Abstract:
A plasticating barrel is provided with a primary heating system having at least one laminated ceramic heater, where the ceramic heater has an electrical insulating layer interposed between the ceramic heater layer and an outer wall of the barrel, and the longitudinal length of the ceramic heater is arranged over a portion of the barrel length. A secondary heating system may also be provided and overlaps, at least in part, the primary heating system.

Description:
This application claims priority to and the benefit of U.S. Provisional Application No. 61/197,719, filed Oct. 30, 2008, the full disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to heating of a metal cylindrical element used in the injection molding or extrusion of feed materials such as plastic resins, formable foodstuffs (i.e. pasta) and appropriate metals (i.e. magnesium). Relevant heated cylindrical elements include barrels, feed pipes or adaptor pipes, dies, nozzles, etc. All such elements are typically heated with resistive contact heaters and are used to plasticate the feed materials by some combination of heating, melting, shearing, mixing, metering and conveying, prior to discharging them under pressure through a nozzle or die. The description of the invention herein focuses on its application to a barrel, but the principles, methods and merits of the invention are equally applicable to any heated cylindrical metal element used in the plasticating of feed materials. 
     2. Description of the Prior Art 
     Referring now to  FIG. 1 , solid plastic feed material, typically in the form of pellets or powder, enters the feed end  1  of the barrel  2  and then is sheared, mixed and metered by a screw  100  that rotates within the barrel  2 . The resulting molten material is then forced out under pressure through a nozzle or die at the discharge end  3  of the barrel. To help melt the material, the barrel  2  is also heated, conventionally with external resistive contact heaters  4  commonly referred to as band-heaters. 
     AC induction has also been used to heat cylindrical plasticating elements such as injection molding and extrusion barrels, by inducing eddy currents within the cylinder&#39;s wall to produce direct resistive heating of the cylinder or barrel  2 . Improved commercialized induction barrel heating systems include a substantial thermal insulating layer between the induction coils and the barrel to increase barrel-heating efficiency and reduce temperature control response time. A suitable such induction barrel heating system is described in U.S. patent application titled “Apparatus and Method for Inductive Heating a Workpiece Using an Interposed Thermal Insulating Layer”, published Jun. 12, 2008 at U.S. Publication No. 2008-0136066, 
     The band-heater&#39;s or induction heating system&#39;s electrical circuitry is usually arranged so that the barrel  2  can be heated in multiple controllable zones  5  along its length (typically three to six zones, but fewer or more are possible), with typically one thermocouple  6  located in the barrel wall per zone to provide temperature measurement feedback. The nozzle or die at the discharge end  3  is usually heated and temperature controlled separately using one or more dedicated band-heaters  7 . 
     Referring still to  FIG. 1 , band-heaters  4  add substantial thermal mass to the barrel  2 , increasing temperature control response times and making it more difficult to control processing temperatures, particularly under changing conditions. The controllability of band-heaters is also further diminished, and they are increasingly prone to overheating and premature failure, if they are covered by thermal insulation  8 . For these reasons, band-heaters  4  are usually left exposed to ambient, which unfortunately leads to significant heat losses and waste of energy. 
     Referring next to  FIGS. 2A and 2B , recent AC induction barrel heating systems  9  eliminate the thermal inertia of the barrel heating means to improve control response. Induction barrel heating systems  9  typically also incorporate a layer of thermal insulation  10  interposed between the barrel  2  and the external induction coils  11  to eliminate heat losses to ambient. However, in spite of their advantages, induction systems  9  have the drawback of specialized components that can be relatively expensive, including high-frequency power supplies  12 , and depending on the application, specialized coil cables  11 . Together, these power supplies  12  and induction coils  11  can also incur heat losses of typically between 5% on high-performance systems and 20% on relatively inexpensive, low-quality systems. And, finally, induction systems  9  require specialized power sources (voltage and number of phases) that often differ from those used by the band-heaters they replace. 
     As described in U.S. Pat. No. 6,285,006 B1 and illustrated in  FIG. 3 , rollers  13  used on sheet manufacturing and conversion processes, which have typically steel cores  14 , can be manufactured with an internal or external laminated ceramic coating  15  (shown applied to the external surface of the roller in  FIG. 3 ) that acts as a resistive heating layer. The laminated coating  15  comprises a first layer of electrical insulating material  16  applied to the inner or outer surface of the core  14  using a suitable method such as plasma spraying. To increase the bond-strength a bonding layer (not shown) can also be previously applied between the core  14  and electrical insulating layer  16 . 
     A ceramic heating layer  17  is then applied on top of the electrical insulating layer  16  by a suitable method such as plasma spraying. A final optional layer or sequence of layers  18  can then be applied over top of the resistive heating layer  17  to provide external electrical insulation, added durability, or a surface sealing function to prevent contamination of the resistive heating layer  17 . This final external layer or series of layers can also be applied by a suitable means such as plasma spraying. Electrodes  19  can then be used to connect an external DC or AC power source  20  to the ceramic heater layer  17  in order to generate resistive heating of the heater layer  17  and hence the roll  13 . 
     As further noted in U.S. Pat. No. 6,285,006 B1, various materials can be used for each layer  16 ,  17 ,  18  and the layer thicknesses can be adjusted to provide various properties. As cited in U.S. Pat. No. 6,285,006 B1, suitable materials and thicknesses for use on a 75 mm (3 inch) diameter×400 mm (16 inch) long steel cylinder would be:
         Optional Bonding layer—100μ (4 mil) Sulzer Metco 480 nickel aluminide bond coat;   Inner electrical insulating layer  16 —250μ (10 mil) Saint Gobin 204 stabilized zirconia;   Ceramic resistive heating layer  17 —12-25μ (0.5 to 1 mil) Eutectic 25040 titanium dioxide; and   ptional outer electrical insulating layer  18 —250μ (10 mil) Saint Gobin 204 stabilized zirconia.       

