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.

Description:
BACKGROUND—PRIOR ART 
       [0001]    The following is a tabulation of some prior art that presently appears relevant: 
         [0000]    
       
         
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
             
           
               
                   
               
             
             
               
                 U.S. Patents 
               
             
          
           
               
                 Pat. No. 
                 Kind Code 
                 Issue Date 
                 Patentee 
               
               
                   
               
               
                 4,256,945 
                 B1 
                 Mar. 17, 1981 
                 Philip S. Carter 
               
               
                 5,003,145 
                 B1 
                 Mar. 26, 1991 
                 Eugen Nolle et al. 
               
               
                 7,942,987 
                 B1 
                 May 17, 2011 
                 S. Scott Crump et al. 
               
               
                 5,121,329 
                 B1 
                 Jun. 9, 1992 
                 S. Scott Crump 
               
               
                 6,238,613 
                 B1 
                 May 29, 2001 
                 John S. Batchelder 
               
               
                 6,142,207 
                 B1 
                 Nov. 7, 2000 
                 Francis Richardot 
               
               
                 7,194,885 
                 B1 
                 Mar. 27, 2007 
                 Daniel J. Hawkes 
               
               
                   
               
             
          
           
               
                 U.S. Patent Application Publications 
               
             
          
           
               
                 Publication Number 
                 Kind Code 
                 Publ. Date 
                 Applicant 
               
               
                   
               
               
                 20120070523 
                 A1 
                 Sep. 22, 2012 
                 Swanson et al. 
               
               
                   
               
             
          
           
               
                 Foreign Patent Documents 
               
             
          
           
               
                 Foreign 
                   
                   
                   
                   
               
               
                 Doc. Nr. 
                 Cntry Code 
                 Kind Code 
                 Pub. Date 
                 App or Patentee 
               
               
                   
               
               
                 2156715 
                 EP 
                 B1 
                 May 2, 2012 
                 Mcdonald 
               
               
                   
               
             
          
           
               
                 Non-patent Literature Documents 
               
               
                   
               
             
          
           
               
                   
                 Jacob Bayless, UBC-Rapid.com, “Induction Heating Extruder”, 
               
               
                   
                 March 2012 
               
               
                   
                 Reprap.org, “Arcol.hu Hot End Version 4”, January 2013 
               
               
                   
                   
               
             
          
         
       
