Thermoplastic-composition resistor and method of manufacture

Abstract: A thermoplastic-carbon resistive rod is made by extr∼ding a heated mixture of carbon black, acetylene black, silica and a polymer resin. The resistive rod is cooled and cut into sections forming resistive bodies. Resistors are formed by fusing terminals to ends of the rods, welding leads to the terminals, and encapsulating the bodies and terminals in an epoxy material.

Description: This invention relates to a resistor capable of dissipating substantial power and to a method for making such a resistor. In particular, the invention relates to a leaded resistor having a thermoplastic-composition body and to the manufacture of such a resistor. The most commonly used resistor is the carbon-composition resistor. It is rugged, is readily available in a wide range of resistance values, will dissipate substantial power, and can be manufactured to a resistance tolerance of about 5%. Despite its popularity, however, the carbon-composition resistor has a number of disadvantages: a. It is difficult to repeatedly make carbon-composition resistors with identical resistance values, because of the many variables involved in their manufacture. The manufacturing process requires applying a precise pressure to compress a predetermined mixture of carbon and silicon particles into a phenolic case of predefined internal dimensions which is heated to a specified temperature. The casing is then sealed, and heated leads are pushed through opposite ends of the casing until they extend into the carbon-silicon mixture to a predetermined depth. Identical resistance values will be obtained only if the pressure, mixture, dimensions, temperature and depth remain constant, regardless of when the resistors are made or which assembly line they are made on. b. The resistance value of a carbon-composition resistor will decrease substantially with use unless it is cured by baking until a point of optimum stability is reached. The requisite baking time and temperature varies with the amount of moisture in the carbon-silicon mixture and is difficult to determine with accuracy. Even after curing, if the resistor is subjected to a high humidity environment, moisture will be reabsorbed through the phenolic casing, which is permeable to water molecules. c. The baking process impairs the solderability of the resistor leads, which were coated with a tin-lead solder before being pushed into the ends of the casing. For as long as baking continues, tin bleeds out of the solder. Despite all of the drawbacks of carbon-composition resistors, no other type of low-cost resistor is available, which has all of its advantages. JP-A-54 152 197 describes a semiconductive resistor body which comprises ethylene vinyl acetate as a thermoplastic resin, mixed with acetylene black and another carbon black. It is an object of the invention to provide a low cost resistor which has all of the advantages of a carbon-composition resistor, and none of the disadvantages. In particular, a resistor is to be provided which has an accurately-predictable resistance, regardless of when or on which assembly line it is made, which has a resistance that does not substantially vary with temperature or humidity at the time of manufacture or in use, and which has leads with a solderability that is not impaired during the manufacturing process. In accordance with the invention, there are provided a resistor body as defined in claim 1, a resistor as defined in claim 6, and methods as defined in claims 3 and 7. The resistor body comprises a rod of predetermined cross-sectional area which is formed from a coextrudated mixture comprising a thermoplastic resin, acetylene black and another carbon black. The ratio of thermoplastic resin to carbon material determines the resistivity of the body. The ratio of acetylene black to carbon black determines the thermal coefficient of resistance of the resistor body. By a proper selection of the amounts of these two blacks, the positive coefficient of resistance of the one black is compensated by the negative coefficient of the other black. Thus resistor bodies can be made having a coefficient of resistance wich does not substantially vary with temperature. The resistor body according to the present invention comprises 3-12 % by weight of acetylene black and 1-5 % by weight of another carbon black, calculated on the total weight of the resistor body. In a preferred embodiment, the weight ratio of acetylene black and another carbon black in the resistor of the present invention lies between 2,5:1 and 3,5:1. According to these preferred embodiments, the coefficient of resistance does not or not noteworthy vary with temperature. A method of manufacturing the resistor body in accordance with the invention involves