Abstract:
Disclosed herein is an apparatus and method with inductive heating of an electrically conductive workpiece such as a barrel used in molding or extrusion, having a layer of thermal insulation interposed between the induction windings and the workpiece, and using alternating current (AC) at an elevated frequency. Further, variable pitch induction windings may be used to generate a non-uniform and calculated heat input profile, such as to compliment the configuration of a screw for transporting material through the barrel.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an apparatus and method for heating an electrically conductive workpiece by inductive heating. More particularly this invention relates to inductive heating of a ferrous workpiece, such as an extrusion or molding barrel, using alternating current (AC) at an elevated frequency. While the application of the invention to barrel heating is described in detail herein, this invention can include the heating of any workpiece through which material flows, provided said workpiece is responsive to AC inductive heating and provided said workpiece can be substantially surrounded by an induction coil and an interposed thermal insulating layer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Referring to  FIGS. 1 and 2 , it is commonly known how extruders and molding machines can take fluids or solids and more commonly the latter, such as plastic or magnesium, in such forms as pellets, powder, granules, or chips, (hereinafter collectively referred as processed “material”  1 ) fed through a feed port  3  in a cylindrical metal tube or barrel  5  and then mixed, heated, and perhaps melted into a homogeneous molten state. Of course, there are various means of molding, such as injection molding, blow molding, injection blow molding, and extrusion blow molding, all of which are herein generally referred to as “molding”, and to all of which the present invention may be applied. With extruders and molding machines, a screw  7  rotates within the barrel  5  to ingest the material and transport it along a helical path toward the exit at the nozzle or die end  9 . Shear heat Q S  is generated by frictional interaction between the material, screw  7  and barrel wall. This shear heat, combined with heat conducted into the material Q P  from the heated surrounding barrel melts the material  1 . The molten material is then mixed and compressed before exiting. 
         [0003]    Electrical contact resistance heaters  11 , of which there are many types, are typically used to heat the barrel  5  by external circumferential contact. Frequently used types of contact resistance heaters include those commonly referred to in the art as mica band-heaters, ceramic band-heaters, and cast aluminum heaters, which are also referred to generally as cast-in heaters. More rarely barrels are heated by other means, such as by hot oil circulated within channels in the barrel wall or within separate contacting elements through which the oil circulates. Due to the added cost and complexity, and the slower control response of the oil&#39;s thermal mass, oil-heated devices are limited to special applications, such as the processing of thermosets, including phenolics, ureas, and rubber. 
         [0004]    Referring still to  FIGS. 1 and 2 , ohmic heat generation within contact resistance heaters  11  is typically accomplished by applying a constant 50-60 Hz AC voltage across an array of resistance heaters  11  electrically connected in series and/or parallel. Closed-loop control of the barrel&#39;s temperature is then accomplished in sequential axial zones  13 , often three to six zones, and sometimes more, of approximately equal length, by means of one or more thermocouples  15  embedded within the barrel wall  17  in each zone  13 , one temperature controller per zone (not shown, typically a stand-alone controller, PLC-based controller, or PC-based controller, employing some level of PID control), and one or more relay-activated power contactors  19  per zone  13 . For practical reasons typical controllers turn power “on” and “off” to the resistance heaters  11 , in thermostatic fashion, in order to maintain the barrel zone temperatures within an acceptable range (as opposed to analog adjustment of the source voltage, which is not cost-effective). 
         [0005]    To prevent them from overheating, resistance heaters  11  are typically left exposed to the surrounding environment  21 , i.e. ambient air or if enclosed, chilled-water or forced-air cooled. The surrounding environment  21  absorbs heat from the resistance heaters  11 , reducing their efficiency, which is defined herein as E H =(Q E −Q L )/Q E (where Q E  is the heat generated in the resistance heaters  11 ; and Q L  is the heat loss from all external surfaces  23 ,  25  exposed to the surrounding environment  21  along the length of the barrel  5 ). More specifically, as illustrated in  FIG. 2 , heat lost Q L  from the exterior exposed surfaces can be defined as Q L =Q H,CV +Q H,RD +Q B,CV +Q B,RD  (where Q H,CV  represents the natural convection losses to the surrounding environment  21  from exposed heater surfaces  23 ; Q H,RD  represents radiation losses to the surrounding environment  21  from exposed heater surfaces  23 ; Q B,CV  represents the natural convection losses to the surrounding environment  21  from exposed barrel surfaces  25 ; and Q B,RD  represents radiation losses to the surrounding environment  21  from exposed barrel surfaces  25 ). 
         [0006]    The remaining components of the overall heat balance can be defined herein as follows: Q H,T  representing the heat absorbed by each heater  11  as its temperature rises; Q H,CO  representing the heat flow across the interface between each heater  11  and the barrel  5 ; Q B,T  representing the heat absorbed by the barrel  5  as its temperature rises; Q P  representing the heat consumed by the process to heat and/or melt the flowing material  1 ; and finally Q CD,A  representing the heat transferred axially through the barrel wall  17  to adjacent cooler regions of the barrel  5  and to the machine housings at both ends of the barrel  5 . 
