Impregnation

Plasti-Seal Corporation, located in the Huntingdon Valley Industrial Center with fast access to all parts of the Delaware Valley and manned by a highly qualified and experienced staff, offers a service in impregnation of metal castings and other porous materials as well as spot testing, 100% testing and consultation by qualified engineers.

The superlative characteristics of our Dapon sealant, our modern equipment and the dedication of our employees to customer service have helped Plasti-Seal Corporation expand in a short time into a position of leadership in our field. The following is a description of the impregnation process and a summation of the properties of Dapon.

Metal Casting Impregnation:

Metal castings are inherently porous. Microscopic porosities and those that will not readily pass water, will leak lower viscosity reagents such as gasoline. Under pressure, these porosities will virtually gush low viscosity liquids and gases. Since even the highest art of casting does not produce wholly non-porous products, it has become the practice of the foundry industry to seal those castings intended to retain fluid or gas pressure or vacuum by a process known as impregnation.

The industry has come to accept impregnation not only as a means of repair of castings but as a standard finishing system for quality low-porosity castings. Castings which require subsequent finishing such as plating or painting and which must exhibit good permanence of this finish without lifting under corrosive conditions are also sealed.

Impregnation has used a variety of solvent and solventless systems of low viscosity that would readily permeate the casting and be dried or cured in place. Early techniques involved the use of materials such as tung oil and sodium silicate which lacked permanence, sealing ability and electrical properties. New systems such as phenolics, polyesters and epoxies are an improvement but all have storage life problems and contain volatile ingredients, such as water or styrene monomer. In the case of phenolics, repeated impregnation cycles are required with considerable difficulty in getting a complete permanent seal. Polyesters and epoxies require monomers to reduce viscosity. Unless these materials are cured at low temperatures these monomers evaporate and there is the hazard of porosities which would require repeated impregnation. The high viscosity of the resins used in these systems causes substantial resin “pick-up” by the casting as it is removed from the system. The removal of this resin from the surface results in a substantial loss of material and efficiency. The solvents required for cleaning cause fire hazards and toxicity. If higher viscosity solvents such as glycols are used, heated tanks are required to get solvency; here too, there is much resin loss.

Dapon Sealant:

Dapon solves many of the problems associated with casting impregnation. It is essentially non-volatile, having a room temperature vapor pressure less than 0.0012 millimeters of mercury. Due to this low vapor pressure and its chemical nature, the flash point of diallyl phthalate impregnation solution is high … 330 deg. F. open cup. There is no fire hazard or need for special precautions as with resin or oil systems.

It has a very faint pleasant odor.

Viscosity adjustments from pure DAP monomer 12 cps. up to the solid state of Dapon resin are easy; viscosity is adjusted by merely adding diallyl phthalate monomer to the system to reduce viscosity and by adding solid Dapon resin to increase viscosity. In either case the chemical nature of the product is unchanged so that it is possible to impregnate from the finest porosity up to macro porosities and achieve the same high quality result. Even at 100% DAP monomer composition having a viscosity of only 12 cps., there is essentially no evaporation and hence no material loss. Furthermore, the boiling point of the pure monomer is higher than the curing temperature … resulting in little or no tendency of the monomer to evaporate within the casting and leave spongy impregnation subject to porosity and failure.

Having no highly volatile components, Dapon sealants do not tend to ooze during their cleaning or baking cycles. The labor required to clean castings is reduced. (Oozing in systems such as polyesters is largely caused by the components alkyd and styrene, which are thermoplastic rather than thermosetting. Both are soft and flow sensitive up to the final cure point. Since plastics expand at several times the rate of metals during the oven cure and since the volatile components cause an internal pressure, the polyester sealant is forced to escape. Reaching the surface, the styrene flashes off. This leaves the pure alkyd which is incapable of complete cure and becomes a sticky ooze or contamination on the resin surface.)

