MXPA00003294A - Molded polymer composite heater - Google Patents

Abstract

A molded polymer composite heater is shown. The use of transfer molding and compression molding allows for the use of thermoset polymers containing very high levels of reinforcement fillers. These improved materials in turn create a heater with thermophysical properties superior to the prior art, including higher heat flux levels, thermal conductivity, impact resistance, and maintenance of mechanical properties at high temperatures (~>300°F). The present invention also allows for wide variety of geometric configurations and the possibility to insert temperature sensors directly in hot zones of the heater.

Description

HEATED COMPOSITE OF MOLDED POLYMER Field of the Invention The present invention relates to electric resistance heaters and more particularly to an electric resistance heater molded of one or more polymeric compounds.

Background of the Invention Electric resistance heaters are common in the industry, and generally comprise a resistance wire, through which an electric current is passed, a ceramic core, around which the same wire is placed, a dielectric ceramic layer , which surrounds the core carrying the current, and a metal alloy lining to complete the assembly. A form of electric resistance heater, known as a cartridge heater, which is used in a very wide range of applications, has a cylindrical liner, which has historically been made of corrosion-resistant metal alloys such as stainless steel or from Ref.119029 incoloy. To improve the thermal performance of the heating element, the above assembly is typically stamped. More recently, the industry has been looking for alternative cartridge heaters that are lighter in weight, less expensive to produce, that can be designed with greater geometric flexibility, and that can be produced in series cost-effectively while still provide lower thermal or mechanical functioning. A solution was proposed in U.S. Pat. No. 5,586,214 to Eckman and jointly assigned to Energy Converters, Inc. of Dallas, Pennsylvania and Rheem Mfg. Co. of New York, New York. Eckman describes a submersion heater, somewhat similar to a cartridge heater in its shape, but that Ae is hollow and has openings in the liner. Instead of being a solid cylinder, the core represents an injection molded polymeric hollow tube over which a liner is injection molded. Therefore, the heater does not have a "core" in the traditional sense. The Eckman heater is shown in Figure 1. The Eckman heater has certain advantages over the prior art, such as a low weight, a low manufacturing cost at high volume, and a resistance mii mtot and elevated to galvanic corrosion and to the deposition of minerals. Still the Eckman heater has many limitations which are undesirable for many different applications of the low temperature and low heat water heating tanks. This is supported by the limitation of the thermoplastic matrices to accept the filter medium. In this context, Eckman describes that the level of the filler in these polymeric matrices can not exceed 40% by weight, which correlates with the research results obtained during the development of the present invention. Providing a solid core (or at least one of substantially larger wall thickness) in the Eckman heater is not easy because it changes the geometry of the polymer, around which the resistance wire is wound. If a core polymer with the same temperature-dependent thermal expansion functions as the external polymer that is used, the heater will be prone to breakage and failure when the energy is applied and brought to the operating temperature. Eckman tes that the external polymeric coating needs to be less than 1.3 cm (0.5 inches) and ideally less than 0.3 cm (0.1 inches), which also sacrifices structural strength. Eckman achieves a somewhat higher conductivity, possibly higher heat fluxes than would be found in a pure polymer, suggesting the use of carbon, graphite, and metallic powder or flakes as an additive. The amount of these additives should however be limited to protect the dielectric strength of the heater. Even then, the thermal conductivity does not become significantly better than 1.0 W / (m * K). Accordingly, it is an object of the present invention to provide a molded polymer composite heater with a level of filler compound substantially greater than 40%. It is also an object of the present invention to provide a molded polymer composite heater with improved structural integrity. It is a further object of the present invention to provide a molded polymer composite heater with a larger core thickness to the end where the hollow space at the center of the element fades. It is still another object of the present invention to provide a molded polymer composite heater with improved thermal performance, especially the thermal conductivity and the maximum heat flow.

Other objects of the invention will become apparent from the specification described hereinafter.

