Abstract:
The present invention provides an electrically heated nozzle for injection molding which is insulated to prevent conduction of electricity and loss of thermal transmission to the casing, with an electric heater and sensor that are wrapped around substantially the same portion of a melt channel of the nozzle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of U.S. application Ser. No. 10/025,767 filed Dec. 26, 2001, which is a continuation of U.S. application Ser. No. 09/520,843 filed Mar. 8, 2000 that issued as U.S. Pat. No. 6,394,784 on May 28, 2002, the entire disclosures of which are hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to injection molding and more particularly to an injection molding nozzle having an integral electrical heating element surrounded by layered dielectric insulation.  
           [0004]    2. Related Art  
           [0005]    Heaters for injection molding and hot runner applications are known in the prior art, as demonstrated amply by the following U.S. Pat. Nos. 2,991,423, 2,522,365, 2,769,201, 2,814,070, 2,875,312, 2,987,300, 3,062,940, 3,550,267, 3,849,630, 3,911,251, 4,032,046, 4,403,405, 4,386,262, 4,557,685, 4,635,851, 4,644,140, 4,652,230, 4,771,164, 4,795,126, 4,837,925, 4,865,535, 4,945,630, and 4,981,431.  
           [0006]    Heaters are of course also amply known in non-injection molding applications, as shown for example in U.S. Pat. Nos. 2,088,586, 2,378,530, 2,794,504, 4,438,322 and 4,621,251.  
           [0007]    There are in general three types of heaters known for use in the hot runner nozzles. The first is so-called “integral heaters” which are embedded or cast in the nozzle body. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 4,238,671, U.S. Pat. No. 4,386,262, U.S. Pat. No. 4,403,405 and EP 765728. The second is so-called “independent external heaters” which have. their own support and that can be removed and replaced. Essentially, in such a design, shown in FIG. 1 a,  the heating element H is external to the nozzle body N. Heating element H comprises a resistance wire W surrounded by electrical insulating material E and is encased in a steel casing C. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 3,553,788, U.S. Pat. No. 3,677,682, U.S. Pat. No. 3,831,004, U.S. Pat. No. 3,912,907, U.S. Pat. No. 4,588,367, U.S. Pat. No. 5,360,333, U.S. Pat. No. 5,411,393, U.S. Pat. No. 5,820,900, EP 748678, EP 963829 and EP 444748. The third is so-called “attached external heaters” which are positioned spirally around the exterior of the nozzle or the nozzle tip but cannot be removed therefrom by reason of being brazed or embedded in the nozzle surface. Referring to FIG. 1 b,  heating element H′ is embedded in a groove G′ in nozzle body N′. Examples of such nozzles are disclosed in the following patents: U.S. Pat. No. 4,557,685, U.S. Pat. No. 4,583,284, U.S. Pat. No. 4,652,230, U.S. Pat. No. 5,226,596, U.S. Pat. No. 5,235,737, U.S. Pat. No. 5,266,023, U.S. Pat. No. 5,282,735, U.S. Pat. No. 5,614,233, U.S. Pat. No. 5,704,113 and U.S. Pat. No. 5,871,786.  
           [0008]    Electrical heaters have been also used in the design of the so-called hot runner probes. Unlike the hot runner nozzles the hot runner probes do not comprise the melt channel. The probes are located inside the melt channel of the nozzle and thus create an annular flow. The melt is heated from the inside and this heating approach is not applicable to all materials and applications. Examples of such nozzles are disclosed in the following U.S. Pat. Nos. 3,800,027 3,970,821, 4,120,086, 4,373,132, 4,304,544, 4,376,244, 4,438,064, 4,492,556, 4,516,927, 4,641,423, 4,643,664, 4,704,516, 4,711,625, 4,740,674, 4,795,126, 4,894,197, 5,055,028, 5,225,211, 5,456,592, 5,527,177 and 5,504,304.  
           [0009]    Injection molding nozzles having integral heaters typically have electrical heating elements, wound spirally around the nozzle, which offer an efficient response to the many critical process conditions required by modern injection molding operations. There has been a continuous effort in the prior art, however, to improve the temperature profile, the heating efficiency and durability of such nozzles and achieve an overall reduction in size. Most of these efforts have been aimed at improving the means of heating the nozzle.  
           [0010]    For example, U.S. Pat. No. 5,051,086 to Gellert discloses a heater element brazed onto the nozzle housing and then embedded in multiple layers of plasma-sprayed stainless steel and alumina oxide. To avoid cracking of the ceramic layers caused by excessive thickness and the differing thermal properties of the ceramic and the stainless steel, Gellert employs alternating thin layers of stainless steel and alumina oxide. The heating element of Gellert is nickel-chrome resistance wire (i.e. see W in FIGS. 1 a and  1   b  herein) extending centrally through a refractory powder electrical insulating material (i.e. see E in FIGS. 1 a  and  1   b ), such as magnesium oxide, inside a steel casing (i.e. see C in FIGS. 1 a  and  1   b ). The heating element is integrally cast in a nickel alloy by a first brazing step in a vacuum furnace, which causes the nickel alloy to flow by capillary action into the spaces around the heater element to metallurgically bond the steel casing of the element to the nozzle body. This bonding produces very efficient and uniform heat transfer from the element to the nozzle body.  
