US2203997A - Molded composition resistor - Google Patents

Description

June 11, 1940. s. E. MEGOW ET AL 2,203,997

' MOLDED COMPOSITION RESISTOR Original Filed July 15, 1937 2 Sheets-Sheet 1 EINZgNTORS 4! M 9 mifiiw Y ATTORNEY June 11, 1940.

G. E. MEGOW ET AL.

HOLDED COMPOSITION RESISII'OR Original Filed July 15, 1937 2 Sheets-Sheet 2 fi XZZVENTORS m2: 7

ATTORNEY Patented June 11, 1940 UNITED STATES PATENT OFFICE Thomson, Milwaukee, Wis., assignor to Allen-Bradley Company, Milwaukee, Wis., a corporation of Wisconsin Original application July 15, 1937, Serial No. 153,746. Divided and this application October 17, 1938, Serial No. 235,388

Claims.

Our invention relates in general to molded composition electrical resistors and, more particularly, to resistors for radio receiving circuits. This application is a division of our co-pending application Serial No. 153,746, filed July 15, 1937.

Our invention resides in a novel article conslsting of a resistor in which head portions only of electrodes are embedded in the body of a resistor, whereby a comparatively small resistor is obtained and one in the preferred form of which surprisingly strong anchorage of the leads is obtained because the splitting effect of a deformable shank within the resistor body is eliminated.

Our invention is easily embodied in a form of resistance unit which has good characteristics with reference to uniformity of resistance, change of resistance with moisture absorption, change of resistance with change of voltage, permanent change of resistance on high electrical load, change of resistance with change in temperature, change of resistance with age, and microphonic noise,

In addition to the good electrical characteristics, our invention provides a resistance unit which is mechanically strong in that the body portion withstands all required breakage tests, being notably resistant to severe stress on the connecting leads in endwise or sidewise'direction.

The resistance unit of our invention may be made of extremely small size as the electrodes are positioned at the very ends of the body portion rather than placed within the body portion with a connecting shank.

The advantages of the resistor of this invention will become more apparent from the description which follows, which is given with the aid of the accompanying drawings in which Fig, 1 shows an enlarged partial cross-section of a resistor of this invention;

Fig. 2 shows an enlarged partial cross-section of a resistor of this invention showing bending of an electrode lead;

Fig. 3 shows an enlarged partial cross-section of another form of the resistor of this invention;

Fig. 4 shows, in perspective, another form of the resistor of this invention;

Fig. 5 shows one method of forming the article shown in Figs. 1, 2, and 3;

Fig. 6 shows one method of forming the article shown in Fig. 4; and

Fig. 7 shows, in perspective, the resistor shown in Fig, 1.

Referring to Fig. 1, the body portion l of a resistor of this invention is shown greatly enlarged, with portions of the body shown in section. Electrode leads l3 are shown comprising relatively flexible shank portions 26, with head portions 21 joined thereto at the shoulders 28. The heads 21, as shown, are slightly tapered to aid in their retention. The particular shape of heads 21, however, is a matter of secondary importance, since any suitable configuration is appropriate as long as the same is sufliciently large and massive with respect to the diameter of the shank portion 26 as to render the same relatively nondeformable. It is also essential that the electrode head will anchor properly in the resistor and will be properly supported by the electrodecarrying plunger during the molding and embedding process. The electrodes 20 in Fig. 6 show one variation in form which is suitable.

In assembling a resistor in a circuit, the shank portions 26 of the leads are frequently bent in some such manner as illustrated at the right of Fig. 2. It is to be noted that the bent portion of the lead forms at a comparatively large radius and rupture or breakage of the lead, as in a small radius, is precluded. The formation of the large radius may be augmented by cold working and/or enlargement of the neck portion of the lead.

The provision of the shoulder 28, or its equivalent, facilitates the embedding of the electrode in the body portion l in the exact position desired, according to the method of parent application Serial No. 153,746, to which reference has been made.

In order to obtain the advantages of this invention it is also possible to embed a portion of the head portion 21 within the resistor body I, as shown clearly in Fig. 3. In Fig. 3 a body portion I' is shown, parts being broken away to show a section. Embedded in the body portion I are leads I3 having head portions 21 joined by shoulders 28' to shank portions 26'. As shown clearly, only a part of the head portion 21' is embedded in the body portion I. Embedding the electrodes in this manner is easily accomplished by counterboring the ends of the bored plunger dies, which cause the embedding of the electrodes, or in any other manner desired.