     Referring still to  FIG. 3 , U.S. Pat. No. 6,285,006 B1 also describes an example in which a 75 mm (3 inch) diameter  21  by 400 mm (16 inch) long  22  steel roller core  14  is coated with the above materials to produce a ceramic heater layer  17  with an electrical resistance of about 29 ohms, resulting in a heat generation rate of about 2000 watts when 240 volts AC is applied across the electrodes  19 . With a roller surface area of about 970 cm 2  (150 inch 2 ) this equates to a heat generation density of about 2.1 watts/cm 2  (13.3 watts/inch 2 ). The roller in this example is then cycled over 800 times from 70° C. (160° F.) to 315° C. (600° F.) without failure, and operated at up to 370° C. (700° F.) before failing. 
     Referring now to  FIGS. 1 and 3 , there is little difference between a steel roller core  14  and a steel cylindrical plasticating element such as a barrel  2 , so it follows that the functional layers  16 ,  17  and  18  described in the above example could be applied in the same manner to the external diameter  23  of a plasticating barrel  2  over any length-wise portion  24 . Furthermore, it follows from basic electrical engineering principles that as the roller core diameter  21  or barrel diameter  23  are changed, the heat generation density (i.e. watts/cm 2 ) will remain essentially the same (provided the thickness  25  of the heater layer  17 , the length of the roller segment  22  or barrel segment  24  between the electrodes  19 , and the applied voltage, all remain unchanged). This is because the axial cross-sectional area of the heater layer  17  increases linearly with roller diameter  21  or barrel diameter  23 , thereby reducing the electrical resistance of the heater layer  17  inversely with the diameter  21 ,  23 , which in turn linearly increases the dissipated power to maintain a constant heat generation density. It also follows that if the length of the roller segment  22  or barrel segment  24  is changed, the thickness  25  of the heater layer  17  must be inversely changed to maintain a constant heat generation density. In practice, the electrical resistance of the heater layer  17  decreases in a non-linear fashion as its thickness  25  is reduced and this relationship must be taken into account when specifying the thickness  25  needed to achieve a given heat generation density. 
     The operating temperature of the heated cylindrical plasticating elements (such as barrels) used in the vast majority of injection molding and extrusion applications is below 315° C. (600° F.). In addition, as indicated in Table 1, the required heat generation density of most barrel heating applications remains essentially constant and below about 2.4 watts/cm 2  during machine startup and 1.2 watts/cm 2  during normal production conditions. Referring again to  FIGS. 1 and 3 , most barrels  2  also have heater control zones lengths  5  of 200 to 1200 mm (8 to 48 inches), meaning that ceramic heater layer thickness  25  of under 4 mils should be adequate in most cases, provided a ceramic heater layer material is used that has similar properties to that used in the example above. 
     Furthermore, most injection molding and extrusion operations shut down and start back up only about once per week, equating to only about 50 full-temperature cycles per year, and therefore well under 1,000 cycles over a 15-year machine life. Plasticating barrel applications are also static, unlike the dynamic rotating loads experienced on roller applications. The external surface  26  of plasticating barrels  2  is also not normally exposed to regular wearing contact, nor is the external surface&#39;s condition critically important to the proper functioning of the barrel  2 . 
     The laminated ceramic coating  15  applied to rollers  14  as described above should, therefore, be equally applicable to plasticating barrels  2  having typical external diameters  23  and operating at typical processing temperatures. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Typical Plasticating Barrel Specifications 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Screw Diameter 
                 mm 
                 20 
                 60 
                 100 
                 140 
                 180 
               
               
                   
                 inch 
                 0.79 
                 2.36 
                 3.94 
                 5.51 
                 7.09 
               
               
                 Barrel approx. length (L/D = 19) 
                 mm 
                 380 
                 1140 
                 1900 
                 2660 
                 3420 
               
               
                   
                 inch 
                 15.0 
                 44.9 
                 74.8 
                 104.7 
                 134.6 
               
               
                 Typical barrel sell price 
                 USD 
                 1389 
                 2385 
                 3212 
                 5508 
                 8124 
               
               
                 Barrel approx. outside diameter 
                 mm 
                 79 
                 154 
                 230 
                 305 
                 380 
               
               
                 Barrel approx. heated surface area 
                 cm 2   
                 943 
                 5526 
                 13705 
                 25481 
                 40854 
               
               
                 Nominal number of zones 
                 per barrel 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                 Barrel approx. mass (incl. screw &amp; resin) 
                 kg 
                 15 
                 167 
                 616 
                 1521 
                 3042 
               
               
                 Band-heaters approximate total rated power 
                 kW 
                 3.2 
                 23 
                 58 
                 105 
                 164 
               
               
                 Band-heater maximum power on startup 
                 % 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                   
                 kW 
                 3.2 
                 23 
                 58 
                 105 
                 164 
               
               
                   
                 watts/inch 2   
                 22 
                 27 
                 27 
                 27 
                 26 
               
               
                   
                 watts/cm 2   
                 3.4 
                 4.2 
                 4.2 
                 4.1 
                 4.0 
               
               
                 Band-heater approx. efficiency on startup 
                 % 
                 60 
                 60 
                 60 
                 60 
                 60 
               
               
                 Band-heater maximum barrel heating rate on 
                 kW 
                 1.9 
                 14 
                 35 
                 63 
                 99 
               
               
                 startup 
                 watts/cm 2   
                 2.1 
                 2.5 
                 2.5 
                 2.5 
                 2.4 
               
               
                 Band-heater power use during production 
                 % 
                 30 
                 30 
                 30 
                 30 
                 30 
               
               
                 (approx. total) 
                 kW 
                 1.0 
                 6.9 
                 17 
                 31 
                 49 
               
               
                   
                 watts/cm 2   
                 1.0 
                 1.3 
                 1.3 
                 1.2 
                 1.2 
               
               
                   
               
             
          
         
       
     