         [0007]    In a typical heat balance equation, heat absorbed Q P  by the processed material, plus heat losses to the machine housings Q CD,A , and from the barrel surface Q L , must substantially equal the sum of the heat generated by process shear Q S  and the heat input from the heaters Q E . For illustration purposes only, referring to  FIGS. 1 ,  2  and  3 , assuming a typical resistance-heated injection molding application known in the art, with a screw diameter in the range of about 50 mm, as an example, about 5 kW of process heat Q P  may be required (which accounts for about 50% of the total required heat generation (Q E +Q S )) to melt the flowing material  1 ; while heat losses Q L  from the exposed external surfaces  23 ,  25  can be about 4 kW (approximately 40% of the total energy consumption (Q P +Q CD,A +Q L )) and heat losses Q CD,A  to the machine housings can be about 1 kW (the remaining approximately 10% of the total energy consumption). On such an application, the heat generated by process shear Q S  between the process material  1 , screw  7  and barrel wall  17  can be about 4 kW (approximately 40% of the total heat generation), thereby requiring the remaining approximately 6 kW heat input Q E  (approximate 60% of the total heat generation) supplied by the heaters  11 . The resistive heating efficiency E H  in this example would therefore be about 33% (E H =(Q E −Q L )/Q E  as described above, or (6 kW−4 kW)/6 kW). If the barrel surface heat loss Q L  is eliminated, the efficiency (as defined herein) would then increase to 100%, and the required heater power consumption Q E  would decrease by 67% (from 6 kW to 2 kW). Therefore, substantially reducing barrel surface heat losses Q L  to significantly improve heating efficiency is an important objective of the present invention and a significant improvement over the prior art. 
         [0008]    Referring now to  FIGS. 2 and 4 , improved efficiency can be achieved by wrapping a layer of effective thermal insulating material  27  around the resistance heaters  11  to virtually eliminate barrel surface heat losses Q L . In practice this has been done using an insulating blanket. However, this corrective action often causes the resistance heaters  11  to overheat and fail. Also, it does not overcome problems caused by the excessive thermal mass of the resistance heaters, more specifically the product of the heaters&#39; mass and heat capacity (i.e. btu/lb-° F. or joules/kg-° C.). High thermal mass slows control response and impedes process uniformity. Therefore, due to their mass, material of construction, and direct contact with the barrel  5 , resistance heaters  11  add a substantial thermal heat sink that further dampens heating response. This is particularly the case with cast-in heaters, which need heavier walls sufficient to permit the channeling of cool air or chilled water. These heavy walls add to the thermal mass of the cast-in heaters, as does the mass of water or air circulating through them. 
         [0009]    Referring next to the graph shown in  FIG. 5 , when raising the barrel temperature  29 , the resistance heater&#39;s temperature  31  must first be raised to create a gradient or differential  33  before the barrel temperature  29  responds. Likewise, when reducing the barrel temperature  29 , the temperature  31  of the resistance heaters  11  must drop below the barrel temperature  29  before it will follow. Therefore, because resistance heaters  11  transfer heat to the barrel  5  by conduction Q H,CO  across an intervening contact area, the heaters must be significantly hotter than the barrel  5  when heating it, and cooler than the barrel  5  before it can be cooled. The thermal mass of resistance heaters  11  and the required temperature differential  33  between them and the barrel  5  therefore effect the response time of the barrel&#39;s temperature control. 
         [0010]    Continuing to refer to  FIG. 5 , resistance heaters  11  are typically controlled by turning power “on” and “off”, using a constant voltage source. Using variable voltage control in each zone is prohibitively complex and expensive. The temperature  31  of resistance heaters  11  must therefore swing between two extremes  35 ,  37  even when the process is stable. In practice this often leads to substantial swings in barrel temperature  29  between two narrower extremes  39 ,  41  with a span  43  of as much as 5% or more above and below the target operating temperature  45 . 
         [0011]    Referring now to test results graphically illustrated in  FIG. 6 , the average temperature  47  of a barrel with three zones  13  of resistance heaters  11  was monitored with thermocouples  15  located at various positions along the barrel&#39;s length during an actual injection molding application producing 40 gm polypropylene parts using a 30 second cycle time  49 . At room temperature, pelletized material  1  was introduced into the feed port  3  at the beginning of each 30-second machine cycle, causing a repeatable drop in average barrel temperature  47  every cycle. Were the resistance band-heaters  11  able to quickly add enough heat Q P  to the process, the average barrel temperature  47  would have oscillated within a narrower band. Instead, as is often the case with resistive-heated injection molding applications, the resistance band-heaters  11  were unable to keep up with the process and fell out of sync. The longer the “on”  51  and “off”  53  portions of the control interval  55 , the more heat is lost, consumed and added per cycle, thereby producing a larger swing in the process temperature. Therefore, in practice, the thermal mass of resistance band-heaters  11  often lengthens the control interval  55  from seconds to minutes. This is substantiated by the example illustrated in  FIG. 6 , where the band-heater control interval  55  exceeds ten minutes, and produces a large cyclical swing  57  of about 20° F. in the average barrel temperature  47 . 
         [0012]    In high electrical demand regions, electricity rates, i.e. cost/kW-hour, typically increase with the peak demands monitored by utility companies. The exact billing basis varies by region, and might for example be based on the peak usage during a billing cycle, or on the ratio of the peak usage to the average usage. Regardless, the peak value is likely to be computed over a period of multiple minutes. For example, with a typical utility company, peak demand might be average over 30-minute intervals, and the billable monthly peak demand will be the highest of all the 30-minute averages for the billing month. Also, if the customer&#39;s use of electricity is intermittent or subject to violent fluctuations, a 5 minute or 15-minute interval may be used instead of the 30-minute interval. Accordingly, a control interval  55  of many minutes may increase peak demand, and thus electricity costs, while a control interval equal to the machine cycle (which is less than a minute in most cases) likely will not, since the machine&#39;s average and peak electricity usage will generally be the same. It is therefore a further objective of the present invention to enable the addition of enough heat to the process Q P  quickly enough to enable the control interval  55  of the molding application to be equal to or less than the machine cycle time  49 , thereby reducing process temperature swings  57  and the electrical peak demand. 
