US2548353A - Casting method - Google Patents

Description

April 0, 1951 R. E. CUNNINGHAM 2,548,353

CASTING METHOD Filed Feb. 25, 1949 4 f v 4 V l -13 1 1' I '1 F f I 5 1 ,1 J 5 7 e :o F]

INVESTOR,

RUSSELL E. CUNNINGHAM Patented Apr. 10, 1951 CASTING METHOD Russell E. Cunningham, Washington, 0., as-

signor to Melpar, Inc., Alexandria, Va.

Application February 25, 1949, Serial No. 78,347

1 Claim. 1

The present invention relates generally to the potting within a casting of resinous plastic material of electrical components generally, and particularly to the potting of fragile electrical components, such as miniature vacuum tubes or the like, for the purpose of protecting the latter against mechanical shock, moisture, fungus and various other effects deleterious to the life and proper functioning thereof.

The invention relates further, broadly, to the potting within a casting of resinous material, of a mechanism having parts which are movable during operation of the mechanism, and more particularly to the potting of electrical variable condensers.

Broadly described, the present invention involves potting of frangible electrical components, such as electronic tubes, or the like, in polyester resins, or other resins which are shockproof, rigid, and relatively non-hydroscopic, but which have a contractibility different from that of the electrical components during cooling or curing of the resins. In general, it has been found that the resinous compounds utilized in fabricating a resinous casting contract during cooling and curing, and, in relation to the contraction of glass or metal, contract volumetrioally to a different extent than the latter. This difference in contractibility may cause mechanical damage to the electrical components, and especially to those which incorporate glass in their structures, during casting or potting, and in the case of miniature vacuum tubes in particular, such mechanical damage results in leakage of gas into the envelope of the tube, destroying the operability of the tube. In the case of metallic bodies, on the other hand, the contraction of the metal is less than that of the casting, resulting in a defective casting rather than in damage to the metallic body, since the casting tends to pull away from the metal.

It is well known, in accordance with prior art practices, to seal vacuum tubes, or other frangible electrical components, within an envelope, container, or covering, prior to potting, the container taking the form of a thin cylinder, and being formed of glass or of a relatively resilient plastic, in order that the envelope, container, or covering may absorb the stresses due to the contraction of the resinous material during casting, thus avoiding damage to the components, or defects in the casting. Various difliculties are found to exist, in practice, in this procedure, as applied particularly to miniature vacuum tubes. Insertion of a vacuum tube into a container is applied,

in the first place, by considerable possibility of occurred, this is not ascertainable until the potting process has been completed, so that considerable economic waste is involved in proceeding according to the known methods. I have, accordingly, disclosed, in a prior application for Letters Patent of the United States, Serial 13,437, filed August 10, 1948, and entitled Potting Electrical Components, a method of potting which involves coating of vacuum tubes, or other electrical components, with a resilient plastic coating prior to casting. This coating may be formulated with standard heat stabilizers and plasticizers and possesses a resilience requisite to enable the coating to absorb the stresses generated by the contraction of the main potting compound during polymerization and cooling of the latter, and is essentially capable of forming a bond with the resinous casting. The coating may be quickly applied, as by dipping, to provide the required protective container.

While the system disclosed in my previous application above referred to is satisfactory for many purposes, it sometimes happens that the total resilience or give of the resilient coating is insufficient to protect the fragile component, such as a miniature vacuum tube, especially after casting has been completed and during normal operation of the component. It will be realized in this connection that electrical components, such as vacuum tubes, are frequently equired to operate under extreme conditions of heat or cold, and that in such cases the differential contraction of the casting and the components which exist during potting recurs during operation of the component. Experience has shown that in extreme cases miniature tubes have cracked after being potted, or while being potted, in accordance with the system disclosed in my prior application.

I have, accordingly, devised a further system for potting fragile electrical components, such as miniature vacuum tubes, by forming about the component, prior to potting, a layer of wax, having a relatively low melting point, and which is compatible with the plastic material which is to form the casting. The casting is then formed about the wax, and after curing of the casting, the wax removed. In this way an air pocket or chamber is formed between the potted component and the casting, which forms the most effective cushion conceivable for absorbing differential 3 contractions or expansion of the casting and of the component.

Still a further problem which is faced in the art of potting or casting electrical components, is that of casting a component which comprises mechanical parts which must be moved during operation of the component, at least intermittent-.

1y. Examples of components of this character are: variable condensers, small electric motors, variable inductances, potentiometers, variable resistances, and the like. I have elected to disclose my invention as applied to the pottingof component having movable mechanical parts, and specifically, a variable condenser. It is to be understood, however, that the principles of the invention may be applied to various other types of components, and that the disclosure as applied to variable condensers is intended to be non-limiting, and for the purpose of providing a specific example only.