One method of forming the articles of this invention as shown in Figs. 1, 2, 3, and 7, is as follows: Carbon black, suitably prepared for molded electrical resistor purposes, is incorporated in the usual proportions with relatively uncondensed resin, such as phenol-formaldehyde, and a filler, such as ground quartz. The mixture is blended upon kneading rolls, or other suitable mixing device, sufiicient heat being employed to partially condense the resin bonding agent and form a weak solid. The material thus prepared is then ground to a powder and fed to a pill machine, or other similar device well known in the art and not shown or described herein, which subdivides the ground mixture into uniform portions of desired mass and compresses the subdivided masses into relatively weak but solid blanks of uniform dimension and mass.

Blank i, prepared as above, is then inserted, as shown in Fig. 5, in the cylindrical hole 2 in the recessed die 3. Recessed die 3 is shown surrounded by a heating coil 4 through which suificient electric current is caused to flow to maintain the die block 3 at a temperature of 300 to 500 degrees F., which limits are appropriate to the particular binder mentioned above, although other binders obviously may require other temperatures,

The die block 3 is furnished with a pair of bearing supports 5, 5, upon which are mounted rocker arms S, 3, joined by toggle links i, i, one each making pivotal connection with the upper ends of the rocker arms 6. G, at the points 3, The toggle linlm E, i, are brought together at a pivoted connection d upon which is mounted the spring-backed presser foot upon which force may be applied.

A pair of centrally bored electrode-carrying plungers i2, it are then inserted in the ends of the hole as shown in Fig. The electrodecarrying plungers i2 carry in their central bores headed wire electrodes i3, having relatively nondeiormable heads 2? and easily flexed wire shanks 26.

Pressure is next applied upon the presser foot 38, forcing the carrier plungers i2, 12 toward one another to the position illustrated in Fig. 5. When this is done the only agency limiting the travel of the plungers i2, 52 is the volume occupied by the body portion 5 of the resistor and, as a consequence, the pressure applied thereto may be maintained at any full value desired from moderate pressure up to 2000 lbs. per sq. in., or heavy pressure up to and beyond 50,000 lbs. per sq. in., thereby performing molding at controlled pressure rather than at constant volume. By the time this state of the process is reached the body 5 has acquired a temperature approximating that of the die block 3 and the condensation reaction moves toward completion,

The electrode inserts, as shown clearly in Fig. 6, are formed from wirepreferably copper wire, solder-coatedof a size permitting easy flexing, with a relatively nondcformable head upset upon the end. For mechanical reasons, the head may be formed, as shown, in frustro-conical form. or other configuration, rendering the same rigid and relatively nondeformable, If desired, the heads may be preliminarily coated with graphite or other finely divided material to reduce the contact resistance at the interface.

In practicing our invention any suitable bonding agent moldable under heat and pressure may be employed, including thermoplastic or heatcondensible bonding agents with or without modifiers. Further, the conducting material dispersed in the bonding agent may be any suitable, finely-divided or dispersed conductor.

In Fig. 6 is shown a form of recessed die adapted for molding from the side, which may be employed in making the form of the article of this invention shown in Fig. 4. It is composed of a block it surrounded by a heating coil through which electric current may be passed to raise the temperature of the die block to the desired point. The block H is recessed to create a rectangular, upwardly-facing slot IE resting upon the bottom. of which is the flat ejector shoe H. The shoe l1 may be raised by the rod l8 to eject the contents from the slot IS. A closely fitting rectangular plunger die I! is arranged to enter the slot 5 and move vertically therein. On either side of the plunger die I! are holes 20, 20, drilled as shown to receive headed wire electrode inserts. Extending above the plunger die I9 is the guiding head 2i upon which is guided the spring-backed presser foot 22. The prcsser foot acting through spring 23 applies a downward pressure, as indicated by the arrow, upon the plunger die IS.

A suitable blank is laid in the recess 16 and full controlled molding pressure is then applied, causing the electrodes 24, 24 to become embedded, l,

as shown. During the application of pressure the bonding agent, under the influence of heat and controlled pressure, fixes the final form of the resistor and presses uniformly against the electrodes, bringing about reliable contact resistance H and good mechanical retention,

Upon completion of molding, the finished resistor is ejected by the shoe ii. The appearance of the finished resistor is illustrated in Fig. 4, wherein the numeral 25 represents the body portion of the resistor and 24, 2d, the embedded wire electrodes.

