METHODS RELATED TO INDUCTIVE HEATING IN EXTRUDERS

Abstract

Methods related to inductive heating in extruders. In some embodiments, a method for heating a feedstock or liquid material can include providing a heating body having inlet and outlet orifices connected via a passage or passages or mixing chamber to define a nozzle, and forming a magnetic loop with a coil of conductive wire wound through the center and around the outside of a core of magnetic but electrically non-conductive or low-conductivity material. The method can further include a high-frequency alternating current applied to the coil, producing a magnetic flux locally heating the nozzle. Some embodiments have passive regulation or limiting of nozzle temperature by selection of a core material with an appropriate Curie temperature.

Claims

1. A method for heating a feedstock of solid or liquid material, comprising: a) providing a heating body having inlet and outlet orifices connected via a passage or passages or mixing chamber to define a nozzle; b) forming a magnetic loop with a coil of conductive wire wound through the center and around the outside of a core of magnetic but electrically non-conductive or low-conductivity material; c) forming a complete magnetic circuit by operating said heating body between two ends of said loop; d) feeding said feedstock in the form of filament, rod, wire, granules, or liquid stream into said inlet orifice or orifices; e) applying a high frequency alternating current to said coil to induce a magnetic flux in said core, the magnetic flux locally heating said heating body to heat said material in said passage or passages; and f) extruding said feedstock from said outlet orifice or orifices.

2. Method of claim 1, providing a temperature sensor or thermostatic device measuring the temperature of said heating body, connected to said alternating current source such as to maintain the temperature of said heating body by varying voltage, current, or frequency of said source, or by cycling said source on and off.

3. Method of claim 1, where said magnetic core is formed from material chosen for a specific Curie temperature, such that said heating body is passively limited to or regulated at a specific temperature,

4. Method of claim 1, where said feedstock is heated at two or more different temperatures in two or more heat zones in said heating body, by means of two or more of said magnetic cores sandwiching said heating body.

5. Method of claim 1, where two or more sources of said feedstock are heated, mixed, and extruded, by feeding said feedstock into two or more inlet orifices connected to a mixing chamber and thence to an outlet orifice defining a mixing nozzle.

6. Method of claim 1, where two or more sources of said feedstock are heated, mixed, and extruded, by feeding said feedstock into two or more inlet orifices, connected to a mixing chamber, with individual heating zones for each feedstock inlet, connecting to an outlet orifice defining a mixing nozzle.

7. Method of claim 1, where two or more sources of said feedstock are heated and extruded, without mixing, by feeding said feedstock into two or more inlet orifices connected to one or more outlet orifices by individual passages defining one or more nozzles in one heating body.

8. Method of claim 1, where said heating body has a flange comprising a heat sink attached to or formed in said heating body to selectively cool portions of said heating body.

9. A method for heating a feedstock of solid or liquid material for additive manufacturing or 3D printing, comprising: a) providing a heating body having inlet and outlet orifices connected via a passage or passages or mixing chamber to define a nozzle; b) forming a magnetic loop with a coil of conductive wire wound through the center and around the outside of a core of magnetic but electrically non-conductive or low-conductivity material; c) forming a complete magnetic circuit by operating said heating body between two ends of said loop; d) feeding said feedstock in the form of filament, rod, wire, granules, or liquid stream into said inlet orifice or orifices; e) applying a high frequency alternating current to said coil to induce a magnetic flux in said core, the magnetic flux locally heating said heating body to heat said material in said passage or passages; and f) extruding said feedstock from said outlet orifice or orifices.

10. Method of claim 9, providing a temperature sensor or thermostatic device measuring the temperature of said heating body, connected to said alternating current source such as to maintain the temperature of said heating body by varying voltage, current, or frequency of said source, or by cycling said source on and off.

11. Method of claim 9, where said magnetic core is formed from material chosen for a specific Curie temperature, such that said heating body is passively limited to or regulated at a specific temperature,

12. Method of claim 9, where said feedstock is heated at two or more different temperatures in two or more heat zones in said heating body, by means of two or more of said magnetic cores sandwiching said heating body.