mixing particulate forms of the above materials in proportions which effect production of a predetermined resistivity. This mixture is then heated until it becomes plastically deformable to ensure distribution of the carbon materials throughout the resin and to facilitate shaping of the mixture. The hot mixture is extruded through an opening in a die to form a hot resistive rod, which is then cooled to hardening and cut into sections to form the resistor bodies. As used herein, the word "rod" is not limited to solid bodies, but also includes other extrudable shapes, such as tubular bodies. A resistor in accordance with the invention comprises a thermoplastic-carbon body and is terminated with electrical leads to which a solder material having a predetermined melting temperature may be applied. The body has a melting temperature which is greater than that of the solder material, and has first and second faces to which are fused corresponding faces of first and second terminals of electrically conductive, weldable material. First and second ones of the electrical leads are welded to respective ones of the terminals, and an electrically insulating material encapsulates the body and the first and second terminals. Preferably the resistor body has the shape and composition described above, but the just-described termination structure may be used advantageously with resistor bodies having other shapes and thermoplastic-carbon compositions. A method of terminating a resistor body in accordance with the invention includes the steps of: a. fusing first and second terminals of electrically conductive, weldable material to respective first and second faces of the body; b. heating the body until the resistivity of the body reaches a predetermined temperature stability; c. welding first and second ones of the electrical leads to respective ones of the terminals; and d. encapsulating the body and the first and second terminals in an electrically insulating material. The invention is explained in detail by means of prefined embodiments and the drawing, in which: Figure 1 is a schematic illustration of an apparatus for making resistor bodies in accordance with a first preferred embodiment of the invention. Figures 2 and 3 are schematic illustrations of collective apparatus for making resistor bodies in accordance with a second preferred embodiment of the invention. Figures 4a through 4f are schematic illustrations of steps for producing a resistor from a thermoplastic-carbon resistive body. Figure 5 is a schematic drawing of a resistor made in accordance with steps of Figures 4a - 4e. Figure 1 shows a first preferred embodiment of an apparatus for making resistive rods in accordance with the invention. The apparatus includes an extrusion machine 10 for forming a hot resistive rod, a cooling bath 12 for cooling the resistive rod, a tension controller 13 and a puller 14 for cooperatively regulating the diameter of the rod, and a resistance-measuring device 16 for measuring the resistivity of the rod. The dashed lines in this drawing figure represent electrical signals flowing in the directions indicated by arrowheads at the ends of the lines. In this exemplary embodiment, the extrusion machine 10 includes an extruder having eight distinct sections (designated 1 through 8) and a drive motor 20, a melt pump 9, a forming die 18, a feed inlet hopper 22, gravimetric feeders 24 and 26, and a control panel 28. The two gravimetric feeders have individually controllable feed rates and supply to the inlet hopper respective components of the particulate materials used to make the resistive rod 30. The extruder comprises a barrel containing a pair of counterrotatable, intermeshing screws, which extend through sections 1 - 8. Section 1 is a feed section, which is water cooled to facilitate the feeding of particulate materials from the hopper 22 into the extruder. Sections 2 - 8 are conveying and mixing sections, which are electrically heated to effect melting of the mixture as it passes through the extruder. A latter one of these sections (e.g. section 7) is vented to allow moisture and gases trapped in the melt to escape, thereby devolatising the melt. The melt pump 9, which is also heated, provides a uniform pressure to force the melt through the forming die 18. The temperatures of sections 2 - 8 and of the melt pump 9 are remotely controlled at the control panel 28. (To avoid complicating the drawing, the temperature control signal lines coupling the control panel to the extruder and the melt pump are not shown). The forming die 18, the cooling bath 12, the tension controller 13, and the puller 14 cooperate to regulate the diameter of the extruded resistive rod 30 produced by the apparatus of Figure 1. The forming die 18 determines the cross sectional shape and