The process of potting an electrical condenser in accordance with the present invention involves the steps of initially potting the condenser in a casting of wax, leaving the rotor shaft of the condenser uncovered. After the wax has solidified, the wax casting may be further cast or potted in a casting of resinous plastic material, which may be provided with an aperture suitable for removing the wax. After the plastic material has cooled and polymerized the entire casting may be reheated for the purpose of melting the wax, and the latter may then be permitted to escape via the aperture above referred to.

Upon removal of the wax from the outer casting there remains a space within the outer casting, which is partially occupied by the variable condenser, and, if the wax originally covered completely all the plates of the condenser, the movable plates thereof will be free to move relative to the stationary plates thereof. Further, the wax casting may be so designed originally as to leave free some portion of the condenser and particularly some portion of the base or supports thereof. These then, come into contact with the plastic materialduring the potting of the outer casting, and form a bond therewith, which subsists after removal of the wax, so that the finished casting forms a support for rigidly securing the condenser.

Obviously rotation of the rotor shaft must be provided for in the arrangement described. This maybe accomplished in two distinct ways, one of which involves merely the provision of an aperture in the resinous casting through which the shaft may protrude, and the other of which involves fitting of a rotary seal or rotary joint about the shaft of the condenser prior to casting thereof in the resinous material. The resinous material is then cast about the seal at its outer diameter, and establishes there a tight joint. Utilization of the rotary seal as described provides a casing for the condenser which is airtight and moistureproof, while elimination of the seal provides merely a casing which is valuable for protecting the condenser against mechanical shock or other mechanically induced damage.

Proper choice must be made of the composition of the wax material which is utilized in the potting process, so that polymerization of the potting compound is not inhibited at the inter-face between the main casting and the wax, which serves to reduce stresses at or adjacent to the inter-face and results in a smooth and uniform casting. If a wax is used which is non-compatible with the resinous compound forming the main 4 casting, polymerization of the latter adjacent to the wax will be prevented over a relatively wide volume of resinous material, and the casting will be unsatisfactory.

It is, accordingly, a broad object of the present invention to provide a novel method for potting fragile objects within a mass of shockproof, rigid and relatively non-hydroscopic material, such as polyester resins, or the like, making provision for an air space between the body of the castin and the objects themselves.

It is a more specific object of the present invention to provide a method of potting electronic vacuum tubes, or other fragile electrical components,- in polyester resins or other casting materials, in a manner which will provide for maximum relative expansion of the casting and the vacuum tube or other component without mutual damage or mechanical contact.

It is more specifically an object of the present invention to provide, as a new article of manufacture, electrical components in potted form, cast within a mass of resinous material, which forms a casing for the components, separated by an air space from selected portions of the components. I

The novel features which are characteristic of my invention are set forth 'in the appended claims. The invention itself, however, will best be understood'by reference o'fthe ronowm 'specification, when considered in conjunction with the accompanying drawings, in which:

Figure 1 illustrates in traverse section, avariable radio condenser, potted in wax preliminary to potting of the wax in resinous casting compound;

Figure 2 is a sectional View Corresponding with Figure 1, wherein the'cast'in'g compound is shown encasing the wax casting;

Figure 3 illustrates in seetional view the s'tru'c ture of Figure 2 with the wax removed to provide a finished product;

Figure 4 illustrates in sections. miniature vacuum "tube coated with wax preliminary to casting thereof in resinous casting compound;

Figure 5 illustrates the structure of Figure '4 of the drawings encased in r'esinous'casting compound, prior to removal of 'the wax from the "interior of the casting; and,

'Figure 6 illustrates a pottedv'acuum tube as manufactured in accordance with the methods of the present invention. 7

Referring now specifically to -Figure l ofth'e accompanying drawings, there is illustrated a variable condenser i of generally conventions character, having fixed plates 2 and variable plates 3, which are mutually inter-meshed in the usual fashion. The variable plates 3 are mounted on and secured to a shaftli,forrotationthereby with respect to the fixed plates 2. The shaft 4 rotates in bearings provided in bracket arms 5 for that purpose, and the bracket arms themselves are secured to and supported by 'a base member t, which further supports one or-more flanges 1, extending parallel to the shaft 4, and which are utilized for supporting and securing the stationary plates- 2 In accordance with the presentinvention, and as 'an'initial step looking to the potting of the condenser within resinous casting compound, the

condenser is cast or potted in a solid block of wax 8, by dipping the condensers into the wax when in a molten state, and then .ipermitting the wax to solidify. While substantially thegentire condenser is coated or covered with wax during this step of the process, it will be clear from the drawings that the shaft 4 is permitted to protrude from the wax casting 8, for purposes which will appear hereinafter, and further that the base member 6 remains uncovered at least in part by the wax of the wax casting 8.

After the wax casting 8 has solidified, a rotary seal 9 is mounted on the shaft 4, in intimate contact with the wax, the seal 9 being any suitable such device which is readily commercially available, and which is capable of permitting free rotation of the shaft 4 therein while sealing the shaft against passage of moisture or gases at the junction of the seal and the shaft.