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to improve upon the conventional methods of heating barrel-like structures, such as plasticating barrels. 
     It is another object of the present invention to provide a heating system for barrel-like structures that includes the advantages of induction heating systems, including fast response and very low heat losses to ambient. 
     It is a further object of the present invention to provide a heating system for barrel-like structures in which the heating system can be protected easily from physical impact or abrasion. 
     It is an additional object of the present invention to reduce differential movement between a heated barrel and its heater. 
     It is still a further object of the present invention to provide heaters for a barrel-like structure to maintain relatively uniform heat density, where the heaters do not have extraordinary operating requirements. 
     It is yet another object of the present invention to provide a heater system for barrel-like structures in which interference between objects on the barrel and the heater system are easily avoided. 
     It is still another object of the present invention to provide a heater system for barrel-like structures wherein the electrical resistance of the heater system is easily predictable, thereby resulting in predictable heat density. 
     It is again an additional object of the present invention to provide a heater system for barrel-like structures, which easily operates in a wide variety of different electrical environments. 
     It is still a further object of the present invention to provide a heater system for barrel-like structures in which substantial heater redundancy and controllability is achieved. 
     It is yet a further object of the present invention to provide a heater system for barrel-like structures in which a high level of heat density control is facilitated. 
     It is still an additional object of the present invention to provide a heater system for barrel-like structures in which temperature variation along the barrel-like structure can be configured as desired, thereby avoiding “hot spots”. 
     It is yet another object of the present invention to provide a heating system for barrel-like structures in which selected temperature levels at various parts of the barrel-like structure can be easily maintained. 
     It is again a further object of the present invention to provide a heating system for barrel-like structures which is simple, inexpensive, and easily controllable, without special power requirements on equipment. 
     It is still another object of the present invention to provide a heating system for barrel-like structures in which the overall system maintains high thermal efficiency while providing desired heat density. 
     It is yet an additional object of the present invention to provide a heater system which is particularly adapted for plasticating barrels. 
     It is still a further object of the present invention to provide a heating system for plasticating barrels in which cooling devices can easily be inserted to control barrel temperature. 
     It is again another object of the present invention to provide a heating system for a plasticating barrel in which the heater remains attached to the barrel under all operating conditions. 
     It is still another object of the present invention to provide a heating system for a plasticating barrel wherein additional heaters and insulation can readily be added to the heated plasticated barrel. 
     It is yet an additional object of the present invention to provide a heater for a plasticating barrel in which electrical redundancy, as well as thermal redundancy, is easily achieved. 
     These and other goals and objects of the present invention are achieved by an apparatus for plasticizing resinous materials. The apparatus includes an electrically conductive barrel having a longitudinal axis along which materials move axially from an inlet to an outlet. A rotatable screw is disposed within the barrel, and cooperates with an inner wall of the barrel for plasticating resinous material fed into the barrel through the inlet. The screw has a longitudinal axis and a main flight having a pitch arranged helically on and extending radially from a core of the screw so as to form a channel. The primary heating system includes at least one laminated ceramic heater having a longitudinal length arranged along the longitudinal axis of the barrel. The ceramic heater has an electrical insulating layer interposed between the ceramic heater layer and the outer wall of the barrel. The longitudinal length of the ceramic heater is arranged over a portion of the screw length. Also included is a secondary heating system arranged at least in part over the primary heating system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having 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 when considered in light of these drawings in which: 
         FIG. 1  is a side elevational view of a plasticating barrel with conventional external resistive heaters typically referred to as band-heaters, and including a partial cut-away showing the plasticating screw within the barrel; 
         FIG. 2A  is a partial view of a plasticating barrel equipped with an induction heating system; 
         FIG. 2B  is a sectional view of the plasticating barrel taken along lines II-II shown in  FIG. 2A ; 
         FIG. 3  is a partial sectional side-view of a roller equipped with break-aways to show parts of a laminated ceramic coating (coating layer dimensions are enlarged for clarity) to provide resistive heating of the roller; 
         FIG. 4  is a sectional view taken along lines IV-IV of the plasticating barrel shown in  FIG. 1  with conventional external resistive heaters typically referred to as band-heaters; 
         FIG. 5A  is a sectional view taken along lines Va-Va of a plasticating barrel with a laminated ceramic coating in  FIG. 5B  to provide resistive heating of the plasticating barrel (coating layer dimensions are enlarged for clarity); 
         FIG. 5B  is a sectional view taken along lines Vb-Vb of the plasticating barrel shown in  FIG. 5A ; 
         FIG. 6A  is a sectional view of the plasticating barrel shown in  FIG. 5A , with the addition of ring-shaped electrodes (coating layer dimensions are enlarged for clarity); 
         FIG. 6B  is a sectional view taken along lines VI-VI of the plasticating barrel shown in  FIG. 6A ; 
         FIG. 7A  is a sectional view of the plasticating barrel shown in  FIGS. 5A and 6A , with the addition of an external thermal insulating layer; 
         FIG. 7B  is a sectional view taken along lines VII-VII of the plasticating barrel shown in  FIG. 7A ; 
         FIG. 8A  is a sectional end-view of the plasticating barrel shown in  FIG. 7A , with the addition of an annular gap between the external thermal insulating layer and the barrel; 
         FIG. 8B  is a sectional side-view taken along lines VIII-VIII of the plasticating barrel shown in  FIG. 8A ; 
         FIG. 9  is a side elevational view of a plasticating barrel with a laminated ceramic coating (external electrical insulating layer not shown for clarity); 
         FIG. 10  is a partial sectional view of the laminated ceramic coating (layer dimensions enlarged for clarity) taken along line X-X in  FIG. 9 , used to heat the plasticating barrel shown therein; 
         FIG. 11  is an unraveled, surface layout of the laminated ceramic coating used to heat the plasticating barrel shown in  FIG. 9 ; 
         FIG. 12  is a side elevational view of the plasticating barrel shown in  FIG. 9 , with the laminated ceramic coating consisting of axial heater stripes; 
         FIG. 13  is a side elevational view of the plasticating barrel shown in  FIG. 9 , with the laminated ceramic coating consisting of spiral heater stripes; 
         FIG. 14  is an unraveled, surface layout of the laminated ceramic coating used to heat the plasticating barrel shown in  FIG. 12 ; 
         FIG. 15  is an elongated, narrowed layout of the heating layer shown in  FIG. 14 ; 
         FIG. 16  shows the surface layout of a single heater layer stripe; 
         FIG. 17  is an unraveled, surface layout of a laminated ceramic coating showing the helix angle of a single heater stripe to the axis of a plasticating barrel; 
         FIG. 18  is an unraveled, surface layout of a laminated ceramic coating having multiple spiral heater stripes; 
         FIG. 19A  illustrates the surface layout of a single heater layer stripe interrupted by a thermocouple hole; 
         FIG. 19B  illustrates the surface layout of two heater layer stripes interrupted by a thermocouple hole; 
         FIG. 19C  illustrates the surface layout of multiple merged heater layer stripes interrupted by a thermocouple hole; 
         FIG. 20  is a representative chart showing how the electrical resistance of a ceramic heater layer typically drops with increasing temperature; 
         FIG. 21  is a side elevational view of the plasticating barrel shown in  FIG. 6B , with the addition of an electric fuse in the power circuit; 
         FIG. 22A  is a partial sectional side-view of the plasticating barrel shown in  FIG. 1  with the addition of ceramic barrel-heating zones; 
         FIG. 22B  is a partial sectional side-view of the plasticating barrel shown in  FIG. 22A  with the addition of an induction heating system installed around the present invention in the feed zone; 
         FIG. 22C  is a partial sectional side-view of the plasticating barrel shown in  FIG. 22B  with the addition of band-heaters installed around the present invention; 
         FIG. 23  is a partial sectional side-view of a plasticating barrel with an air-cooling system; 
         FIG. 24  is a side elevational view of a plasticating barrel with water-cooling jackets; and 
         FIG. 25  is a side elevational view of the plasticating barrel shown in  FIG. 21 , with the addition of air-cooling and water-cooling equipment installed around the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention uses a laminated ceramic coating comprised of multiple layers and applied directly to the exterior of a cylindrical plasticating element (i.e. barrel). The laminated ceramic coating is preferably applied using a process such as plasma spraying and then a layer of thermal insulation may be applied to, wrapped around, or sleeved over the laminated ceramic coating to thermally insulate the barrel. An annular gap may also be formed between the thermal insulation and the ceramic coating to permit forced-air cooling of the barrel. 
     Of the multiple layers, a ceramic heater layer may be formed in the laminated ceramic coating by applying a ceramic-metal blend (commonly known as a “cermet” material) whose coefficient of thermal expansion is much closer to that of steel than that of a pure ceramic layer. By matching the coefficients of thermal expansion between the heater layer and the underlying steel, cracking of the heater layer at elevated temperatures can be avoided or minimized. By minimizing the incidence of large-scale cracking, the life of the heater layer can be maximized. Also, by minimizing the incidence of micro-cracking, changes in the electrical resistance of the heater layer (versus temperature) can be minimized to produce a heater that has a predictable and sufficient heating rate across the operating temperature range. 
     The ceramic (or ceramic-metal) heater layer can also be applied in multiple axial stripes with intervening gaps. This enhancement reduces the overall surface area of the heater layer to increase its electrical resistance and allows the gaps between heater stripes to accommodate obstructions. The heater layer striping approach can be further optimized by employing one or more parallel spiral stripes. This spiral approach lengthens the path length for the electrical current to further increase the heater layer&#39;s electrical resistance, while still forming gaps that may accommodate obstructions. This preferred spiral striping approach has many advantages, including;
         the ability to map the path of the stripes during manufacturing in order to bypass obstructions such as thermocouple holes;   allowing the applied heater layer to be thicker, thereby making it easier to apply consistently and making its resistance more predictable; and   allowing the helix angle of the spirals to be adjusted to maintain a reasonable heat generation density (i.e. approximately 2 watts/cm 2 ) across a wide range of supply voltages. (This versatility is needed to permit the invention&#39;s application around the world in a wide range of electrical environments, i.e. from a low of about 100V in Japan to a high of about 600V in Canada).       