Brief Description of the Invention According to the objects listed above, the present invention is a heater composed of molded polymer having highly filled polymers, such that the polymers are more suitable for either transfer molding or compression molding. Compared with the prior art, which refers specifically to injection molding, the present invention allows much higher filler levels. The higher filler levels, which exceed 50% by weight and can reach values as high as 90% by weight, provide polymeric compounds with better mechanical properties such as strength and impact resistance, superior thermal properties, such as higher service temperatures, specific heat, and thermal conductivity, as well as improved electrical properties, such as dielectric strength and insulation resistance. The polymer core of the heater has lead terminals inserted into it that make contact with an electrical resistance wire placed around them. The present invention also preferably utilizes a larger core and liner thickness up to and including a solid core, which allows for a greater number of geometric variations and the possibility of including additional features in the heater. For example, the sensors may be included at a particular point in the heater, where the temperature measurement is more critical, or microcircuits may be interspersed within the heater that provide the control means integrated with the heater. The thermosetting polymers are preferably used, although some selected thermoplastics may also be used. The polymers are filled with reinforcing additives, which increases the viscosity of the raw materials and the processable molding compound. For best results, the level of reinforcement must exceed 50%. The structural integrity of thermoplastic materials decreases rapidly once the reinforcement levels exceed 40%, this is due to the preference for thermosetting polymers which can exceed the 50% reinforcement level. Different fillers can be used depending on the particular need of an application. 5 Some applications will not need as much thermal conductivity, but will require high mechanical strength and impact resistance. Others may require high chemical resistance, low moisture absorption, etc. The reinforcement filler can be made from a large number of materials, however many applications will require good thermal conductivity of the polymer backing. For such applications, it has been found that the fillers of materials ceramic particulates or polymeric hairs, such as magnesium oxide or boron nitride, work well, in addition to many forms of carbon. Caution must be exercised in the use of the carbon reinforcement, because it decreases the dielectric strength of the lining and of the nucleus. The present invention incorporates techniques that allow high fill levels (at least 60%) of carbon fibers without significant loss of dielectric strength, but provide good thermal conductivity and excellent mechanical strength. > According to one aspect of the present invention, the solid core is made of a polymeric compound, as described above, formed into two halves that are interfused. The halves can be made from the same mold, and they have a self-coupling feature, thus reducing the manufacturing cost. The complete core will have holes for two or more bolts. For power connector pins, the core will have sections that expose the holes, so that a resistance wire can be welded to the bolts. Preferably, an exposed point of the power connector pins will be toward one end of the remote heater as far as the power connector pins exit from heater. Another exposed point should be near the end where the pins' connectors exit the heater. This allows a single coiled resistance wire, which is desirable over the resistance wires with loops (double coils) that are more prone to short circuits with high potential. Over the core, a polymeric liner is added. The liner is made primarily of the same polymeric compound as the core, although the exact composition may vary, particularly when desired different coefficients of thermal expansion, for f "- high temperature applications (-> 149 ° C (300 ° F)) Most of the lining is added by transfer or compression molding, however, for applications requiring high dielectric strength, a thin layer Additional polymer can be added by submerging, spraying, or screen printing, either to the assembled core or to the heater with a liner.

Brief Description of the Drawings So that the manner in which the characteristics, advantages and objects identified above of the present invention are achieved and can be understood in detail, a more particular description of the invention, summarized briefly above, may have to be referred to the mode of the same which is illustrated in the attached drawings. However, it was pointed out that the appended drawings illustrate only a typical embodiment of this invention and therefore will not be considered limiting of its scope, so that the invention can accept other equally effective modalities. Reference is made to the accompanying drawings, where: Figure 1 is an isometric view of a polymeric heater of the prior art as described in U.S. Pat. No. 5,586,214 to Eckman. Figure 2 is a bottom view of a half-cylinder core composed of molded polymer for use in the present heater. Figure 3 is a front view of the core half cylinder in Figure 2. Figure 4 is a right side view of the core half cylinder of Figure 2. Figure 5 is a left side view of the half cylinder of Figure 2. Figure 6 is an isometric view of a cylindrical core composed of molded polymer with a resistance wire placed around it and the power connecting pins inserted therein. Figure 7 is an isometric view of one embodiment of the cartridge heater of the present heater composed of molded polymer. Figure 8 is an isometric view of a flat, folded core, composed of molded polymer, with a resistance wire placed around it and connecting pins positioned therein.

