Apparatus and method for thermoplastic extrusion

An electrically operated apparatus and method for altering the flow resistance experienced by a thermoplastic passing through a flow channel using one or more thermal valves having a short response time is described. The extrudate configuration of thermoplastic emerging from one or more extrusion orifices is alterable by selectively opening one or more of thermal valves that supply thermoplastic to the one or more extrusion orifices. Each thermal valve is cooled by a heat sink and has an associated heater responsive to control signals. Each heater is independently activated in synchronization with movement of the apparatus in a predetermined spatial pattern with respect to a base. By extruding thermoplastic onto a base layer-by-layer in this manner, a three-dimensional object may be formed.

CROSS-REFERENCE TO RELATED APPLICATION(S)
 None
 BACKGROUND OF THE INVENTION
 This invention relates to thermoplastic extrusion technologies. More
 particularly, this invention relates to extrusion of thermoplastic in a
 predetermined spatial pattern under computer control.
 Thermoplastic extrusion technologies perform rapid prototyping of
 three-dimensional objects by selectively extruding a molten thermoplastic
 from an extrusion head while moving the extrusion head in three dimensions
 with respect to a base. The thermoplastic is extruded in "beads" or
 "roads" that solidify after being deposited. Movement of the extrusion
 head with respect to the base is performed under computer control, in
 accordance with design data provided from a computer aided design (CAD)
 system. U.S. Pat. Nos. 5,121,329 and 5,764,521, commonly assigned to
 Stratasys, Inc., the assignee of the present invention, describe the rapid
 prototyping deposition modeling technology and are hereby incorporated by
 reference as if set forth fully herein.
 In existing thermoplastic extrusion technologies, the configuration of the
 extrudate is adjustable in quantity and flow rate but not in spatial
 configuration. The flow rate of material out of an orifice is carefully
 controlled, but the spatial configuration (e.g., road width) of the flow
 is not readily alterable.
 As the size of the element of additive material shrinks, a prototype part
 built with those additive elements will typically represent its CAD model
 parent with greater fidelity. For example, depositing layers of
 thermoplastic half as thick using extruded roads that are half as wide
 will improve the feature detail and surface finish of a model by about a
 factor of two. Unfortunately, with a constant deposition velocity, the
 time to build the model with this factor of two increase in resolution
 increases by about a factor of eight.
 This speed/resolution conflict has been resolved in other rapid prototyping
 technologies by replacing vector motion of a single source with raster
 motion of multiple ink jets. An example is the Actua.TM. ink jet rapid
 prototyping system from 3D Systems, Inc., which forms three-dimensional
 models from a wax-like material. The ink jets are individually controlled
 so that any number of the jets will deposit the modeling material at a
 given time. Ink jet-based technologies are attractive for extruding
 discrete quantities of relatively low viscosity materials, however, ink
 jetting techniques have difficulty with high viscosity materials (such as
 thermoplastics) and particulate or fiber-filled materials. These materials
 tend to clog the jets. Also, in thermoplastic extrusion, "wetting" of the
 base by the extruded thermoplastic serves to separate the thermoplastic
 from the extrusion head, while ink-jetted materials break free due to the
 jetting momentum.
 There is an unmet need for a computer-controlled extrusion apparatus
 suitable for dispensing thermoplastic in an extrudate configuration that
 may be varied quickly during deposition, in accordance with movement of
 the apparatus in a predetermined spatial pattern relative to a base.
 BRIEF SUMMARY OF THE INVENTION
 Thermoplastic is extruded in a varying extrudate configuration in
 synchronization with movement of a dispensing apparatus in a predetermined
 spatial pattern with respect to a base, by using the temperature dependent
 properties of the thermoplastic to valve the thermoplastic flow. A heat
 sink cools a valve region of a flow channel within the apparatus to a
 temperature below the lowest flowable temperature of the thermoplastic. A
 heater thermally contacting the valve region creates a thermal valve. The
 heater is capable of receiving heat generation signals from a control so
 as to selectively generate heat. A flow of thermoplastic provided to the
 flow channel is selectively allowed to flow through the thermal valve for
 extrusion in a varying extrudate configuration, by selectively heating the
 valve region to a temperature at which the thermoplastic is flowable in
 accordance with movement in the predetermined spatial pattern. A second
 thermal valve is optionally created by placing a second, independently
 controlled heater in a second valve region along the flow channel. The
 second thermal valve may be used to vary the pressure of the thermoplastic
 extrudate.