Service Characteristics of Dapon:

Dapon resins are recognized for their excellent chemical resistance, electrical properties and heat resistance. These characteristics present tremendous advantages in casting impregnation. Dapon is suitable for continuous service at 400 deg. F. and intermittent service at 500 deg. F. and over. It has excellent resistance to solvents (including the chlorinated solvents), acid, and five times the resistance to caustics of polyesters (one hundred times the resistance to caustics of epoxies).

The electrical properties and the water absorption properties of impregnating systems are important because of the possibility of the galvanic corrosion in metal parts, such as occurs when two dissimilar metals are in contact with inserts, in this characteristic. Dapon is unexcelled among resins. The electrical resistivity of Dapon after two hundred hours of exposure at 100% relative humidity is unchanged. Other resins experience resistivity drop-offs of as much as 90%. This means that Dapon impregnated castings will not experience internal corrosive effects owing to moisture pick-up and will, therefore, show far superior stability as compared to other systems in high humidity atmospheres.

Dapon in its cured form is tough rather than brittle. In the casting it will tend to conform to flexing and expansion where more brittle resins would break and permit leakage. This is particularly valuable in non-metallic materials such as fibre in which resin systems would tend to embrittle the parts rather than toughen them.

Stability:

Since Dapon resins are heat stable, chemically stable, non-solvent systems, their stability is outstanding. There is nothing to evaporate and no tendency for Dapon to continue to shrink and open passages. The dimensional change for the life of the resin is .0007 of an inch per inch in a solid resin casting. This excels over all known resin systems including epoxies.

Impregnating Process:

Castings are usually impregnated using a vacuum-pressure system, followed by baking in accordance with Military Specification MIL-STD-276, “Military Standard, Impregnation of Porous Non-Porous Castings.” The materials used meeting this standard comply with MIL-1-6869B. Conforming to this process, castings are often machined to remove surface skin and open up voids which would not be sealed if the rough casting were impregnated. This prevents opening up unimpregnated areas during machining. For ferrous, light metal and copper alloys, sealing after finish machining is generally specified.

A typical impregnation cycle consists of cleaning and drying of the casting and loading in an empty pressure vessel. The vessel is closed and the air exhausted until a vacuum of 27 inches of mercury is obtained. This vacuum is maintained and the diallyl phthalate impregnating solution admitted until the castings are totally covered. The vacuum is held long enough to assure that all of the gases in the casting have been removed. After the vacuum has stabilized, pressure is admitted to the tank on the order of 100 psi and maintained for thirty minutes, after which it is released and the resin drained from the system. Castings are removed and cleaned, placed in the oven and cured. A test is normally specified … complying with MIL-STD-276.

Applications:

Plasti-Seal impregnation is applied to aluminum, zinc, magnesium and various alloys … also grey and malleable iron, brass, bronze and sintered, chrome and nickel plated metals, fibre, asbestos, ceramics, phenolics, laminates, woods, etc.

Users of impregnation are too numerous to list but include foundries, forge shops, machine shops, painting and finishing operations and the wood, fibre and carbon industries as well as manufacturers of pressure vessels, valves, fitting gauges, regulators and tubing.

Powder Metal Parts:

Their porosity makes chemical corrosion contamination a serious problem and plating a near impossibility. Plasti-Seal impregnation makes possible the production of complete parts of this type.

Metal Sprayed Parts:

Initial sealing by Plasti-Seal prevents galvanic corrosion when castings are sprayed with another alloy. Some corrosion preventing solutions actually cause reactions but Dapon sealants have been successful in inhibiting these failures.

Die Castings:

Contrary to conventional opinion, die castings tend to be quite porous with the porosities concealed by a continuous skin on the surface of the casting. When pressure or vacuum is applied the micro porosities that exist in the skin allow transmission of fluid or gases and ultimately, failure of the part. When parts must be plated, cleaning solutions and moisture, condensed during atmospheric changes, lodge in the porosities. After plating there is a tendency to internal corrosion and lifting of the plating, such as is common on certain automotive parts. Plasti-Seal impregnation results in excellent life expectancy in each of these instances.