           [0011]    Nozzles with this type of electrical heaters, however, are often too big to be used in small pitch gating due to the size of the insulated heater required. These heaters are also generally expensive to make because of complex machining required. Also, the manufacturing methods to make these nozzle heaters are complex and therefore production is time consuming.  
           [0012]    U.S. Pat. No. 5,955,120 to Deissler which discloses a hot runner nozzle with high thermal insulation achieved by coating the electrical heater with layers of a thermally insulation materials (mica or ceramic) and high wear resistance material (titanium). Like Gellert, the heater element of Deissler has its own electrical insulation protection and thus can be placed in direct contact with the metallic nozzle body (see FIG. 2 of Deissler). Also the heater element of Deissler is attached to the nozzle by casting (brazing) a metal such as brass. Deissler is thus similar to Gellert in that it discloses an insulated and brazed heater element. Again, as with Gellert, such a device requires many additional steps to braze and insulate the heater and is therefore time consuming. Also, as with Gellert, the use of an insulated element makes the size of the heated nozzle not well suited for small pitch applications.  
           [0013]    In an attempt to reduce nozzle size, U.S. Pat. No. 5,973,296 to Juliano shows a thick film heater applied to the outside surface of an injection nozzle. The nozzle heater comprises a dielectric film layer and a resistive thick film layer applied directly to the exterior cylindrical surface of the nozzle by means of precision thick film printing. The thick film is applied directly to the nozzle body, which increases the nozzle&#39;s diameter by only a minimal amount. Flexibility of heat distribution is also obtained through the ability to apply the heater in various patterns and is, thus, less limited than spiral designs.  
           [0014]    There are limitations to the thick film heater, however. Thermal expansion of the steel nozzle body during heating can cause unwanted cracking in the film layers due to the lower thermal expansion of the film material. This effect is particularly acute after a large number of injection cycles. The cracks could affect the resistive film heater because it is not a continuous and homogeneous material (as is a wire), but rather the fine dried powder of the conductive ink, as disclosed in Juliano &#39;296.  
           [0015]    Another heated nozzle design is disclosed in U.S. Pat. No. 4,120,086 to Crandell. In one embodiment, Crandell &#39;086 discloses an electrically heated nozzle having an integral heater comprising a resistance wire heater disposed between two ceramic insulating layers. The Crandell &#39;086 nozzle is made by wrapping a metal nozzle body with flexible strips of green (i.e., unsintered) ceramic particles impregnated in heat dissipatable material, subsequently winding a resistance wire heating element around the wrapped green layer, wrapping a second layer of the flexible strips of green ceramic particles thereover, heat treating the assembly to bake out the heat dissipatable material and sinter the ceramic particles together, and then compacting the assembly to eliminate air voids in the assembly. In U.S. Pat. No. 4,304,544, also to Crandell, the inventor further describes the flexible green ceramic strips as comprising a body of green ceramic insulator particles which are impregnated in a heat dissipatable binder material. In the green state, such strips are pliable and bendable, permitting them to be wrapped around the metal nozzle core, but when baked, the strips become hard and the particles agglomerate into a mass.  
           [0016]    The Crandell &#39;086 and &#39;544 nozzle has relatively thick ceramic layers, employs an awkward process for applying the ceramic layers and requires additional heat treatment steps in fabrication, Crandell &#39;086 concedes that the baking step is time consuming (see column 5, lines 20-25) and therefore admits that the design is less preferable than other embodiments disclosed in the patent which do not utilize this method. Also, as mentioned above, it is desirable to reduce nozzle size, which is not possible with the thick ceramic strips of Crandell &#39;086 and &#39;144 .  
           [0017]    The use of ceramic heaters for both hot runner nozzle heaters and hot runner probe heaters is also disclosed in U.S. Pat. No. 5,504,304 to Noguchi. Noguchi, like Juliano, uses a printing method to form an electrical resistive wire pattern of a various pitch from a metal or a composite paste. A ceramic heater embodiment for a nozzle probe (shown in FIG. 1 of Noguchi) is made by printing various electrical resistive patterns shown in FIGS. 3-4 of Noguchi. Noguchi discloses a method whereby a mixture of insulating ceramic powder such as silicon carbide (SiC), molybdenum silicide (MOSi 2 ) or alumina (A 1   2 O 3 ) and silicon nitride (SiN), and electrically conductive ceramic powder such as titanium nitride (TiN) and titanium carbide (TiC is sintered and kneaded into a paste, which is then printed in a snaking manner on the external surface of a cylindrical insulating ceramic body, as shown in FIG. 3 of Noguchi. The printing state is made denser in certain areas and, by so controlling the magnitude of the so-called “wire density,” a temperature gradient is given to the heater. The heater pattern can be formed using metals such as tungsten, molybdenum, gold and platinum. A ceramic heater embodiment for a hot runner nozzle is also disclosed in Noguchi (see FIG. 14 of Noguchi). This self-sustained ceramic heater is also made by wire-printing using the same paste or metals. The heater is placed over the nozzle body and is then sintered and kneaded into a paste comprising a mixture of insulation ceramic powder such as silicon carbide, molybdenum silicide or alumina and conductive ceramic powder such as titanium nitride and titanium carbide. The paste is printed in a single snaking line on the part where, again, the heater pattern is formed by applying temperature gradients by varying the magnitude of wire density across the part.  