 In an alternative embodiment, multiple thermal valves are integrated into
 compact arrays within an extrusion head of the present invention, to
 provide a varying extrudate configuration and high speed extrusion. The
 heaters associated with the various thermal valves in the array are
 selectively and independently controlled in accordance with the
 predetermined spatial pattern.

DETAILED DESCRIPTION
 The present invention alters the flow resistence of a channel using a
 thermal valve technique. The temperature dependant viscosity and
 elasticity of a flowable thermoplastic allows the channel to be valved on
 and valved off by controlling temperature in the channel. The present
 invention is explained in detail below with reference to various
 embodiments. In the explanation of the various embodiments, the same
 reference numerals are used where appropriate to denote the same
 functional elements. Use of the same reference numerals in the various
 embodiments is done for convenience, and it is not intended to limit the
 present invention to any specific embodiment.
 A thermoplastic extrusion apparatus according to the present invention
 showing a first embodiment of a thermal valve is illustrated in FIGS. 1a
 and 1b. The thermoplastic extrusion apparatus 1 is comprised of a body 2
 having a flow channel 4. A plenum 5 is flowably connected to an inlet end
 of flow channel 4, and an outlet end of flow channel 4 forms a nozzle 7
 having an orifice 9. The plenum 5 connects a source of flowable
 thermoplastic with the flow channel 4. Flowable thermoplastic under
 pressure flows in the direction of the arrow from plenum 5 through flow
 channel 4. The cross sectional area for flow in the plenum is preferably
 greater than the cross sectional area for flow in the flow channel, to
 maintain a steady flow pressure in the channel. The thermoplastic emerges
 from orifice 9 having an extrudate configuration 10.
 At a selected position along flow channel 4 are a heater 6, a heat sink 8
 and a thermal resistor 12, which together with the adjacent region of flow
 channel 4 that is thermal contact with heater 6 and heat sink 8 (i.e., the
 "valve region"), form a thermal valve 14. Heater 6 is controlled by
 electrical signals received from a control 16, to selectively generate
 heat. The heater 6 may be a standard surface mount resistor (SMT),
 comprised of, for example, carbon, gallium arsenide, germanium,
 molybdenum, platinum, ruthenium, oxide, silicon or tungsten; or it may be
 another of numerous known selectively heat producing elements, for
 example, a diode, a spark gap or a transistor. In the embodiment shown,
 the heater 6 forms a portion of the flow channel 4, but alternatively a
 conductive channel wall member may separate heater 6 from channel 4. Where
 heater 6 forms a portion of the flow channel, heater 6 optionally has a
 protective layer isolating it from the flow channel 4 to inhibit corrosion
 and maintain smooth channel walls.
 The heat sink 8 functions to remove heat generated by heater 6. The heat
 sink 8, shown as a fin-type heat sink, is maintained at a temperature
 below the lowest flowable temperature of the thermoplastic. The heat sink
 may be maintained at a desired low temperature using a flowing thermal
 fluid, such as water (shown in FIGS.6a and 6b), or using any other active
 cooling technique known to those skilled in the art, such as air cooling,
 thermoelectric cooling, refrigeration or conduction cooling. Passive
 cooling may alternatively be used, if the ambient air temperature is
 sufficiently low. The heater 6 and the heat sink 8 are thermal conductors.
 Thermal resistor 12 is comprised of thermally resistive material, for
 example, ceramic, epoxy, graphite, Kapton, silicon, silicone or Teflon,
 which material provides a thermal resistance between the heat sink 8 and
 the heater 6. In this embodiment, the heat sink 8 thermally contacts the
 flow channel 4 through the heater 6.
 In the absence of heat generated by the heater 6, the thermoplastic in the
 valve region is not flowable, and the flow channel 4 is said to be closed
 or valved off (i.e., thermal valve 14 is closed). As heater 6 provides an
 increasing amount of heat, the temperature drop across the thermal
 resistor 12 increases with the heat flow from the heater 6 to the heat
 sink 8; the temperature of the thermoplastic in the valve region rises,
 and the resistance to flow of the thermoplastic material in the valve
 region drops. When the heat produced by the heater 6 times the thermal
 resistance of the thermal resistor 12 equals the temperature difference
 between the lowest flowable temperature of the thermoplastic and the
 temperature of the heat sink 8, the flow channel 4 is unobstructed in the
 valve region (i.e., thermal valve 14 is open), and the flow channel is
 said to be open or valved on.