dimensions of the extrudate forced through it by the melt pump. For a circular cross section, the extrudate leaving the die typically has a diameter which is 10 to 20% larger than the final diameter of the rod. This final diameter is achieved by the puller 14, which employs a succession of opposed roller pairs for pulling the extrudate through the cooling bath 12 at a controlled rate. The cooling bath gradually lowers the temperature of the extrudate, causing it to change from a soft deformable state to a hardened state. The tension controller 13 includes a laser micrometer having a beam transmitter and a beam receiver (not individually shown) which are disposed on opposite sides of the hardened rod leaving the cooling bath. The laser micrometer optically measures the diameter of the rod and applies to the control panel 28 a signal representative of the measurement. The control panel includes a comparator which compares the measurement with a preset nominal diameter and applies a variable output signal to the puller 14 to control the speed at which the puller rollers draw the extrudate out of the forming die 18. The output signal is delayed by a period equivalent to the time required for the diameter of the rod leaving the die 18 to travel to the laser micrometer part of the tension controller 13. If the measured rod diameter is larger than the nominal diameter, the rotary speed of the rollers is increased to reduce the diameter of the rod being hardened in the cooling bath. Conversely, if the measured rod diameter is smaller than the nominal diameter, the roller speed is decreased to enlarge the diameter of the rod being hardened in the cooling bath. The resistance measuring device 16, the control panel 28, the gravimetric feeders 24,26 and the extruder drive motor 20 cooperate to regulate the resistivity of the rod 30 produced by the apparatus. Principally, this resistivity is determined by the composition of the particulate matter received in section 1 of the extruder from the gravimetric feeders. Other factors which affect the resistivity are the melt pump pressure and any other factor which affects the properties of the extrudate. Feeder 24 contains particles of a thermoplastic resin, while feeder 26 contains a particulate mixture of acetylene black and another carbon black. Alternative arrangements may be used for supplying these three materials to the extruder. For example, a separate feeder may be provided for each material. The relative rates at which the feeders convey their respective materials to the feed inlet hopper 22 are determined by the control panel 28 in conjunction with the resistance measuring device 16. These feed rates are manually adjusted at the control panel until a desired resistivity is measured by the resistance measuring device 16. This device electrically measures the resistivity of the rod 30 by determining the resistance between spaced-apart electrical contacts 32 which press against the rod at a fixed separation distance. The device 16 applies an input signal to the control panel 28 indicating changes in the resistivity of the rod being produced. If the resistivity increases above the desired magnitude, the feed rate of the gravimetric feeder supplying the carbon materials is increased at the control panel. Conversely, if the resistivity decreases below the desired magnitude, the feed rate of the same feeder is decreased. Such increases and decreases delayed by a period equivalent to the time required for the carbon materials leaving feeder 26 to reach the resistance measuring device 16. The speeds of the extruder drive motor 20 and the melt pump 9 are manually controlled at the control panel between an upper and a lower limit. Above the upper limit the extruder screws will generate sufficient heat to effect decomposition of the thermoplastic resin. Below the lower limit the screws will not effect uniform mixing of the carbon and thermoplastic components. The speed of the melt pump is adjusted to a speed compatible with the rate at which the particulate matter is received in the hopper 22. A number of extruded resistive rods 30 have been made by the apparatus of Figure 1. In each case, gravimetric feeder 24 contained a mixture of silica and a polymer resin. Examples of the polymer material are liquid crystal polymers, polyetherketones, polyetheretherketones, polyethersulfones and polyphenylene sulfides. However, liquid crystal polymers are preferred, as they showed the best heat tolerance during the soldering of the resistors into circuitry. Gravimetric feeder 26 contained a mixture of acetylene black and another carbon black, and may further contain a quantity of one or more inorganic filler materials. Examples of preferable filler materials are silica, alumina and talc. The temperatures of the heated