         [0013]    Referring still to  FIGS. 5 and 6 , because resistance heaters  11  must be hotter than the barrel  5  when heating it, the heater temperature  31  must be raised beyond the melt point of the material  1  being extruded or molded. This temperature elevation further increases system heat losses Q L  to the surrounding environment  21  and to the upstream and downstream machine housings Q CD,A  in contact with the barrel  5 , reducing efficiency, as well as the reliability and life of the contacting equipment at the machine housings at ends of the barrel  5 . Therefore, an objective of the present invention is to reduce the maximum barrel surface temperature  59  by preventing the heating device from itself getting hot and maintaining the exposed surfaces  23 ,  25  at temperatures safe to the touch. 
         [0014]    Referring again to  FIGS. 1 and 2 , uniform contact between resistance heaters  11  (particularly band-heaters) and the barrel  5  is important to prevent hot-spots and heater failures. Band-heaters  11 , therefore, most commonly have a relatively small “length to diameter” ratio,  61 ,  63  respectively. This often means three or more interconnected band-heaters  11  are required per control zone  13 , thereby in such cases totaling nine or more band resistance heaters  11  at select portions over the length of the barrel  5 , and frequently as many as 15 to 30. This more common system arrangement makes it difficult to promptly detect and replace a single failed band-heater  11 . However, any delay in detection and replacement can produce defective product and/or constrained throughput. In addition to labor and parts costs associated with replacement, production is also lost while waiting for the barrel  5  to cool, and then disassembling and replacement, and finally waiting for the barrel  5  to re-heat. In practice, band-heaters  11  can also become covered with excess plastic emanating from the manufacturing process, such as through excessive clearances around dies or nozzles, or at connections to screen changers on extrusion machines, or at vent holes that can be located along the length of the barrel on vented extrusion machines, thereby making proximate band-heaters more susceptible to overheating and premature failure. It is therefore yet another objective of the present invention to minimize the number of individual heating units, while inherently increasing their reliability and reducing susceptibility to overheating due to material overflow. 
         [0015]    Referring still to  FIGS. 1 and 2 , sufficient and uniform contact pressure between resistance heaters  11  and the barrel  5  is critical to facilitate the desired heat flow across the interface Q H,CO  and to prevent overheating and failure of the resistance heaters  11 . As resistance heaters  11  and the fasteners that constrain them age, the contact pressure and its uniformity can diminish, which can gradually reduce the heater&#39;s life and/or the machine&#39;s throughput rate, if the rate is constrained by heating capacity. It is therefore another objective of the present invention to eliminate the need for any contact pressure between the heating device and the barrel  5 , as well as to effectively eliminate sensitivity of the heating device&#39;s heat transfer performance and reliability to small variations in the clearance therebetween. 
         [0016]    Referring next to  FIG. 7 , resistance heaters  11  in the prior art are often constructed in two semi-circular halves  65 ,  67  that bolt together, or that hinge open and closed via diametrically-opposed longitudinal seams  69 ,  71 . The regions near these seams  69 ,  71 , and possibly near the electrical connection terminals  73 , are unheated. The barrel within these regions  75  is therefore not directly heated, thereby wasting surface area, across which heat transfer could otherwise occur. This reduces the capacity of such two-part resistance heaters  11  and introduces circumferential barrel temperature variations. In practice, this problem can be overcome by offsetting the seams of adjacent resistance heaters  11  to avoid development of a continuous cool seam along the length of the barrel  5 . However, installers and maintenance personnel will occasionally overlook this design flaw and not position the heaters correctly, producing a relatively cool streak along part or all of the barrel&#39;s length which can diminish the temperature uniformity of the molten material stream. It is therefore another objective of the present invention to provide uniform heating around the entire circumference of the barrel  5 . 
         [0017]    Referring now to  FIGS. 1 ,  2  and  4 , regarding the resistance heaters  11  currently in use, embedded electrical heating elements do not extend right to their upstream and downstream edges  77  and, as previously discussed, to reduce the risk of inadequate or non-uniform contact pressure, these resistance heaters  11  often come in relatively short lengths. Therefore, there are often multiple unheated gaps  79  between adjacent heaters  11 . More specifically, there are typically three or more unheated gaps  79  per control zone  13 . These gaps  79  represent wasted surface area across which heat transfer would ideally occur but cannot, further reducing the heaters&#39; capacity. In addition, the application of heat in a plurality of discontinuous segments is not ideal for process uniformity. In order to maintain the processed material at optimal averaged temperatures along the barrels length, the processed material must be exposed to higher than optimal temperatures at the center of resistance heaters  11 , to compensate for exposure to lower temperatures at the gaps  79 . It is therefore another objective of the present invention to minimize the number of unheated gaps  79  along the length of the barrel  5 , preferably to less than or equal to the number of control zones  13 . 
         [0018]    Referring now to  FIGS. 8   a - c  and  9   a - c , the typical process of mixing, heating, and/or melting material  1  within a barrel  5  includes a helical screw  7  whose geometry is often optimized for the process, based on multiple factors, including but not limited to the material&#39;s thermal and physical properties, as well as the desired throughput rate. The screw geometry includes such parameters as a screw core with a root depth  81  and main helical flight  83  that can have a constant or variable pitch  85 . Of course, the screw geometry affects the amount and distribution of heat generated within the process by shear Q S , and therefore the amount and optimal distribution of heat Q E  that must be supplied externally to the process to satisfy the overall heat balance as discussed. In practice heat Q E  is conventionally applied in discrete zones  13  using resistance heaters  11 , producing a step-wise heat input profile that may not optimally complement the varying requirements of the process along various sections of the barrel&#39;s length L. 
         [0019]    Extrusion and molding screws  7  commonly include multiple functional sections, such as feed “A”, transition “B”, metering “C”, mixing “D”, barrier “E”, reorientation “F”, and vent “G” sections, as are well known in the extrusion and molding art. Were a more smoothly varying means available to add heat Q E  to the barrel  5 , those skilled in the art would have the freedom and opportunity to optimize the axial heat distribution Q E  in concert with the screw geometry, to improve upon the performance of extruding and molding operations. More specifically, the ability to smoothly and contiguously profile the heat input Q E  would allow those skilled in the art to better profile the screw&#39;s functional sections, and/or to more optimally transition from one functional section to another. It is therefore another objective of the present invention to enable a more smoothly and contiguously varying heat input profile Q E  along the axis of the barrel  5 . 