13. Method of claim 9, where two or more sources of said feedstock are heated, mixed, and extruded, by feeding said feedstock into two or more inlet orifices connected to a mixing chamber and thence to an outlet orifice defining a mixing nozzle.

14. Method of claim 9, where two or more sources of said feedstock are heated, mixed, and extruded, by feeding said feedstock into two or more inlet orifices, connected to a mixing chamber, with individual heating zones for each feedstock inlet, connecting to an outlet orifice defining a mixing nozzle.

15. Method of claim 9, where two or more sources of said feedstock are heated and extruded, without mixing, by feeding said feedstock into two or more inlet orifices connected to one or more outlet orifices by individual passages defining one or more nozzles in one heating body.

16. Method of claim 9, where said heating body has a flange comprising a heat sink attached to or formed in said heating body to selectively cool portions of said heating body.

17. A method for heating a feedstock of solid or liquid material for adhesive application, comprising: a) providing a heating body having inlet and outlet orifices connected via a passage or passages or mixing chamber to define a nozzle; b) forming a magnetic loop with a coil of conductive wire wound through the center and around the outside of a core of magnetic but electrically non-conductive or low-conductivity material; c) forming a complete magnetic circuit by operating said heating body between two ends of said loop; d) feeding said feedstock in the form of filament, rod, wire, granules, or liquid stream into said inlet orifice or orifices; e) applying a high frequency alternating current to said coil to induce a magnetic flux in said core, the magnetic flux locally heating said heating body to heat said material in said passage or passages; and f) extruding said feedstock from said outlet orifice or orifices.

18. Method of claim 17, providing a temperature sensor or thermostatic device measuring the temperature of said heating body, connected to said alternating current source such as to maintain the temperature of said heating body by varying voltage, current, or frequency of said source, or by cycling said source on and off.

19. Method of claim 17, where said magnetic core is formed from material chosen for a specific Curie temperature, such that said heating body is passively limited to or regulated at a specific temperature,

20. Method of claim 17, where said heating body has a flange comprising a heat sink attached to or formed in said heating body to selectively cool portions of said heating body.

Description

DRAWINGS

[0018] Figures

[0019] FIGS. 1A and 1B show embodiments illustrating different nozzle shapes.

[0020] FIG. 1C is a cross-sectional view of the first embodiment.

[0021] FIGS. 2A, 2B, and 2C show embodiments illustrating different shaped magnetic cores.

[0022] FIGS. 3A, 3B and 3C show embodiments illustrating different nozzle orifices.

[0023] FIG. 4 shows a dual heat zone embodiment.

[0024] FIGS. 5A and 5B show cross-sectional views illustrating tapered nozzle embodiments.

[0025] FIG. 6 shows a dual wire coil embodiment.

[0026] FIGS. 7A and 7B show embodiments incorporating temperature sensing and control.

[0027] FIG. 8 shows one embodiment in a 3-D printer.

DRAWINGS

Reference Numerals

[0028] 10filament, rod or other feedstock, omitted in some figures for clarity [0029] 20insulated wire coil or coils, omitted in some figures for clarity [0030] 30electrically and thermally conductive nozzle or nozzles [0031] 31inlet orifice or orifices [0032] 32outlet orifice or orifices [0033] 33passage or passages, omitted in some figures for clarity [0034] 34heat sink flange present in some embodiments [0035] 40magnetic non-conductive core [0036] 41air gap present in some embodiments [0037] 42path of magnetic flux in magnetic core and nozzle [0038] 50temperature sensor, omitted in some figures for clarity [0039] 51thermostat, omitted in some figures for clarity [0040] 60high-frequency alternating current source, omitted in some figures for clarity [0041] 70temperature control circuit, omitted in some figures for clarity [0042] 71signal from temperature control circuit to alternating current source

DETAILED DESCRIPTION

First EmbodimentFIGS. 1A, 1B and 1C

[0029] The embodiment shown in FIGS. 1A to 1C is an inductively heated extruder heater. The nozzle 30 consists of a heating body made of an electrically and thermally conductive material, such as steel, with an inlet orifice 31 and an outlet orifice 32. The inlets and outlets are connected by a passage 33 (not visible in FIGS. 1A-1C). The nozzle 30 fits into a hole or gap cut or formed through a loop of high-permeability soft magnetic material such as ferrite or pressed iron powder, forming a core 40.

