The non-circular portion may comprise the first segment or section and/or second segment or section. 
     The liquefier or body may be deformed, e.g. by crushing, a first region to provide a first segment of the non-circular portion. The liquefier or body may be deformed, e.g. by crushing, a second region, downstream of the first region, to provide a second segment of the non-circular portion downstream of the first segment. The first segment and/or second segment may be non-circular. In examples, deforming or crushing the liquefier or body comprises compressing the liquefier or body. Deforming or crushing the liquefier or body may comprise compressing the liquefier or body in a first region, e.g. to provide a first segment or section, and compressing the liquefier or body in a second region, e.g. to provide a second segment or section. 
     In embodiments, the method may comprise compressing, deforming or crushing the liquefier or body to a first extent in a first region, and/or compressing, deforming or crushing the liquefier or body to a second extent, e.g. greater than the first extent, in a second region, e.g. so as to provide a tapered profile. 
     The second region or second segment or section may be downstream of the first region or first segment or section. 
     The non-circular portion of the liquefier or body may extend along a portion of the length, e.g. along a longitudinal axis or principal axis, of the liquefier, liquefier tube or body. 
     The non-circular portion of the liquefier or body may comprise a tapered cross-sectional profile. The flow area of the passageway of the non-circular portion of the liquefier or body may comprise a tapered profile. The non-circular portion may taper along a portion of the length, e.g. along a longitudinal axis or principal axis, of the liquefier or body. The non-circular portion may have a major dimension and a minor dimension. The minor dimension may taper, e.g. along a portion of the length of the non-circular portion. The minor dimension may decrease, e.g. along a portion of the length of the non-circular portion. 
     Providing a taper, as per embodiments of the invention, is advantageous in that as the filament is advanced along the liquefier or liquefier tube, the molten portion (located adjacent the wall of the tubular body) is moved toward the centre of the flow path or passageway. Further, the distance the heat has to travel to reach the centre of the passageway or flow path is decreased. 
     The non-circular portion of the liquefier or body may comprise a plurality of segments or sections. Each of the plurality of segments or sections may comprise a deformed or crushed segment of the liquefier or body. Each of the plurality of segments or sections may be arranged, e.g. in series, along the length of, or principal axis of, the liquefier, liquefier tube or body. Each of the plurality of segments or sections may be arranged along a longitudinal axis of the liquefier, liquefier tube or body. 
     Each of the plurality of segments or sections, hereinafter segments, may have a different cross-sectional shape, configuration or cross-sectional profile, e.g. from one another. One, e.g. a first, of the plurality of segments may have a different cross-sectional shape, configuration or cross-sectional profile from another, e.g. a second, of the plurality of segments. The flow area of the passageway of or in the plurality of segments may have a different cross-sectional shape, configuration or cross-sectional profile, e.g. from one another. The flow area of the passageway of or in one, e.g. a first, of the plurality of segments may have a different cross-sectional shape, configuration or cross-sectional profile from the passageway of or in another, e.g. a second, of the plurality of segments. 
     A minor dimension of a first of the segments may be greater than a minor dimension, e.g. a corresponding minor dimension, of a second of the segments. A major dimension of the a first of the segments may be less than a major dimension, e.g. a corresponding minor dimension, of the a second of the segments. The second segment may be downstream of the first segment. 
     The first segment may be deformed so as to have a minor dimension which is greater than a minor dimension of the deformed second segment. 
     The method may comprise compressing the liquefier or body to a first extent or magnitude in a first region to provide the first segment, The method may comprise compressing the liquefier or body to a second extent or magnitude in a second region to provide the second segment. The second extent or magnitude may be different from, e.g. greater or less than, the first extent or magnitude. 
     In some examples, the first segment and the second segment, e.g. their major and/or minor dimensions, are rotationally offset or skewed from one another. The first segment and the second segment may be skewed or twisted from, or relative to, one another. The first segment and second segment may comprise a skew angle described therebetween. The first segment may comprise a portion of the liquefier or body deformed or crushed in a direction that is skewed or rotationally offset relative to a direction in which the second segment is deformed or crushed. 
     Corresponding minor and/or major dimensions of the first segment and second segment may be rotationally offset or skewed from one another. 
     The liquefier or body may be deformed such that a minor and/or major dimension of the first segment and a corresponding minor and/or major dimension of the second segment are rotationally offset from one another. 
     Providing rotational offset, or skew, helps improve the thermal transmission characteristics of the liquefier, liquefier tube or body by creating a tortuous flow path for the molten material. Such a flow path promotes mixing of the melted or molten material as it is advanced along the liquefier, liquefier tube or body, allowing for more even thermal distribution through the material. 
     The method may comprise compressing, deforming or crushing the liquefier or body in the first region in a first direction and compressing, deforming or crushing the liquefier or body in the second region in a second direction, the second direction being skewed or rotationally offset relative to the first direction. 
     The cross-section, e.g. the cross-sectional shape, of first segment (or the passageway thereof or therein) and/or second segment (or the passageway thereof or therein) may comprise an axis of symmetry. An axis of symmetry of the first segment (or the passageway thereof or therein), e.g. the cross-sectional shape thereof, and a corresponding axis of symmetry of the second segment (or the passageway thereof or therein), e.g. the cross-sectional shape thereof, may be rotationally offset or skewed from one another. 
     In embodiments, the liquefier or body comprises a substantially circular portion intermediate two or more segments. The liquefier or body may comprise a substantially circular portion intermediate the first segment and the second segment. 
     The liquefier or body may be deformed so as to comprise a substantially circular portion intermediate the first segment and second segment. 
     The non-circular cross-section of the non-circular portion (or the passageway thereof or therein), or the liquefier or body may be deformed such that, one or more segments thereof, may be substantially stadium shaped, disco-rectangular or obround. 
     The non-circular cross-section of the non-circular portion, or one or more segments thereof, (or the passageway thereof or therein) may be oval, elliptical, regular polygonal, irregular polygonal, simple convex polygonal or simple concave polygonal. 
     The non-circular cross-section of the non-circular portion, or one or more segments thereof, (or the passageway thereof or therein) may be star-shaped. 