           [0018]    Although Noguchi introduces a wire-printing method to achieve a certain heat profile along the nozzle it does not teach or show how this wire-printing method is actually implemented. More detailed information about this wire-printing method is provided by the patentee&#39;s (Seiki Spear System America, Inc.) catalogue entitled “SH-1 Hot Runner Probe” (undated). According to the catalogue, the circuit pattern, which provides the resistance for heating, is screen printed direction onto a “green” or uncured ceramic substrate. The flexible “green” substrate with the printed circuit is wrapped around an existing ceramic tube and the complete unit is fired and cured to produce a tubular heater. The resistive circuit pattern is encased within the ceramic between the tube and the substrate and has no exposure to the outside atmosphere. The thermocouple is inserted through the center of the tubular heater and positioned in the tip area. Thermocouple placement in the probe tip gives direct heat control at the gate. The ceramic heater unit is then fixed outside the probe body. Thus, this Seiki Spear method of making a ceramic heater body according to Noguchi including a printed-wire is similar to the method disclosed in Crandell &#39; 086 , with the exception that Crandell uses a self-sustained resistance wire wound spirally around the nozzle between two “green” ceramic layers. As with Crandell, as well, an additional sintering step is required to sinter the green ceramic layers.  
           [0019]    Accordingly, there is a need for a heated nozzle which overcomes these and other difficulties associated with the prior art. Specifically, there is a need for a heated nozzle which is simpler to produce and yields a more compact design.  
         SUMMARY OF THE INVENTION  
         [0020]    The present invention provides an injection molding nozzle which is smaller in diameter than most prior art nozzles but which does not sacrifice durability or have the increased manufacturing costs of previous small diameter nozzles. Further the nozzle of the present invention is simpler, quicker and less costly to produce than prior art nozzles and minimizes the number of overall steps required in production. In particular, the need for heat treating the dielectric materials of the heater is removed entirely, saving time, money and hassle in fabrication. Further, the apparatus of the present invention provides a removable and/or replaceable cartridge heater design which offers the advantage of low-cost repair or replacement of a low cost heater component, rather than wholesale replacement of an intricately and precisely machined nozzle. The methods of the present. invention similarly provide reduced and simplified steps in manufacturing, as well as permitting precise temperature patterns to be achieved a nozzle more simply than with the prior art.  
           [0021]    In one aspect, the present invention provides an injection molding nozzle comprising a nozzle body having an outer surface and at least one melt channel through the body, a first insulating layer having a chemical composition, the first insulating layer disposed on the nozzle body outer surface so as to substantially cover at least a portion of the nozzle body, at least one wire element disposed exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the wire element, a second insulating layer having a chemical composition, the second insulating layer disposed over the first insulating layer and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer, and wherein the chemical compositions of the first and second insulating layers remain substantially unchanged once the layers are disposed on the nozzle body.  
           [0022]    In a second aspect, the present invention provides an injection molding nozzle comprising a nozzle body assembly having an outer surface and at least one melt channel through the assembly, the assembly having a core and a surface layer disposed around the core, the surface layer forming at least a portion of the nozzle body assembly outer surface, the core being composed of a first metal and the surface layer being composed of a second metal, the second metal having a higher thermal conductivity than the first metal, a first insulating layer disposed on the nozzle body assembly outer surface so as to substantially cover at least a portion of the outer surface, at least one wire element disposed exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the wire element and a second insulating layer disposed over the first insulating layer and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer.  
           [0023]    In a third aspect, the present invention provides an injection molding nozzle comprising a nozzle body having an outer surface and at least one melt channel through the body, a first insulating layer disposed on the nozzle body outer surface so as to substantially cover at least a portion of the nozzle body, at least one wire element disposed exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the wire element, a second insulating layer disposed over the first insulating layer and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer, and wherein the first insulating layer is between 0.1 mm and 0.5 mm in thickness.  