[0030] Electrically conductive wire is coiled around and through this loop to form one or more coils 20. An high-frequency alternating current source 60 applies a high-frequency alternating current to the wire coil or coils 20. There may optionally be small air gaps 41A and 41B present between the nozzle 30 and the magnetic core 40.

[0031] A filament, rod, wire or other feedstock 10 of meltable or flowable material is introduced to inlet orifice 31 when the nozzle 30 has reached operating temperature. The force required to push feedstock 10 into the extruder heater is provided by external mechanisms. The melted material exits outlet orifice 32 after traveling through the passage 33 (not visible in FIGS. 1A-1C).

OperationFIGS. 1A, 1B, and 1C Embodiment

[0032] The high-frequency alternating current flowing in the wire coil or coils 20 creates a strong magnetic field within the core 40 of high-permeability material, around path 42. Because it is a closed loop, the magnetic field is nearly all contained within the loop. Very little electromagnetic radiation leaks from the coil to cause interference to nearby electronics or radio devices, a problem we have observed with prior art inductive heater designs. Ferrite, iron powder and other known magnetic core materials exhibit only very small internal energy losses, because the magnetic particles are very small and insulated from each other by extremely thin layers of non-magnetic, non-conductive material. The conductive nozzle 30 inserted into the loop, however, will have high losses (in the form of heat) from eddy currents created by the magnetic field. In the case of nozzles 30 formed from ferrous materials, additional heating takes place from hysteresis losses. These losses are used by this embodiment to melt the filament, rod, or other feedstock 10 to be extruded. The loop of magnetic material forming core 40 will often be in the general shape of a toroid, although other shapes can also work, as long as they form a closed magnetic circuit.

[0033] In some embodiments, there will be present air gaps 41A and 41B, either due to manufacturing variations in the core 40 or the nozzle 30, or by design. The air gaps 41A and 41B will lower the permeability and increase the reluctance of the magnetic circuit through core 40 and nozzle 30. A higher alternating current amplitude from alternating current source 60 or more turns of wire in coil 20 will maintain a sufficiently high magnetic field to heat nozzle 30 to the desired temperature.

[0034] Non-magnetic nozzle materials that could work in some embodiments might include tungsten, graphite, copper, or aluminum. Additional electrically and thermally conductive materials are possible.

[0035] In some embodiments, a flange 34 is formed at the top of nozzle 30 to reduce the flow of heat up the filament 10. The flange 34, if present, will radiate some of the heat flowing up the filament 10 by conduction, keeping down the temperature of filament 10 before it enters inlet orifice 31. The flange 34 could also be formed near the outlet orifice 31 to cool the molten material as it exits. Flange 34 could also be formed elsewhere on nozzle 30 to provide selective or localized cooling as desired.

DescriptionAdditional EmbodimentsFIGS. 2-6

[0036] A circular toroidal shape of core is not the only possible configuration. FIG. 2A shows a rectangular shaped magnetic core 40. Any shape is possible, as long as it forms a continuous magnetic circuit. The soft magnetic material can be made in bulk and cut to the desired shape, or can be pressed, molded, or sintered in the final shape. The magnetic core 40 could be fabricated in segments and fused or held together by high temperature adhesives or mechanical methods. The nozzle 30 may be inserted in a hole in core 40 that does not completely sever the core. FIG. 2B is a cross-section illustrating such an embodiment. FIG. 2C shows an embodiment with a more complicated magnetic circuit. There is still a continuous magnetic path 42 through core 40 and nozzle 30. Magnetic flux, created by the high frequency current from source 60 flowing in coil 20 will substantially follow magnetic path 42 to heat nozzle 30 by induced eddy currents.

