Patent Publication Number: US-11383417-B2

Title: Filament heaters configured to facilitate thermal treatment of filaments for extruder heads in three-dimensional object printers

Description:
PRIORITY CLAIM 
     This disclosure application is a divisional of and claims priority to U.S. patent application Ser. No. 15/334,851, which was filed on Oct. 26, 2016, is entitled “Filament Heaters Configured To Facilitate Thermal Treatment Of Filaments For Extruder Heads In Three-Dimensional Object Printers,” and which issued as U.S. Pat. No. 10,814,544 on Oct. 27, 2020. The disclosure of this document is hereby expressly incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure is directed to extruders used in three-dimensional object printers and, more particularly, to heaters that melts filaments for extrusion in three-dimensional object printers. 
     BACKGROUND 
     Three-dimensional printing, also known as additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Many three-dimensional printing technologies use an additive process in which an additive manufacturing device forms successive layers of the part on top of previously deposited layers. Some of these technologies use extruder heads that soften or melt extrusion material, such as ABS plastic, into thermoplastic material and then emit the thermoplastic material in a predetermined pattern. The printer typically operates the extruder head to form successive layers of the thermoplastic material that form a three-dimensional printed object with a variety of shapes and structures. After each layer of the three-dimensional printed object is formed, the thermoplastic material cools and hardens to bond the layer to an underlying layer of the three-dimensional printed object. This additive manufacturing method is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
     These additive manufacturing devices can produce highly functional three-dimensional (3D) parts, but typically the time of manufacture can be quite lengthy. One improvement that speeds the manufacturing process was the development of a multi-channel extruder. In this type of extruder, a filament of extruder material is fed to a heater that either melts or softens the filament to form thermoplastic material that flows into a manifold that is fluidly connected to an array of nozzles for the extrusion of the thermoplastic material. A valve is interposed between each nozzle and the manifold to control thermoplastic flow from the manifold to one of the nozzles. This configuration enables the valves to be activated selectively so the thermoplastic material can be extruded from a single nozzle, a group of nozzles, or all of the nozzles fluidly connected to the manifold. In order to supply the thermoplastic material on demand as the valves are operated requires high pressure within the manifold and the production of thermoplastic material at a much faster than is available from a heater for a standard extruder. The volume of thermoplastic material produced for an extruder head can be increased by processing larger diameter filament, but the amount of time needed to thermally treat the thicker filament would also be increased. A filament heater that can increase the production of thermoplastic material for an extruder head without extending the length of thermal treatment time would be beneficial. 
     SUMMARY 
     A new additive manufacturing system incorporates a heater that is configured to enable extrusion material filaments to be thermally processed without extending the length of time required to thermally treat the filament. The additive manufacturing system includes an extruder head having a manifold configured to store thermoplastic material and at least one nozzle through which thermoplastic material from the manifold can be emitted, a mechanical mover configured to move extrusion material from a supply of extrusion material along a path, the extrusion material having a first cross-sectional shape, and a heater having a channel positioned along the path of the extrusion material to receive the extrusion material and at least one heating element configured to melt the extrusion material in the channel to form thermoplastic material, the channel in the heater being fluidly connected to the manifold in the extruder head to enable the thermoplastic material to enter the manifold and the channel in the heater is configured with the first cross-sectional shape at a first position and with a second cross-sectional shape at a second position, the second cross-sectional shape being different than the first cross-sectional shape, to enable the channel in the heater to increase a surface area of the filament. 
     A heater has been configured to enable extrusion material filaments to be thermally processed without extending the length of time required to thermally treat the filament. The heater includes a body having a channel, the channel in the heater body being configured with a first cross-sectional shape at a first position and with a second cross-sectional shape at a second position, the second cross-sectional shape being different than the first cross-sectional shape, to enable the channel in the heater to increase a surface area of extrusion material that passes through the channel from the first position to the second position, and at least one heating element positioned in the body to generate heat in the channel and melt the extrusion material to form thermoplastic material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a heater that thermally processes filaments into thermoplastic material for extruder heads are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  depicts an additive manufacturing system that includes a heater that improves the thermal treatment of filaments for the provision of thermoplastic material to a manifold of an extruder head. 