         [0020]    Referring now to  FIGS. 10 and 11 , as will be described in more detail in the preferred embodiment, magnetic induction heating  87  of an electrically conductive workpiece, such as the barrel  5 , can be used with or without contact between the induction windings  89  and the barrel  5 . Electrical current “I” passed through the induction windings  89  will generate a magnetic field whose flux lines  91  pass through the barrel wall  17 . When the current&#39;s direction is alternated at high frequency, eddy currents  93  are generated within the wall  17  of the barrel  5 , producing localized, direct heating Q E  of the barrel  5 . The fact that induction heating is not dependent upon direct contact between the windings  89  and the barrel  5  permits further improvements that are exploited by the present invention to meet the many objectives listed above. 
         [0021]    Although the use of magnetic induction using alternating current to heat electrically conductive workpieces is known, including induction heating of barrels  5  used to heat materials such as plastic or metals in extrusion and molding applications, the present invention provides many distinct advantages over the prior art. For example, British Pat. No. 772,424 to Gilbert discloses a plurality of induction units assembled around a barrel, each consisting of a single multi-turn coil or winding. Although the winding is enclosed in a heat resisting and electrically protective sheath, each is surrounded by a magnetisable ferrous shell, and no effective thermal insulating layer is interposed between the barrel and each winding unit. In fact, between adjacent windings the magnetisable shell of each unit makes direct contact with the barrel. Therefore, the windings and magnetisable shell described therein are thermally coupled to the barrel, increasing the thermal mass of the system, as well as providing a path for dissipation of heat to the environment through radiation and natural convection. Further, Gilbert&#39;s use of windings having multiple turns with a relatively low frequency alternating current (25 to 100 Hz), would mandate a larger number of winding turns (10 to 30 times more) than would otherwise be the case with higher operating frequencies (10 to 40 kHz). 
         [0022]    U.S. Pat. No. 5,025,122 to Howell discloses an induction coil assembly for heating associated workpieces inserted therein, using a plurality of interleaved, selectable induction coils to control the operable power and heated length in discrete increments. This invention also does not use interposed thermal insulating material between the coil and workpiece to reduce the apparatus&#39; thermal mass and heat losses to the environment. 
         [0023]    U.S. Pat. No. 5,799,720 to Ross, et al., pertains to transferring molten metal from a reservoir by gravity to a mold for casting molten metal. The Ross assembly uses a casting nozzle having an electrically conductive top wall and bottom wall. An inductive heater is positioned to heat the top and bottom walls of the nozzle. A layer of insulation is positioned between the inductive heater and the wall surfaces and a magnetic shield is provided to partially surround the inductive heater to direct magnetic flux into the nozzle. Also, like Gilbert&#39;s British Pat. No. 772,424 and Howell&#39;s U.S. Pat. No. 5,025,122, this patent does not envision varying the pitch of the induction windings to complement a screw&#39;s flow profile. 
         [0024]    Finally, U.S. Pat. Nos. 6,717,118, 6,781,100 and 7,041,944, all to Pilavdzic et al, describe and favor an apparatus that combines inductive and contact resistance heating of a workpiece, such as a barrel. A layer of thermal insulation interposed between the coil and barrel is not suggested, and the invention specifically favors a coiled electrical conductor that is in thermal communication with the heated article in order to directly transfer any resistive heat generated in said coiled electrical conductor to said article. As such, among other things, Pilavdzic&#39;s inventions do not use or envision an interposed layer of thermal insulation to reduce the apparatus&#39; thermal mass and heat losses to the environment. 
         [0025]    The present invention is directed to overcoming the numerous limitations and problems set forth above. 
       SUMMARY OF THE INVENTION 
       [0026]    The apparatus and method described herein use one or more induction coils, each comprising a helically wound electrical conductor surrounding a thermal insulating layer of non-electrically conductive material to heat an enclosed electrically conductive workpiece, such as a metal extrusion or molding barrel  5 . The helical winding is commonly referred to as a “tunnel coil”. The induction tunnel coil heats the workpiece through the interposed thermal insulating layer. 
         [0027]    By interposing a thermal insulating layer between the induction coils and the heated workpiece, heat generated within the workpiece cannot substantially escape through the insulation to the environment. This raises the heating efficiency and protects the external induction windings from elevated temperatures. Maintaining the induction coil&#39;s windings at lower temperatures reduces their electrical resistance to further reduce resistive losses, which in turn increases the system&#39;s overall energy efficiency. 