           [0024]    In a fourth aspect, the present invention provides an injection machine for forming a molded article, the machine comprising a mold cavity, the mold cavity formed between a movable mold platen and a stationary mold platen, at least one injection molding nozzle connectable to a source of molten material and capable of feeding molten material from the source to the mold cavity through at least one melt channel therethrough, the at least one nozzle injection molding having a nozzle body having an outer surface and the at least one melt channel through the body, a first insulating layer having a chemical composition, the first insulating layer disposed on the nozzle body outer surface so as to substantially cover at least a portion of the nozzle body, at least one wire element disposed exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the wire element, a second insulating layer having a chemical composition, the second insulating layer disposed over the first insulating layer and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer, and wherein the chemical compositions of the first and second insulating layers remain substantially unchanged once the layers are disposed on the nozzle body.  
           [0025]    In a fifth aspect, the present invention provides an injection mold to form an article, the mold comprising a mold half capable of communication with a mold manifold, at least one injection molding nozzle in flow communication with the mold half through at least one melt channel, the at least one nozzle injection molding having a nozzle body having an outer surface and the at least one melt channel through the body, a first insulating layer having a chemical composition, the first insulating layer disposed on the nozzle body outer surface so as to substantially cover at least a portion of the nozzle body, at least one wire element disposed exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the wire element, a second insulating layer having a chemical composition, the second insulating layer disposed over the first insulating layer and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer, and wherein the chemical compositions of the first and second insulating layers remain substantially unchanged once the layers are disposed on the nozzle body.  
           [0026]    In a sixth aspect, the present invention provides an injection molding nozzle comprising the steps of providing a nozzle body, the nozzle body having an outer surface and at least one melt channel through the body providing a first insulating layer on the outer surface of the nozzle body, the first insulating layer having a chemical composition, the first insulating layer substantially covering at least a portion of the nozzle body outer surface, positioning at least one wire element exterior to and in contact with the first insulating layer, the at least one wire element being connectable to a power supply capable of heating the at least one wire element, providing a second insulating layer on the first insulating layer and the at least one wire element, the second insulating layer having a chemical composition, the second insulating layer substantially covering the at least one wire element and at least a portion of the first insulating layer, and wherein the chemical compositions of the first and second insulating layers remain substantially unchanged once the layers are provided on the nozzle body.  
           [0027]    In a seventh aspect, the present invention provides an injection molding nozzle comprising the steps of providing a nozzle body, the nozzle body having an outer surface and at least one melt channel through the body positioning a self-supporting insulating sleeve around the nozzle body, the sleeve substantially covering at least a portion of the nozzle body outer surface positioning at least one wire element exterior to and in contact with the insulating sleeve, the at least one wire element being connectable to a power supply capable of heating the at least one wire element, providing a second insulating layer on the insulating sleeve and the at least one wire element, the second insulating layer substantially covering the at least one wire element and at least a portion of the insulating sleeve.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0028]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings.  
         [0029]    The drawings show articles made according to a preferred embodiment of the present invention, in which:  
         [0030]    [0030]FIGS. 1 a  and  1   b  are partial sectional views of heated nozzle configurations according to the prior art;  
         [0031]    [0031]FIG. 2 is a sectional view of a portion of an injection molding system showing a heated nozzle according to a preferred embodiment of the present invention;  
         [0032]    [0032]FIG. 3 is an enlarged sectional view of the nozzle of FIG. 2;  
         [0033]    [0033]FIG. 4 is a further enlarged and rotated (90′ counter-clockwise) sectional view of the heater assembly of the nozzle of FIG. 2;  
         [0034]    [0034]FIG. 5 is an enlarged sectional view, similar to FIG. 4, of an alternate embodiment of a nozzle heater assembly according to the present invention;  
         [0035]    [0035]FIG. 6 is an enlarged sectional view, similar to FIG. 4, of another alternate embodiment of a nozzle heater assembly according to the present invention;  
         [0036]    [0036]FIG. 7 is an enlarged sectional view, similar to FIG. 4, of a further alternate embodiment of a nozzle heater assembly according to the present invention;  
         [0037]    [0037]FIG. 8 is an enlarged sectional view, similar to FIG. 4, of a yet further alternate embodiment of a nozzle heater assembly according to the present invention;  
         [0038]    [0038]FIG. 9 is an exploded isometric view of an alternate embodiment of the nozzle heater of the present invention;  
         [0039]    [0039]FIG. 10 is a sectional view of a further embodiment of the nozzle heater of the present invention;  
         [0040]    [0040]FIG. 11 is an enlarged sectional view of another nozzle embodiment employing a heater according to the present invention;  
         [0041]    [0041]FIG. 12 a  is an isometric view of a straight wire element for use as a heater element of the present invention;  
         [0042]    [0042]FIG. 12 b  is an isometric view of a coiled wire element for use as a heater element of the present invention;  
         [0043]    [0043]FIG. 13 a  is an isometric view of a doubled and twisted straight wire element for use as a heater element of the present invention; and  
         [0044]    [0044]FIG. 13 b  is an isometric view of a doubled, coiled wire element for use as a heater element of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    A multi-cavity injection molding system made in accordance with the present invention is shown in the Figures generally at M. Referring to FIG. 2, a portion of injection molding system M is shown. A melt passage  10  extends from a common recessed inlet  12  in a manifold extension  14  to an elongated manifold  16  where it branches out to a number of outlets  18 . As can be seen, each branch  20  of melt passage  10  extends through steel nozzle  22 , having a central melt bore  24  in communication with melt passage outlet  18  from manifold  16  to a gate  26  leading to each cavity  28 . Nozzle  22  is a heated nozzle having a heater  30  according to a preferred embodiment of the invention, as described in greater detail below.  