 The orifice 9 is an aperture, slot, or pinhole or the like that marks the
 transition from the flow channel 4 to the external environment of the
 extrusion apparatus 1. In rapid prototyping applications, the external
 environment is a chamber in which prototypes are built and in which a
 robot moves an extrusion head carrying an extrusion apparatus of the
 present invention in a predetermined spatial pattern with respect to a
 three-dimensional object being built. Rapid prototyping systems to which
 the present invention is applicable are disclosed, for example, in U.S.
 Pat. No. 5,121,329 and U.S. Pat. No. 5,764,521, assigned to Stratasys,
 Inc., which are hereby incorporated by reference.
 The extrudate configuration 10 refers to the spatial configuration (e.g.,
 cross-sectional dimensions) and temporal character (e.g., flow rate) of
 the thermoplastic extrudate that emerges from the orifice 9.
 The temporal extrudate configuration varies as a function of the amount of
 heat released by heater 6. In other words, the rate that thermoplastic is
 extruded from the nozzle 7 at a given time is dependent on the amount of
 heat produced by the heater 6. Multiple thermal valves may be arranged in
 a series along a flow channel, in parallel to valve multiple flow
 channels, or a combination thereof to further vary the temporal extrudate
 configuration.
 The spatial extrudate configuration may be varied using an array of thermal
 valves 14 arranged in parallel (e.g., embodiments described below).
 Likewise, a single thermal valve 14, such as the embodiment shown in FIGS.
 1a and 1b, can be implemented to vary the spatial extrudate configuration
 by placing the thermal valve at a position contiguous with orifice 9.
 While a thermal valve generally is referred to herein in a binary sense of
 being open or closed, it is clear that the valve can have intermediate
 states. A thermal valve is made relatively more open (decreasing its flow
 resistance) by increasing the heat released by the heater 6, and is made
 relatively more closed (increasing its flow resistance) by decreasing the
 heat released by the heater 6. When thermoplastic adjacent the heater 6 is
 relatively warmer and thermoplastic adjacent the heat sink 8 is relatively
 colder, flow adjacent the heater 6 is constrained to a reduced cross
 section. With the thermal valve located at orifice 9, the spatial
 extrudate configuration at a given time is dependent on the amount of heat
 produced by the heater 6.
 A thermal valve of the present invention may also be used to reduce the
 response time of a liquifier used in thermoplastic extrusion. It is known
 in the art that the response time of a liquifier having a
 pressure-controlled thermoplastic flow (i.e., time for a change of
 pressure at the orifice following a change in thermoplastic feed pressure)
 increases with liquifier length. Similarly, the maximum thermoplastic flow
 rate increases with liquifier length as well. It is desirable in rapid
 prototyping applications to simultaneously reduce liquifier response time
 while increasing the flow rate. Short response times allow an extrusion
 head to accelerate quickly, and large flow rates allow the extrusion head
 to move at a high velocity. Both are important for rapidly creating
 prototypes. Adding a thermal valve near the orifice of a liquifier reduces
 response time independent of liquifier length.
 EXAMPLE 1
 An example is provided of the thermal power required to open a single
 thermal valve of the embodiment shown in FIGS. 1a and 1b. In this example,
 we assume the following: the thermoplastic is ABS (acrylonitrile butadiene
 styrene); the width of flow channel 4 is W=40 mils (0.040 inches) wide;
 the height of flow channel 4, which is the characteristic cross-sectional
 dimension for heat flow in this configuration, is d.sub.c =5 mils high;
 the heater 6 is a ruthenium oxide film on an alumina substrate (a standard
 surface mount (SMT) resistor); the heat sink 8 is copper held at
 80.degree. C.; and the thermal resistor 12 is formed of a Kapton film
 having a thickness d.sub.r =1 mil thick and a length L=60 mils long along
 the flow direction. We take the lowest flowable temperature of ABS to be
 220.degree. C. and the highest non-flowable temperature of ABS to be
 110.degree. C. Between these two temperatures ABS exhibits creep flow. The
 thermal conductivity of Kapton is k.sub.kapton =0.14 Watts/(meter .degree.