         [0028]    Another unique characteristic of induction heating using a helical tunnel coil in this invention is that the distribution of transferred energy along the length of the workpiece is inversely proportional to the pitch of the helix. By varying the pitch of the windings additional embodiments of the present invention are envisioned that can profile the heat generation along the length of an enclosed workpiece, in an intentionally non-uniform, predictable manner to complement and optimize transition of the processed material from solid to molten phases. In other words, with extruder and molding applications this invention allows the distribution of heat along the barrel length within a controlled zone to optimally match the geometry of the conveying screw and processing objectives. For example, the conveying screw might be designed in concert with the winding profile to produce an optimal temperature profile along the flow path of the material being processed. Establishing the optimal axial temperature profile can minimize shear and reduce screw drive horsepower, while also reducing internal barrel wear to increase screw, barrel and drive motor life, and/or improves the uniformity of material properties influenced by temperature to produce extruded or molded parts of more uniform quality. In the case of more stable and predictable process applications, this invention may use a single profiled controlled heating zone over the entire barrel, where previously three or more controlled zones would conventionally be required. And, where more uniform full-length heating is needed to permit relatively uniform and fast barrel preheating, and/or where more flexible response to process disturbances is needed, two controlled zones might be sufficient, where three or more were conventionally used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings used to illustrate and describe the preferred embodiments thereof. Further, these and other advantages will become apparent to those skilled in the art from the following detailed description of the embodiments described herein when considered in the light of these drawings in which: 
           [0030]      FIG. 1  is an elevational view of a typical molding barrel using conventional resistance heaters in the prior art; 
           [0031]      FIG. 2  is a sectional view showing the overall heat balance of a lengthwise segment of the typical molding barrel shown in  FIG. 1 ; 
           [0032]      FIG. 3  is a graphical illustration of the typical time-averaged power distribution during production conditions, using three separate discrete control zones along the length of the barrel shown in  FIG. 1 ; 
           [0033]      FIG. 4  is a sectional view showing the overall heat balance of a lengthwise segment of the typical molding barrel shown in  FIG. 1 , but in this case with the barrel and conventional resistance heaters shown surrounded by a thermal insulating blanket; 
           [0034]      FIG. 5  is a graph showing temperature response versus time for a resistance heater thermally communicating with the adjacent barrel of the molding apparatus shown in  FIG. 1 ; 
           [0035]      FIG. 6  graphically shows barrel temperature measurements obtained during operation of an injection molding machine, and shows response and control differences between heating the barrel using contact resistance heaters versus induction heaters; 
           [0036]      FIG. 7  is a sectional end view of the conventional resistance heaters used with the molding barrel shown in  FIG. 1 ; 
           [0037]      FIGS. 8   a ,  8   b  and  8   c  are partial sectional lengthwise views of a typical molding barrel with the external heaters removed and various molding screws shown therein; 
           [0038]      FIGS. 9   a ,  9   b  and  9   c  are partial sectional lengthwise views of a typical extrusion barrel with the external heaters removed and various extrusion screws shown therein; 
           [0039]      FIG. 10  is a lengthwise view of a segment of a molding or extrusion barrel surrounded by an induction tunnel coil; 
           [0040]      FIG. 11  shows a sectional view of the lengthwise barrel segment with screw shown in  FIG. 10 , illustrating the orientation of the magnetic flux field generated by the induction tunnel coil; 
           [0041]      FIG. 12  is a sectional view illustrating the heat balance in the barrel segment shown in  FIGS. 10 and 11  without a thermal insulating layer interposed between the induction tunnel coil and the barrel; 
           [0042]      FIG. 13  is a sectional-view illustrating the heat balance of a lengthwise segment of the first preferred embodiment of the present invention, comprising an extrusion or molding barrel heated by an induction tunnel coil, with a thermal insulating layer interposed between the windings of the induction coil and the barrel; 
           [0043]      FIG. 14   a  is a partial sectional lengthwise view of a suitable insulating winding template employed by the present invention, and  FIG. 14   b  is a lengthwise view of the same winding template with suitable induction windings wrapped around it; 
           [0044]      FIG. 15   a  is a lengthwise view of an insulating sleeve with induction windings wrapped around it, with a partial cutaway showing the interposed relationship between the barrel and insulating sleeve, and  FIG. 15   b  is a sectional lengthwise view of another insulation embodiment with a thinner similar insulating sleeve surrounded by a separate winding template; 
           [0045]      FIG. 16  is a lengthwise view of the molding barrel shown in  FIG. 1 , but heated by three zones employing the first preferred embodiment of the present invention, rather than by conventional resistance heaters; 
           [0046]      FIG. 17  is a graphical illustration of the time-averaged power distribution using induction heating along the length of the barrel shown in  FIG. 16  during production conditions; 
           [0047]      FIG. 18  is a graph showing the relationship of a tunnel coil&#39;s generated heat input density to winding density, where the heat input density substantially equals the power transferred to the workpiece per unit length of coil, and the winding density equals the number of winding turns per unit length of coil; 
           [0048]      FIG. 19  is a lengthwise view of the molding barrel also shown in  FIG. 16 , but illustrating the barrel being heated by a second embodiment of the present invention; 
           [0049]      FIG. 20  is a graphical illustration of the time-averaged power distribution during production conditions of a contiguous step-wise power profile, using a single zone along the length of the barrel shown in  FIG. 19 ; 
           [0050]      FIG. 21  graphically shows the normalized profile  133  of the heater power distribution illustrated in  FIG. 20 , the normalized cumulative power input profile  135  derived by integrating  133 , a smoothed normalized cumulative power input profile  137  derived by a least-squares curve-fit of  135 , and a continuous heater power distribution profile  139  derived by taking the derivative of  137 ; 
           [0051]      FIG. 22  graphically shows the normalized continuous power distribution profile  139  shown in  FIG. 21 , in relationship with the normalized continuous winding density profile  141  derived by taking the square root of  139 , and the normalized continuous pitch profile  143  derived by taking the reciprocal of  141 , thereby illustrating the continuous, variable winding pitch principle of a third embodiment of the present invention; 
           [0052]      FIG. 23  is a lengthwise view of the molding barrel shown in  FIGS. 16 and 19 , but illustrating the barrel being heated by the fourth embodiment of the present invention; and 
           [0053]      FIG. 24  is a graphical illustration of the time-averaged power distribution combining the profile of one zone having one pitch, with a second zone having multiple pitches, along the length of the molding barrel shown in  FIG. 23 , during both production conditions and initial barrel heat-up. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    This discussion begins with reference to  FIGS. 2 and 12  to make a comparison of some of the primary differences between heating of workpieces, such as barrels  5 , with conventional resistance heaters  11  versus induction heaters with windings  89 . Notably, the induction heater generates heat Q E  directly within the workpiece, while resistance heating must drive heat Q H,CO  across the contact interface between the resistance heater  11  and the barrel  5 . In practice, this allows induction heating to heat the barrel  5  more quickly, even when the windings  89  are in thermal contact therewith. However, the induction windings  89  being in thermal contact with the barrel  5  will create additional thermal mass in the apparatus that will, like that of resistance heaters  11 , absorb heat Q I,T , thereby slowing the thermal response of the system. Even if the windings  89  do not generate significant resistive heat within themselves, heat Q I,CO  will conduct across the interface between the heated barrel  5  and the contacting windings  89 . Still further, the induction windings  89 , being in thermal contact with the barrel  5 , will also get hot, causing their exposed surfaces  95  to dissipate heat through convection Q B,CV  and radiation Q B,RD  to the surrounding environment  21 . Further, the exposed barrel surface  25  between the windings  89  will lose heat through convection Q I,CV  and radiation Q I,RD  to the environment. 