         [0046]    Manifold  16  is heated by a heating element  32 , which may be integrally brazed into it. Manifold  16  is held in place by a central locating ring  34  and insulating pressure pads  36 . Locating ring  34  bridges an insulative air space  38  between manifold  16  and a cooled spacer plate  40 . Pressure pads  36  provide another insulative air space  42  between manifold  16  and a cooled clamp plate  44 . Spacer plate  40 , clamp plate  44  and cavity plate  46  are cooled by pumping cooling water through a plurality of cooling conduits  48 . Clamp plate  44  and spacer plate  40  are secured in place by bolts  50  which extend into cavity plate  46 . Manifold extension  14  is held in place by screws  52  and a locating collar  54  which is secured to the clamp plate  44  by screws  56 .  
         [0047]    Each nozzle  22  is seated in a well  58  in spacer plate  40 . An insulative air space  64  is provided between heated nozzle  22  and the surrounding cooled spacer plate  40 .  
         [0048]    Referring to FIGS. 2 and 3, nozzle  22  has a body  68  having a steel central core portion  70 , an outer surface  72 , and a tip  74 , which is seated in gate  26 . Tip  74  has a flow channel  76  which is aligned with central melt bore  24 . Nozzle  22  is seated and secured in manifold  16  by a threaded portion  78 . Heater assembly  30  has an electrical resistive wire heating element  80 , having a cold pin connections  82  for connecting wire element  80  to a power supply (not shown). Heater assembly  30  also has a first insulating layer  84  and a second insulating layer  86  disposed on either side of wire element  80 , so as to “sandwich” element  80  therebetween. First layer  84  is positioned on core  70 , with wire element  80  wrapped therearound, and second layer  86  positioned thereover. An outer steel layer  88  is provided to finish nozzle  22 . These layers are provided in a manner as will be described in more detail below.  
         [0049]    Wire element  80  is a simple, bare, electrically and thermally uninsulated wire, preferably of thirty (30) gauge chromium nickel, though any wire material having resistive heating characteristics may be employed. Wire element  80  is preferably wrapped around nozzle  22 , and may be provided in any arrangement which provides the temperature distribution desired for a particular application. For example, in the embodiment of FIG. 3, successive windings of wire element  80  are closer together at the ends of nozzle  22 , where more heat is typically required, with a more spaced distribution occurring in the central portion of nozzle  22 .  
         [0050]    According to the present invention, first layer  84  and second layer  86  are dielectric materials which can be applied in a “finished” (i.e. “non-green”) state to the nozzle body. In other words, the dielectric material does not require additional heat treating steps once it is applied to the nozzle assembly, and thus has a chemical composition which does not change after it is applied to the apparatus and the material does not require heat treating of sintering to achieve its “finished” state. In addition to this constraint, first layer  84  is also preferably a dielectric material which can withstand the high operating temperatures and heater wattages experienced in hot runner injection molding. As one skilled in the art will understand, the dielectric is preferably a good thermal conductor with low heat capacity, a combination which encourages rapid heating (and cooling) with maximum efficiency. The dielectric should also be a good electrical insulator, since wire element is otherwise uninsulated from nozzle  22 . The choice of material depends also on the temperature target for the molten material which will flow through the melt channel of the nozzle.  
         [0051]    Illustrative of the dielectric materials which can be used in the practice of this invention are: aluminum oxide; magnesium oxide; mica coatings; Vespel™ (trade mark of E. I. Du Pont de Nemour &amp; Company), graphite; alumina; alumina-silica; zirconia-based materials, such as tetragonal zirconia polycrystals (TZP) partially stabilized zirconia (PSZ), fully stabilized zirconia (FSZ), transformation toughened ceramics (TTC), zirconia toughened alumina (ZTA) and transformation toughened zirconia (TTZ); Cerama-Dip™ 538N (trade mark of Aremco Products Inc.), a zirconium silicate-filled water-based high temperature dielectric coating for use in insulating high-power resistors, coils and heaters; and Ceramacoat™538N (trade mark of Aremco Products Inc.) is a silica based, high temperature dielectric coating for use in insulating induction heating coils. Aluminum oxide is a preferred material because of its relatively high thermal conductivity.  