extruder sections 2 - 8 and of the melt pump 9 were above the melting temperature of the polymer and were below the temperature where the polymer will degrade. These temperatures ranged from about 290 to 330 degrees C. for a typical liquid crystal polymer. In a specific example, feeders 24 and 26 contained mixtures of the above-described materials and were operated at feed rates which resulted in a mixture in feeder 22 of 26.67% by weight of silica, 66.67% by weight of liquid crystal polymer composed of an aromatic copolyester, (trade name Vectra; Hoechst Celanese) 1.66% by weight of carbon black, and 5.00% by weight of acetylene black. A .270 inch long, .10 inch diameter (1 inch = 25.4 mm) circular resistive rod produced by the apparatus of Figure 1 had a measured resistance (from one end to the other) of 46,200 ohms. Figures 2 and 3 show a second preferred embodiment of apparatus for making resistive rods in accordance with the invention. The apparatus of Figure 2 is utilized to form batches of pellets of different resistivities. The apparatus of Figure 3 is utilized to form, from the pellets, rods of different resistivities. The apparatus of Figure 2 includes an extrusion machine 34 for producing an extrudate of soft resistive material and a pelletizer 36 for forming the extrudate into hard pellets. In this exemplary embodiment, the extrusion machine is substantially identical to that of Figure 1, except that the melt pump 9, the forming die 18 and the control panel 28 have been eliminated. The melt pump is not needed because the screws of the extruder themselves provide sufficient pressure to convey the soft extrudate to the pelletizer 36 through a flanged connecting pipe 38. The pelletizer forms small resistive pellets 40 by cutting the soft extrudate into small cylindrical shapes, cooling them with a flow of water, drying them and conveying them to an outlet opening where they can be collected. The resistivity of the pellets 40 produced by the apparatus of Figure 2 is determined principally by the composition of the particulate matter received in section 1 of the extruder 34 from the gravimetric feeders 24 and 26. Other factors which affect the resistivity are the pressure developed by the extruder and any other factor which affects the properties of the extrudate. Batches of pellets of different resistivities can be obtained by locally adjusting the feed rates of the feeders and/or the relative amounts of the two carbon materials in feeder 26. If a separate feeder is provided for each different particulate material, only the feed rates need be adjusted. The resistivity of each batch of pellets is determined experimentally by adjusting the feed rate of feeder 26 and measuring the resistivity of individual pellets in different batches. The apparatus of Figure 3 includes an extrusion machine 42, a cooling bath 12, a tension controller 13 and a puller 14. Again, the dashed lines represent electrical signals flowing in the directions indicated by the arrowheads at the ends of the lines. In this exemplary embodiment, the extrusion machine 42 includes an extruder having four distinct sections (designated A through E) and an extruder drive motor 44, a melt pump E, a forming die 18, a feed inlet hopper 22, and a control panel 26. All of the elements in figure 3 with reference numbers 30 or lower are substantially identical to the elements in Figure 1 with the same numbers. The extruder comprises a barrel containing a single screw, which extends through sections A - D. The screw has three successive portions, including a feed portion for mixing and conveying resistive pellets received from the hopper 22, a compression portion for compacting and encouraging melting of the pellets, and a metering section for controlling the quantity, steadiness and homogeneity of the melt supplied to the melt pump E. Section A of the extruder, which substantially corresponds to the feed portion of the screw, is water cooled to facilitate feeding of the pellets from the hopper 22 into the extruder. Sections B - D are electrically heated to effect melting of the pellet mix as it passes through the extruder. The melt pump E is also heated. The forming die 18, the cooling bath 12, the tension controller 13, and the puller 14 cooperate, as was described with reference to Figure 1, to regulate the diameter of the rod 30. The resistivity of the rod is determined by the relative quantities of pellets of different resistivities which are deposited in the hopper 22. The speeds of the extruder drive motor 44 and of the melt pump E are manually controllable at the control panel. Typically these speed of the melt pump is set to maximize productivity and the speed of the motor 44 is adjusted to maintain a constant pressure differential across the melt pump. The extruder