         [0055]    Referring now to  FIGS. 12 and 13 , when the induction windings  89  are separated from the barrel  5  by an interposed layer of thermal insulation  97 , the windings  89  are thermally isolated from the barrel  5 , essentially eliminating heat absorption Q I,CO  by the windings  89 , as well as heat losses Q B,CV , Q B,RD , Q I,CV , Q I,RD  from their exposed surfaces  95  to the environment  21 . The unique non-contact principle of induction heating allows the induction windings  89  to generate heat within the barrel  3  through the interposed thermal insulating layer  97 , thereby effectively eliminating not only the heat losses Q B,CV , Q B,RD , Q I,CV , Q I,RD , to the environment  21 , but also any thermal mass otherwise attributable to the induction windings  89 . 
         [0056]    In contrast, referring again to  FIGS. 2 and 4 , resistance heaters  11  must be in direct contact with the barrel  5 . Therefore, thermal insulation  27  intended to effectively eliminate heat losses Q B,CV , Q B,RD , Q H,CV , Q H,RD , to the environment  21  cannot be interposed between the resistance heaters  11  and the barrel  5 . Instead, the thermal insulation  27  must surround the resistance heaters  11 . Accordingly, insulated resistance heaters  11  are not thermally decoupled from the barrel  5 , thereby attributing thermal mass which slows the thermal response of the system. 
         [0057]    Referring now to  FIGS. 14   a  and  14   b , this embodiment of the present invention employs an insulating winding template  99  that will surround the barrel  5  and serve one or a combination of three purposes. The first and most critical purpose is to thermally insulate the barrel  5  from the windings  89  and the environment  21 . The second purpose is to support the windings  89 , and the third is to set and constrain the pitch  101  of the windings  89  by means of winding grooves  103 . The insulating winding template  99  will typically be cylindrical in shape, with an insulating wall thickness  105  of between 5 and 35 mm, made from a thermal insulating material that is sufficiently durable, and which has a suitably low thermal conductivity of typically less than 1 btu-inch/hr-ft 2 -° F. The preferred insulating material  107  will also be cost-effectively moldable or machineable, allowing incorporation of the winding grooves  103 . A suitable insulating material  107  of this type, for example, would be Gemcolite, manufactured by Refractory Specialties Incorporated, wherein the insulating winding template  99  would be vacuum-formed from a slurry of the material. Of course, other moldable refractory materials having similar physical and thermal properties can be used. 
         [0058]    Referring to  FIGS. 14   a  and  14   b  versus  15   a  for comparison, as an alternative to winding template  99 , an insulating sleeve  109  could be used made of an insulating material  111  of a uniform wall thickness  113 , that is sufficiently durable, and which has the same suitably low thermal conductivity as described above, that is available in bendable sheet form or semi-rigid tube form, around which the windings  89  would be manually or machine-wound at any desired pitch  101 . A suitable insulating material  111  for such use would be Minwool 1200 pipe insulation, manufactured by IIG, Minwool LLC. Of course, other insulating sleeve material having similar geometric, physical and thermal properties can be used. 
         [0059]      FIG. 15   b  shows in its combined entirety  115 , yet another alternative form of the insulating winding template shown in  FIG. 15   a , but with a thinner uniform wall thickness  117  and a separate winding template  119 . The separate winding template  119  need not, but could have, thermal insulating properties, which would surround the insulating material  111  and incorporate machined or molded winding grooves  103 , so as to set and constrain the winding pitch  101 . 
         [0060]    Notably, the use of sleeves  109  or winding templates such as  99  or  115  would essentially eliminate heat losses Q L  from exposed longitudinal surfaces, leaving only axial heat losses Q CD,A  to the upstream and downstream machine housings. By example, as discussed previously with reference to  FIGS. 1 ,  2  and  3 , using a typical resistance-heated injection molding application known in the art, with a screw diameter in the range of about 50 mm, about 5 kW of process heat Q P  may be required to heat and/or melt the flowing material  1 , while heat losses Q L  from the exposed external surfaces  23 ,  25  could be about 4 kW. In comparison, were the present invention as shown in  FIGS. 13 and 16  applied to the same application, heat losses Q L  from its relatively cool exposed winding surfaces  95  and thermal insulation surfaces  121  would approach 0 kW. Heat losses to the machine housings Q CD,A  would be about the same—about 1 kW. Likewise, in this representative example, the shear energy Q S  generated by friction between the processed material  1 , screw and barrel wall  17 , is calculated to be the same in both cases, or about 4 kW, assuming the process operating conditions remain the same. Accordingly, and as indicated by comparing  FIGS. 3 and 17 , the application of the preferred embodiments of the present invention would reduce the required heating system power Q E  from about 6 kW to 2 kW, for a reduction in heating system energy consumption of 4 kW, or about 67%. This demonstrates that by virtually eliminating the barrel surface heat losses Q L  shown in  FIGS. 1 and 2 , this invention significantly improves heating efficiency and accomplishes the many objectives stated above. 