     The transition or change from the substantially circular portion to the non-circular portion, or one or more segments thereof, may comprise a continual transition, gradual transition and/or tapered transition. 
     The transition or change from the substantially circular portion to the non-circular portion, or one or more segments thereof, may comprise a staged, discretised and/or stepped transition. 
     The liquefier or liquefier tube may comprise a substantially circular portion downstream of the non-circular portion, or a segment thereof. 
     The first segment (or the passageway thereof or therein) may have a different cross-sectional shape, configuration or cross-sectional profile from the second segment (or the passageway thereof or therein). 
     In examples, the non-circular portion may comprise a first non-circular portion and a second non-circular portion. The first non-circular portion and/or second non-circular portion may comprise a plurality of segments or sections. Each of the plurality of segments or sections may comprise a deformed or crushed segment of the liquefier or body. 
     Each of the plurality of segments or sections may be arranged, e.g. in series, along the length of, or principal axis of, the liquefier, liquefier tube or body and/or first and/or second non-circular portion. Each of the plurality of segments or sections may be arranged along a longitudinal axis of the liquefier, liquefier tube or body. 
     The first non-circular portion and/or second non-circular portion, e.g. their major and/or minor dimensions, may be rotationally offset, twisted or skewed from one another. The respective segments or sections of the first non-circular portion and/or second non-circular portion, e.g. their major and/or minor dimensions, may be rotationally offset or skewed from one another. 
     The first non-circular portion and second non-circular portion may comprise a skew angle described therebetween. The skew angle may comprise 90°, 60°, 45°, 30° or another angle. The first non-circular portion or plurality of segments or sections thereof may comprise a portion of the body deformed or crushed in a direction that is skewed or rotationally offset relative to a direction in which the second non-circular portion or plurality of segments or sections thereof is deformed or crushed. 
     Corresponding minor and/or major dimensions of the first non-circular portion and second non-circular portion may be rotationally offset or skewed from one another. The liquefier or body may be deformed such that a minor and/or major dimension of the first non-circular portion and a corresponding minor and/or major dimension of the second non-circular portion are rotationally offset from one another. 
     The first non-circular portion and second non-circular portion may comprise a transition region or zone therebetween. The transition region or zone may extend along a portion of the length of the liquefier, liquefier tube or body. The transition region or zone may comprise or accommodate the skew of the first non-circular portion and second non-circular portion, or one or more segments thereof, relative to one another. 
     The method may comprise deforming, crushing or compressing the liquefier or body in the first region to form a first segment (or the passageway thereof or therein) having a first cross-sectional shape. The method may comprise deforming, crushing or compressing the liquefier or body in the second region to form a second segment (or the passageway thereof or therein) having a second cross-sectional shape, e.g. different from the first cross-sectional shape. 
     Deforming, crushing or compressing the liquefier or body may comprise placing the liquefier or body in a press. Deforming, crushing or compressing the liquefier or body may comprise placing the liquefier or body in a forming machine, e.g. between a punch and die or between a pair of dies. The liquefier or body may be deformed, crushed or compressed by rollers, e.g. a pair of opposed rollers, for example so as to form a continuous transition between the inlet portion and the non-circular portion. The press may comprise one or more, e.g. opposed rollers. Deforming, crushing or compressing the liquefier or body may comprise deforming, crushing or compressing the liquefier or body in segments, e.g. discrete segments. 
     The liquefier, body or inlet portion thereof may have an internal diameter, which may be between 1 and 4 mm, for example between 1.5 and 3.5 mm, e.g. between 1.75 and 3.25 mm. The liquefier or body may have a diameter of 2.0 or 3.0 mm. 
     The liquefier or body may comprise a thin-walled tube. The liquefier or body or thin-walled tube may comprise a, or be formed of, a thermally conductive material. 
     The liquefier or body may have a wall thickness of less than 1 mm, preferably less than 0.75 mm, for example between 0.1 and 0.5 mm. 
     The liquefier tube or body may have a length of 100 mm or less, for example 90 mm or less or 80 mm or less, e.g. 70 mm or less. The liquefier or body may have a length of between 10 and 100 mm, for example between 10 and 70 mm, e.g. between 15 and 60 mm. 
     The liquefier, liquefier tube or body may comprise a heating zone. The heating zone may be configured to be heated, in use. The heating zone and non-circular portion may coincide or overlap. The heating zone may encompass or span the non-circular portion. 
     The heating zone may be a portion or length of the liquefier or body at or to which heat is applied, e.g. so as to melt a filament material being advanced therealong. The heating zone may have a length of less than 100 mm, for example less than 90 mm, less than 80 mm, less than 70 mm or less than 60 mm. The heating zone may have a length of 56 mm, 26 mm or 16 mm. 
     The extrusion tip or nozzle may have an internal diameter which is smaller than that of the liquefier or body. The extrusion tip or nozzle may have a diameter of 3 mm or less, for example 2.5 mm or less. The extrusion tip or nozzle may have a diameter of between 0.05 and 2.5 mm, e.g. between 0.1 and 1.5 mm or between 0.1 and 1.4 mm. 
     The liquefier or liquefier tube may comprise a substantially circular portion at or towards the outlet. The extrusion tip or nozzle (or the passageway thereof or therein) may comprise a substantially circular portion or substantially circular cross-section. 
     The non-circular portion, or one or more segments thereof, may be intermediate the inlet and outlet. The non-circular portion or a segment thereof may splay or flare towards the outlet. 
     In embodiments, the substantially circular portion (or the passageway thereof or therein), e.g. inlet portion, and the non-circular portion, or one or more segments thereof, may have substantially equal cross-sectional areas. The non-circular portion, or one or more segments thereof, (or the passageway thereof or therein) may have a substantially constant cross-sectional area. The passageway of or in the non-circular portion, or one or more segments thereof, may have a substantially constant flow area. 
     The liquefier or body may be deformed such that the substantially circular portion (or the passageway thereof or therein) and the non-circular portion (or the passageway thereof or therein) have substantially equal cross-sectional areas. The liquefier or body may be deformed such that the passageway of or in the substantially circular portion and the non-circular portion have substantially equal flow areas. 
     The liquefier or body and/or passageway may have or comprise a constant or substantially constant hydraulic diameter. The non-circular portion, first non-circular portion and/or second non-circular portion may have or comprise a constant or substantially constant hydraulic diameter. 
     The hydraulic diameter is defined by the following equation: 
     
