Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article – Agglomeration or accretion
Patent
1987-08-19
1990-08-28
McCamish, Marion C.
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
Agglomeration or accretion
26427213, 26427218, 1562733, 1562755, 522 14, 522170, 522174, 522 83, 524428, 524458, B32B 110, B32B 3128
Patent
active
049523422
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to electrical/mechanical devices of the type comprising a housing in which a mechanism is potted by a cured resinous mass, and to a method of making such devices.
BACKGROUND ART
In the manufacture of electrical/mechanical devices wherein a mechanism is disposed in a housing and potted with a cured mass of a resinous composition to provide a barrier between the mechanism and the external environment of the device, a number of curable compositions have been employed.
For example, Japanese patent application No. 57/134,923 discloses the manufacture of an electrolytic capacitor in which the capacitor element is positioned in an aluminum case and sealed with rubber prior to potting of the assembly with an epoxy or phenolic resin.
West German Offenlegungsschrift No. 2,649,070 teaches the encapsulation of polystyrene capacitors with a thermosetting epoxy in a mold at elevated temperature.
Thermosetting epoxy resins have come into broad usage in such potting applications due to their favorable properties, including high strength, hardness, solvent resistance, good adhesion to conventionally employed housing materials, thermal stability, gas impermeability, and ready curability by a variety of mechanisms and numerous curatives over a wide range of elevated temperature cure conditions.
Despite their significant advantages, however, thermosetting epoxy resins have associated disadvantages which have limited their use, or otherwise rendered their employment in the aforementioned potting applications difficult in the context of commercial high volume manufacturing operations.
A primary disadvantage of thermosetting epoxy resins in the potting of electrical/mechanical assemblies is that their viscosities decrease with increasing temperature under curing conditions until the onset of gellation is reached.
Thus, as the resinous mass dispensed into the housing of the device is subjected to elevated temperature, e.g., on the order of 150-400 degrees F., as in a cure oven, the viscosity of the epoxy resin progressively decreases. The positional stability of the resinous mass then is lost and it migrates or "runs" within the housing.
Such migratory behavior is deleterious due to its tendency to foul the mechanism in the housing. When the mechanism is mechanical or electromechanical in character, and thus includes moving parts, the fouling may interfere with or prevent the proper operation of the mechanism, rendering it useless for its intended purpose. When the mechanism is electrical in character, the fouling may change the conductivity, capacitance, resistance or other critical characteristics of the mechanism's components, ruining the device and requiring its rejection in the manufacturing operation.
Another significant disadvantage of the use of thermosetting epoxy resins in the potting of electrical/mechanical devices is associated with air or other ambient gas which is trapped around the mechanism when the resin is dispensed into the housing, particularly when the mechanism contains voids from which the ambient gas is not readily displaced when the liquid resin is introduced.
The trapped gas in the housing during the curing of the epoxy resin then may bubble through the curing mass, particularly since its viscosity is reduced at this stage, and produce gas channeling void formation in the finally cured composition mass. Frequently, this gas bubbling effect is extensive enough to create continuous leak path channels in the finally cured mass, with the result that the intended barrier function of the cured mass is defeated.
The gas bubbling effect is also present when ambient cure epoxy resins are employed, although to a lesser extent.
In some electrical/mechanical devices, the mechanism is interiorly disposed in the housing and the housing is provided with an opening through which a structural element is inserted and joined to the interior mechanism after the latter has been potted and the resinous mass fully cured. An example is a trimming potentiometer, wherein the lead screw
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Drain Kieran F.
Nativi Larry A.
Hultquist Steven J.
Koeckert Arthur H.
Loctite Corproration
McCamish Marion C.
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