     Roller applications (such as those described in U.S. Pat. No. 6,285,006 B1) require extremely uniform heat generation across the width of the roller and around its circumference (in order to produce sheets with sufficiently uniform properties in both the cross-direction and machine-direction). By comparison, heating of plasticating elements such as barrels does not require extreme uniformity, as evidenced by the design of conventional band-heaters. Referring now to  FIGS. 1 and 4 , there are gaps  27  between band-heaters  4 , and they typically incorporate features such as hinges  28  and latches  29  so they can be tightened around the heated element (i.e. barrel  2 ) to reduce the contact heat-transfer resistance between them. In practice, the resulting contact pressure is not entirely uniform, so the heating rate varies over the band-heater&#39;s length  30  and around its circumference  31 . Also, the resistive heating element embedded within the band-heater  4  cannot extend all the way around its circumference, so there is no heat generation in the immediate vicinity of the band-heater&#39;s electrical terminations  31  and fastening components, such as its hinges  28  and latches  29 . By comparison, the present invention&#39;s preferred use of a spiral striped heater layer will produce inherently more uniform heating due to the continuous spiral path along the length of the zone  5  and around the circumference  31  of the heated element  2 . 
     Referring now to  FIG. 3 , to maximize heating uniformity the external surfaces of rollers  13  used for sheet manufacturing or conversion are generally intentionally unobstructed, so gaps in the heater layer  17  are typically not required to accommodate obstructions. Roller heated widths  22  are also relatively large compared to their diameter  21 , so their end-to-end electrical resistances are relatively high for a given range of ceramic heater layer thickness  25 . Therefore, spiral striping of the heater layer  17  is generally not needed or desirable on roller applications for the various reasons listed above. 
     Referring again to  FIGS. 1 and 4 , on most cylindrical plasticating elements  2  the ratio of the zone length (“Lz”)  5  to the element&#39;s outside diameter (“OD”)  23  is relatively small, i.e. Lz/OD z≈1 to 3. Consequently, the voltage drop per axial length increment on cylindrical plasticating elements  2  (i.e. barrels) usually must be relatively large, thereby requiring a spiral heater layer path to produce a sufficient electrical resistance. 
     For all the various reasons described above, therefore, the use of spiral heater layer striping is much more appropriate and beneficial for the heating of cylindrical plasticating elements, as needed in this case. 
     Referring again to  FIGS. 1 ,  2 A and  2 B, cylindrical plasticating elements such as barrels  2  are conventionally heated by various means, including by resistive band-heaters  4  and AC induction coils  11 . Band-heaters  4  can be particularly suited to use on small diameter sections such as nozzles  74 , or on relatively short sections of different diameter, such as those commonly referred to in the art as barrel “heads” or “end-bells”  75 . The relatively small external surfaces of these shorter sections  74 ,  75  offer minimal energy savings potential, as well as minimal space for the present invention&#39;s electrical connections. AC induction can also be of unique advantage in the first zone  68  of an injection barrel (commonly referred to in the art as the “feed” zone) where maximizing the heat input rate can permit the throughput rate of the process to be increased. For the various reasons listed above, it can therefore be desirable to concurrently employ band-heaters  4  and/or AC induction coils  11  on the same cylindrical plasticating element  2  (i.e. injection or extrusion barrel) as the present invention. It is therefore an objective of the present invention to be able to concurrently use band-heaters  4  and/or AC induction coils  11  alongside the present invention on the same cylindrical plasticating element  2 . 
     In the event an embodiment of the ceramic heating system fails during operation, it is also an objective of this invention to allow band-heaters  4  and/or AC induction coils  11  to be subsequently installed and operated in place without having to remove the ceramic heating system. 
     It is also an objective of the present invention to allow the superimposed use of a means for cooling cylindrical plasticating elements, such as air-cooling using blowers (fans) with surrounding sheet-metal shrouds, and also water-cooled jackets. 
     More specifically, referring to  FIGS. 5A and 5B , a first preferred embodiment of the present invention utilizes a ceramic heater layer  17  of thickness  25  applied to the external surface  32  of a cylindrical plasticating element, such as a barrel  2 , in order to heat the barrel  2 . An electrical insulating layer  16  is applied between the ceramic heater layer  17  and the barrel  2 . Another electrical insulating layer  18  may also be applied over the ceramic heater layer  17 . Optional bonding layers (not shown) may be applied between any two layers, but they are typically not needed. The various layers  16 ,  17 ,  18  may be applied by multiple methods, including plasma spraying. In this first embodiment, electrical current  33  will flow longitudinally (along the axis  34  of the barrel  2 ) through the ceramic heater layer  17  between two longitudinally-spaced electrodes  19  that are connected to an external power source  20 . 
     Referring now to  FIGS. 5A ,  5 B,  6 A and  6 B, electrodes  19  can be constructed in various ways, including as rings  35  that clamp around the barrel  2  using hinges  36  and simple screw fasteners  37  that can be employed to both fix the electrodes  19  in place and terminate electrical wires  38  to an external power source  20 . The external insulating layer  18  can also be applied in sections  39  in order to create exposed regions  40  of the heating layer  17 , thereby allowing the electrodes  19  to make electrical contact with the heater layer  17 . When the power source  20  is connected across paired electrodes  41 ,  42 , a longitudinal electrical current  33  then flows through the heat layer  17 . To facilitate separately controllable heating zones  43 , the heater layer  17  can also be applied in sections  44  to create gaps  45  between adjacent zonal heater layers  17 . 