[0037] The nozzle 30 must have at least one inlet orifice 31 and one outlet orifice 32 to extrude feedstock material 10. FIG. 3A illustrates an embodiment with two inlet orifices 31A and 31B and two outlet orifices 32A and 32B with two separate passages 33A and 33B to extrude two beads of material simultaneously and independently. Two inlets 31A and 31B and one outlet 32, connected by passages 33A and 33B, shown in FIGS. 3B and 3C, embody a blending arrangement to extrude one bead from two feedstock filaments 10A and 10B. Passages 33A and 33B can take different forms in different embodiments, or be combined into one mixing chamber, to achieve specific mixing characteristics. In another embodiment represented by FIG. 3B and FIG. 3C two different feedstocks 10A and 10B are alternately fed into inlets 31A and 31B, such that only one at a time is extruded from outlet orifice 32. FIG. 3C is a cutaway view of FIG. 3B making passages 33A and 33B visible.

[0038] Multiple magnetic cores 40A and 40B can share a common nozzle 30 for purposes of multi-zone heating. FIG. 4 illustrates such an embodiment. This is advantageous for feedstock materials 10 that require a preheating step to alter some material properties, such as viscosity or moisture content, before final melting. Multiple cores 40A and 40B may also provide faster heating response time. Core 40A will be wrapped with coil 20A and connected to high-frequency alternating current source 60A. Core 40B will be wrapped with coil 20B and connected to high frequency alternating current source 60B, which could have a different amplitude or frequency than source 60A. Coil 20B could have a different number of turns than coil 20A, and core 40B could have a different Curie temperature than core 40A.

[0039] In one embodiment, the air gaps 41A and 41B due to dimensional variations that could occur in manufacturing magnetic core 40 and nozzle 30 are eliminated by foaming the nozzle 30 and the gap in core 40 with matching tapers, as shown in FIGS. 5A and 5B. Variability of magnetic field from heater assembly to heater assembly during manufacturing may be reduced with air gaps 41A and 41B eliminated.

[0040] Another embodiment, FIG. 6, has more than one coil of wire. Two coils 20A and 20B may permit a two-phase alternating current drive circuit 60A and 60B with fewer components than a typical single-phase circuit. Three coils could permit a three-phase alternating current drive circuit, which may have some efficiency benefits. Embodiments with additional coils are possible. An embodiment with a single coil with a center-tap may permit simplified drive electronics, equivalent to the two-coil circuit illustrated in FIG. 6.

DescriptionAdditional EmbodimentsFIG. 7A

[0041] One embodiment includes a temperature sensor 50, such as a thermocouple, resistive temperature device, or thermistor, to measure the temperature of the nozzle 30, and communicate that temperature to a control circuit 70, which controls the alternating current source 60 by signal 71.

OperationFIG. 7A Embodiment

[0042] In the embodiment of FIG. 7A, the alternating current source 60 has adjustable frequency or amplitude. The adjustment is performed by signal 71 from temperature control circuit 70 in response to changes in the temperature of nozzle 30 as measured by sensor 50. A person skilled in the art is familiar with suitable temperature control circuits. The magnetic field strength in magnetic core 40 is directly related to and controlled by the amplitude and frequency of the alternating current in coil 20.

Description and OperationFIG. 7B Embodiment

[0043] Another embodiment uses a thermostatic device 51 in contact with the nozzle 30 to turn the alternating current on and off in coil 20 to control the temperature in nozzle 30. The thermostat 51 may either disconnect the supply of high-frequency alternating current to the coil 20, as shown in FIG. 7B, or it may alternatively disconnect the power source to the alternating current source 60.