         [0061]    Referring to  FIG. 16 , the first preferred embodiment of the present invention could employ the sleeve  109  or winding templates  99 ,  115  wound with suitable windings  89 , such as Litz cabling. Litz cable is well known as an effective induction heating winding and is commonly used at high frequencies because of its high current-carrying capacity with minimal electrical resistance. As a result, resistive losses in the windings can be reduced to less than 5%, thereby substantially eliminating heating in the coil and raising its overall heating efficiency to over 95%. By comparison, conventional resistance heaters  11  that lose heat to the surrounding air, or to a flow of water or forced-air, typically have an overall heating efficiency of 30-60%. 
         [0062]    With this invention, the windings  89  can be electrically powered by one or more accompanying inductive power supplies  123  designed to generate the desired amount of dissipated power Q E (equal to Q E,1 +Q E,2 +Q E,3 +Q E,n , where n is equal to the number of zones) within the barrel  5 , by the application to the windings  89  of a proper electrical voltage and total amperage at an appropriate frequency, preferably greater than 60 Hz, and more preferably between 10 to 40 kHz, although lower and higher frequencies can be used. Notably however, high frequency induction in the preferred range will reduce the number of tunnel coil turns needed to transfer a given amount of power, thereby reducing the required length of the winding  89 , and the associated electrical resistance losses therein, to further improve efficiency. This will also reduce the total cost of the winding  89 , including the labor required to wrap it around the sleeve  109  or winding templates  99 ,  115 . 
         [0063]    As a result, the improved efficiency of the present invention can be used to reduce energy consumption and resulting electricity costs, and/or it enables higher throughput in cases where the throughput was previously limited by the capacity of the prior heating means. 
         [0064]    With reference to  FIG. 18 , an advantage of induction heating using the helical tunnel coil of the present invention is that the distribution of generated heat along the axial length L of the barrel  5  is more substantially proportional to the winding pitch  101 . Specifically, the relative amount of heat generated at a given position is in effect inversely proportional to the square of the winding pitch  101  at that position. The relationship between the desired distribution of generated heat and the required winding pattern can thus be defined by: 
         [0000]      Q E =ΣQ E,n ; 
         [0000]      Q E,n =∫q E,x,n dx for x=0 to L n ; 
         [0000]      q E,x,n =QR x,n x q E,M,n ; 
         [0000]        QR   ,x,n ≈( WR   x,n ) 2 ≈(1 /PR   x,n ) 2 ; and 
         [0000]        WR   x,n   =W   x,n   /W   M,n =1 /PR   x,n   =P   m,n   /P   x,n ; where 
         [0000]        W   x,n =1 /P   x,n ; and 
         [0000]        W   M,n =1 /P   m,n    
         [0065]    In this case, Q E  is the total heat generated in the barrel  5  by the induction heating system; Q E,n  is the heat generated within each “n th ” zone  13  in the barrel  5 ; q E,x,n  is the heat generated as a function of the axial position “x” within the length “L n ” of the “n th ” zone; q E,M,n  is the maximum heat generated per unit length within the “n th ” zone; QR x,n  is the heat generation ratio at position “x” along the length “L n ” within each “n th ” zone  13 ; P x,n  is the winding pitch  101  at position “x” along the length “L n ” within each “n th ” zone; P m,n  is the minimum winding pitch within each “n th ” zone  13 ; W x,n  is the winding density at position “x” along the length “L n ” within each “n th ” zone  13  (equal to the number of winding turns per unit length); W M,n  is the maximum winding density within each “n th ” zone  13 ; WR x,n  is the winding density ratio at position “x”, as described above; and PR x,n  is the pitch ratio at position “x”, as described above. 
         [0066]    Based on the relationship between the distribution of power consumption, and in this case heat generation, versus the winding density, as described above and illustrated in  FIG. 18 , the winding pitch  101  (which is the reciprocal of the winding density) can then be varied to achieve a desired heat input profile. This then is the basis of the second preferred embodiment of the present invention, which is best illustrated with reference to  FIGS. 19 and 20 . Referring thereto, in comparison to  FIGS. 16 and 17 , instead of using three separately controlled zones  13  as shown in  FIG. 16 , each having the same uniform winding pitch  101  to produce the discontinuous or broken step-wise power transfer profile shown in  FIG. 17 , the second preferred embodiment of the present invention illustrated in  FIG. 19 , employs a single contiguous winding  125 , using three different pitches  101  (P),  127  (2.6×P) and  129  (1.9×P) within a single controllable zone  131 , to transfer the same total power Q E , but with the contiguous step-wise power profile as illustrated in  FIG. 20 . Of course, the various pitches suggested here, and the ratio between them, are merely exemplary, as the optimal pitches may differ in practice from one application to another. 
         [0067]    Referring still to  FIG. 19 , while the example illustrated here uses a contiguous winding  125  with three different discrete pitches  101 ,  127  and  129 , it should be understood that the second preferred embodiment of the invention may use any number of continuously or discretely varying pitches over the length “L C ” of the contiguous winding  125 . 
         [0068]    Referring now to  FIG. 21 , the contiguous step-wise power profile produced by the second preferred embodiment of the present invention can be normalized (power at position x versus maximum power over length L C  of the contiguous winding) to 1 and re-plotted as line  133  on a 0-to-1 scale, versus position (represented here as the percentage of L C —from 0 to 100). The step-wise normalized power profile  133  can then be integrated from 0 to L C , and then normalized again, to plot the normalized cumulative power profile  135  from 0 to L C . 