     Referring still to  FIGS. 5A ,  5 B,  6 A and  6 B, instead of the external insulating layer  18  and heater layer  17  being applied in sections  39 ,  44  as described above, they can also be applied contiguously, and then sections of them removed by a suitable method such as machining or grit blasting to produce the exposed regions  40  and gaps  45 . 
     Referring again to  FIG. 5A , the surface of one or more layers  16 ,  17 ,  18  can be coated, chemically treated or mechanically finished to improve characteristics such as adhesion or resistance to contamination, but these added steps are typically not needed. Additional functional external layers (not shown) can also be applied without altering the spirit or scope of the invention. For example, referring now to  FIG. 6B , a highly-conductive layer (not shown), can be applied by suitable means (such as spray coating of molten copper) as a narrow strip in the exposed region  40  over top of the heater layer  17  in order to improve the electrical contact between the electrode  19  and the heater layer  17 . 
     It should also be understood that the designs of the electrodes  19  illustrated in  FIGS. 6A and 6B  are merely representative and that many suitable electrode designs can be envisioned within the scope of this invention. It should also be understood that while plasma spraying is the readily envisioned manner in which the layers  16 ,  17 , and  18  are applied, other methods of application would also fall within the spirit of the invention. 
     As illustrated next in  FIGS. 7A and 7B , a cylindrical plasticating element (such as a barrel  2 ) having an external laminated ceramic coating  15  (comprising the functional layers  16 ,  17 ,  18  described previously) can then be easily thermally insulated with one or more wrapped layers of flexible insulation  46 , such as Superwool 607™ insulating sheet manufactured by Thermal Ceramics, Inc., a division of the Morgan Crucible Company plc, (having its main office Thermal Ceramics de France S.A.S., 5 boulevard Marcel Pourtout, F-92563 Rueil-Malmaison Cedex, France at boulevard Marcel Pourtout, F-92563 Rueil-Malmaison Cedex, France) or alternatively with a cylindrical rigid sleeve of molded insulating material  46  such as can be vacuum-formed using a mix of Superwool 607 fiber and a suitable binder. Wrapping the hot external surface  26  of the ceramic coated barrel  2  can then virtually eliminate heat losses from the heated barrel  2  to the surrounding ambient environment. 
     In addition, as shown in  FIGS. 8A and 8B , a sufficiently rigid and adequately supported thermal insulating sleeve  46  can also be used to form an annular gap  47  between itself and the ceramic coating  15 , thereby allowing the passage of cooling air  48  between the sleeve  46  and the barrel  2 , wherein said cooling air  48  can be forced through the annular gap  47  under pressure or drawn through it by means of an applied vacuum. The purpose of this cooling air flow  48  is to either facilitate steady-state cooling of the barrel  2  during production, as is often needed on extruders in certain zones, or to reduce the temperature of the barrel  2  when needed, as is sometimes necessary on both extruders and injection molding machines following planned or unplanned process disturbances. 
     As discussed in the “Summary of the Invention,” differences between the coefficient of thermal expansion (“CTE”) of a ceramic coating, such as a plasma-sprayed titanium dioxide (“titania”) coating, and that of steel (the material normally used for cylindrical plasticating elements such as barrels), result in tensile stresses in the ceramic coating. Referring again to  FIGS. 5A through 8B , preferred embodiments of the present invention, therefore, use a ceramic-metal mix for the heater layer  17  commonly referred to as “cermet” (discussed in detail below), to better match the CTE of the heater layer  17  to that of the underlying steel element  2 . 
     When a typical ceramic coating such as titania is used as a heater layer  17  on a cylindrical plasticating element  2 , the tensile stress in the titania increases as the temperature increases, because the ceramic layer  17  does not expand as much as the underlying steel  2 . Plasma-sprayed ceramic coatings can withstand very high stresses in compression but are weak in tensional stress, with an ultimate elongation of only about 1%. At increasing temperatures, the stresses developed in the titania coating cause increased micro-cracking of the heater layer  17 . This micro-cracking reduces the number of available electrically conductive pathways in the heater layer  17  and permanently increases the electrical resistance of the coating  17 . In the 300° F. to 400° F. range, this increase in electrical resistance is minor, but in the 500° F. to 600° F. range, it is significant, and can increase the electrical resistance of the coating  17  by more than 50% after just a few thermal cycles, i.e. heating from ambient (approximately 70° F.) to &gt;500° F. then allowing the temperature to cool back down before repeating the cycle. 
     One way to reduce thermally-induced tensile stresses in the heater layer  17  to prevent or minimize micro-cracking, is to blend the ceramic (such as titania) with a suitable material (ductile metals preferred) that has a significantly higher CTE value. When metal is used, the resulting ceramic-metal formulation is commonly referred to as a “cermet”. Use of a cermet increases the CTE of the heater layer while the metal component lends ductility to the blend to further reduce micro-cracking. The cermet&#39;s metal particles also interrupt the structure of the titania to spread out the remaining thermal stresses. 
     In Table 2, the CTE values of various ceramics and metals are listed, along with that of a suitable cermet blend. Referring again to  FIGS. 5A through 8B , alumina and zirconia are typically used for the electrical insulating layers  16 ,  18 , while titania is a particularly suitable ceramic component for the heater layer  17 . 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Coefficients of Thermal Expansion 
               