The resistor formed in accordance with this invention and as illustrated in the drawings may be formed with the embedded electrodes entering g,

from side or end and, in such cases as occasion may require, the resistor may be formed with but one embedded electrode. The resistor as shown in greater detail in Figs. 1, 2, and 3, however, is

characterized by a main body portion, preferably headed electrodes to the body portion of the resistor. These advantages arise in any case, but are particularly marked when the body portion has the high strength characteristic of a resistor molded under controlled pressure.

The enlarged electrode head also has the effect of enhancing the heat-dissipating capacity of the resistor. The high conductivity of the electrode material rapidly carries heat from the interior of the resistor to the exterior, having a direct in-- fluence upon the heat-dissipating capacity and, therefore, increases the wattage rating of the resistor.

No excess material need be included in the blank, as all of the material is formed into the resistor body. The plungers employed in this invention fit the recessed die as closely as possible and the finished resistor is practically free of molding flash. In contrast to this, in the old, constant volume molding, it was essential that a slight excess of material be included in the blank so that the mold would be certain to be filled. Such a mold, further, cannot be closed completely for the reason that the excess material of the blank must be permitted to escape into the flash of the mold. The resistors of this invention, illustrated in Figs. 1, 2, and 3-, have a straight, cylindrical body portion of nonvarying cross-section extending the entire distance between the electrodes. The only molding flash borne by the resistor is confined to two peripheral, circular flashes ll disposed one at each end and disposed exteriorly to the portion of the resistor which carries the current and determines the resistance.

The resistance unit of our invention may be made of comparatively small size in that accuracy of the position of electrodes and leads with reference to body portion of resistor is attained. Also, as each part of the resistance unit is identically formed and processed, a high uniformity of product is assured.

It is desired that the protection of letters patent afforded hereby extend to the full extent of the inventive concept hereof as represented by the scope of the claims appended hereto without being limited by statements herein which are made in connection with the setting forth of specific instances of practice or use.

What we claim as our invention and wish to secure by Letters Patent is:

1. A resistance unit comprising a molded resistor body, and two wire electrodes each having a flexible solid metallic wire shank portion and a rigid head portion as a continuous integral part thereof, said heads being embedded in opposite ends of said resistor body with the said flexible shank portions entirely exterior of said resistor body.

2. A resistance unit comprising an endwise,

molded resistor body, two wire electrodes each having a flexible solid metallic wire shank portion and a relatively nondeformable head formed as a continuous integral part thereoi, said heads being embedded in opposite ends of said resistor body with said shanks entirely exterior to said body, and a peripheral molding flash at an end of said body portion.

3. A resistance unit comprising a. molded resistor body, and two wire electrodes each having a flexible shank portion, a shoulder and a rigid head portion, said shank, shoulder and head being composed of one solid continuous piece of metal, the parts thereof being arranged coaxialiy with one another with said shoulder intermediate said shank and head, said electrode heads being embedded in opposite ends of said resistor body with said shoulders substantially flush with the surface of said resistor body, said shanks being disposed in substantial alignment with one another along an axis of and entirely exterior to said resistor body.

4. In a resistor having a metallic electrode emerging from a molded resistor body, the combination comprising a molded body composed of v insulating binder with conductor particles dispersed therein, said molded body being provided with an electrode-embedment socket, and a metallic electrode having a section of greater thickness forming a head adapted to resist deformation and a section of lesser thickness forming an easily bent solid shank, said shank being integrally joined to said head at a zone of marked change in thickness, said head being embedded in said embedment socket in said molded body in intimate electrical and mechanical contact therewith to such a depth that said zone of change in thickness remains exposed with said shank entirely exterior to said body.

5. In a resistor having a metallic electrode emerging from a molded resistor body, the combination comprising a molded body composed of insulating binder with conductor particles dispersed therein said molded body being provided with an electrode embedment socket, and a metallic electrode having a section of greater thickness forming a head adapted to resist deformation and a section of iesser thickness forming an easily bent solid shank, said shank being integrally joined to said head at a zone

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