     
       INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
       [0002]    Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. This application is a divisional of U.S. application Ser. No. 13/843,843 filed Mar. 15, 2013, entitled “Inductively Heated Extruder Heater”. 
         [0003]    One class of 3-D printers or additive manufacturing systems uses thermoplastic filament or rod heated to a softened, molten, or liquid state and extruded through a small hole in a nozzle to build up a part or model. The extruder nozzle is moved relative to a platform, under computer control, to lay down a bead of the thermoplastic on the platform as a feeder mechanism pushes the filament or rod into the extruder heater. The computer interprets a file of movement instructions to drive three axes of motion while starting and stopping the flow of heated plastic. The part or model is built up layer by layer on the platform. 
         [0004]    Prior art heater designs for 3-D printers fall into two categories. The vast majority of filament-type 3-D printers use simple resistance heaters wrapped around or encased in a metal nozzle or heating body (often simply called the “hot end”). The resistance heating element is supplied with direct current or line-frequency (50 or 60 Hz) alternating current, turned on and off by an electronic or mechanical thermostat device to maintain proper temperature. The heating body assembly must be physically large to accommodate a suitably high-wattage resistance heater element. The heater/nozzle assembly is wrapped in insulation to prevent other components in the printer from overheating. The Stratasys U.S. published patent application 2012/0070523 is typical of this approach. Another typical resistively heated extruder nozzle assembly is the Arcol unit. 
         [0005]    Resistance heated extruders are by nature relatively heavy. We have found that the weight of the extruder heater, the large heated zone and the slow response time to temperature set point changes are major limitations on the speed and accuracy of current 3-D printers. 
         [0006]    If the temperature sensor, thermostatic device, or control circuit in a prior art conventional resistive extruder heater fails, we have observed that the heater may overheat or even catch fire. Extra circuitry is needed to detect heater control failure. 
         [0007]    A few printer designs have used or proposed to use an induction heating method (also sometimes called “eddy current heating”). Conventional induction heaters consist of a helical coil of wire surrounding an electrically conductive metal heating block. An oscillator creates a high-frequency alternating current that is applied to the wire coil. The magnetic field created by this current couples to the metal heating block, which heats up due to eddy currents in its internal resistance. We have determined that the magnetic field may also radiate all around the outside of the coil of wire, causing electromagnetic interference and undesired heating of nearby metallic objects. The plastic filament to be melted is fed into an orifice in the heater block. Because the heater block is entirely surrounded by the wire coil, it is difficult to make direct temperature measurements of the heater block so as to properly control the melt temperature. A thermocouple, resistive temperature device, or thermostat placed on the heater block inside the straight-line coil will experience eddy current and hysteresis heating itself, causing errors in temperature measurement. If the heater block is extended far enough beyond the ends of the coil to provide a measurement location not adversely affected by the magnetic field of the straight-line coil, the temperature measured will not accurately reflect the temperature at the center of the heater block where the plastic filament is melted. 
         [0008]    Resistance heaters and straight-coil induction heaters are also the current state of technology in hot-glue adhesive dispensers, both manual hand-operated dispensers and industrial automatic dispensers. We have observed that the large heater blocks necessitated by resistance heating make it difficult to regulate the temperature at the nozzle tip. We have found that heating is slow, and cooling is also slow, leading to dripping of adhesive after the dispenser is turned off. 
         [0009]    We have also observed that the large, hot blocks of metal in conventional resistance heaters in 3-D printers and adhesive dispensers are hazardous to operators because of the large area of exposed nozzle and their long cool-down time after power is removed. 
       SUMMARY 
       [0010]    One embodiment of our inductively heated extruder heater or adhesive dispenser uses an electrically conductive nozzle of minimal size, with an inlet orifice and an outlet orifice connected by a passage, inserted into a gap or hole through a magnetic core formed in the shape of a loop. A high-frequency magnetic field is created in the core by a helical coil of wire wrapped through the center and around the core and connected to a source of high-frequency alternating current. The high-frequency magnetic field in the core gap induces eddy currents in the metal nozzle, rapidly heating it to the melting temperature of the filament or feedstock to be extruded. Another embodiment uses a ferrous material for the nozzle. The magnetic field will cause heating of the nozzle from both eddy current losses due to the electrical conductivity, and hysteresis losses due to the magnetic properties of the ferrous material. 
         [0011]    The soft magnetic core material is selected to have a Curie temperature below the maximum safe operating temperature of the extruder or dispenser. 
       Advantages 
       [0012]    Because there is no excess mass in the inductively heated nozzle of an embodiment of our extruder heater, the time to heat up and cool down is very short, and the power required is much lower than conventional resistively heated extruders or dispensers. In 3-D printers using prior art extruder heaters, we have observed that the slow rate of heating and cooling causes the melted plastic to begin to flow after the extruder head or build platform has begun to move, and continues to flow after the motion has ceased. This lag causes inaccuracies in the parts printed with prior art extruder heaters. 
         [0013]    In addition, the combined mass of the nozzle, magnetic core, and wire in the present invention is much lower than prior art conventional resistive extruder heaters, allowing much higher acceleration of a print head for higher 3-D printing speeds. 
         [0014]    The Curie temperature property of the magnetic core material, selected below the maximum safe operating temperature of the extruder or dispenser makes an embodiment of the heater passively safe in the event of temperature sensor or control circuit failure. No extra circuitry is needed to monitor the temperature sensor or controller. 
         [0015]    In one embodiment, the small mass of the inductively heated nozzle cools off quickly when the high-frequency alternating current is removed, eliminating the dripping and oozing problems we have observed with conventional 3-D printer extruders and adhesive dispensers. Conventional extruders must pull back the filament to prevent dripping or oozing, which adds mechanical complexity and undesirable changes in plastic properties. The present invention can be handled by operators much sooner after turning off, with reduced danger of burns. 
         [0016]    Because the magnetic field induced by the coil is concentrated by the magnetic core onto two small areas on either side of the nozzle heating body, in one embodiment, there are areas not within the magnetic field for easy measurement of the nozzle temperature. Thermocouples or resistive temperature devices attached to the nozzle in these areas outside of the magnetic field region will not experience eddy current or hysteresis heating effects, and thus will provide an accurate indication of the temperature inside the nozzle. Because the nozzle heating body can be made very small, the temperature at the surface being measured will also be very close to the temperature inside the nozzle. 
         [0017]    The inductively heated nozzle in one embodiment has such a small surface area that only a small amount of thermal insulation is required to protect the operator of the 3-D printer or adhesive dispenser and keep the temperature of adjacent components of a 3-D printer cool, reducing the size and cost. 
     