             
          
           
               
                   
                 CTE 
                 100% 
                   
                   
                 Volume 
                 Bulk CTE 
               
               
                 Material 
                 (10{circumflex over ( )}−6 in/in/° F.) 
                 Density 
                 Parts 
                 Volume 
                 Fraction 
                 (10{circumflex over ( )}−6 in/in/° F.) 
               
               
                   
               
             
          
           
               
                 Alumina 95% 
                 4.3 
                 3.7 
                   
                   
                   
                   
               
               
                 Alumina 99% 
                 4.7 
                 3.7 
               
               
                 Titania 
                 5.0 
                 4.6 
                 49 
                 10.65 
                 0.64 
                 3.2 
               
               
                 Zirconia 8% Yttria 
                 5.6 
                 6.0 
               
               
                 Various Steels low 
                 6.2 
                 7.8 
               
               
                 Various Steels high 
                 6.7 
                 7.8 
               
               
                 Mild Steel 
                 6.7 
                 7.9 
               
               
                 Nickel 200 
                 7.4 
                 8.9 
               
               
                 80/20 Nickel Chrome 
                 9.6 
                 8.4 
                 51 
                 6.06 
                 0.36 
                 3.5 
               
               
                 Totals 
                   
                   
                   
                 16.72 
                   
                 6.7 
               
               
                   
               
             
          
         
       
     
     Referring still to Table 2 and  FIGS. 5A through 8B , the CTE of a 100% titania heater layer is approximately 5.0 micro-inches per inch of length per ° F., compared to about 6.7 for mild steel. As seen in Table 2, 49% titania and 51% percent (by weight) of an 80/20 nickel chromium (NiCr) steel alloy are blended to produce a plasma-sprayed heater layer with a bulk CTE approximately equal to that of mild steel. It is not necessary to exactly match the CTE of the steel element  2  to produce a sufficient improvement in the micro-cracking resistance of the heater layer  17  (in order to sufficiently stabilize the electrical resistance throughout thermal cycles). A cermet consisting of 30% by weight of nickel and 70% of titania provides a CTE of about 5.5 that produces a significant improvement in the stability of the electrical resistance compared to that of 100% titania with a CTE of 5.0. Notably, the CTE value of this 30/70 blend is relatively low because the CTE of nickel is significantly lower than 80/20 NiCr. It is also possible to produce a blended cermet heater layer  17  that has a higher CTE than the underlying steel element  2 , such as, for example, 35% titania and 65% NiCr. This would theoretically put the heater layer  17  into compression at elevated temperatures. As illustrated in Table 2, the bulk CTE value of the blended cermet heater layer  17  is calculated by converting the weights of the constituents into volumes. By calculating the volume fraction of each material in the blend, the CTE of the mixture can be calculated. For example, if the volumes of the ceramic and metal were equal, they should contribute equally to the final CTE values. 
     As summarized previously, a preferred embodiment of the invention uses spiral striping of the heater layer to increase its electrical resistance. Referring now to  FIGS. 9 and 10 , a ceramic heater layer  17  applied over top of an insulating layer  16  (for clarity, the exterior electrical insulating layer  18  is not shown in  FIG. 9 , but is shown in  FIG. 10 ) to a cylindrical plasticating element  2 , essentially forms a tube having a wall thickness  25  of only a few mils thick that can then be portrayed as an unraveled, flattened heater layer sheet  49  as shown in  FIG. 11 . 
     Referring still to  FIGS. 9 ,  10  and  11 , as well as to  FIGS. 5A through 6B , on each end of the heater layer  17  narrow electrode bands  50 ,  51  of a highly-conductive sprayed metal (i.e. copper) can also be applied to form particularly low-resistance surfaces that will eliminate the possibility of arcing between the heater layer  17  and the subsequently installed power supply electrodes  19 . Normally, the voltage drops by 5-15 volts per inch of distance between the electrode bands  50 ,  51  of a plasma-sprayed titanium dioxide (titania) ceramic heater layer  17  that is 1-2 mils thick. For example, referring to  FIG. 3 , on a typical laminator roller  13  that is 8 inches diameter and 32 inches long, with a heater layer  17  that is 31 inches long, the power supply voltage  20  connected between the electrodes  19  is about 240 volts, resulting in a voltage drop per inch of heated roller width  22  of about 8 volts. 
     Referring still to  FIGS. 5A through 6B , and  9  through  11 , on a cylindrical plasticating element  2  such as a barrel, the lengths  43  of the temperature control zones are typically only about two times the outside diameter  23  of the barrel  2 , even though the applied voltage  20  may be up to nearly 600 volts (i.e. in Canada). This results in a voltage drop of up to 40-50 volts per inch of distance  52  between electrodes  19  (or electrode bands  50 ,  51 ). To compensate for this relatively high voltage drop and limit the current flow  33  between the electrodes  19  (or electrode bands  50 ,  51 ) to a reasonable level, the electrical resistance of the heater layer  17  needs to be raised, which can be achieved by reducing its thickness  25 . For a titania heater layer  17  with a nominal 2 mil thickness, this means its thickness  25  must be further reduced by at least 4 times. However, ceramic heater layers less than 1 mil thick are not practical from a manufacturing control or reliability point of view. 
     Referring now to  FIGS. 11 and 12 , one alternative is to make the heater layer thickness  25  in the normal range (i.e. ≧2 mils thick), but to decrease the effective flow path width  53  of the electrical current  33  between the electrode bands  50 ,  51  by employing multiple axial heater layer stripes  54  parallel to the axis  34  of the barrel  2 . By reducing the effective flow path width  53  of the heater layer  17 , the total electrical resistance of the heater layer  17  is increased. The non-conductive gaps  57  between the heater layer stripes  54  can then be used to accommodate obstructions such as thermocouple holes  6 . The multiple axial heater stripes  54  are then typically electrically connected in parallel by the electrode bands  50 ,  51  and the widths  56  of the heater stripes  54 , and the widths  58  of the intervening gaps  57 , are then sized to produce the desired electrical resistance between the electrode bands  50 ,  51 . 
     Referring now to  FIGS. 9 and 12 , the use of axial heater stripes  54  (as shown in  FIG. 12 ) is an improvement over that of a contiguous heater layer  17  (as shown in  FIG. 9 ). However, referring now to only  FIG. 12 , with axial heater layer stripes  54  there is still a key limitation, which is that the length of the current flow still cannot be increased beyond that of the distance  52  between the electrodes  50 ,  51 . 
     Referring now to  FIG. 13 , a further enhancement is then to increase the current flow path length between the electrode bands  50 ,  51  by spiraling the heater layer stripes  54  (as few as one spiral heater stripe may be used) with helix angles  55  around the barrel  2 . Multiple spiral heater stripes  54  are then typically electrically connected in parallel by the electrode bands  50 ,  51  and are typically of equal widths  56  with gaps  57  of minimum width  58  between them. Series or series/parallel connections between multiple heater stripes  54  can also be envisioned as can different stripe widths  56  and gap widths  58 . 
     Striping of a sprayed heater layer on a roller is described in U.S. Pat. No. 6,596,960 B1 using a masking approach during thermal spraying of the heater layer. However, referring again to  FIGS. 10 ,  11 ,  12  and  13 , the use of heater layer striping, and more so, spiral striping, is particularly relevant and beneficial on a cylindrical plasticating element  2 , for multiple reasons, including;
         the ability to adjust the pitch or helix angle  55  of the heater stripes  54  and the width  58  of the gaps  57  between them to bypass obstacles, such as thermocouple holes  6  and mounting holes for barrel covers and supports, etc.   allowing the heater layer thickness  25  to be increased, thereby making it easier to apply consistently and making its electrical resistance more predictable.   allowing the helix angle  55  of the spiral heater stripes  54  to be adjusted to maintain a reasonable heat generation density (i.e. approximately 2 watts/cm 2 ) across a wide range of supply voltages. This versatility is needed to permit the invention&#39;s application around the world in a wide range of electrical environments (i.e. from a low of about 100V in Japan to a high of about 600V in Canada).   improving the reliability of the heater layer  17  by providing redundancy. With multiple stripes  54 , if one suffers a failure (i.e. due to cracking, arcing, physical damage, etc.), the failure will typically be limited to a single stripe. That stripe will cease to heat but the other stripes  54  will continue to carry current and provide heating of the barrel  2 . For example, if one stripe  54  out of eight fails, the power consumption and heat generation rate is only reduced by 12.5%.   the ability to power each stripe  54  separately, for even greater redundancy and controllability. Referring now to  FIG. 13 , the examples below assume that all stripes  54  are the same length  52  and width  56  and are electrically connected as resistors in parallel. In reality, each stripe  54  can be powered and fused separately, if desired, giving an extra layer of safety, reliability and power control. For example, half the stripes  54  can be turned off completely for a half-power application with minimal effect on overall temperature uniformity, allowing the remaining stripes  54  to be held in reserve in case of failures.       