       
         
           
             
               D 
               H 
             
             = 
             
               
                 4 
                 A 
               
               P 
             
           
         
       
     
     Where D H  is the hydraulic diameter, A is the flow area and P is the perimeter described by the tubular body. 
     The provision of a liquefier or liquefier tube having a constant or substantially constant hydraulic diameter provides, in use, a substantially constant pressure as a filament material is being advanced therealong. 
     Therefore, in the case of a liquefier or liquefier tube, the change in cross-sectional shape in the non-circular portion, first non-circular portion and/or second non-circular portion allows for more effective heat transfer to a filament material as is being advanced, but whilst without increasing the required applied pressure to generate a given flow rate. 
     The liquefier or body may be made or formed of or comprise metal. The liquefier or body may be made of or comprise stainless steel. In embodiments, the liquefier or body may be made of or comprise brass, copper, tungsten, titanium, molybdenum, beryllium copper or any other suitable metal or alloy. 
     The liquefier or body may be made of or comprise a polymeric material, e.g. a thermally conductive polymeric material. Such polymeric material preferably has a melting point and/or glass transition temperature substantially higher than the filament material to melted. 
     The liquefier, liquefier tube or body may comprise a heating zone. The heating zone may be configured to be heated, in use. The heating zone and non-circular portion may coincide or overlap. The heating zone may encompass or span the non-circular portion. 
     According to another aspect of the invention, there is provided a liquefier assembly for use in an extrusion-based additive manufacturing system, the liquefier assembly comprising a liquefier or liquefier tube as described above, and a heating means, e.g. a heater, heating element, heater element or heater cartridge, for heating a filament of material advanced, in use, into and/or along the liquefier or liquefier tube. 
     The heating means may comprise a heating block or heater block. The heating block or heater block may comprise a heating element or heating cartridge. The heating means may at least partially surround the liquefier or liquefier tube and/or may surround or lie adjacent a heating zone of the liquefier or liquefier tube. The heating element or heating cartridge may extend along the length of the heating block or heater block. 
     The heating means, e.g. the heater, heating block or heater block, may clamp around the liquefier or liquefier tube. The heating means, heating block or heater block may comprise a temperature sensor. The heating cartridge or heating element and/or temperature sensor may each be operatively connected with a controller, e.g. so as to provide closed-loop temperature control. 
     According to a further aspect of the invention, there is provided an extrusion head or extruder comprising a liquefier or liquefier tube as described above, or a liquefier assembly as described above. 
     The extrusion head or extruder may comprise a feeder or feeding mechanism. The feeder or feeding mechanism may be located upstream of the liquefier, liquefier tube or liquefier assembly. The feeder or feeding mechanism may be configured to advance a filament of material, in use, into and/or along the liquefier or liquefier tube. 
     The feeder or feeding mechanism may comprise one or more rotatable members or gears, which may be configured to contact, in use, a surface of a filament being fed or advanced into the liquefier, liquefier tube or liquefier assembly. 
     The feeder or feeding mechanism may comprise a pair, e.g. of opposed, rotatable members or gears, which may be configured to contact, in use, a surface of a filament being fed or advanced into the liquefier, liquefier tube or liquefier assembly. 
     The feeder or feeding mechanism or one or more rotatable members may comprise a feed roller. The feeder or feeding mechanism or one or more rotatable members may comprise a pair of feed rollers. 
     At least one or each of the feed rollers may be driven, e.g. by a stepper motor. 
     One or more of the rotatable members may comprise one or more surface features, which may be configured to engage with or grip, in use, a surface of a filament of material received within or being fed or advanced into the liquefier, liquefier tube or liquefier assembly. 
     The surface feature(s) may comprise one or more topographical feature(s). The surface feature(s) may comprise one or more groove(s), protrusion(s) or rib(s). 
     According to another aspect of the invention, there is provided an additive manufacturing system comprising a liquefier or liquefier tube as described above, a liquefier assembly as described above or an extrusion head as described above. 
     The additive manufacturing system may be a fused filament fabrication (FFF) system. 
     For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. For example, the liquefier or liquefier tube may comprise any one or more features of the liquefier assembly, extrusion head or additive manufacturing system relevant to the liquefier or liquefier tube or vice-versa. Similarly, the method may comprise any one or more features or steps relevant to one or more features of the liquefier, liquefier tube, liquefier assembly, extrusion head or the additive manufacturing system. 
     Another aspect of the invention provides a computer program element comprising and/or describing and/or defining a three-dimensional design for use with a simulation means or a three-dimensional additive or subtractive manufacturing means or device, e.g. a three-dimensional printer or CNC machine, the three-dimensional design comprising an embodiment of the liquefier, liquefier tube, liquefier assembly, extrusion head and/or additive manufacturing system described above. 
     A further aspect of the invention provides a computer program element comprising computer readable program code means for causing a processor to execute a procedure to implement one or more steps of the aforementioned method. 
     A yet further aspect of the invention provides the computer program element embodied on a computer readable medium. 
     A yet further aspect of the invention provides a computer readable medium having a program stored thereon, where the program is arranged to make a computer execute a procedure to implement one or more steps of the aforementioned method. 
     A yet further aspect of the invention provides a control means or control system or controller comprising the aforementioned computer program element or computer readable medium. 
     For purposes of this disclosure, and notwithstanding the above, it is to be understood that any controller(s), control units and/or control modules described herein may each comprise a control unit or computational device having one or more electronic processors. The controller may comprise a single control unit or electronic controller or alternatively different functions of the control of the system or apparatus may be embodied in, or hosted in, different control units or controllers or control modules. As used herein, the terms “control unit” and “controller” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) or control module(s) to implement the control techniques described herein (including the method(s) described herein). The set of instructions may be embedded in one or more electronic processors, or alternatively, may be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement. In any event, the set of instructions described herein may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. For the avoidance of doubt, the terms “may”, “and/or”, “e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG.  1    is a schematic of an additive manufacturing system incorporating aspects of the invention; 
         FIG.  2    is a perspective view of a liquefier tube according to a first example; 
         FIG.  3    is a side view of the liquefier tube of  FIG.  2   ; 
         FIG.  4    is a sectional view of the liquefier tube of  FIGS.  2  and  3   ; 
         FIG.  5    is an end view of the liquefier tube of  FIGS.  2  to  4   . 
         FIG.  6    is a perspective view of a liquefier tube according to another example; 
         FIG.  7    is a side view of the liquefier tube of  FIG.  6   ; 
         FIG.  8    is a top view of the liquefier tube of  FIGS.  6  and  7   ; 
         FIG.  9    is an end view of the liquefier tube of  FIGS.  6  to  8   ; 
         FIG.  10    is a perspective view of a split-block liquefier; 
         FIG.  11    is a perspective view of one half of the split-block liquefier of  FIG.  10   ; 
         FIG.  12    is a cross-sectional view through the split block liquefier of  FIG.  10   ; 
         FIG.  13    is a perspective view of a liquefier tube according to another example; 
         FIG.  14    is a side view of the liquefier tube of  FIG.  13   ; 
         FIG.  15    is a sectional perspective view of the liquefier tube of  FIGS.  13  and  14   ; 
         FIG.  16    is an end view of the liquefier tube of  FIGS.  13  to  15   ; 
         FIG.  17    is a perspective view of a liquefier tube according to another example; 
         FIG.  18    is a side view of the liquefier tube of  FIG.  17   ; 
         FIG.  19    is a top view of the liquefier tube of  FIGS.  17  and  18   ; 
         FIG.  20    is a sectional perspective view of the liquefier tube of  FIGS.  17  to  19   ; and 
         FIG.  21    is an end view of the liquefier tube of  FIGS.  17  to  20   . 
     