includes the drive motor 44, sections A-D, and heating elements for these sections which are both locally and remotely controllable. A number of extruded resistive rods 30 have been made by the apparatus of Figure 3 from resistive pellets made by the apparatus of Figure 2. In each case, gravimetric feeder 24 contained a mixture of silica and a polymer resin. Examples of the polymer material are liquid crystal polymers, polyetherketones, polyetheretherketones, polyethersulfones and polyphenylene sulfides of which the liquid crystal polymers are preferred. Gravimetric feeder 26 contained a mixture of carbon acetylene black and another carbon black, and may further contain a quantity of one or more inorganic filler materials. Examples of preferable filler materials are silica, alumina and talc. The temperatures of the heated extruder sections 2 - 8 in Figure 2 were above the melting temperature of the polymer and were below the temperature where the polymer will degrade. These temperatures ranged from about 280 to 300 degrees C. for a typical liquid crystal polymer. The temperatures of the heated extruder sections A - D and of the melt pump E in Figure 3 were also above the melting temperature of the polymer and below the temperature where the polymer will degrade. These temperatures ranged from about 295 to 300 degrees C. for a typical liquid crystal polymer. In a specific example, feeders 24 and 26 (Figure 2) contained mixtures of the above-described materials and were operated at feed rates which resulted in a mixture in feeder 22 of 7.0% by weight of silica, 82.5% by weight of liquid crystal polymer, on the basis of an aromatic polyester 2.6% by weight of carbon black, (more general from 1-5%) and 7.9% by weight of acetylene black. (more general from 3-12%) A .270 inch long, .10 inch diameter (1 inch = 25.4 mm) circular resistive rod produced by the apparatus of Figure 3 from pellets made from the above mixture had a measured resistance (from one end to the other) of 1,000 ohms. The resistance did not or not substantially vary with temperature. Figures 4a through 4f illustrate a method of making resistors by terminating and encapsulating thermoplastic-carbon resistive bodies 45 cut from the extruded resistive rods 30. Preferably the bodies are cut to a standardized length and the resistance of each batch of resistors is determined by cutting the bodies from rods of corresponding resistivity. Alternatively, a rod of one resistivity may be cut into bodies of different lengths to form resistors having different resistances. As another alternative, even if a standardized length is used for all resistances, the length to which the bodies are cut can be adjusted within a desired nominal range to bring the resistances within a specified tolerance. The following lettered paragraphs describe the method steps illustrated in Figures 4a - 4f, with the paragraph letters corresponding to the suffix letters of the figures: a. A porous metal disk 46 of a sintered ferromagnetic material is brought into contact with a first end of the resistive body 45 to be attached as a first terminal of the resistor. The disk is formed by conventional sintering techniques from a powdered, electrically conductive material (such as nickel) which is both weldable and has a resistance that is small in comparison to that of the body. The disk has a thickness sufficient to prevent bending, act as a heat sink during soldering of a subsequently-attached lead, and to withstand welding of the lead thereto (e.g. 1 mm (.040 inch)). The disk has a diameter substantially corresponding to that of the resistive body. b. The resistive body 45 is gently held against the first end of the disk 46 as both of these elements are passed through an RF electromagnetic field produced by a coil 50. The electrical current in the coil is adjusted to a magnitude which causes eddy currents induced in the disk by the field to heat the disk to a temperature which slightly exceeds the melting temperature of the body and causes the disk to fuse to the body. This temperature varies with the thermoplastic material used, but for the materials of the examples the melting temperature is approximately 280 degrees C. c. Steps a and b are repeated at the second end of the body 45 to effect attachment of a second disk 48 as a second terminal of the resistor. To avoid reheating the first end, the second end is passed across the coil 50 in a direction transverse to a longitudinal axis x-x of the body. Alternatively, the disks 46,48 can be successively fused to both ends of the body in one step by passing it axially through the coil. d. A quantity of resistor subassemblies formed in steps a through c are baked in an oven 52 until the resistive body portions 45 shrink to stable dimensions which will not change