         FIG. 2A  is a cross-sectional view of the heater of  FIG. 1  that improves the thermal treatment of filaments for the provision of thermoplastic material to the manifold of the extruder head of  FIG. 1  that is taken along lines  2 A- 2 A. 
         FIG. 2B  is a cross-sectional view of the view of the heater shown in  FIG. 2A  taken along lines  2 B- 2 B. 
         FIG. 2C  is a cross-sectional view of the view of the heater shown in  FIG. 2A  taken along lines  2 C- 2 C. 
         FIG. 3A  is an end view of the heater shown in  FIG. 1  taken at the position of lines  2 C- 2 C in  FIG. 2A . 
         FIG. 3B  is an end view of an alternative embodiment of the heater at the same position at which  FIG. 3A  was taken that reduces the width of the heater. 
         FIG. 3C  is an end view of an alternative embodiment of the heater at the same position at which  FIG. 3A  was taken that reduces the width of the heater. 
         FIG. 4  is a side view of a longitudinal cross-section of an alternative embodiment of the heater that improves the thermal treatment of filaments for the provision of thermoplastic material to a manifold of an extruder head that can be used in the system of  FIG. 1 . 
         FIG. 5  is a side view of a longitudinal cross-section of another alternative embodiment of the heater that improves the thermal treatment of filaments for the provision of thermoplastic material to a manifold of an extruder head that can be used in the system of  FIG. 1 . 
         FIG. 6  is a side cross-sectional view of a heater that can be formed with an additive manufacturing or casting process with cross-sectional views of the channel through the heater at different positions along the channel. 
         FIG. 7  is a diagram of a prior art three-dimensional object printer having a multi-nozzle extrusion printhead that does not have the solid extrusion material feeding system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the device disclosed herein as well as the details for the device, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
     As used herein, the term “extrusion material” refers to a material that is softened or melted to form thermoplastic material to be emitted by an extruder head in an additive manufacturing system. The extrusion materials include, but are not strictly limited to, both “build materials” that form permanent portions of the three-dimensional printed object and “support materials” that form temporary structures to support portions of the build material during a printing process and are then optionally removed after completion of the printing process. Examples of build materials include, but are not limited to, acrylonitrile butadiene styrene (ABS) plastic, polylactic acid (PLA), aliphatic or semi-aromatic polyamides (Nylon), plastics that include suspended carbon fiber or other aggregate materials, electrically conductive polymers, and any other form of material that can be thermally treated to produce thermoplastic material suitable for emission through an extruder head. Examples of support materials include, but are not limited to, high-impact polystyrene (HIPS), polyvinyl alcohol (PVA), and other materials capable of extrusion after being thermally treated. In some extrusion printers, the extrusion material is supplied as a continuous elongated strand of material commonly known as a “filament.” This filament is provided in a solid form by one or more rollers pulling the extrusion material filament from a spool or other supply and feeding the filament into a heater that is fluidly connected to a manifold within the extruder head. The heater softens or melts the extrusion material filament to form a thermoplastic material that flows into the manifold. When a valve positioned between a nozzle and the manifold is opened, a portion of the thermoplastic material flows from the manifold through the nozzle and is emitted as a stream of thermoplastic material. As used herein, the term “melt” as applied to extrusion material refers to any elevation of temperature for the extrusion material that softens or changes the phase of the extrusion material to enable extrusion of the thermoplastic material through one or more nozzles in a printhead during operation of a three-dimensional object printer. The melted extrusion material is also denoted as “thermoplastic material” in this document. As those of skill in the art recognize, certain amorphous extrusion materials do not transition to a pure liquid state during operation of the printer. 