Figure 9 is an isometric view of one embodiment of the submersion heater of the flat element of the present heater composed of the molded polymer.

Detailed Description of the Drawings The present invention is an electric heater made of a polymer compound, which is preferably either transfer molded or compression molded. Previous attempts in the production of polymer heaters have always used injection molding, which limits the possible filling levels in the polymer, which in turn severely impairs the commercial uses of polymer heaters in almost the simplest of applications. The present invention can be used in many different applications, due in part to increases in heat flow and mechanical strength. The use of higher fill levels also allows a wider range in the physical properties of polymer composites, which in turn allows greater flexibility in the geometrical configuration of the heater. In addition to doing or make heavier, more durable polymer heaters that work thermally at a higher level, this allows the addition of extra features within the heater itself. Referring now to Figure 1, a polymer heater 1 of the prior art is shown as taught by U.S. Pat. No. 5,586,214 to Eckman. The Eckman heater has a plurality of holes 2 in the heater liner, and a hollow hole 3 in place of a core. In contrast, for this, the preferred embodiment of the present invention is shown as a cylindrical polymeric heater 10 in Figure 7. The preferred embodiment includes a liner 12 incorporating the molded screw 14 and a hexagonal projection 16 (both used for the montage) . Exiting the end 18 of the heater 10 close to the mounting features 14, 16 are a plurality of energy connecting bolts 20. The liner 12 and the mounting characteristics 14, 16 are made of a polymer and formed either by transfer molding or compression molding. Hidden beneath the liner 12 is a complete core 22, shown in Figure 6. The complete core comprises the energy connecting pins 20, a resistance wire 24 welded to the energy connecting pins 20 at the welding points 26, and optionally formed of two core sections 28 (see also Figures 2 and 3). The preferred core sections 28 are identical and substantially cylindrical and semicircular in cross section except for an end portion 30 on either side. Figures 2-5 show a preferred core section 28. Each preferred core section 28 has a long longitudinal notch 36 extending along the entire length thereof and two short longitudinal notches 38 running parallel to the long notch 36 extending an equal distance from any end portion 30. , a short notch 38 extending from each end portion 30. The notches are located on the flat face 44 of the core section 28 (which is semicircular in cross-section). Accordingly, when the two identical core sections 28 are placed together, making contact on their flat faces 44, the notches 36, 38 from a section 28 of the core correspond to the notches 36, 38 from the other core section forming a plurality of holes parallel to the cylinder axis.