         [0052]    Second layer  86  is provided to protect wire element  80  from the deleterious effects of the atmosphere, such as oxidation and corrosion, and to insulate the exterior of nozzle  22  electrically and thermally, so as to direct the output of heater assembly  30  towards the melt in flow channel  76 . Second layer  86  may be made from the same dielectric material as first layer  84  or a different material. In some applications, it may be desirable to use different materials. For example, the first layer  84  may be fabricated from a material having good electric insulating properties but high heat conductive characteristic, while the second layer  86  is of a material having high electric insulating properties and high heat insulating properties, so that the heat is directed to the central melt bore  24  within body  68 , while outer layer  88  remains cooler. The use of the same material, preferably aluminum oxide, for first layer  84  and second layer  86  is preferred.  
         [0053]    First layer  84  and second layer  86  may be provided as particles or a liquid sprayed onto the nozzle apparatus, as a liquid “painted” onto the apparatus or as a solid, pre-fabricated, self-supporting sleeve, as described in more detail below. The layers may be provided in thicknesses as desired to suit a particular application. Thicknesses of the layers can range from 0.1 mm to 3 mm, and thicker, depending on the amount of insulating, overall nozzle diameter and method of fabrication desired, as will be described further below. Thicknesses in the range of 0.1 mm to 0.5 mm are preferred.  
         [0054]    Outer layer  88  may be applied by spraying or by shrink-fitting a sleeve on second layer  86 . Outer layer  88  may have any desired thickness, though a thickness of about 1.5 mm is preferred.  
         [0055]    Referring to FIGS.  4 - 7 , other embodiments of a nozzle heater according to the present invention are shown. In the embodiment of FIG. 5, a secondary wire element  90  is provided around second layer  86 , protected by a third insulating layer  92 . In this three-layer embodiment, second layer  86  is preferably a good heat conductor and electrical insulator while third layer  92  is a dielectric having good thermal insulating characteristics. Third layer  92  can be chosen from the same set of materials as described above for layers  84  and  86 . This embodiment permits a higher wattage heater to be obtained, at the obvious expense of a slightly larger nozzle diameter. Alternatively, secondary wire element  90  can provide redundancy for operational, use if and when the primary wire element fails. FIG. 6 shows a configuration similar to FIG. 4, but with integral temperature sensors or thermocouple wire  94  and  96  positioned between first layer  84  and second layer  86 , wound spirally around nozzle  22  adjacent wire element  80 . Inclusion of thermocouples  94  and  96  allow for exacting temperature control in nozzle  22 , as will be understood by one skilled in the art. The thermocouples may be disposed immediately adjacent wire element  8 , as shown in FIG. 6, or may be provided between second layer  86  and third insulating layer  92 , as depicted in FIG. 7. In this embodiment, second layer  86  and third layer  92  preferably have similar characteristics as described above for the FIG. 5 embodiment.  
         [0056]    Referring to FIG. 8, in a further alternate embodiment, a metal surface layer  98  is provided on outer surface  72 , between nozzle core  70  and first layer  84 . Surface layer  98  is a layer of a metal having a higher thermal conductivity than steel nozzle body  68 , such as copper and alloys of copper. Surface layer  98  thus promotes a more even distribution of heat from heater assembly  30  to the pressurized melt in central melt bore  24 . Surface layer  98  may be applied by spraying or by shrink-fitting a sleeve on core  70 . Surface layer  98  may have a thickness of between 0.1 mm to 0.5 mm, or greater if desired.  
         [0057]    Referring to FIG. 9, in an alternate embodiment of the present invention, nozzle  22 ′ has a core  70 ′, a surface layer  98 ′ and a heater assembly  30 ′, which is composed of a first layer  84 ′, a wire element  80 ′, a second layer  86 ′ and an outer layer  88 ′. In this embodiment, surface layer  98 ′, first layer  84 ′, second layer  86 ′ and outer layer  88 ′ in fact, self-supporting, substantially rigid, annular telescoping sleeve components  98   a,    84   a,    86   a,  and  88   a,  respectively, which are pre-fabricated, prior to assembly of nozzle  22 ′, according to a method of the present invention, described below. This sleeve construction permits a heater assembly  30 ′ configuration which is selectively removable in part or in whole, depending on the design, from nozzle  22 ′ for periodic inspection, repair and/or replacement. Also, this sleeve construction permits the nozzle body to expand independently from the insulating layers, by virtue of the separate and self-supporting nature of the heater sleeves. Thus, when thermal expansion occurs in the nozzle, nozzle body  68  is free to grow longitudinally while the insulating sleeves and wire, which typically have lower thermal expansion characteristics, will not be subject to a mechanical stress induced by this nozzle body expansion. This feature has beneficial implications for increased heater durability.  
         [0058]    The self-supporting annular sleeves of this embodiment may be made of any suitable dielectric material, as described above, that can be machined, molded or extruded into a thin-walled tube. As with the previous embodiments, it is desirable that the coefficient of thermal transfer to be higher for inner sleeve than the outer sleeve. Both sleeves are preferably made of the same materials.  