     As used herein, the term “manifold” refers to a cavity formed within a housing of an extruder head that holds a supply of thermoplastic material for delivery to one or more nozzles in the printhead during a three-dimensional object printing operation. As used herein, the term “extruder head” refers to a component of a printer that extrudes melted extrusion material received from a manifold through one or more nozzles. Some extruder heads include a valve assembly that can be electronically operated to enable thermoplastic material to flow through nozzles selectively. The valve assembly enables the independent connecting of one or more nozzles to the manifold to extrude the thermoplastic material. As used herein, the term “nozzle” refers to an orifice in an extruder head that is fluidly connected to the manifold in an extruder head and through which thermoplastic material is emitted toward an image receiving surface. During operation, the nozzle extrudes a substantially continuous linear arrangement of the thermoplastic material along the process path of the extruder head. A controller operates the valves in the valve assembly to control which nozzle connected to the valve assembly extrudes thermoplastic material. The diameter of the nozzle affects the width of the line of extruded thermoplastic material. Different extruder head embodiments include nozzles having a range of orifice sizes with wider orifices that produce ribbons of thermoplastic material having widths that are greater than the widths of ribbons produced by narrower orifices. 
     As used herein, the term “arrangement of extrusion material” refers to any pattern of thermoplastic material that the extruder head forms on an image receiving surface during a three-dimensional object printing operation. Common arrangements of thermoplastic material include straight-line linear arrangements of the thermoplastic material and curved arrangements of the thermoplastic material. In some configurations, the extruder head extrudes the thermoplastic material in a continuous manner to form the arrangement with a contiguous mass of the thermoplastic material while in other configurations the extruder head operates in an intermittent manner to form smaller groups of thermoplastic material that are arranged along a linear or curved path. The three-dimensional object printer forms various structures using combinations of different arrangements of thermoplastic material. Additionally, a controller in the three-dimensional object printer uses object image data and extruder head path data that correspond to different arrangements of thermoplastic material to operate the extruder head and form each arrangement of the extrusion material. As described below, the controller optionally adjusts the operation of the valve assembly to form multiple arrangements of thermoplastic material through one or more nozzles during a three-dimensional printing operation. 
     As used herein, the term “process direction” refers to a direction of relative movement between an extruder head and an image receiving surface that receives thermoplastic material extruded from one or more nozzles in the head. The image receiving surface is either a support member that holds a three-dimensional printed object or a surface of the partially formed three-dimensional object during an additive manufacturing process. In the illustrative embodiments described herein, one or more actuators move the extruder head about the support member, but alternative system embodiments move the support member to produce the relative motion in the process direction while the extruder head remains stationary. 
     As used herein, the term “cross process direction” refers to an axis that is perpendicular to the process direction in the plane of the process direction and is also perpendicular to the surface of the three-dimensional object being produced. The process direction and cross-process direction refer to the relative path of movement of the extruder head and the surface that receives the thermoplastic material. In some configurations, the extruder head includes an array of nozzles that extend along the cross-process direction. Adjacent nozzles within the extruder head are separated by a predetermined distance in the cross-process direction. In some configurations the system rotates the extruder head to adjust the effective cross-process direction distance between adjacent nozzles in the extruder head to adjust the corresponding cross-process direction distance between arrangements of the thermoplastic material extruded from the nozzles in the extruder head. 
     During operation of the additive manufacturing system, an extruder head moves in the process direction along both straight and curved paths relative to a surface that receives thermoplastic material during the three-dimensional object printing process. Additionally, an actuator in the system optionally rotates the extruder head about the Z axis to adjust the effective cross-process distance that separates nozzles in the extruder head to enable the extruder head to form two or more arrangements of thermoplastic material with predetermined distances between each arrangement of the thermoplastic material. The extruder head moves both along the outer perimeter to form outer walls of a region in a layer of the printed object and within the perimeter to fill all or a portion of the region with thermoplastic material. The extruder head forming the arrangements can move through any planar or rotational degree of freedom provided the processing of the image data for the three-dimensional object is adequate to generate the data for operating the extruder head. 