OperationFIGS. 7A and 7B Embodiments

[0044] The magnetic permeability of ferrite and iron powder materials varies somewhat with temperature. As the temperature of the material rises, it eventually reaches a point called the Curie temperature. Above the Curie temperature, the permeability drops to negligible levels. This causes the magnetic field to also drop to very low levels. A thin layer of the soft magnetic core that is in contact with the nozzle will heat up to the temperature of the nozzle by thermal conduction. When this exceeds the Curie temperature, the permeability of this thin layer will drop. The magnetic field will then drop, reducing the eddy current and hysteresis losses that are heating the nozzle. Inductive heaters for soldering irons have used this property to regulate the temperature of their heating elements. In the embodiments shown in FIGS. 7A and 7B, the Curie temperature is used as a safety measure. If the control circuitry 70 or sensor 50 or thermostat 51 malfunctions, the magnetic core 40 temperature cannot exceed the Curie temperature because the magnetic field in magnetic core 40 will drop, lowering the eddy and hysteresis currents in nozzle 30, which will lower the temperature in nozzle 30 to a temperature close to the Curie temperature of core 40. Choosing a core material with a Curie temperature lower than the maximum safe temperature of the heater assembly and feedstock material makes this embodiment passively safe from overheating or fire, which we have found to be a serious problem with prior art extruder heaters.

Description and OperationFIG. 8 Embodiment

[0045] A 3-D printer or additive manufacturing system may consist of a build bed 80, where the part is printed or formed, layer by layer, the filament feeder 90, the extruder heater 100, and a mechanism 110 to move the extruder relative to the build bed 80. A control circuit 70 actuates the movement of the extruder relative to the build bed 80, the temperature of the extruder 100, and the feed rate of the filament feeder 90. The smaller the extruder heater 100 the smaller the printer can be, and the lighter the extruder heater 100, the faster extruder heater 100 can be moved relative to the build bed 80. The smaller the mass being heated in the extruder 100, the faster the filament feed rate can be changed. Printing a 3-D part requires the filament feed to be started and stopped many times for each layer deposited. Our inductive extruder heater focuses the heating energy to the smallest possible mass in the nozzle, permitting much faster operation than prior art 3-D printers. Because the heating body in some embodiments of our extruder heater is very small, with a very short passage for the filament 10 to pass through, much less force is required to push the filament 10 into and through the nozzle (not shown in this FIG. 8). Less force required permits smaller feed mechanisms than necessary for prior art extruder heaters.

[0046] We have found it desirable to have multiple filament feeders 90 and extruder heaters 100 in 3-D printers, permitting a part to be formed with more than one color or type of plastic filament 10. Prior art extruders were too heavy and bulky to permit multiple filaments in a compact printer. An embodiment of our extruder heater is small enough that multiple extruders can be easily installed in even very compact 3-D printers.

CONCLUSION, RAMIFICATIONS, AND SCOPE

[0047] Accordingly, at least one embodiment of this inductively heated extruder heater is much lighter, more compact, and more energy efficient than conventional extruder heaters, reaches operating temperature in far less time, and responds to temperature set point changes much quicker, while possessing inherent safety not present in prior art extruder heaters. The material costs to produce this design are lower than conventional resistance heaters, and the components are well suited to low-cost, automated manufacturing.

[0048] Despite the specific details present in our descriptions above, these should not be construed as limitations on the scope. Rather they serve as exemplification of several embodiments. Many other variations are possible. For example, the tapered nozzle may be used with either circular or non-circular soft magnetic cores. The inlet and outlet orifices in the nozzle do not have to be concentric. The nozzle does not need to be positioned perpendicular to the plane of the toroidal core. The nozzle may be inserted into a hole through the core, without the core being completely severed. The wire used in the coil may be of round or rectangular cross-section, and may have any type of insulation between turns, including air, that is compatible with the operating temperatures. The shape and size of the inlet and outlet orifices may be adjusted to suit the materials being extruded. Instead of filament or rod feedstock, a tube may deliver granular or viscous material to the heater, which will be melted or heated to a reduced viscosity condition before exiting the outlet. The soft magnetic core may have a complex three-dimensional shape, resulting in a magnetic path that does not lie in a plane. The heat sink flange, if present, may be in many different forms and shapes, as needed, to radiate heat away from the feedstock.

[0049] Accordingly, the scope should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.