     The benefits of heater-layer spiral striping and how it can be optimized can be better understood by the following discussion: 
     Spiral stripe widths and lengths. Referring again to  FIGS. 9 and 11  where the heater layer  17  is represented as an unraveled, flattened sheet  49 , assume that the heated zone of a plasticating barrel  2  with a 10 inch distance  52  and 10 inch circumference  53  has a heater surface area of 100-square inches and an electrical resistance between electrode bands  50 ,  51  of 1 ohm. 
     The surface resistivity of this heater layer sheet  49  is therefore, 1.0 ohm per square. The resistance of any other size square (1×1, 1.36×1.36, 2×2, 5×5, 10×10 inches, etc.) using the same heater layer material and thickness will also be 1.0 ohm. This is because the width  53  of the electrical current path increases at the same time as the length  52  of the current path, thereby resulting in no net change in the electrical resistance. This feature is useful for calculating heater layer stripe lengths and resistances as described below. 
     As illustrated next by comparing  FIGS. 11 and 14 , cutting the heater layer sheet  49  into ten stripes  54  of 1 inch width  56  with 10 inch length  52  has no effect on the overall electrical resistance of the zone (assuming the gaps  57  between the heater layer stripes  54  have an infinitely narrow width  58 ). The resistance of each 1×10 inch stripe  54  will have a resistance of 10 ohms, and, therefore, the resistance of each square inch will be 1 ohm. The resistance of ten, parallel-connected 1×10 inch stripes  54  with resistances of 10 ohms each is therefore 1 ohm (10/10) based on the rules for equal resistances in parallel. Another way to look at this is that the resistance of a 1×1 inch square is 1 ohm, so ten 1×1 inch squares in series equals a total of 10 ohms for a 1×10 inch stripe  54 . 
     Referring now to  FIG. 15 , if the heater layer sheet  49  is rearranged into five heater layer stripes  54  of 20 inches length the electrical resistance of each 1×20 inch stripe  54  will be 20 ohms. A heater layer sheet  49  consisting of five 1×20 inch resistors in parallel will therefore total 4 ohms (20/5 or four 5×5 squares at one ohm each connected in series). The overall electrical resistance of the heater layer sheet  49  therefore increases with the square of the increase in stripe length  52  because the total area of the zone is fixed, and hence the stripe width  53  must decrease at the same rate as the stripe length  52  increases. Increasing the length  52  and decreasing the width  56  of the stripe  54  both work to increase the electrical resistance of the stripe  54 . 
     Referring next to  FIGS. 13 and 16 , if the heater layer sheet  49  is rearranged into one stripe  54  of 1 inch width  56 ×100 inch length  52 , the resistance of the heater layer sheet  49  will total 100 ohms. This longer and narrower heater stripe  54  must therefore be spirally wrapped around the barrel  2  for it to fit within the shorter zone length  43 . A heater layer stripe  54  of 100 inch length  52  will therefore have to make about ten rotations around the barrel  2  in a zone of only 10 inch length  43  (assuming infinitely narrow gaps  57  between the stripes  54 ). 
     The electrical resistance of the 10 inch×10 inch heater layer sheet  49  illustrated in  FIG. 11  can therefore be increased by up to 100 times by converting it to a spiral stripe  54  of 1 inch width  56  wrapped up to 10 times around the barrel  2  (as illustrated in  FIGS. 13 and 16 ). And, with even narrower stripe widths  56 , even higher electrical resistances are possible. 
     Gaps between spiral stripes. Referring again to  FIG. 13 , the heater layer stripes  54  are formed by making gaps  57  in the heater layer  17 . Realistic gap widths  58  range from as narrow as 20 mils to as wide as 0.5 inch or more. The width  58  of the gaps  57  is not particularly important, except that relatively wide gaps  57  will significantly reduce the overall surface area of the ceramic heater layer  17 , requiring a higher heat generation density in the remaining area to achieve an acceptable overall heating rate. 
     Forming the spiral stripes. Referring now to  FIGS. 10 and 13 , the spiral stripes  54  of the heater layer  17  are preferably of equal width  56  to ensure uniform resistance and uniform heating, assuming the application of a common voltage to all the stripes  54 . The heater layer  17  can be striped by masking during the thermal spray process, by using thermal spray tape, or wire or other metallic fixtures to prevent the heater layer  17  from adhering to the base insulator layer  16  at the selected gap locations. The gaps  57  between the heater layer stripes  54  can also be cut into the heater layer  17  after it is applied, using a narrow grinding or cut-off wheel, by tooling, or by grit blasting. The preferred striping method is by using a micro-grit blasting unit (or pencil blaster) such as is made by Comco Inc., Burbank, Calif., and Vaniman Manufacturing Co., Brandon Fallbrook, Calif., designed for processing jewelry and dental fixtures. These units can produce narrow channels  57  in the ceramic heater layer  17  as small as 20-30 mils in width  58 . Because blasting leaves the ceramic surfaces textured and uncontaminated, additional ceramic sprayed over the heater layer  17  (such as a top insulator layer) will adhere properly. Very narrow gaps  57  between stripes  54  are possible because the voltage difference between the stripes at adjacent points (equal resistors in parallel) is typically only a few volts. Calculation of stripe length and other factors. For calculating the appropriate stripe length, all of the required parameters are known (refer to  FIGS. 5B ,  6 B,  11  and  13 ):
         1) Zone applied voltage  20 ;   2) Heater layer ohms/square value (measureable function of material and thickness);   3) Desired wattage per square inch or centimeter;   4) Total zone amperage (maximum allowable); and   5) Heated zone area (distance  52 ×circumference  53 , less the total area of the gaps  57 ).       