       FIG. 7  depicts a prior art three-dimensional object additive manufacturing system or printer  100  that is configured to operate an extruder head  108  to form a three-dimensional printed object  140 . Although the printer  100  is depicted as a printer that uses planar motion to form an object, other printer architectures can be used with the extruder head and mechanical mover of extrusion material described in this document. These architectures include delta-bots, selective compliance assembly robot arms (SCARAs), multi-axis printers, non-Cartesian printers, and the like. The printer  100  includes a support member  102 , a multi-nozzle extruder head  108 , extruder head support arm  112 , controller  128 , memory  132 , X/Y actuators  150 , an optional Zθ actuator  154 , and a Z actuator  158 . In the printer  100 , the X/Y actuators  150  move the extruder head  108  to different locations in a two-dimensional plane (the “X-Y plane”) along the X and Y axes to extrude arrangements of thermoplastic material that form one layer in a three-dimensional printed object, such as the object  140  that is depicted in  FIG. 7 . For example, in  FIG. 7  the X/Y actuators  150  translate the support arm  112  and extruder head  108  along guide rails  113  to move along the Y axis while the X/Y actuators  150  translate the extruder head  108  along the length of the support arm  112  to move the printhead along the X axis. The extruded patterns include both outlines of one or more regions in the layer and swaths of the thermoplastic material that fill in the regions within the outline of thermoplastic material patterns. The Z actuator  158  controls the distance between the extruder head  108  and the support member  102  along the Z axis to ensure that the nozzles in the extruder head  108  remain at a suitable height to extrude thermoplastic material onto the object  140  as the object is formed during the printing process. The Zθ actuator  154  controls an angle of rotation of the extruder head  108  about the Z axis (referenced as Zθ in  FIG. 4 ) for some embodiments of the extruder head  108  that rotate about the Z axis. This movement controls the separation between nozzles in the extruder head  108 , although some extruder heads do not require rotation during the manufacturing process. In the system  100 , the X/Y actuators  150 , Zθ actuator  154 , and the Z actuator  158  are embodied as electromechanical actuators, such as electric motors, stepper motors, or any other suitable electromechanical device. In the illustrative embodiment of  FIG. 7 , the three-dimensional object printer  100  is depicted during formation of a three-dimensional printed object  140  that is formed from a plurality of layers of thermoplastic material. 
     The support member  102  is a planar member, such as a glass plate, polymer plate, or foam surface, which supports the three-dimensional printed object  140  during the manufacturing process. In the embodiment of  FIG. 7 , the Z actuator  158  also moves the support member  102  in the direction Z away from the extruder head  108  after application of each layer of thermoplastic material to ensure that the extruder head  108  maintains a predetermined distance from the upper surface of the object  140 . The extruder head  108  includes a plurality of nozzles and each nozzle extrudes thermoplastic material onto the surface of the support member  102  or a surface of a partially formed object, such the object  140 . In the example of  FIG. 7 , extrusion material is provided as a filament from extrusion material supply  110 , which is a spool of ABS plastic or another suitable extrusion material filament that unwraps from the spool to supply extrusion material to the extruder head  108 . 
     The support arm  112  includes a support member and one or more actuators that move the extruder head  108  during printing operations. In the system  100 , one or more actuators  150  move the support arm  112  and extruder head  108  along the X and Y axes during the printing operation. For example, one of the actuators  150  moves the support arm  112  and the extruder head  108  along the Y axis while another actuator moves the extruder head  108  along the length of the support arm  112  to move along the X axis. In the system  100 , the X/Y actuators  150  optionally move the extruder head  108  along both the X and Y axes simultaneously along either straight or curved paths. The controller  128  controls the movements of the extruder head  108  in both linear and curved paths that enable the nozzles in the extruder head  108  to extrude thermoplastic material onto the support member  102  or onto previously formed layers of the object  140 . The controller  128  optionally moves the extruder head  108  in a rasterized motion along the X axis or Y axis, but the X/Y actuators  150  can also move the extruder head  108  along arbitrary linear or curved paths in the X-Y plane. 