         [0059]    Further, as one skilled in the art will appreciate, the various layers of a particular heater need not all be applied in an identical manner but rather a combination of layer types may be employed. One will further appreciate that the removability benefit of the sleeve embodiment requires that only at least one of the layers be a self-supporting sleeve, to permit it to be slidably removed from the nozzle assembly. For example, if first layer  84 ′ is provided as a self-supporting sleeve, second layer  86  may be applied directly to first layer  84  (and over wire element  80 , as well) by spraying or other coating method, as described further below. Conversely, in a particular application, it may be desirable to spray or otherwise coat a first layer  84  onto the nozzle body, and provide second layer  86  in a sleeve format In such a configuration, wire element  80 ′ may be integrally provided on the interior of the second layer sleeve element, so as to be removable therewith. Other combinations of layer construction are equally possible, as described below.  
         [0060]    Referring to FIG. 10, in an alternate nozzle embodiment, heater assembly  30 ″ is disposed centrally within nozzle  22 ″. Heater  30 ″ has a core  70 ″, first layer  84 ″, wire element  80 ″, second layer  86 ″ and outer layer  88 ″. A removable nozzle tip  74 ″ is provided to permit heater assembly  30 ″ to be removed from nozzle  22 ″ for inspection, repair or replacement, as described above.  
         [0061]    The present invention may be employed in any known injection molding nozzle design. Referring to FIG. 11, a two-part nozzle configuration according to the present invention is shown. A forward nozzle  100  has a heater assembly  102  according to the present invention, as described above, and a rearward nozzle  104  has a heater  106  according to the prior art, such as, for example, as.is described in U.S. Pat. No. 5,051,086 to Gellert, incorporated herein by reference. Heater assembly  102  has a wire element  110 , a first insulating layer  112  and second insulating layer  114 , similar to that described above.  
         [0062]    It will be apparent to one skilled in the art at the present invention can be employed using a straight wire  120 , as shown in FIG. 12 a,  as element  80  to be wound spirally around the nozzle body, as described above. Equally, however element  80  may be a coiled wire  122 , as shown in FIG. 12 b,  spirally wound around the nozzle. “Coiled” in this application means helical or spring-like in nature, as illustrated in FIG. 12 b.  Coiled wire heating elements are well-known in the heating art as allowing for a reduction in heater power for a given operating temperature.  
         [0063]    Similarly, referring to FIG. 13 a,  it will be appreciated that the length of element  80  can be effectively doubled by folding over the wire element, and optionally twisted, to create a unitary element  124 . Element  124 , as expected, has twice the length of wire for a given element  80  length, and is twice as thick. Referring to FIG. 13 b,  a coiled and doubled element  126  can equally be provided.  
         [0064]    Referring again to FIG. 3, in use wire element  80  is energized by a power source (not shown). As current flows through wire element  80 , resistance to the electrical flow causes the wire to heat, as is well understood in the art. Heat generated by the element is preferably channelled and expelled substantially inwardly, by the presence first insulating layer  84  and second layer  86 , to heat the pressurized melt in central melt bore  76 . First layer  84  and second layer  86  also provide electrical insulation to electrically isolate wire element  80  from the surrounding metal components of the nozzle.  
         [0065]    The uninsulated resistive wire heating element according to the present invention permits a cheaper heater to be obtained while permitting more exacting temperature distribution and control through more precise and flexible positioning of the element. Unlike the prior art, complex machining of the nozzle body and the need for integrally brazing the heating. element to the nozzle body are removed, permitting savings in cost and time in fabricating the nozzle. Likewise, special and complex film printing techniques, materials and machinery are not required. Further, and perhaps most importantly, the present invention permits smaller diameter heated nozzle designs to be more easily achieved and more reliably operated than is possible with the prior art.  
         [0066]    The heated nozzles of the present invention may be fabricated according to the method of the present invention. In a first embodiment of this method, steel nozzle body  68  is provided as the substrate for spraying first layer  84  thereon. First layer  84  may be provided by spraying, “painting” or otherwise coating in a thickness of between 0.1 mm and 0.5 mm. While greater thicknesses are possible, little benefit is attained by providing a thickness greater than 0.5 mm and, since it is generally desirable to minimize nozzle diameter, greater thicknesses are not typically preferred. First layer  84  is provided on outer surface  72  of nozzle body  68  so as to substantially cover, and preferably completely cover, outer surface  72  over the region where wire element  80  is to be located. After layer  84  is dry, wire element  80  is then positioned around first layer  84 , preferably by winding wire element  80  spirally around the exterior of the nozzle. Although any wire pattern is possible, winding is typically preferred because, among other things, it requires the simplest operation in automated production. With wire element  80  around first layer  84 , second layer  86  is then provided so as to substantially cover, and preferably completely cover, wire element  80  and thereby sandwich and encase wire element  80  between first layer  84  and second layer  86 . Second layer  86  is preferably applied by spraying, “painting” or otherwise coating to a thickness of between 0.1 mm and 0.5 mm (for reasons described above), though any other method of applying second layer  86  may be employed, including providing a sleeve as described below. Once second layer  86  is dry, metal outer layer  88  is provided. Metal outer layer  88  may be applied in any known manner, such as by spraying or by shrink-fitting a sleeve, with spraying being preferred in this embodiment to minimize the overall diameter of the nozzle. With the outer layer applied, the assembly is then typically swaged to compact the assembly and bring the overall nozzle diameter to within desired dimensional tolerances.  