Porous electrical Insulation:

Due to porosity, fibre, asbestos, and ceramic parts used as electrical insulators experience problems from mildew and severe degrading of electrical properties caused by moisture entrapment. Plasti-Seal impregnation overcomes this.

Vacuum-proof Housings:

Plasti-Seal impregnation makes possible tight castings under vacuum or low pressure conditions. Leak rates as low as cc’s per year are achieved.

Hydraulic Parts:

Plasti-Seal impregnation using Dapon is advised for both ferrous and non-ferrous hydraulic parts to eliminate leakage during the life of the part. Where the hydraulic systems use corrosive chemicals or high temperatures such as in oil cooler housings, the heat and chemical resistance and stability of Dapon make it of great value.

Forgings:

When forgings incorporating multiple parts such as inserts, studs and the like, must be pressure or vacuum tight. Plasti-Seal impregnation is the ideal solution.

Weldments:

The manufacturing of parts incorporating gas-tight welded seams is difficult, but with Plasti-Seal impregnation the difficulty is eliminated while making possible economies in design and production.

Pressure Bonding:

When similar or dissimilar materials are joined, Plasti-Seal impregnation creates a mechanical interlocking between the insert and the base metal. There is no leakage and sheer strength is increased. Metallic and non-metallic materials can be bonded to cast or wrought metals.

CHARACTERISTICS OF CASTINGS SEALED BY PLASTI-SEAL

Resist chemicals including acids, alcohol, ammonia, ammonium hydroxide, carbon dioxide, freon, fruit acids, fuel oils, gas (manufactured and synthetic), gasoline and glycols,
Heat resistant
Increased dielectric resistivity
Free of finish deterioration, spotting out
Free of corrosion
Tougher rather than more brittle
Tolerances or machined areas not effected
Free of bacterial or fungus growth
Non-toxic (can be used with food and will not injure skin, lungs, etc.)
Easily machined (milling, drilling, buffing, grinding and lathe work will not foul tooling or the machined surface of the castings. Only rarely will machining open up porosity after Plasti-Seal impregnation.)
Life of parts lengthened
Impregnant is invisible on the casting
MILITARY SPECIFICATIONS:

Plasti-Seal processes and materials are all in accordance with military specifications and a certificate of compliance will be issued whenever required. Plasti-Seal processing is performed in accordance to the specifications set forth in our approved Quality Control Manual. Copies of this manual are available on request.

TESTING:

Plasti-Seal Corporation is equipped to do most tests which use pressures. All testing is in accordance with MIL-STD-276 or other approved specifications.

Hydrostatic Testing is used to test to extreme pressures using water, special oils or other specified liquids.

Pneumatic Testing is done in the higher ranges using air, helium, nitrogen and other gases which are not highly combustible.

Flow Testing of gases and liquids is performed using flow meters and gauges.

Vacuum Testing is done in the micron range.

Special Test Work includes measuring the accuracy of pressure actuated devices. This testing is done to individual specifications.

THERMOSEAL:

This process is used when impregnation is not feasible. A mixture of self-setting resins and metallic powder is used to fill blow holes, large surface porosity and irregularities in the castings. Application is done by hand. The powders used are of the same metals to which they are applied, i. e. brass powder for brass castings, aluminum powder for aluminum castings, etc.

Thermoseal is a must for castings with large leaking holes. It precedes impregnation to prevent the impregnant from flowing out during the curing state. Where critical paint finishes are desired, thermoseal improves the surface of the casting.

Thermoseal is used only if recommended by Plasti-Seal Corporation and approved by the customer. It has been approved by the government for special jobs on missile work.

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    Plasti Seal Corporation
    1647 Republic Road
    Huntingdon Vly, PA 19006-1807
    (215) 355-4555