    
    
     Referring now to  FIG.  1   , there is shown a schematic of a extrusion-based additive manufacturing system  1  including a print bed  2 , a gantry  3  located above the print bed  2  and an extrusion head  4  carried by, and movable along, the gantry  3 . The gantry  3  is in the form of a guide rail system configured to allow movement of the extrusion head  4  in a horizontal plane within a boundary described by the print bed  2 . The gantry  3  is supported above the print bed  2  by a structural frame F. 
     The extrusion head  4  includes a liquefier assembly having a liquefier tube  5  in this example (shown in  FIGS.  2  to  5   , in particular), a heating means H in the form of a heating element in this example, for heating a filament of material received within the liquefier tube  5 , and a feeding mechanism  6  for advancing a filament of material along the liquefier tube  5 . 
     Referring now to  FIGS.  2  to  5   , there is shown a liquefier tube  5  for use in the manufacturing system  1  of  FIG.  1   . 
     The liquefier tube  5  includes a tubular body  50  providing a passageway or flow-path (hereinafter passageway) P extending therealong and having sidewall of substantially constant wall thickness t (as shown in  FIG.  5   , in particular) that extends along a longitudinal axis L. The tubular body  50  is formed of metal, in particular stainless steel, in this example. 
     A first, upstream end of the liquefier tube  5  is provided with an inlet portion  51  of substantially circular cross-section. A second, downstream end  52  of the liquefier tube  5 , opposite the inlet portion  51 , is also provided with a substantially circular cross-section. The inlet portion  51  and second end  52  are located at opposite ends of the tubular body  50 . 
     Intermediate the inlet portion  51  and second end  52  is a non-circular portion  53  having a non-circular cross-section, in the form of an obround cross-section in this example. In the present example, the non-circular portion  53  extends uninterrupted along a portion of the length of the tubular body  50 , and has a substantially constant cross-sectional area. A first end portion  57 , having a substantially circular cross-section extends along a portion of the length of the tubular body  50  between the inlet portion  51  and non-circular portion  53 . 
     In the present example, the non-circular portion  53  has four discrete segments: a first segment  53   a , second segment  53   b  downstream of first segment  53   a , third segment  53   c  downstream of second segment  53   b  and fourth segment  53   d  downstream of third segment  53   c . Each of the segments  53   a : 53   d  are arranged in series along the longitudinal axis L of tubular body  50 . 
     The tubular body  50  is deformed by crushing to provide the non-circular portion  53 . In the present example, each segment  53   a : 53   d  corresponds to a crush point/zone at which the tubular body  50  is deformed. A substantially continuous transition is provided between each segment  53   a : 53   d  and between the first end portion  57  and non-circular portion  53 . 
     The tubular body  50  includes a continual transition when viewed downstream from the substantially circular cross section of the inlet portion  51  to the non-circular cross section of the first segment  53   a . The cross sectional area at the inlet portion  51  is substantially equal to the cross sectional area of the first segment  53   a  in this example. 
     Each of the segments  53   a : 53   d  has a major dimension M1 and a minor dimension M2. When viewed downstream from the inlet portion  51  toward the second end  52 , major dimension M1 increases whilst the minor dimension M2 decreases. The major dimension M1 reaches its maximum and the minor dimension M2 reaches its minimum at the fourth segment 53d. 
     Throughout the non-circular portion  53 , the minor dimension M2 is less than the diameter of the inlet portion  51  and second end  52 . Therefore, the distance from the sidewall of the tubular body  50  to the centre of the passageway P is reduced in the non-circular portion  53  when compared to the substantially circular cross-section of the inlet portion  51  and second end  52 . 
     Downstream of the fourth segment  53   d  the non-circular portion  53  transitions from a non-circular cross section to a second end portion  54  having a substantially circular cross section at the downstream end  52 . The second end portion  54  extends along a portion of the length of the tubular body  50  between the non-circular portion  53  and downstream end  52 . 
     The second end  52  is provided with an extrusion tip  55  in the form of a nozzle for dispensing filament material (not shown) in a molten state. The extrusion tip  55  provides an outlet of the liquefier tube  5  and has a substantially circular passage  56  (shown in  FIG.  5   , in particular) extending therealong. The extrusion tip  55  is welded to the tubular body  50  at the second end  52  in this example. As shown most clearly in  FIG.  5   , the circular passage  56  of the extrusion tip  55  is of a smaller cross-sectional area than that of the tubular body  50 . 
     In use, the liquefier tube  5  is received within an extrusion head  4  of an extrusion-based additive manufacturing system  1  (as shown in  FIG.  1   , in particular). Filament material is fed into the inlet portion  51  by a feeding mechanism  6 . The substantially circular cross-section of the inlet portion  51  is configured to receive, in use, a filament of material having a circular cross section from a feeding mechanism  6 . 
     The filament of material is advanced along the tubular body  50 . Heating means H (as shown in  FIG.  1   , in particular), in the form of one or more heating element(s) in this example, is located within the extrusion head  4  and adjacent the liquefier tube  5 . The one or more heating element(s) heat an external surface of the liquefier tube  5 , which by virtue of thermal transfer, in turn heats the filament material as it is advanced. 
     As the filament material is advanced from the inlet portion  51  toward the non-circular portion  53 , it becomes molten as a result of the heating. The cross-sectional shape of the filament material then conforms to the cross-sectional shape of the non-circular portion  53 . 
     In the non-circular portion  53 , the distance from the heating means H to the centre of the passageway P, and therefore centre of the filament material, or filament flow-path is reduced, allowing heat to more effectively reach the centre of the filament material. This allows for more effective heat transfer from the heating means H to the filament material. 
     Extrusion pressure is created by the feeding of filament upstream. The molten filament is extruded from the extrusion tip  55  and onto the print bed  2  (as shown more clearly in  FIG.  1   , in particular). 