         [0067]    This embodiment of the method permits smaller diameter and more durable nozzles to be obtained than is possible with the prior art.  
         [0068]    Further, the method is advantageous over the prior art since no additional heat treating step is required, thereby simplifying manufacture.  
         [0069]    In an alternate embodiment of the method of the present invention, first layer  84  is provided as a pre-fabricated, self-supporting, substantially rigid, annular sleeve component which is telescopically, slidably positioned concentrically over core  70 . The sleeveless element may be cast, machined, molded or extruded into a thin-walled tube, and may be provided in any desired thickness, though thicknesses in the range of 1.5 mm to 2 mm are preferred to optimize thickness and durability of the sleeve component. The inside diameter of the first layer sleeve is preferably as small as possible while still permitting a sliding installation over core  70 , so as to minimize any air space between the two components. The next step is to position wire element  80  around the first layer sleeve and, as one skilled in the art will understand, it is not important whether the wire element is positioned around the first layer sleeve prior or subsequent to the sleeve&#39;s installation on the nozzle body. In fact, an advantage of the method of this embodiment is that the wire element can be pre-wired on the first layer sleeve prior to installation, which can offer flexibility and simplification in manufacturing. Once wire element  80  has been provided around the first layer sleeve, second layer  86  is then applied to substantially cover, and preferably completely cover, wire element  80  so as to sandwich and encase wire element  80  between the first layer sleeve and second layer  86 . Second layer  86  may be applied as a sleeve or by spraying, with the sleeve form being preferred in this embodiment. Again, it is not important whether second layer  86  is applied prior or subsequent to the installation of the first layer sleeve on the nozzle body. Second layer  86 , if applied in sleeve format, is sized to fit as closely as possible over wire element  80  on the first layer sleeve to minimize the air space between the first and second layers. A metal outer layer  88  is then applied to the outside of second layer  86  and may be applied by any known means, such as by spraying or by shrink-fitting a sleeve, with shrink-fitting a sleeve being preferred in this embodiment. Again, as will be understood by one skilled in the art, if a second layer sleeve is used, the outer layer may be applied to the second layer sleeve either pre- or post-installation of the second layer sleeve on the first layer sleeve or the nozzle assembly. With the outer layer applied, the assembly is then typically swaged to compact the assembly and bring the overall nozzle diameter to within desired dimensional tolerances. The assembly is then finished as required. Such finishing steps may include providing removable nozzle tip  74  to the nozzle assembly, if necessary in the particular application.  
         [0070]    This embodiment of the method permits a removable heater assembly to be achieved. The first layer sleeve and/or second layer sleeve can be selectively removed from the nozzle body for inspection and/or replacement, if the heater is damaged or worn, without the need to replace the entire nozzle. Further, the independent nature of the sleeve elements permits the order of assembly to be varied as necessary, for example, by allowing the wire element to be provided on the first layer sleeve prior to installation on the nozzle body. Similarly, the second layer may be provided on first sleeve, over the installed wire, prior to installation of the first layer sleeve on the nozzle body. This advantage offers not only flexibility in manufacture but also permits the wire element to be more precisely placed on the first layer sleeve. For example, laying the wire over the sleeve and then spinning the sleeve so as to wind the wire onto the sleeve permits a precisely controlled pitch and pitch variation. A further advantage of the method is that no additional heat treating step is required, thereby simplifying manufacture.  
         [0071]    It will be understood in the previous embodiment that, if desired, wire element  80  can equally be pre-installed in the interior of a second layer sleeve, rather than the outside of first layer sleeve.  
         [0072]    In both of the above embodiments of the method of the present invention, a metal surface layer  98  of copper or other highly thermally conductive metal may be applied with advantage to the nozzle body prior to providing the first insulating layer, as described above with respect to the apparatus. In one aspect, the surface layer is applied by spraying. In another aspect, the surface layer is provided by shrink-fitting a sleeve onto core  70  of nozzle body  68 . As described above, the surface layer promotes thermal transfer between heater  30  and nozzle body  68 .  
         [0073]    While the above description constitutes he preferred embodiment, it will be appreciated that the present invention is susceptible to modification and change without parting from the fair meaning of the proper scope of the accompanying claims.