 C.), and the thermal resistance of the thermal resistor 12 is calculated
 as R=d.sub.r /(k.sub.kapton LW)=117.degree. C./Watt. The thermal power
 that needs to be produced by the heater 6 to open the thermal valve is
 then (220.degree.-80.degree.)/R=1.2 Watts in steady state.
 EXAMPLE 2
 FIGS. 2a and 2b illustrate temperature profiles in stationary thermoplastic
 for the thermal valve of the first embodiment based on one-dimensional
 heat flow in a semi-infinite solid. Time is plotted along the horizontal
 axis, and temperature in the flow channel is plotted along the vertical
 axis. It is assumed here that the body 2 has the same thermal properties
 as the thermoplastic. In FIG. 2a, at time zero the thermoplastic and the
 insulating material 2 are at 180.degree. C. and the heater 6 is turned on.
 In FIG. 2b, at time zero, the thermoplastic and the insulating material 2
 are at 270.degree. C. and the heater 6 is turned off. The temperature
 profile in the flow channel is shown in both figures at distances of 1
 mil, 2 mils, 3 mils, 4 mils and 5 mils from the heater. In both cases, the
 thermoplastic in the immediate vicinity of the heater/heat sink
 combination has the most rapid initial temperature change.
 EXAMPLE 3
 An example is provided in FIGS. 2c and 2d of the flow rate and switching
 time, respectively, for a hypothetical thermoplastic that abruptly changes
 from infinite viscosity to 500 Poise at 220.degree. C. The volumetric flow
 rate through a channel varies with the cube of the height of the channel,
 d.sub.c.sup.3. A pressure of 500 psi is applied to drive this
 thermoplastic through the thermal valve embodiment shown in FIGS. 1a and
 1b having the dimensions given in Example 1. FIG. 2c shows the flow rate
 time evolution as the channel is valved on and valved off. As shown, the
 channel valves off more quickly than it valves on, and the channel takes
 0.1 seconds to fully open. FIG. 2d charts the time for the channel to
 fully open, as a function of the height of the flow channel 4. The
 switching time quickly increases to more than a second at a relatively
 modest channel dimension of 15 mils.
 A real thermoplastic will have a gradual change in viscosity with
 temperature characteristic, slowing the response times from those of
 Example 3. Additionally, a flowing thermoplastic will carry heat into and
 create pressure in region 14, which was not accounted for in Example 3,
 thus decreasing the switch-on time and increasing the switch-off time. The
 thermal valve switch-on time (i.e., time from valve closed to valve open)
 will be somewhat shorter than the hypothetical switch-on time, and the
 switch-off time (i.e., time from valve open to valve closed) will be
 longer.
 A momentary over-drive and under-drive technique can be used to improve the
 flow rate time response. The rate of opening can be increased by
 momentarily producing substantially more heat from the heater 6 than is
 required to achieve the desired steady state increasing flow rate.
 Similarly, the rate of closing can be increased by momentarily reducing or
 eliminating the amount of heat generated by the heater 6 that is required
 to achieve the desired steady state decreasing flow rate.
 FIG. 3 shows a second embodiment of a thermal valve of the present
 invention, in which the heater 6, resistor 12 and heat sink 8 each
 comprise two elements, mirrored on opposite sides of the flow channel 4.
 The amount of heat absorbed and generated in this embodiment increases
 over that of the first embodiment, thereby increasing the response time of
 the thermal valve 14 for a given channel height. If the height of the
 channel d.sub.c for FIG. 3 is twice that for FIGS. 1a and 1b, the
 switching time for the thermal valve of FIG. 3 will be equal to that of
 FIGS. 1a and 1b, while the flow rate will be increased eight-fold (since
 the volumetric flow varies as d.sub.c.sup.3). For the embodiment of FIG.
 3, the calculated switch-on time associated with d.sub.c =2 mils is 4
 milliseconds. (One-dimensional heat diffusion into a slab, as described in
 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, Oxford,
 1959).
 Geometric constraints are imposed on the flow channel so that a thermal
 valve will switch from open to closed and from closed to open in a time
 useful for thermoplastic extrusion. It is difficult to filter most viscous
 thermoplastics to eliminate impurities smaller than about 2 mils. For
 rapid prototyping applications, as a practical matter it is necessary for
 an extrusion head to deposit material at a linear speed of at least about
 0.1 inches per second, and to have a 50 mil resolution of object features
 along the direction of travel. The maximum switch-on time for a thermal
 valve of the present invention for rapid prototyping applications is thus
 about 500 milliseconds. Taking the embodiment of FIG. 3, the calculated
 channel height associated with a 500 milliseconds switch-on time is
 d.sub.c =22 mils. Accordingly, for rapid prototyping applications, it is
 desirable that the flow channel be between 2 mills and 22 mills high.