     The liquefier tube  5  of  FIGS.  2  to  5    is manufactured by providing a tubular body  50  having a substantially constant wall thickness t and substantially circular cross-section. The tubular body  50  is deformed or crushed at locations defined by segments  53   a : 53   d , so as to provide an inlet portion  51  and first end portion  57  having a substantially circular cross-section for receiving a filament of material (not shown), and a non-circular portion  53 , formed of segments  53   a : 53   d , each having a non-circular cross-section. A substantially continuous transition is provided between each segment  53   a : 53   d . 
     In this example, deforming or crushing the tubular body  50  is carried out by compressing the tubular body  50  at discrete points along its length using a press. 
     An extrusion tip  55  is connected to the tubular body  50  at the other end  52 , opposite the inlet portion  51 , to form an outlet for dispensing filament material in a molten state. The extrusion tip  55  is welded to the tubular body  50  in the present example. 
     Referring now to  FIGS.  6  to  9   , there is shown a liquefier tube  105  according to another example of the invention, for use in the manufacturing system  1  of  FIG.  1   . The liquefier tube  105  according to this example is similar to liquefier tube  5  according to the first example, wherein like features will be denoted by like references incremented by ‘ 100 ’. 
     The liquefier tube  105  according to this example includes a tubular body  150  providing a passageway P extending therealong and having sidewall of substantially constant wall thickness t (as shown in  FIG.  9   , in particular) that extends along a longitudinal axis L. As in the liquefier tube  5  according to the first example, the tubular body  150  is formed of metal, in particular stainless steel, in this example. 
     A first, upstream end of the liquefier tube  105  is provided with an inlet portion  151  of substantially circular cross-section. A second, downstream end  152  of the liquefier tube  105 , opposite the inlet portion  151 , is also provided with a substantially circular cross-section. The inlet portion  151  and second end  152  are located at opposite ends of the tubular body  15   0 . 
     Intermediate the inlet portion  151  and second end  152  is a first non-circular portion  153  having a non-circular cross-section, in the form of an obround cross-section in this example. The first non-circular portion  153  is similar to non-circular portion  53  of liquefier tube  5 , and extends uninterrupted along a portion of the length of the tubular body  150 , and has a substantially constant cross-sectional area. A first end portion  157 , having a substantially circular cross-section extends along a portion of the length of the tubular body  150  between the inlet portion  151  and first non-circular portion  153 . 
     In this example, like the non-circular portion  53  of liquefier tube  5 , the first non-circular portion  153  has four discrete segments: a first segment  153   a , second segment  153   b  downstream of first segment  153   a , third segment  153   c  downstream of second segment  153   b  and fourth segment  153   d  downstream of third segment  153   c . Each of the segments  153   a : 153   d  are arranged in series along the longitudinal axis L of the first non-circular portion  153 . 
     Downstream of the first non-circular portion  153 , between the first non-circular portion  153  and the second end  152  is a second non-circular portion  158 . The second non-circular portion  158  is similar to the first non-circular portion  153 , but rotationally offset by  90  degrees. 
     The second non-circular portion  158  is similar to non-circular portion  53  of liquefier tube  5 , and extends uninterrupted along a portion of the length of the tubular body  150 , and has a substantially constant cross-sectional area. A second end portion  154 , having a substantially circular cross-section extends along a portion of the length of the tubular body  150  between the second end  152  and second non-circular portion  158 . 
     In this example, like the first non-circular portion  153 , the second non-circular portion  158  has four discrete segments: a first segment  158   a , second segment  158   b  downstream of first segment  158   a , third segment  158   c  downstream of second segment  158   b  and fourth segment  158   d  downstream of third segment  158   c . Each of the segments  158   a : 158   d  are arranged in series along the longitudinal axis L of the second non-circular portion  158 . 
     Between the first non-circular portion  153  and second non-circular portion  158  is a transition point or zone T at which the orientation of the non-circular part of the liquefier tube  105  is rotated by 90 degrees. 
     The tubular body  150  is deformed by crushing to provide the first non-circular portion  153  and second non-circular portion  158 . In the present example, each segment  153   a : 153   d  and  158   a : 158   d  corresponds to a crush point/zone at which the tubular body  150  is deformed. A substantially continuous transition is provided between each segment  153   a : 153   d  and  158   a : 158   d  and between the first non-circular portion  153  and second non-circular portion  158  via the transition point or zone T. 
     The tubular body  150  includes a continual transition when viewed downstream from the substantially circular cross section of the inlet portion  151  to the non-circular cross section of the first segment  153   a . The cross sectional area at the inlet portion  51  is substantially equal to the cross sectional area of the first segment  153   a  in this example. 
     Each of the segments  153   a : 153   d  and  158   a : 158   d  has a major dimension M1 and a minor dimension M2. When viewed downstream from the inlet portion  151  toward the second end  152 , within the first non-circular portion  153 , major dimension M1 increases whilst the minor dimension M2 decreases. The major dimension M1 reaches its maximum and the minor dimension M2 reaches its minimum at the fourth segment  153   d . The same as above applies to the second non-circular portion  158  when viewed from the transition point or zone T toward the second end  152 . 
     In this example, the respective first segments  153   a ,  158   a , respective second segments  153   b ,  158   b , respective third segments  153   c ,  158   c  and respective fourth segments  153   d ,  158   d  correspond to one another in terms of cross-sectional shape, but are offset by 90 degrees. Therefore, the major dimensions M1 and minor dimensions M2 correspond to one another in each of these corresponding segments, but offset by 90 degrees. 
     Throughout the first non-circular portion  153  and second non-circular portion  158 , the minor dimension M2 is less than the diameter of the inlet portion  151  and second end  152 . Therefore, the minimum distance from the sidewall of the tubular body  150  to the centre of the passageway P is reduced in the first non-circular portion  153  and second non-circular portion  158  when compared to the substantially circular cross-section of the inlet portion  151  and second end  152 . 