substantially during usage of the resistors which are the final product of this method. A typical temperature and time for baking the resistive bodies are 300°C. for 16 to 32 hours. e. First and second electrical leads 56 and 58 are held against and resistance welded to the respective first and second terminals 46 and 48. Welding is achieved by means of first and second power supplies 60 and 62 which are momentarily connected to the first terminal/lead 56/46 and to the second terminal/lead 58/48, respectively. The power supplies produce respective current pulses which generate sufficient heat at contact points where the leads touch the terminals to cause fusion. Because the heat generated by the pulses is localized and of very short duration, it will not cause melting or softening of the thermoplastic body material, even adjacent to the terminals 46, 48. A typical current pulse which has been utilized successfully is a substantially square-wave pulse having a duration of 4 to 6 milliseconds and an amplitude of 600 to 800 amperes. f. The resistive body and the terminals are encapsulated in an electrically insulating, moisture resistant material (e.g. epoxy) by a process such as the conformal coating process illustrated. In this process, each terminated resistive body 45 is coated by rolling it across a rotating wheel 64 which is continuously coated with the epoxy material 66. The coating is hardened by curing in a heated area (e.g. under heat lamps). The resulting encapsulated resistor is illustrated in Figure 5. An alternative encapsulation process which may be used is transfer molding.

Claims: A resistor body (45) comprising a rod (30) of predetermined cross-sectional area formed from a coextrudated mixture comprising a thermoplastic resin, acetylene black and another carbon black, characterized in that the mixture comprises acetylene black in an amount of 3 - 12% by weight and an another carbon black in an amount of 1 - 5% by weight, said weight % being calculated on the total amount of the resistor body (45). A resistor body (45) as in claim 1 where the thermoplastic resin comprises a liquid crystal polymer. A method of manufacturing a resistor body (45) according to claim 1 or 2, comprising the steps of: a. mixing particulate materials to effect production of a mixture comprising a thermoplastic resin, acetylene black and another carbon black; b. heating the mixture until it becomes plastically deformable; c. extruding the heated mixture through an opening in a die (18) to form a hot resistive rod (30); d. cooling the resistive rod (30); and e. cutting the resistive rod (30) into sections to form bodies (45) of predetermined resistance. A method as in claim 3 where the particulate materials include first and second extrudates having known resistivities, and where said materials are mixed in a predetermined ratio to effect production of said bodies (45) of predetermined resistance. A method as in claim 4 where each of said first and second materials comprises a thermoplastic resin, acetylene black and another carbon black. A resistor including electrical leads (56,58) to which a solder material having a predetermined melting temperature may be applied, said resistor further comprising: a. a resistor body (45) according to claim 1 or 2 having a melting temperature which is greater than said predetermined melting temperature, said body (45) having first and second faces; b. first and second terminals (46,48) of electrically conductive, weldable material, each of said terminals having a face fused to a respective one of the faces of the body (45); c. first and second ones of said electrical leads (56,58) welded to respective ones of the terminals (46,48); d. an electrically insulating material (66) encapsulating the body (45) and the first and second terminals (46,48); A method of terminating a resistor body according to claim 1 or 2 with electrical leads (56,58) to which a solder material may be applied, said method comprising the steps of: a. fusing first and second terminals (46,48) of electrically conductive, weldable material to respective first and second faces of the body; b. heating the body until the resistivity of the body reaches a predetermined stability; and c. welding first and second ones of said electrical leads (56,58) to respective ones of the terminals (46,48), A method as in claim 7, further including the step of encapsulating the body and the first and second terminals in an electrically insulating material (66).

Inventors: AYCOCK ROBERT FIELDING, BOWLIN STANLEY, MARKVICKA MAX, MARSHALL GILBERT LEE, STRANDIN PAUL ROBERT, AYCOCK, ROBERT FIELDING, BOWLIN, STANLEY, MARKVICKA, MAX, MARSHALL, GILBERT LEE, STRANDIN, PAUL ROBERT
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01C7
IPC codes: H01C17, H01C7, H01C1