 FIG. 4 shows a third embodiment of a thermal valve of the present invention
 in which the heater 6 and the heat sink 8 straddle the flow channel 4. An
 advantage of this configuration is that the thermal resistor 12 is
 eliminated, with the thermoplastic acting as a thermal resistor. The
 switch-off time for this configuration is shorter than that of the FIGS.
 1a and 1b configuration with the same component dimensions, but the turn
 on time is longer.
 FIGS. 5a-5c show a fourth embodiment of a thermal valve of the present
 invention implemented in a manner analogous to a printed circuit board.
 Thermal valve 14 comprises a cooling block 36 surrounded on three sides
 and a top portion by an insulator 28 and contacting the heat sink 8 on a
 fourth side. A thermally conducting hollow cylinder 24 containing a fitted
 tube 22 extend through the cooling block 36 and define the flow channel 4.
 The cylinder 24 is preferably formed of a stack of copper pads with
 co-axial plated-through holes; this configuration in a printed circuit
 board is a via. The tube 22 forms the outside walls of flow channel 4, and
 can be any thermal conductor. Thin-wall stainless steel is particularly
 suitable. The tube 22 is not necessary, however, it helps to seal the
 channel 4 and can be used to define the nozzle. A top end of the thermally
 conducting cylinder 24 extends past the top of cooling block 36 and
 connects to two conductive pads 30, supported on insulator 28. The pads 30
 are preferably copper films formed by standard printed circuit
 lithography, etching and plating techniques. The insulator 28 is made of a
 thermally insulating material, such as glass-filled epoxy laminate. Heater
 6, comprised of two heating elements formed by a surface mount resistor 32
 having two metalized contacts 34, is mounted in thermal and electrical
 contact with conductive pads 30. While two heating elements are shown in
 FIGS. 5a and 5b, use of one to four heating elements is preferred and any
 number may be used. The conductive pads 30 carry heat from heater 6 to the
 flow channel 4 and carry electrical signals from the control 16 to the
 heater 6. A cylindrical spacer 26, made of thermally insulating material,
 isolates the cylinder 24 from the cooling block 36. The cooling block 36
 is preferably metallic, and is preferably copper. The heat sink 8
 maintains the cooling plate below the highest non-flowing temperature of
 the thermoplastic. If desired, feedback on the temperature in the flow
 channel 4 can be provided by temperature sensors. A convenient method for
 detecting the temperature of the embodiment shown in FIGS. 5a-5c is to
 detect the temperature dependent resistance of the surface mount resistors
 32. Advantages of this embodiment are that it uses existing design tools
 and embedded control circuitry, patterns may be laid out lithographically,
 and heat diffusion is two-dimensional.
 FIGS.6a-6c show an extrusion head 70 according to the present invention
 formed of an array of thermal valves configured to provide a varying
 extrudate configuration 56. FIG. 6a shows thermoplastic material 40 forced
 into the extrusion head 70 from a source 74 as the extrusion head 70 is
 moved in a predetermined spatial pattern (according to methods known in
 the art) with respect to a base 72. Electrical signals from the control 16
 synchronize activation and deactivation of heaters 6 with the motion
 between the extrusion head 70 and the base 72, causing extrusion of
 thermoplastic 40 in extrudate configuration 56, such as shown in FIGS. 6b
 and 6c. Extrudate can be applied over or adjacent to previously extruded
 material, as well as directly onto the base 72. In this manner, extrusion
 head 70 is used to make patterns, features or physical object 74 rapidly
 and with high resolution.
 The body of extrusion head 70 is formed of two cooled jaws 60 having
 opposing interior faces mounted in a "V" formation with a small gap
 therebetween, a heated keel 46 positioned between the jaws 60, a heated
 sleeve 42 mounted on top of the jaws 60, and endplates 66 attached at
 opposite ends of jaws 60. A pair of flex circuit boards 58 (FIG. 6b),
 which act as thermal insulators, line the interior faces of jaws 60 and
 extend to exterior edges of jaws 60. Thermally insulating material 44
 isolates the heated keel 46 from the cooled jaws 60 and endplates 66.