     The controller  128  is a digital logic device such as a microprocessor, microcontroller, field programmable gate array (FPGA), application specific integrated circuit (ASIC) or any other digital logic that is configured to operate the printer  100 . In the printer  100 , the controller  128  is operatively connected to one or more actuators that control the movement of the support member  102  and the support arm  112 . The controller  128  is also operatively connected to a memory  132 . In the embodiment of the printer  100 , the memory  132  includes volatile data storage devices, such as random access memory (RAM) devices, and non-volatile data storage devices such as solid-state data storage devices, magnetic disks, optical disks, or any other suitable data storage devices. The memory  132  stores programmed instruction data  134  and three-dimensional (3D) object image data  136 . The controller  128  executes the stored program instructions  134  to operate the components in the printer  100  to form the three-dimensional printed object  140  and print two-dimensional images on one or more surfaces of the object  140 . The 3D object image data  136  includes, for example, a plurality of two-dimensional image data patterns that correspond to each layer of thermoplastic material that the printer  100  forms during the three-dimensional object printing process. The extruder head path control data  138  include a set of geometric data or actuator control commands that the controller  128  processes to control the path of movement of the extruder head  108  using the X/Y actuators  150  and to control the orientation of the extruder head  108  using the Zθ actuator  154 . The controller  128  operates the actuators to move the extruder head  108 , 
       FIG. 1  depicts an additive manufacturing system  100 ′ that includes a valve assembly  204  in the extruder head  108  that is operatively connected to the controller  128  to enable control of the operation of the valves and the material emitted from the plurality nozzles in the extruder head  108 . Specifically, the controller  128  activates and deactivates different valves in the valve assembly  204  connected to the nozzles in the extruder head  108  to emit thermoplastic material and form arrangements of the thermoplastic material in each layer of the three-dimensional printed object  140 . System  100 ′ also includes an extrusion material dispensing system  212  that feeds filament from the supply  110  to the heater  208  at a rate that maintains the pressure of the thermoplastic material in the manifold  216  within a predetermined range during operation of the system  100 ′. The dispensing system  212  is one embodiment that is suitable for regulating pressure of the thermoplastic material in the manifold. Additionally, the controller  128  is operatively connected to an actuator in the dispensing system  212  to control the rate at which the dispensing system  212  delivers solid filament to a heater  208 . The heater  208  melts extrusion material filament  220  fed to the heater  208  via drive roller  224 . Actuator  240  drives the roller  224  and is operatively connected to the controller  128  so the controller can regulate the speed at which the actuator drives the roller  224 . Another roller opposite roller  224  is free-wheeling so it follows the rate of rotation at which roller  224  is driven. While  FIG. 1  depicts a feed system that uses an electromechanical actuator and the driver roller  224  as a mechanical mover to move the filament  220  into the heater  208 , alternative embodiments use one or more actuators to operate a mechanical mover in the form of a rotating auger or screw. The auger or screw moves solid phase extrusion material in the form of extrusion material powder or pellets into the heater  208 . 
     In the embodiment of  FIG. 1 , the heater  208  has a body that is formed from stainless steel and in which one or more heating elements  228 , such as electrically resistive heating elements, are positioned about a channel  232 . The heating elements  228  are operatively connected to the controller  128  so the controller  128  can connect the heating elements  228  to electrical current selectively to melt the filament of extrusion material  220  in channel  232  within the heater  208 . While  FIG. 1  shows heater  208  receiving extrusion material in a solid phase as solid filament  220 , in alternative embodiments, it receives the extrusion material in solid phase as powdered or pelletized extrusion material. Cooling fins  236  attenuate heat in the channel  232  upstream from the heater  208 . A portion of the extrusion material that remains solid in the channel  232  at or near the cooling fins  236  forms a seal in the channel  232  that prevents thermoplastic material from exiting the heater from any other opening than the connection to the manifold  216 . The extruder head  108  can also include additional heating elements to maintain an elevated temperature for the thermoplastic material within the manifold  216 . In some embodiments a thermal insulator covers portions of the exterior of the extruder head  108  to maintain a temperature within the manifold  216 . 
     To maintain a fluid pressure of the thermoplastic material within the manifold  216  within a predetermined range, avoid damage to the extrusion material, and control the extrusion rate through the nozzles, a slip clutch  244  is operatively connected to the drive shaft of the actuator  240 . As used in this document, the term “slip clutch” refers to a device applies frictional force to an object to move the object up to a predetermined set point. When the range about the predetermined set point for the frictional force is exceeded, the device slips so it no longer applies the frictional force to the object. The slip clutch enables the force exerted on the filament  220  to remain constant no matter how many valves are opened or how fast the actuator  240  drives roller  224 . This constant force can be maintained by either driving the actuator  240  at a speed that is higher than the fastest expected rotational speed of the filament drive roller  224  or by putting an encoder wheel  248  on the roller  224  and sensing the rate of rotation with a sensor  252 . 