    
     
       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]      10 —filament, rod or other feedstock, omitted in some figures for clarity [0029]  20 —insulated wire coil or coils, omitted in some figures for clarity [0030]  30 —electrically and thermally conductive nozzle or nozzles [0031]  31 —inlet orifice or orifices [0032]  32 —outlet orifice or orifices [0033]  33 —passage or passages, omitted in some figures for clarity [0034]  34 —heat sink flange present in some embodiments [0035]  40 —magnetic non-conductive core [0036]  41 —air gap present in some embodiments [0037]  42 —path of magnetic flux in magnetic core and nozzle [0038]  50 —temperature sensor, omitted in some figures for clarity [0039]  51 —thermostat, omitted in some figures for clarity [0040]  60 —high-frequency alternating current source, omitted in some figures for clarity [0041]  70 —temperature control circuit, omitted in some figures for clarity [0042]  71 —signal from temperature control circuit to alternating current source 
       DETAILED DESCRIPTION 
     First Embodiment—FIGS.  1 A,  1 B and  1 C 
       [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  41 A and  41 B 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 ). 
       Operation—FIGS.  1 A,  1 B, and  1 C 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  41 A and  41 B, either due to manufacturing variations in the core  40  or the nozzle  30 , or by design. The air gaps  41 A and  41 B 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. 
       Description—Additional Embodiments—FIGS.  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  31 A and  31 B and two outlet orifices  32 A and  32 B with two separate passages  33 A and  33 B to extrude two beads of material simultaneously and independently. Two inlets  31 A and  31 B and one outlet  32 , connected by passages  33 A and  33 B, shown in  FIGS. 3B and 3C , embody a blending arrangement to extrude one bead from two feedstock filaments  10 A and  10 B. Passages  33 A and  33 B 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  10 A and  10 B are alternately fed into inlets  31 A and  31 B, such that only one at a time is extruded from outlet orifice  32 .  FIG. 3C  is a cutaway view of  FIG. 3B  making passages  33 A and  33 B visible. 
         [0038]    Multiple magnetic cores  40 A and  40 B 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  40 A and  40 B may also provide faster heating response time. Core  40 A will be wrapped with coil  20 A and connected to high-frequency alternating current source  60 A. Core  40 B will be wrapped with coil  20 B and connected to high frequency alternating current source  60 B, which could have a different amplitude or frequency than source  60 A. Coil  20 B could have a different number of turns than coil  20 A, and core  40 B could have a different Curie temperature than core  40 A. 
         [0039]    In one embodiment, the air gaps  41 A and  41 B 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  41 A and  41 B eliminated. 
         [0040]    Another embodiment,  FIG. 6 , has more than one coil of wire. Two coils  20 A and  20 B may permit a two-phase alternating current drive circuit  60 A and  60 B 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 . 
       Description—Additional Embodiments—FIG.  7 A 
       [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 . 
       Operation—FIG.  7 A 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 Operation—FIG.  7 B 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 . 
       Operation—FIGS.  7 A and  7 B 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 Operation—FIG.  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.