 Heaters 6 are mounted on the circuit boards 58 at the bottom edges of the
 interior faces of jaws 60, creating an array of thermal valves and
 defining an elongated nozzle 7 having an orifice 9 shaped as an elongated
 slot. Heaters 6 are preferably formed by a pair of opposing surface mount
 resisting having metalized contacts (shown in FIG. 5a). Jaws 60 are
 chilled by cooling channels 62 carrying flowing water or other coolant,
 and act as a heat sink. Heated sleeve 42 provides pressurized and flowable
 thermoplastic to keel 46, heated by two heaters 50. Plenum 48, a large
 lateral passageway within keel 46, distributes flowable thermoplastic to
 the flow channel 4, formed of nozzle 7 and an array of narrow channels 54
 within keel 46 leading thereto. Channels 54 are flow regulators,
 delivering nearly the same pressure of thermoplastic along the nozzle 7
 independent of the instantaneous extrudate configuration. The keel 46
 maintains a constant pressure source of thermoplastic along the length of
 the plenum 48 independent of where thermoplastic is flowing in the
 elongated nozzle; the pressure drop for thermoplastic material passing
 through the channels 54 is designed to be larger than the largest pressure
 drop that can occur along the length of the plenum 48.
 The extrusion head 70 of FIGS. 6a-6c has ten heaters 6; each pair of
 heating elements straddling a different section of the nozzle 7. A thermal
 valve of the embodiment shown in FIG. 3 is created by each of the ten
 heaters 6, together forming an array of thermal valves in parallel. Each
 heater 6 is individually controlled to activate and deactivate
 independently, thereby selectively opening and closing portions of the
 nozzle 7. If only the leftmost heater 6 of FIG. 6c is activated, then the
 thermoplastic is flowable through the nozzle 7 only at that location,
 thereby producing extrusion configuration 56 corresponding to the geometry
 of the nozzle orifice adjoining the leftmost heater 6.
 For some applications, it is desirable to have multiple thermal valves in
 series along a flow channel. For example, if the latent heat of the
 thermoplastic is sufficiently low, valving-off the flow channel may result
 in the thermoplastic being nearly solid when it comes in contact with the
 base 72 or with previously extruded material. In such a case, a solid
 bridge of thermoplastic tends to form from the previously extruded
 material, through the newly extruded and solidified thermoplastic, to
 solidified thermoplastic in the flow channel 4. This solid bridge can
 immobilize the extrusion head 70 with respect to the base 72. Adding a
 downstream thermal valve near the orifice 9 can prevent formation of a
 solid thermoplastic bridge.
 An extension of the thermal valve embodiment of FIG. 3 to two thermal
 valves in series is shown in FIG. 8. An upstream and a downstream heater 6
 create two sequential valves 14 in the flow channel 4. As shown, a single
 heat sink 8 and resistor 12 (each having two opposing elements) straddle
 the two heaters 6. Alternatively, two separate heat sinks 8 and resistors
 12 may be used. To prevent formation of a solid thermoplastic bridge, the
 downstream valve, placed near the orifice 9, is maintained open while the
 upstream valve is closed. The downstream valve is closed when extrusion
 will cease for a time substantially longer than the switching time. A fast
 switching upstream valve may be used with a slower response downstream
 valve that can seal against creep. Thermal valves in series may also be
 used to vary orifice size, while gating the flow. The upstream valve gates
 flow while the downstream valve sets orifice size.
 FIG. 9 shows an embodiment of a series configuration of thermal valves as
 an extension of the printed circuit board technology of FIGS. 5a-5c. An
 upstream tube 22 and a downstream tube 22, each having an associated
 thermally conducting cylinder 24, define two valve regions in flow channel
 4. The top of the upstream thermally conducting cylinder 24 connects to an
 upstream pair of conductive pads 30, and the bottom of the downstream
 thermally conducting cylinder 24 connects to a downstream pair of
 conductive pads 30. Each pair of conductive pads 30 has a heater 6,
 comprising a pair of heating elements mounted thereon. A gap 116 between
 the upstream and downstream thermally conducting cylinders 24 and tubing
 22 is filled with insulating material 28, providing thermal isolation
 between the two valves.