The core 22 may incorporate a self-attaching feature, wherein an end portion 30 of the core section 28 has one or more hooks 32 integrally molded thereon, and the other end portion 30 has an equal number of grooves 34 therein. The slots 34 are adapted to receive the hooks 32 located on the other section of the core 28. This allows the core sections 28 to be produced in series in a cost-effective manner with a simple mold. It is also possible to form the core by directly inserting the bolts into a one-piece core. This literally involves molding the core around the bolts and could allow a less complicated and delicate winding or winding operation, more suitable for automation. When the core sections 28 are coupled together by their respective hooks 32 and the slots 34, the bolts 20 are inserted into the holes formed by the notches 36 and 38. A resistance wire 24, made of any material known in the art, it is then wound around the coupled core sections 28 starting in a welding groove 42 next to the wires of the extension bolt, (which provide access to the bolt 20 in the notch 38) and ending in the other welding groove 40. remote with respect to the wires of the extension pin (which provides access to the bolt 20 in the groove 36). The resistance wire 24 thus covers a substantial portion of the core 22. It is preferable to wind the resistance wire 24 around the core 22 only as a single strand. Due to geometrical limitations of injection molded polymer heaters, the prior art resistance wire has to be wound around the core like a double thread or thread, turning it around a hook near the end of the heater away from the power connector pins. This configuration of the prior art increases the likelihood of high voltage short circuits, which can potentially lead to shorter life extensions of the heater or even to immediate failure and rejection of the product. The present thread or single strand does not suffer from the same limitations. The present invention also allows the resistance wire to be completely replaced by a resistive ink, which could be printed on the outside of the core. A typical ink for this use is a series of polymer resistors sold by Electro-Science Laboratories, Inc. of King of Prussia, Pennsylvania. Transfer molding and compression molding are already known in the plastics art, and the techniques are described in Molded Thermosets, by Ralph E. Wright, which is incorporated herein by reference. In injection molding, which was used in the prior art, a screw and compaction barrel assembly receives the granular raw material from a cooper and melts it by a heating band aided by the shearing action of the screw and barrel. The reciprocating rotary and reciprocating movement of the screw pushes the dose or load through a nozzle and into the mold itself. In transfer molding, on the other hand, a non-compactor screw preplasticizes the thermosetting compound of raw material by the use of heating bands. Here, the action of the screw only serves the purpose of transporting the material from the cooper to the outlet of the non-reduced barrel where the dose or load is cut and automatically transferred to a cylindrical cavity. A piston continues to apply a large force (~ 40 tons) to the dose or pasty charge, causing a tremendous increase in pressure and temperature. In turn, the viscosity decreases dramatically and the threshold of the reaction temperature is exceeded while the material is pushed through the nozzle into the mold cavity. Another advantage of transfer (and compression) molding is a more effective per-crosslinking, which encompasses the thermal bridging of the high thermal conductivity particulates by the fibers. Yet another advantage of transfer (and compression) molding is that the interleaved fibers added to the raw material polymer maintain their best lengths during these molding processes when compared to injection molding. This is largely due to the fact that injection molding is a more traumatic process than others, causing the fibers to break, imposing an intense shearing action on them. Additionally, the longer the fibers are in the matrices, the more effective the crosslinking in them is. The molding of the liquid composite ("resin transfer molding"), which is a variation of transfer molding, can also be used in the present invention. In this latter "fiber-friendly" process, the mold cavity is pre-filled with the filler material and the pure polymer matrix is transferred into the cavity thereafter.