     The signal generated by the sensor  252  indicates the angular rotation of the roller  224  and the controller  128  receives this signal to identify the speed of the roller  224 . The controller  128  is further configured to adjust the signal provided to the actuator  240  to control the speed of the actuator. When the controller is configured to control the speed of the actuator  240 , the controller  128  operates the actuator  240  so its speed is slightly faster than the rotation of the roller  224 . This operation ensures that the torque on the drive roller  224  is always a function of the slip clutch torque. If one valve/nozzle combination is open, the filament  220  moves slowly. If all of the actuator/valve combinations in the assembly  204  are opened, the filament begins to move more quickly and the controller  128  immediately operates the actuator  240  to increase its speed to ensure that the output shaft of the actuator is turning faster than the speed of the roller  224  indicated by the sensor  252 . A delay inherently exists between the force applied to the filament and the pressure of the thermoplastic material in the nozzle region of the extruder header. Empirical data of these delays enable set points to be defined for the slip clutch that enable the slip clutch to be operated to provide more uniform pressure of the thermoplastic material in the nozzle region of the extruder head. 
       FIG. 2A  is a longitudinal cross-sectional view of the heater  208  from a position past the cooling fins  236  to the exit position for the thermoplastic material. While the embodiments shown in  FIG. 2A to 2C  and in  FIG. 3A to 3C  are depicted as two piece configurations assembled with fasteners, the heaters could be formed as an integral unit. For example, the heaters could be formed as an integral unit using an additive manufacturing system, such as direct metal laser sintering (DMLS) system, or by a known casting process.  FIG. 2A  shows the lower half of the channel  232  that lies above one of the heating elements  228 . The filament moves through the channel  232  as indicated by the arrow in the figure. As shown in the figure, the channel  232  is narrower where the filament enters the heated zone of the heater  208  and is wider at a position past the entrance. As depicted in  FIG. 2B , one of the heating elements  228  is positioned beneath the lower half of the channel  232 , which is circular at the entrance to the channel  232  when the upper half of the heater  208  is joined to the lower half depicted in the figure. As depicted in  FIG. 2C , the heating element  228  remains beneath the channel  232 , but the channel has increased in width, while its height is reduced. This change in the configuration of the channel  232  increases the surface area of the filament exposed to the heat generated by the heating element  228 . The same is also true with regard to the upper half of the channel  232 . This increase in surface area enables the heating elements  228  to thermal treat the filament more effectively so more of the cross-section of a filament can be thermally processed, which reduces the amount of time previously required for thermally treating the filament. The fasteners  230  join the two halves of the heater  208 . 
       FIG. 3A  shows the assembled heater  208  at the position depicted in  FIG. 2C . Again, the rectangular shape at this location in the channel  232  has a height that is less than the diameter of the circular entrance and a width that is greater than the diameter of the circular entrance. Thus, the rectangular channel shape of  FIG. 3A  increases the surface area of the filament in channel  232  to increase the exposure of the filament to heat over the heat exposure of the filament that occurs if the channel remained circular as it is at the entrance. Although the channel in  FIG. 3A  is depicted as being rectangular, it could be any polygonal shape as long as the height of the polygon is less than the diameter of the circular entrance and the width of the polygon is greater than the diameter of the circular entrance. To reduce the dimensions of the heater  208  further, the channel  232  is configured with a non-circular curved shape, such as the semi-circular shape shown at the position depicted in  FIG. 3A  to widen the channel  232  at that position as shown in  FIG. 3B . As used in this document, the term “non-circular curved shape” means any shape formed by a radius from the center of the channel in the heater that varies in length and is perpendicular to the longitudinal axis of the channel. Because the distance from one end of the semi-circular shaped channel to the other end of the semi-circular shaped channel is less than the end-to-end distance of the rectangular channel shown in  FIG. 3A , while the height of the heater remains the same, the heater  208  in  FIG. 3B  consumes less space than the heater in  FIG. 3A .  FIG. 3C  shows another alternative embodiment in which channel  232  is configured with a pair of semi-circular shaped channels to further increase the surface area of the filament and enhance the exposure of the increased surface area of the filament to the heat provided by the two heating elements  228 , while further reducing the width of the heater  208 . The two semi-circular shaped channels are joined together at intersection  236  to enable the extrusion material to spread into each semi-circular shaped channel. 