         [0069]    Referring still to  FIG. 21 , a suitable least-squares curve-fit (such as a 3-degree polynomial) of the normalized cumulative power profile  135  can be used to derive a smoother, continuous cumulative power profile  137 , which is the basis for the third preferred embodiment of the present invention. The derivative of this smooth cumulative power profile  137  can then be developed and re-normalized to draw a smooth, continuous normalized power profile  139  that is a close fit to the original step-wise normalized power profile  133 . A smoothly varying contiguous pitch profile can then be employed by the third preferred embodiment of the present invention to produce this smooth normalized power profile  139 . 
         [0070]    Referring now to  FIGS. 18 ,  21  and  22 , and in keeping with the relationship defined in  FIG. 18 , the square-root of the normalized power profile  139  can be computed and re-normalized to develop the normalized winding density profile  141 , the normalized reciprocal of which is the normalized pitch profile  143 . Either of the normalized winding density or pitch profiles,  141 ,  143  respectively, can be employed by the third preferred embodiment to produce an insulated winding template with a continuously varying pitch that will produce a predictable heating profile along the length of the barrel  5 . The above-described modeling procedure describes one rational means to easily compute the preferred continuously varying pitch profile of the winding employed by the third preferred embodiment of the present invention. Of course, insubstantial variations may be made to the model to derive the substantially same pitch profile. 
         [0071]    Referring now to  FIGS. 23 and 24 , still a fourth preferred embodiment of the present invention combines one individually controllable zone  145  having one pitch pattern  147 , with one or more additional zones  149  having one or more other pitch patterns  151 . The unique advantage of this embodiment can be best understood by considering how the second and third embodiments of the present invention will affect the temperature of the barrel  5  during initial heat-up. 
         [0072]    For comparison, reference is made to  FIGS. 16 ,  17 ,  19  and  20 . Intentional non-flat power distribution profiles, such as those shown in  FIGS. 17 and 20 , are primarily intended to satisfy non-flat process heat input requirements during normal production conditions; i.e. when the material being processed  1  needs to be heated. However, during start-up conditions, when there is no material flow and the sole objective is to heat the barrel  5  to the desired, uniform initial operating temperature, either of the power profiles shown in  FIGS. 17 and 20  (which graphically illustrate heater power distribution of a discontinuous step-wise power transfer profile along the length of the barrels shown in  FIGS. 16 and 19 , respectively) will provide a non-flat initial temperature profile in the barrel that may be undesirable. This is more likely the case with molding applications where the heating zone  13  in proximate relationship to the screw&#39;s feed section “A” provides most of the total heat input Q E  during production (i.e. typically 60-80%), hence requiring a substantially non-flat power transfer profile along the length L of the barrel  5 . 
         [0073]    Now, referring back to  FIGS. 23 and 24 , the problem described above can be largely overcome by the fourth embodiment of the present invention which combines one individually controllable zone  145  having one pitch pattern  147 , with one or more additional zones  149  having one or more pitch patterns  151 . While the pitch patterns  147 ,  151  in either zone  145 ,  149  may be fixed, varied in steps, or varied continuously, this fourth preferred embodiment uses a fixed pitch  147  in the zone  145  nearest the feed port  3 , and a step-wise varying pitch  151  in the remaining, longer zone  149 . Particularly with respect to injection molding applications, during continuous production this arrangement can produce a desirable, highly non-flat power transfer profile as graphically shown in  FIG. 24 , yet during startup, when the barrel  5  is heating up, independent control of the longer zone  149  can raise the heat input profile over the adjacent length of the barrel to generate a far more flat initial temperature profile than would otherwise be possible. In practice, the result is an essentially concave power profile  153  that provides additional valuable advantages during startup. Among others benefits, this concave heating profile  153  can be applied during heat-up conditions to symmetrically deliver more heat towards the ends of the barrel  5 , to better compensate for initial heat losses Q CD,A  to the upstream and downstream machine housings. 
         [0074]    The ability to profile the heat input Q E  to the barrel  5  along its axial length L, within a controlled zone  13 , and/or across the transition from one zone to the next, during start-up and normal process conditions, offers many advantages. Multiple screw designs are used for extrusion and molding, such as, for example, those commonly referred to as general purpose screws, mixing screws, barrier screws, and vented screws.  FIG. 8   a  shows a commonly used general purpose screw, while  FIG. 8   b  shows a mixing screw; both used for injection molding.  FIG. 8   c  is an example of a barrier screw used for injection molding, and  FIGS. 9   a  and  9   b  show different barrier screws used for extrusion. Finally,  FIG. 9   c  is an example of a vented screw used in extrusion machines. It is well known that the optimum temperature profile differs with screw design, material  1  and between extrusion and molding applications. For example, polyethylene and ABS typically prefer the temperature to ramp up along the length L of the barrel  5 , while polypropylene and nylon generally prefer a reverse temperature profile, and with a barrier screw application the desired maximum temperature is typically near the middle. 
         [0075]    Currently, these different requirements are only partially satisfied using discrete resistance heaters. The flexibility and predictability available with the present invention, to produce a continuously varying heat input pattern, can be used by molding and extrusion machine designers to better optimize the process. One example of how profiled heating can improve extruder or molding machine performance relates to the elimination or lessening of process temperature constraints encountered with discrete resistance-heated control zones  13 . Take, for example, the situation where the throughput might be limited by a minimal allowable temperature at one location along the barrel  5  being apt to cause excessive shear Q S . With discrete resistance heaters it may not be possible to simply add more heat Q E  to the relevant zone  13 , as doing so may cause overheating and compositional degradation and/or burning of the process material  1  elsewhere within the zone  13 , or downstream of the zone. The solution can be to use one of the several embodiments of the present invention to better profile the heat input Q E  upstream and downstream of the zone  13 , and/or variably within the zone, during start-up and/or normal process conditions, to permit an increase in throughput, and thereby productivity. 
         [0076]    In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope. It is intended that all such modifications and alterations be included insofar as they come within the scope of the appended claims or the equivalents thereof.