     Downstream of the fourth segment  158   d  the second non-circular portion  158  transitions from a non-circular cross section to the second end portion  154  having a substantially circular cross section at the downstream end  152 . The second end portion  154  extends along a portion of the length of the tubular body  150  between the second non-circular portion  158  and downstream end  152 . 
     ???As in the case of liquefier tube  5 , the length of the perimeter described by the tubular body  150  in the first non-circular portion  153  and second non-circular portion  158  is greater than that at the first end portion  157  and second end portion  154 . This provides a greater contact area between the tubular body  150  and filament (not shown), increasing the rate of heat transfer. 
     The second end  152  is provided with an extrusion tip  155  in the form of a nozzle for dispensing filament material (not shown) in a molten state. The extrusion tip  155  provides an outlet of the liquefier tube  105  and has a substantially circular passage  156  (shown in  FIG.  9   , in particular) extending therealong. The extrusion tip  155  is welded to the tubular body  150  at the second end  152  in this example. As shown most clearly in  FIG.  9   , the circular passage  156  of the extrusion tip  155  is of a smaller cross-sectional area than that of the tubular body  15   0 . 
     In use, as is the case with liquefier tube  5 , liquefier tube  105  is received within an extrusion head  4  of an extrusion-based additive manufacturing system  1  (as shown in  FIG.  1   , in particular). Filament material is fed into the inlet portion  151  by a feeding mechanism  6 . The substantially circular cross-section of the inlet portion  151  is configured to receive, in use, a filament of material having a circular cross section from the feeding mechanism  6 . 
     The filament of material is advanced along the tubular body  15   0 . Heating means H, in the form of one or more heating element(s) in this example, is located within the extrusion head  4  and adjacent the liquefier tube  105 . The one or more heating element(s) are configured to heat an external surface of the liquefier tube  105 , which by virtue of thermal transfer, in turn heats the filament material as it is advanced. 
     As the filament material is advanced from the inlet portion  151  toward the first non-circular portion  153 , it becomes molten as a result of the heating. The cross-sectional shape of the filament material then conforms to the cross-sectional shape of the first non-circular portion  153 . 
     In the first non-circular portion  153 , the distance from the heating means H to the centre of the passageway P, and therefore centre of the filament material, or filament flow-path is reduced, allowing heat to more effectively reach the centre of the filament material. This allows for more effective heat transfer. 
     The filament is then advanced from the first non-circular portion  153  toward the second non-circular portion  158 , via the transition point or zone T. The rotational offset of the first non-circular portion  153  and second non-circular portion  158  results in mixing of the filament material, improving the heat transfer therethrough. 
     Extrusion pressure is created by the feeding of filament upstream. The molten filament is extruded from the extrusion tip  155  and onto the print bed  2  (as shown in  FIG.  1   , in particular). 
     In the present example, the liquefier tube  105  of  FIGS.  6  to  9    is manufactured by providing a tubular body  150  having a substantially constant wall thickness t and substantially circular cross-section. The tubular body  150  is deformed or crushed in a first direction at locations defined by segments  153   a : 153   d , so as to provide an inlet portion  151  and first end portion  157  having a substantially circular cross-section for receiving a filament of material (not shown), and a first non-circular portion  153 , formed of segments  153   a : 153   d , each having a non-circular cross-section. A substantially continuous transition is provided between each segment  153   a : 153   d . 
     Downstream of the first non-circular segment  153 , the tubular body  150  is deformed or crushed in a second direction, offset by 90 degrees from the first direction. The tubular body  150  is deformed or crushed in the second direction at locations defined by segments  158   a : 158   d  so as to provide transition point or zone T, and second end portion  152  having a substantially circular cross-section. 
     In the present example, deforming or crushing the tubular body  150  is carried out by compressing the tubular body  150  at discrete points along its length using a press. 
     An extrusion tip  155  is connected to the tubular body  150  at the other end  152 , opposite the inlet portion  151 , to form an outlet for dispensing material in a molten state. The extrusion tip  155  is welded to the tubular body  150  in the present example. 
     Referring now to  FIGS.  10  to  12   , there is shown a liquefier tube  205  according to another example of the invention, for use in the manufacturing system  1  of  FIG.  1   . 
     The liquefier tube  205  has a similar profile to liquefier tube  105 , but, instead of the passageway P being described by a tubular body  150  it is described in part by a split heater block  7 . The split heater block  7  has two halves  7   a ,  7   b  (as shown in  FIG.  10   , in particular). Each half  7   a ,  7   b  describes one half of the passageway P of liquefier tube  205 . 
     The halves  7   a ,  7   b  are configured to be secured together to form the split heater block  7  and liquefier tube  205 . The halves  7   a ,  7   b  are configured to be secured together using screws (not shown) and are each formed of metal in this example. 
     In the present example, the first non-circular portion  253 , transition point or zone T and second non-circular portion  258  form a first element A of the liquefier tube  205  and are described by the halves  7   a ,  7   b . The inlet portion  251  and first end portion  257  form a second element B of the liquefier tube  205 , separate from the first element A. The second end  252 , second end portion  254  and extrusion tip  255  form a third element C, separate from the first element A and second element B. 
     The second element B is secured to a threaded aperture described in first end  70  of the split heater block  7  by virtue of a threaded connection (not shown). Similarly, the third element C is secured to a threaded aperture described in the second end  71  of the split heater block  7  by virtue of a threaded connection (not shown). 
     The first half  7   a  includes a cylindrical heater cartridge  71   a  including electrical wires  72   a  extending therefrom. The second half  7   b  includes a temperature sensor  71   b  including electrical wires  72   b  extending therefrom. Each of the cylindrical heater cartridge  71   a  and temperature sensor  71   b  are located adjacent the first element A and extend along the entire length of the heater block  7  from the first end  70  to the second end  71  (as shown in  FIG.  12   , in particular). 