 FIGS.10a and 10b show an extrusion head 70 formed by a two-dimensional
 array of 104 thermal valves in parallel, based on the printed circuit
 board technology of FIGS. 5a-5c. The orifice diameter and relative
 positions are chosen so that a single pass of the array over a region
 (traversing vertically as the array is oriented in FIG. 10a) can solidly
 fill a region with a layer of thermoplastic. For orifices 9 having an
 inner diameter of 13 mils and a deposited road of 15 mils wide and 10 mils
 high per nozzle, the array would span 1.6 inches and would deposit at a
 peak rate of about a pint of thermoplastic per hour at an inch per second
 velocity. A redundant orifice configuration (e.g., in FIG. 10a, a second
 array displaced a small distance vertically on the page from the existing
 first array) could be added to allow continued extrusion with a partially
 clogged first array, given a mechanism to detect clogging in the orifices.
 FIG. 10b shows the left most eight valves and orifices, viewed along the
 section 10b--10b of FIG. 10a. An upper housing 124 having a horizontal top
 section and four vertical walls, cooling block 36 mounted beneath the
 walls of housing 124, a bottom heated steel plate 134 and insulating
 material 28 form the body of extrusion head 70. Insulating material 28
 lines the walls of housing 124, separates a lower face of cooling block 36
 from steel plate 134 and separates an upper face of cooling block 36 from
 a heated keel plate 126. Space in the housing 124 above keel plate 126
 forms the plenum 5. The 104 flow channels 4 each extend from plenum 5
 through keel plate 126, through cooling block 36 and through steel plate
 134. Near the bottom surface of steel plate 134, the flow channels 4 each
 terminate in an associated nozzle 7. Additionally, each flow channel 4 has
 an upstream and a downstream cylindrical spacer 120. The upstream
 cylindrical spacer extends from the keel plate 126 to the cooling block
 136, and the downstream cylindrical spacer extends from the cooling block
 36 to the steel plate 134. Spacers 120 form the portion of the flow
 channels leading to and from the thermal valves 14, which valves are
 located along the flow channels where the flow channels pass through the
 cooling block 36. Polytetra-fluoroethylene washers may be used as the
 cylindrical spacers 120. The flow channels 4 are preferably constricted at
 the position of the thermal valves 14, to decrease the response time of
 the valves. The relatively larger diameter of other portions of the flow
 channels 4 maintain a desired low pressure drop through the channels.
 Circuit board 58, mounted on top of cooling block 36, carries contact pads
 30. Contact pads 30 and circuit board 58 exit the side of the housing 124
 to allow electrical connection with external drive and sensing circuits.
 In the absence of heat generated by heaters 6 (i.e., pairs of opposing
 resistors 32 placed on contact pads 30), the thermal valves are cooled by
 cooling block 36 to a temperature below the highest non-flowing
 temperature of the thermoplastic.
 A section of tubing 132 is swaged into each nozzle 7 in the steel plate
 134, thereby defining the orifices 9. The housing 124 and the heated steel
 plate 134 should be rigid enough not to experience significant distortion
 when the plenum 5 is pressurized to several hundred psi.
 FIG. 7 shows an alternate thermal valve array for use in an extrusion head
 70 such as shown in FIGS.6a-6c. Heaters 6 are integrated into two silicon
 chips 150. The chips 150 mounted in "v"-formation with a gap between their
 bottom inside edges create a nozzle region having an orifice 9 in the
 shape of a slot. The bottom outside edges of the chips 150 are ground to
 make the orifice 9 the lowest point of the assembly. Conductors 154
 implanted into or deposited on the silicon chips 150 carry signals to the
 nozzle region, where heaters 6 (e.g., pairs of diodes, resistors or
 transistors) convert the signals to heat. The thermal conductivity and
 thickness of the silicon chips 150 are selected so that the silicon acts
 as a thermal resistor. The silicon chips 150 are thus suitable for
 mounting directly onto heat sinks. An advantage of this design is that it
 can be made spatially dense. Another advantage is that additional
 circuitry, such as temperature sensors and signal de-multiplexers, can be
 included in the silicon chips without a significant cost increase.
 In order to utilize the high deposition rates of which an extrusion head
 having an array of thermal valves is capable, the extrusion head must be
 supplied with high flow rate sources of pressurized liquified
 thermoplastic. The thermoplastic source should not liquify more
 thermoplastic than will be used in a relatively short time, due to the
 finite pot life typical of most thermoplastics.