       FIG. 6  depicts a side cross-sectional view of an embodiment of a heater  208  that is formed as an integral unit with an additive manufacturing or casting process. These processes are capable of forming the channel with walls having varying curvatures. Beneath the side cross-sectional view of the heater in  FIG. 6  are cross-sectional views of the channel  232  through the heater at different positions A-A to G-G along the channel. Thus, a heater  208  formed as an integral unit can have a channel  232  that varies in curvature along at least a portion of the length of the channel from one end of the channel to the other end of the channel to further improve the thermal processing of the filament within the heater. As used in this document, the term “integral unit” refers to a heater formed with single piece construction that does not require fasteners to assemble the heater. Also, as used in this document, “varies in curvature” means that the slope of the wall or walls of a channel within a heater changes in slope in either the longitudinal direction or cross-longitudinal direction or both directions simultaneously. 
     The depiction presented in  FIG. 6  shows a variety of channel cross-sections that can be formed with additive manufacturing or casting processes. The reader should note that the channel in general begins with a circular cross-sectional area that receives extrusion material filaments. The cross-sectional area of the channel then changes to increase the surface area for facilitating the transfer of heat to the filament for the production of thermoplastic material. In most embodiments, the cross-sectional area of the channel then returns to a cross-sectional area shape and size that is compatible with the port in the extruder head that receives the thermoplastic material and directs it to the manifold in the head. This configuration enables the output of the heater  208  to be coupled directly to the extruder head  108  without an intervening fitting. 
     Another alternative embodiment of a channel configuration in a heater is shown in  FIG. 4 . Again, a channel  232  receives a filament  404  having a circular cross-section as the filament is moved in the direction indicated by the arrow in the figure. The heating elements  228  are not shown in the figure to simply the depiction, but are present as previously shown on either side of the channel. The upper wall  408  of the channel  232  is parallel to the upper surface of the filament  400 , but the channel is also configured to begin increasing the width of the filament as described above. That is, the channel is configured at that position to change the filament of extrusion material to have a height that is less than the height of the filament when it entered the heater  208  and a width that is greater than the width of the filament when it entered the heater  208 . Additionally, the lower wall  412  of the channel  232  is canted at a predetermined angle with regard to the upper surface of the filament to urge the lower surface of the filament toward the upper surface of the channel. This urging of the stiff center portion of the filament  404  toward the wall  408  exposes the center portion to the heated wall  408  so it bends under the pressure. Consequently, the center portion of the filament  404 , which is the coldest portion of the filament, continues to move toward the upper wall  408  of the channel  432 , which reduces the time required to transition the filament into thermoplastic material. 
     Another alternative embodiment of a channel configuration in a heater is shown in  FIG. 5 . Again, a channel  232  receives a filament  404  having a circular cross-section as the filament is moved in the direction indicated by the arrow in the figure. The heating elements  228  are not shown in the figure to simply the depiction, but are present as previously shown on either side of the channel. The upper wall  408  of the channel  232  is parallel to the upper surface of the filament  400 , but the channel is also configured to begin increasing the width of the filament as described above. That is, the channel is configured at that position to change the filament of extrusion material to have a height that is less than the height of the filament when it entered the heater  208  and a width that is greater than the width of the filament when it entered the heater  208 . Additionally, the structure of the channel  232  shifts inwardly and outwardly in a direction perpendicular to the regions of the filament  404  that are being flattened. The shifting in the walls of the channel causes different portions of the filament to move toward the walls of the channel. The movement of the different portions use the feeding force on the filament and the stiffness of the cold portion to induce mixing of the thermoplastic material in the channel to convert the filament to thermoplastic material more efficiently. Specifically, the downwardly sloping wall  408  helps push thermoplastic material to the side of the channel  232  while the colder center portion of the filament  404  continues in a straight line. This displacement of the thermoplastic material to the sides of the channel continues until the lower wall  412  begins to slope downwardly as well. As the thermoplastic material falls away from the colder center portion of the filament, the center portion is exposed to the heat from at least the upper wall  408 . This exposure hastens the transformation of the center portion of the filament into thermoplastic material. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.