     The temperature sensor  71   b  and heater cartridge  71   a  together with a controller (not shown) allow for closed-loop feedback control of temperature via respective electrical wires  72   a ,  72   b . 
     In use, the heater cartridge  71   a  is configured to provide heat around the entire circumference of the liquefier tube  205 , in particular first element A, by virtue of conduction through the metal material of the halves  7   a ,  7   b . 
     To assemble the liquefier tube  205  of  FIGS.  10  to  12   , each of the halves  7   a ,  7   b  are secured together using screws (not shown), in this example. The two halves  7   a ,  7   b  describe the first element A. 
     The second element B is screwed into the threaded aperture described in the first end  70  and the third element C is screwed into the threaded aperture described in the second end  71 . 
     As is the case with liquefier tubes  5  and  105 , liquefier tube  205  is configured to be received within an extrusion head  4  of an extrusion-based additive manufacturing system  1  (as shown in  FIG.  1   , in particular). The substantially circular cross-section of the inlet portion  251  is configured to receive, in use, a filament of material having a circular cross section from a feeding mechanism  6 . 
     The filament of material is advanced along the tubular body  25   0 . Heating means H, in the form of the cylindrical heater cartridge  71   a  transfers heat to the filament through the material of the first half  7   a . The heater cartridge  71   a  heats the filament material as it is advanced along the entire length of the heater block  7 . Temperature output of the heater cartridge  71   a  is controlled via the closed-loop feedback controller discussed above. 
     The effects on the filament material as it is advanced along the liquefier tube  205  is similar to that of liquefier tube  105  and, for the sake of brevity, will not be described further. In the present example, the heater block  7  begins heating the filament material upstream of the first non-circular portion  253 , such that the polymer of the filament material begins liquifying prior to having to change shape. 
     In the present example, the liquefier tube  205  of  FIGS.  10  to  12    is manufactured by providing either a pair of metallic blocks, or a single metallic block and splitting it in two, and machining the profile of the first element A into a planar face of each so as to provide halves  7   a ,  7   b . 
     The second element B is formed by providing a tubular body and forming a thread along a portion thereof. The third element C is formed in a similar manner to that of the second element B, with the additional step of welding the extrusion tip  255  to a free end thereof, to from an outlet for dispensing material in a molten state. 
     Referring now to  FIGS.  13  to  16   , there is shown a liquefier tube  305  according to another example of the invention, for use in the manufacturing system  1  of  FIG.  1   . 
     The liquefier tube  305  is similar to liquefier tube  5 , and like features will be denoted by like references incremented by ‘ 300 ’. 
     The liquefier tube  305  differs from the liquefier tube  5  in the cross-sectional profile in the non-circular portion  353 . 
     A first, upstream end of the liquefier tube  305  is provided with an inlet portion  351  of substantially circular cross-section. A second, downstream end  352  of the liquefier tube  305 , opposite the inlet portion  351 , is also provided with a substantially circular cross-section. The inlet portion  351  and second end  352  are located at opposite ends of the tubular body  35   0 . 
     Intermediate the inlet portion  351  and second end  352  is the non-circular portion  353  having a non-circular cross-section. 
     In the present example, the non-circular portion  353  has six discrete segments: a first segment  353   a , second segment  353   b  downstream of first segment  353   a , third segment  353   c  downstream of second segment  53   b , a fourth segment  353   d  downstream of third segment  353   c , a fifth segment  353   e  downstream of the fourth segment  353   d  and a sixth segment  353   f  downstream of the fifth segment  353   e . Each of the segments  353   a : 353   f  are arranged in series along the longitudinal axis L of tubular body  35   0 . 
     Moving toward the second end  352  from the inlet portion  351 , the cross-sectional profile of the tubular body  350  is progressively deformed such that it has a cross or cruciform shape at the fourth segment  353   d . Moving toward the second end  352  from the fourth segment  353   d , the cross-sectional profile of the tubular body has a gradual or continual transition toward a substantially circular profile at the second end  352  and second end portion  354 . In the non-circular portion  353 , the distance from the heating means H (as shown in  FIG.  1   , in particular) to the centre of the passageway P, and therefore centre of the filament material, or filament flow-path is reduced, allowing heat to more effectively reach the centre of the filament material. 
     Further, the cross or cruciform profile provides a lower flow area (shown more clearly in  FIG.  16   , in particular), allowing for more effective heat transfer into a filament material being conveyed along the passageway P. The restriction in flow area, in particular in the fourth segment  353   d , also manipulates the filament material, promoting mixing. 
     The advancement of filament material, in use, is similar to that described above in relation to liquefier  5 ,  105  and  205  and, for the sake of brevity, will not be described further. 
     In the present example, the liquefier tube  305  of  FIGS.  13  to  16    is manufactured by providing a tubular body  350  having a substantially constant wall thickness t and substantially circular cross-section. The tubular body  350  is deformed or crushed at locations defined by segments  353   a : 353   f , so as to provide an inlet portion  351  and first end portion  357  having a substantially circular cross-section for receiving a filament of material (not shown), and a non-circular portion  353 , formed of segments  353   a : 353   f , each having a non-circular cross-section. A substantially continuous transition is provided between each segment  353   a : 353   f . 
     In the present example, deforming or crushing the tubular body  350  is carried out by compressing the tubular body  50  at discrete points along its length using a press. In particular, in the present example, the tubular body  350  is deformed using a punch and die, or press brake. 
     Referring now to  FIGS.  17  to  21   , there is shown a liquefier tube  405  according to another example of the invention, for use in the manufacturing system  1  of  FIG.  1   . 
     The liquefier tube  405  is similar to liquefier tube  5 ,  105 ,  205  and  305  and like features will be denoted by like references incremented by ‘ 400 ’. 
     The liquefier tube  405  differs from the above-described liquefier tubes in that the flow path or passageway P described thereby has a constant hydraulic diameter D H . The hydraulic diameter is defined by the following equation: 
     