Thereafter the casting 8, and also the seal 9, are dipped Within liquid resinous casting material contained in a suitable form, the clipping process being accomplished with the shaft 4 in a vertical orientation, and the surface of the resinous casting compound being arranged parallel to the wall ii) of the wax casting 8, and at a slightly higher level than the Wall IU of the wax casting 8, so that the resinous casting material at least partially covers the outer diameter of the seal 9, but does not attain to or enclose any portion of the shaft 4 as it extends beyond the seal 9. The form containing the resinous casting material is provided, moreover, with a projection designed to reach from the bottom of the form to the wall H of the wax casting 8, which may, for this purpose, rest on the projection. Accordingly, when the resinous compound has been permitted to cure and polymerize to form a casting I2, an aperture 13 is provided in the wall H of the resinous casting, which otherwise completely encloses the wax casting 8, and seals the external diameter of the rotary seal 9.

After the main resinous casting l2 has cured and polymerized, it may be heated to a temperature sufficient to melt completely the enclosed wax casting, the wax being then permitted to escape via the aperture l3. After all wax has been removed, the aperture [3 may be plugged by a plastic plug I4, whereupon the process of enclosing the condenser is complete.

It will be noted that by following the steps above described, a product results which consists of a variable condenser having movable plates 3, which are free to move with respect to the stationary plates 2, by rotation of the shaft 4, the variable condenser being, however, completely enclosed by the resinous casting material, and, accordingly, protected against mechanical shock or .damage of any type, and further isolated from the external atmosphere by seal 9, and, accordingly, maintained free of fungus, moisture, and the like.

It will be clear from the process described and from the description and illustration of the resulting product that the process may be applied to other structures than variable condensers, and particularly to electric motors, potentiometers, variable inductances, and the like, which comprise mechanically moving parts and a rotatable shaft, and whether that shaft is used for providing output power, or for transferring power to the mechanism internally of the resinous casting.

A modification of the present invention involves the casting of relatively fragile objects, which do not comprise mechanically movable parts, but which it is desired to protect against shock or mechanical impact. One obvious object of this character is a vacuum tube having a glass envelope. While vacuum tubes have been potted in accordance with procedures now understood by those skilled in the art, and particularly in ac cordance with a method illustrated and .described in my prior application, Serial 13,437, filed August 10, 1948, to which reference has been made hereinabove, these methods all have the inherent defect that the tube is essentially in contact with the resinous casting, despite the fact that the contact may be via a relatively flexible or soft protective layer. It is a primary virtue of the present invention that the protective layer may be dispensed with entirely, which results in optimum provision for differential expansion or contraction as between the tube and its resinous casting, and, further, in that the space provided between the casting and the tube may be made of an .desired dimensions with extreme simplicity and rapidity.

Referring now more specifically to Figure 4 of the drawings, I have illustrated a miniature Vacuum tube 20 having a glass envelope 2 I, and a base 22, from which protrude leads 23 which are utilized for energizing the tube and deriving signal therefrom. It will become clear, as the description proceeds, that while I have chosen to illustrate a vacuum tube 20 which is provided with a base 22, that provision of the base is not essential to the present invention and that vacuum tubes of any type may be utilized in the practice thereof, and especially vacuum tubes having no bases, but which consist essentially of glass envelopes from which protrude the requisite leads.

The glass envelope 2|, may, in accordance with the present invention, be coated with wax 24 to any desired thickness, either by a casting process or by a simple dipping process. After the coated wax has solidified, the wax coated tube 20 may be enclosed in resinous compound, preferably in liquid form, and preferably by a molding process, the mold being provided with a projection designed to introduce an aperture 25 in the finished casting. The resultant casting 26 may be of any desired shape or size provided only that it covers the wax envelope 24, and at least a portion of the base 22 of the vacuum tube 20, in the event the latter is provided with a base, or encloses the entire tube in the event the tube has no base, but in any event permitting protrusion of the requisite leads so that the vacuum tube may be connected in circuit. After the resinous casting has cured and polymerized, the wax 24 may be heated and drawn 01f Via the aperture 24, leaving free space between the tube envelope and the casting. Thereafter the aperture 25 may be plugged up in any convenient manner, as by a plug 2?, and the structure is ready for use.

While I have described and illustrated two specific embodiments of my invention, it will be clear that variations of structure and method may be concerned by those slnlled in the art without departing from the true spirit and scope of my invention.

What I claim and desire to secure by Letters Patent of the United States is:

A method of potting an electronic tube having a frangible envelope and a non-frangible support, comprising, dipping only said envelope in liquified wax, removing said envelope from said liquid wax, hardening said wax, enclosing said wax coated frangible element totally and said non-frangible support partially in a hot resinous casting compound, cooling and solidifying said resinous casting compound to provide a casting surrounding said wax coated frangible envelope totally and said non-frangible support partially, and removing gum-ass to leave -.a. seal between gaid Lsuppoltt and said \casting.

RUSSELL AREEERENCES CITED The following :references are of record in sthe file of this patent; 7

:Name 113%? liipharsi W -Feb- 1 1903

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