     Computing the heater layer as a single spiral heater stripe  54  should be done first. Selecting a value for the stripe length and gap width  58  then automatically determines the stripe width  56 , stripe resistance, stripe amperage, stripe wattage, and watt density (watts per square inch or centimeter). Once the stripe length has been determined, the single stripe  54  can be divided into multiple narrower stripes compensating only for the area lost due to the gaps  57  between stripes  54 . 
     Calculation errors in stripe resistance. Referring still to  FIG. 13 , the stripe(s)  54  are at a helix angle  55  to the axis  34  of the barrel  2 , so the electrode bands  50 ,  51  are actually not perpendicular to the axis of the stripes  54 . 
       FIG. 17  now provides an unraveled, flattened representation of a single wide heater layer stripe  54 , showing the helix angle  55  between it and the barrel axis  34  (which is perpendicular to the electrode bands  50 ,  51 ). It is apparent from the flattened representation in  FIG. 17  that asymmetric current flow paths are created, whereby the current path  60  from A to D is shorter than the current path  61  from B to C. Accordingly, less electrical current flows along the long path  61 , producing less heat generation in the vicinity of points B and C, resulting in relatively cooler regions  62 ,  63 . This geometry problem also causes the measured electrical resistance between the electrode bands  50 ,  51  to be ten or more percent lower than the calculated resistance based on the surface resistivity (ohms per square) value of the ceramic heater layer, and thus also causes the current flow to be higher than expected. 
     Improved temperature uniformity with multiple stripes. By dividing the wide stripe  54  shown in  FIG. 17 , into several narrower stripes  54   a  through  54   d  as illustrated in  FIG. 18 , the cooler areas  62   a  through  63   d  are still present but are much smaller in area and more distributed around the circumference  53  of the barrel. The temperature uniformity throughout the heated zone is therefore much improved. 
     Routing stripes around obstacles. Referring now to  FIGS. 1 and 10 , cylindrical plasticating elements such as barrels  2  typically have at least one thermocouple temperature sensor (“TC”)  6  per zone  5 , typically installed in ⅜ th  inch (NPT threaded) holes. There may be other discontinuities in the external surface  32  of the barrel  2  that must be circumnavigated by the heater layer  17 . The thermally-sprayed internal insulator layer  16  will extend nearly to the edge of the TC hole  6 , but the heater layer  17  must be masked for at least ¼ inch away from the edge of the TC hole  6  to prevent the electrical current from arcing to ground. This makes the area to be avoided by the heater layer  17  significantly larger than the actual TC hole  6 . 
     As illustrated next in  FIG. 19A , if the TC hole  6  lies within a single narrow stripe  54  the narrowed areas  64 ,  65  of heater layer remaining on either side of the TC hole  6  will carry the entire current flow  33  and so will likely overheat. The same situation is true to a lesser extent if the TC hole  6  is straddled by two stripes  54   a ,  54   b  as illustrated in  FIG. 19B . The way to minimize this potential overheating problem is to spread the current flow  33  over a merged, larger area  66 , as illustrated in  FIG. 19C . With this approach multiple heater layer stripes  54   a - 54   c  are merged into a shared heating layer area  66  in the immediate vicinity of the TC hole  6 . Of course, for this method to work, the stripes  54   a - 54   c  must be connected as parallel resistors. 
     A unique characteristic of electrically-conductive ceramic or metal-ceramic (i.e. cermet) coatings is that their electrical resistance reduces with temperature. This phenomenon is illustrated in  FIG. 20  where the resistance of a representative heating layer (2 mil thick titania) is charted versus temperature. Referring now to  FIG. 21 , for a given supply voltage  20 , the electrical current  33  passing between the electrodes  19  and through the heater layer  17  will therefore increase with temperature. A given process temperature can then be equated to a given electrical current (amperage)  33 , and so a circuit breaker or fuse  67  can be incorporated into the power circuit in the present invention to protect the cylindrical plasticating element  2  from reaching an undesirable, excessive temperature. 
     Referring now to  FIG. 22A , zones  68 ,  69 ,  70  that are heated solely by the ceramic heating system  71  (i.e. referring to herein, in combination, the ceramic heating and insulating layers  16 ,  17 ,  18 , electrodes  19  with terminals  37 , and power supply  20 ) can be combined on the same cylindrical plasticating element  2  (such as a barrel) with zones  72 ,  73  that are heated entirely or partially by conventional band-heaters  4 ,  7 . This mixed approach allows the ceramic heating system  71  to be preferably used on substantial, contiguous sections of the barrel  2 , while band-heaters  4 ,  7  can be retained on shorter sections where lower energy savings are possible. As an example, referring still to  FIG. 22A , band-heaters  7  can be retained on the nozzle  74  for control of the discharge zone  73 , and on sections with over-sized diameters, such as the end-bell  75 , where the band-heaters  4  can even be electrically connected in parallel with the ceramic heating system  71  for control of a hybrid zone  72 . 
     Referring next to  FIGS. 2A and 22B , because the ceramic materials used in the ceramic heating system  71  are generally not ferrous or magnetic, induction heating system coils  11  can be installed around the exterior thermal insulting layer  10 ,  46  to further enhance heating of the barrel  2 , or in the event there is a failure in the underlying ceramic heating system  71 . Referring now to  FIGS. 21 and 22C , because the outer surface of the present invention&#39;s external electrical insulating layer  18  is preferably smooth, band-heaters  4  may also be installed around the electrical insulating layer  18  of enhanced heating. 
     Cooling of the cylindrical plasticating barrel  2  is often required, particularly on extruders where internal viscous heating of the processed materials can produce excess heat. As illustrated in  FIG. 23 , one conventional way to cool a barrel  2  and its surrounding band-heaters  4  is to use an air-cooling system  76  that typically comprises at least one or more air blowers (fans)  77  and constraining a sheet-metal enclosure  78  or shroud to circulate cooling air  79  around the barrel  2  prior to discharging it to ambient through discharge vents  80 . As shown next in  FIG. 24 , another conventional way to cool a barrel  2  is to use a water-cooling system  81  that typically comprises water-cooled band-heaters or jackets  82  that incorporate flow-channels through which cooling water  83  can be circulated. Referring now to  FIG. 25 , because the outer surface of the ceramic heating system&#39;s external electrical insulating layer  18  is preferably smooth, conventional barrel cooling systems  76 ,  81  can be installed around the electrical insulating layer  18  of the present invention to cool the barrel  2 . 
     It should be noted that the present invention can be practiced otherwise than as specifically illustrated and described herein without departing from its spirit or scope. It is intended that all such modifications and alterations be included insofar as they are consistent with the objectives and spirit of the invention.