A. * - The polymers that can be formed are generally classified as either thermoplastic or thermosetting (also known as chemically curable polymers). The thermoplastic materials can be melted and, during the reduction of the temperature, brought back to the solid state. In the solidification process, the polymer chains are contracted by bending one inside the other creating physical unions such as might occur when freshly cooked hot spaghetti is served and if left to rest until it dries. Theoretically, it is possible to impose infinitely many cycles of fusion / solidification on the material. In general, thermoplastic materials are highly resistant to impacts due to the loose arrangement of the polymer chains, still, allowing a higher degree of moisture absorption for the same reason. Returning to the idea of spaghetti, the reader should have no difficulty in contemplating the dramatic reduction of the mechanical properties of thermoplastic materials at elevated temperatures. On the other hand, thermosetting materials can only solidify once while subsequent melting is not possible. This curiosity can be explained by the creation of '*' chemical cross-links between the polymer chains in the solidification process of the chemical reaction. Not surprisingly, the raw material for the production of the thermosetting material consists of properly sized chemical reaction ingredients whose reaction temperature threshold is intentionally exceeded in the molding process. These crosslinks restrict the movement of the polymer chains to each other, which results in a more brittle character compared to other thermoplastic characteristics. In addition, at higher temperatures the same chemical cross-links maintain the mechanical properties. Another advantage of thermosetting materials is that they typically rewet better than thermoplastic materials. That is, before the thermosetting materials are fully cured, more thermosetting polymer can be molded thereon, and the bond between the two layers will be strong and less permeable because the chemical crosslinks will form across the boundary of the layer. As described by Wright, most thermosetting plastics are not suitable for injection molding due to high viscosity. Injection molding also limits the amount of reinforcement that may be contained within the polymeric compound to a value of no greater than about 40% by weight. Fill levels well beyond 40% by weight produce plastics that are too viscous for injection molding when thermosetting materials are used (thermoplastic materials begin to lose their structural integrity at fill levels well beyond 40% by weight ). In addition, the opposite is also true because with many plastics, the filler levels much lower than 40% by weight produced a composite material that is not viscous enough for transfer to the mold. The inventors of the present invention have discovered that it is not until the fill levels within the thermoset polymeric compounds exceed 50% by weight that the thermoplastic properties are drastically improved. It has also been found that thermosetting materials generally provide better thermophysical properties for heaters than thermoplastic materials, particularly once the filler levels exceed 50% by weight due to significantly improved impact resistance and maintenance properties. mechanical at higher temperatures. Thermosetting plastics with high filling levels, as a general rule, are not very suitable for molding by i ^ yeccJ ^ Sfe, therefore the present invention transfer or compression molding. Thermosetting materials can also accept higher total fill levels than thermoplastic materials. As already mentioned, thermoplastic polymers lose structural integrity if they are filled beyond 40% by weight. Thermosetting materials, on the other hand, can accept filling levels as high as 90% by weight. The present invention also produces a better heater by the use of high performance reinforcements. The specific reinforcement fillers provide better thermal conductivity than the fillers used in the polymeric heaters of the prior art. Eckman teaches the use of some thermally conductive materials, such as graphite or metallic dust, but warns specifically against the excessive use of such fillers, because of the loss in the dielectric strength of the heater. This limitation can be overcome by the use of an intermediate dielectric layer (not shown) between the resistance wire 24 and the outer shell 12. The dielectric layer is made of a polymer similar to the rest of the heater, however it lacks a filler reinforcement. The dielectric inks of Electro-Science Laboratories, Inc., are very suitable for this purpose. Any interest on the dielectric strength of the outer shell 12 is debatable. To maximize the efficiency and thermal conductivity of the heater, the intermediate dielectric layer should be ultra thin, approximately 100 microns thick, however the thicknesses up to 1 millimeter can also be be suitable for this invention. This can be applied to the core by the implementation of a submerging, spraying, or stenciling operation prior to overmolding the outer skin 12. Another method of increasing the conductivity thermal is by the use of carbon fibers as a reinforcing filler. Carbon fibers significantly improve the thermophysical properties of the heater, but they conduct thermal energy much better in its longitudinal direction, than in its transversal direction. However, because the fibers behave like logs during molding, the alignment by itself in the direction of mold flow, its natural tendency is to end up parallel to the surface of the heater (perpendicular to the flow of heat). The orientation M < * ¡UmM & < *, the desired rf can be obtained by applying an electric field to the mold flow during manufacturing. The energy connector pins 20 can act as an electrode, and the mold itself can act as the other. Other desirable fillers that have been found are magnesium oxide (MgO), aluminum nitride (AIN), and boron nitride (BN). The inventors have found by means of the rapid method of laser beam (ASTM E1461), in the special application of which all measured quantities are likely to be found directly in the standards of the National Bureau of Standards ("NBS"), that such fillers provide thermal conductivity in excess of 2.0 W / (m * K), and approach 5.0 W / (m * K). On the other hand, it is highly probable that the polymeric heaters of the prior art, such as those described in the Eckman patent, could be significantly exceeded 1.0 W / (m * K) using the same standard. The desirable polymeric bases for the composite material consist of allyls, amines, epoxies, phenolics, silicones, and thermosetting polyesters. The reinforcement fillers desired for the particular heater are selected and added to the base polymer before transfer molding (or compression).