 The embodiment of FIG. 11 addresses potential problems of sudden increases
 and decreases in pressure in an extrusion head of the present invention.
 Solidified thermoplastic generally expands as it is heated. Temperature
 increases without a corresponding release of pressure could therefore
 cause the extrusion head to become destructively over-pressurized. For
 example, at a transition between rapidly extruding along the length of the
 nozzle as solid thermoplastic is rapidly fed into the extrusion assembly,
 to not extruding at all, the relatively colder thermoplastic in the
 extrusion assembly will expand as it is heated. This effect is amplified
 by absorbed materials in the thermoplastic, such as water, which will
 vigorously expand as the thermoplastic is heated. The generation of gas by
 water in the extrusion assembly poses the converse concern that a gas
 pocket will exit suddenly out of the nozzle, creating a pressure drop.
 Unless the thermoplastic feed mechanism can supply additional material to
 quickly refill the extrusion assembly with thermoplastic, the pressure
 drop will result in an incorrect extrudate configuration.
 In FIG. 11, the extrusion head 70 of FIG. 6a is modified to include a
 pressure regulator. Thermoplastic 40 is forced into the heated sleeve 42
 by thermoplastic source 74 under the control of control 16, generating
 pressurized and flowable thermoplastic. The pressurized thermoplastic
 flows into a chamber 218 created between two accumulator pistons 210. The
 accumulator pistons 210 are forced towards each other by a pair of
 opposing springs 214. Two sets of magnetic sense coils 216 monitor the
 lateral positions of the pistons 210, providing pressure feedback
 information to the control 16. Thermoplastic flows from the chamber 218
 into the plenum 48 of keel 46. In the event that the pressure feedback
 indicates that pressure in the extrusion head is excessive, a relief valve
 212 located in plenum 48 opens and releases thermoplastic into the ambient
 environment. The relief valve 212 can be a spring-loaded ball valve or
 other apparatus well known to those skilled in the art. Also, the
 spring-operated accumulator pistons 210 can be replaced with a sealed gas
 bellows, a bladder, or other accumulator design well known to those
 skilled in the art. Since most engineering plastics thermally degrade with
 exposure to heat, it is desirable that the accumulator not have pockets or
 crevices that retain thermoplastic for long periods of time.
 FIGS. 12a through 12c show the extrusion head 70 of FIGS. 6b and 6cmodified
 to include one possible source 74 of thermoplastic under pressure, using
 motors to actively regulate pressure. A heated housing 220 has two access
 ports 238, each leading to a heated chamber 228. Each access port 238 has
 an associated piston 234 and a driveshaft 224 driven by a rack gear 226
 and a remote motor. Access ports 238 receive cylindrical slugs of solid
 thermoplastic 222. One of the pistons 234 motor drives the slug into the
 heated chamber 228, where the slug 222 is liquified by thermal contact
 with the heated housing 220. The heated housing 220 further has two flow
 channels 230 which connect each heated chamber 228 to an associated
 one-way valve 232. The one-way valves lead to a single feed channel 233
 within housing 220. Once the slug 222 is partially liquified, the piston
 is advanced by the remote motor, forcing pressurized thermoplastic through
 one of the flow channels 230, through the associated one-way valve 232,
 and into the feed channel 233. The feed channel 233 terminates in plenum
 48. Torque required from the motor to advance the pistons 234 is used to
 control the pressure of the thermoplastic; in this manner thermoplastic
 can advance either quickly or slowly into the plenum 48, depending on how
 many thermal valves are open at a given time. Several of these piston
 assemblies can be mounted over the extrusion head in order that
 thermoplastic slugs can simultaneously be heating in some and delivered as
 pressurized fluid from others.
 The disclosed thermoplastic extrusion apparatus and method have
 applicability outside of rapid prototyping. For example, apparatus of the
 present invention can be used to deposit patterns of thermoplastic solder
 paste on circuit boards, or to dispense patterns of hot melt adhesive for
 assembly of clothing and other cloth articles.
 Although the present invention has been described with reference to
 preferred embodiments, workers skilled in the art will recognize that
 changes may be made in form and detail without departing from the spirit
 and scope of the invention. It should be understood that while this
 description is made by way of preferred example, the invention is defined
 by the scope of the claims.