       
         
           
             
               D 
               H 
             
             = 
             
               
                 4 
                 A 
               
               P 
             
           
         
       
     
     Where D H  is the hydraulic diameter, A is the flow area and P is the perimeter described by the tubular body  45   0 . 
     Hydraulic diameter D H  allows pipework of non-circular cross-section to be approximated as circular for the purposes of pressure drop and fluid flow rate calculations. Having a liquefier tube  405  having a constant hydraulic diameter D H  provides, in use, a substantially constant pressure as a filament material is being advanced. 
     Therefore, in the case of liquefier tube  405 , the change in cross-sectional shape in the non-circular portion  453  allows for more effective heat transfer to a filament material as is being advanced, but whilst without increasing the pressure applied to generate a given flow rate. 
     A first, upstream end of the liquefier tube  405  is provided with an inlet portion  451  of substantially circular cross-section. A second, downstream end  452  of the liquefier tube  405 , opposite the inlet portion  451 , is also provided with a substantially circular cross-section. The inlet portion  451  and second end  452  are located at opposite ends of the tubular body  45   0 . 
     Intermediate the inlet portion  451  and second end  452  is the non-circular portion  453  having a non-circular cross-section. The first end portion  457 , second end portion  454  and non-circular portion  453  have a constant hydraulic diameter D H . 
     In the present example, the non-circular portion  453  has four discrete segments: a first segment  453   a , second segment  453   b  downstream of first segment  453   a , third segment  453   c  downstream of second segment  453   b  and a fourth segment  453   d  downstream of third segment  453   c . Each of the segments  453   a : 453   d  are arranged in series along the longitudinal axis L of tubular body  45   0 . 
     The tubular body  450  includes a continual transition when viewed downstream from the substantially circular cross section of the inlet portion  451  to the non-circular cross section of the first segment  453   a . The cross sectional area at the inlet portion  451  is substantially equal to the cross sectional area of the first segment  453   a  in this example. 
     Like liquefier tube  5 , each of the segments  453   a : 453   d  has a major dimension M1 and a minor dimension M2. When viewed downstream from the inlet portion  451  toward the second end  452 , major dimension M1 increases whilst the minor dimension M2 decreases. The major dimension M1 reaches its maximum and the minor dimension M2 reaches its minimum at the fourth segment  453   d . 
     Throughout the non-circular portion  453 , the minor dimension M2 is less than the diameter of the inlet portion  451  and second end  452 . Therefore, the distance from the sidewall of the tubular body  450  to the centre of the passageway P is reduced in the non-circular portion  453  when compared to the substantially circular cross-section of the inlet portion  451  and second end  452 . 
     Downstream of the fourth segment  453   d  the non-circular portion  453  transitions sharply from a non-circular cross section to the second end portion  454  having a substantially circular cross section at the downstream end  452 . 
     In the present example, the liquefier tube  405  of  FIGS.  17  to  21    may be manufactured in a similar manner to that of liquefier tube  205  described above, i.e. by providing either a pair of metallic blocks, or a single metallic block and splitting it in two, and machining the profile of one half of the passageway P into a planar face of each so as to provide halves that, when brought together, describe passageway P. 
     As an alternative, the liquefier tube  405  may be manufactured by providing a sheet of metal and carrying out a hydroforming process so as to form one half of the tubular body  45   0 . A pair of halves are then attached to one another so as to form the tubular body  45   0 . 
     As a further alternative, the liquefier tube  405  may be manufactured by carrying out a hydroforming process on a tubular body  45   0 . Deforming the tubular body  450  by hydroforming may comprise placing the tubular body  450  between a pair of dies and injecting fluid under pressure into passageway P. The fluid under pressure causes the tubular body  450  to deform such that it conforms to a profile described by the forming tool or pair of dies and forms non-circular portion  453 . 
     An extrusion tip  455  is connected to the tubular body  450  at the other end  452 , opposite the inlet portion  451 , to form an outlet for dispensing material in a molten state. The extrusion tip  455  is welded to the tubular body  450  in the present example. 
     It will be appreciated by those skilled in the art that several variations to the aforementioned examples are envisaged without departing from the scope of the invention. For example, the non-circular portion of the tubular body need not have an obround cross-section. Instead, the non-circular portion may have any other suitable non-circular cross section, for example, rectangular, star-shaped, elliptical, square, oval, regular polygonal, irregular polygonal, simple convex polygonal or simple concave polygonal.. 
     Although the tubular member is described as being formed of metal, in particular stainless steel, this need not be the case. Instead, the tubular member may be made of brass, copper, tungsten, titanium, molybdenum, beryllium copper or any other suitable metal or alloy. 
     Alternatively, the tubular member may be made of a polymeric material, e.g. a thermally conductive polymeric material. 
     Although the transition or change from the substantially circular inlet portion to the non-circular portion is described as being continual, the skilled person will appreciate that this need not be the case. Instead, the transition may comprise a staged, discretised and/or stepped transition. 
     In examples, the extrusion tip or nozzle may be removably connected to the liquefier tube or tubular body. Instead of being welded, as described above, the extrusion tip or nozzle may be brazed to the liquefier tube or tubular body. 
     The non-circular portion  53  is described as having a taper along a portion of the length of the tubular body  50 . Alternatively, the non-circular portion of the tubular body may comprise a plurality of segments or sections. Each of the plurality of segments or sections may comprise a deformed or crushed segment of the tubular body. Each of the plurality of segments or sections may be arranged, e.g. in series, along the length of, or principal axis of, the liquefier tube or tubular body. Each of the plurality of segments or sections may be arranged along a longitudinal axis of the liquefier tube or tubular body. 
     Each of the plurality of segments or sections, hereinafter segments, may have a different cross-sectional shape, configuration or cross-sectional profile, e.g. from one another. One, e.g. a first, of the plurality of segments may have a different cross-sectional shape, configuration or cross-sectional profile from another, e.g. a second, of the plurality of segments. 
     In some examples, the first segment and the second segment, e.g. their major and/or minor dimensions, are rotationally offset or skewed from one another. The first segment and the second segment may be skewed or twisted from, or relative to, one another. The first segment and second segment may comprise a skew angle described therebetween. The first segment may comprise a portion of the tubular body deformed or crushed in a direction that is skewed or rotationally offset relative to a direction in which the second segment is deformed or crushed. 
     Corresponding minor and/or major dimensions of the first segment and second segment may be rotationally offset or skewed from one another. 
     Although the non-circular portion  53  is described as being uninterrupted, this need not be the case. Instead, the tubular body  50  may have a substantially circular portion intermediate two or more segments of the non-circular portion. 
     It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.