He? AMMfa To use a solid core 22 for high temperature applications, it may be necessary to compensate the coefficient of thermal expansion ("CTE") for the material of the CTE liner of the core material. This is due to the fact that the core material will naturally be hotter than the lining material. The CTE for the lining material must be matched (falling within a specific range) with the temperature of a particular application and the CTE of the core material. The CTE of the materials can be adjusted by controlling the levels of the filler. For example, the higher filler levels in the core material can counteract the imbalance in the expansion. Another example of changing the CTE of the core to overcome this imbalance or decompensation is the use of reinforcement fillers in the core which has lower CTEs than the reinforcement fillers used in the lining material. The improved thermophysical properties of the materials used in the present invention, combined with the ability to use solid cores, allow the heaters to withstand temperatures and heat flow levels significantly higher than those allowed by the prior art. The prior art, which uses thermoplastic polymers, may not be heated well beyond 82 ° C (180 ° F). The prototypes of the heaters of the present invention have been measured at 204 ° C (400 ° F) (with a core temperature * 'of 243 ° C (470 ° F)), and it is conceivable that temperatures as high as 399 ° C (750 ° F) may be possible with the selection of the correct fillers and filler levels. Prototypes of the present invention have handled heat flux levels of 39 W / cm2 (6 W / in.2) in the air of natural convection, and 194 W / cm2 (30 W / inch2) in forced convection fluids. A thermosetting composite material that has been found to be suitable for the present invention is sold as AB1000F by Cuyahoga Plastics of Cleveland. Ohio. After molding, the resulting heater can withstand continuous operation up to 538 ° C (1000 ° F) without losing physical integrity even when the organic substance burns completely at 399 ° C (750 ° F). Another benefit of the present invention is the ability to be used in a wide variety of geometric configurations. Heaters of different shapes work better for different applications. For example, flat surface heaters provide better convective heat transfer when oriented vertically, than cylindrical heaters. The preferable geometry will depend on the particular characteristics of an application. However, the present invention allows this flexibility. For example, Figures 8 and 9 show a flat surface embodiment 100 of the present invention. The heater with flat surfaces 100 has the same mounting characteristics 114, 116 as the cylindrical heater 10. The liner 112 is of the same material. The core 122, however, is compression molded or transfer in a flat surface shape with two bends 146 positioned at about 90 °, leading to a fork turn. The same type of resistance wire 124 is used, which is coupled to the energy connecting pins 120 at the welding points 126. The energy connecting pins 120 then exit from the finished heater 100 through the end 118. The other advantage of the present invention is the ability to mold temperature sensors such as thermocouples directly into core 22 in any desired position. The previous technique shows a thermistor located at the same end of the heater (near the mounting position). This is located in a "cold zone". Therefore, the temperature readings obtained are not indicative of the actual temperature of the heater and are further compromised by the generally low thermal conductivity of the polymer matrix. By placing thermocouples in the core in the "hot zones", a true exact temperature reading can be obtained, which is preferable. Although the foregoing is directed to the preferred embodiments of the present invention, other embodiments and some additional embodiments of the invention may be contemplated without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates.

Having described the invention as above, property is claimed as contained in the following

Claims (10)

1. A molded polymer composite heater, characterized in that it comprises: a core of polymeric composite; a heating element that conducts electricity, placed around the core, the heating element has two ends adapted to receive an electric current; and a liner surrounding the heating element, the liner comprises a polymeric composite material containing more than 50% by weight of a thermally conductive filler, such that the liner can withstand a continuous heat flux of at least 20 watts per square centimeter.

2. The molded polymer composite heater according to claim 1, characterized in that the material of the polymer composite of the liner contains an amount greater than 60% by weight of a thermally conductive filler.

3. The molded polymer composite heater according to claim 1, characterized in that the thermally conductive filler is a ceramic material selected from the group comprising magnesium oxide, aluminum nitride, aluminum oxide, and boron nitride.

4. The molded polymer composite heater according to claim 1, characterized in that the thermally conductive filler comprises carbon fibers.

5. The molded polymer composite heater according to claim 4, characterized in that the carbon fibers are oriented predominantly parallel to the axis of the core.

6. The molded polymer composite heater according to claim 4, characterized in that the carbon fibers are oriented predominantly perpendicular to the axis of the core.

7. The molded polymer composite heater according to claim 6, characterized in that it further comprises a dielectric layer placed between the heating element and the liner, the dielectric layer is less than 1 millimeter in thickness.

8. The molded polymer composite heater according to claim 7, characterized in that the dielectric layer is less than 100 microns thick.

9. The molded polymer composite heater according to claim 1, characterized in that the core is solid.

10. The molded polymer composite heater according to claim 1, characterized in that the polymer composite materials of the core and the liner are transfer moldable. . .. ...- s ____-_______ ^ _ »__ a | i