How Impregnation Makes Porous Parts Pressure Tight

A PROBLEM AS OLD AS METAL ITSELF

Ever since metal casting was first discovered, porosity, an area of sponge-like internal structure in an otherwise sound metal part, has been a problem. Porosity may be caused by internal shrinkage, gas cavitation, oxide films, inclusions and combinations thereof. It can be found in virtually any type of metal casting or part, and is a particular problem in castings made from aluminum, zinc, bronze, iron, magnesium, and other alloys. Porosity is always present in powdered or sintered metal parts because of their structural nature.

Various methods have been used to attempt filling porous openings in parts designed to contain liquids or gases under pressure. One of the first materials used for impregnation was “water-glass” or sodium silicate. In addition to sodium silicate, tung oil, linseed oil, pitch gum and many other materials were used with little success. Shortly after World War II, the development of thermosetting plastics, to be used as impregnants, became an effective and economical means of sealing porosity within the walls of metal castings, especially when used in conjunction with vacuum pressure impregnation techniques.

UNDERSTANDING IMPREGNATION TECHNOLOGY

Impregnation in metal castings and powdered metal parts refer to the sealing of leaks resulting from porosity. The impregnating material, as a liquid, is introduced into the voids or porosity within the wall of the part usually using vacuum and pressure. The material is then solidified, filling the porous openings and making the part pressure tight.

Impregnation of powdered metal parts not only seals parts for pressure applications, but also improves plating or finishing, since bleedout or spotting due to entrapment of plating solutions in the pores is eliminated. Extended tool life is another benefit when machining powdered metal parts.

At left: 

Coated parts

Part on left was impregnated prior to coating

Part on right was not impregnated prior to coating.

Note coating breakdown on part at right.

When castings have blind or continuous porosity areas, impregnation prior to painting or plating improves and protects the final surface finish from bleedout and blistering.

Impregnation technology seals leaks on all ferrous and nonferrous metals, including die castings, sand castings, investment castings, pressure castings, powdered metal parts as well as forgings or weldments. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metal, as well as alloys of these metals can be impregnated. Other non-metallic materials, such as wood, plastic, and ceramics can also be impregnated.

IMPREGNATION IMPROVES OVERALL PART QUALITY

When porosity in a metal part causes leakage problems, “bad” parts are often sorted out by testing and inspection. The “good” parts that are sent to production are often as porous as the “bad” parts, but the porosity is blind and not completely interconnected. Subsequent machining, mechanical or thermal shock, or stress often breaks the thin membrane which keeps the blind porosity from being continuous, thus causing a “leaker”. Impregnation fills porosity from both sides preventing leaks even if the membrane does break. Therefore, impregnation improves and enhances quality, while inspection only sorts out leakers.

ECONOMIES OF IMPREGNATION

The value added to metal parts by machining, handling, and assembly may range into the hundreds or even thousands of dollars. This value is lost when a metal part is scrapped because of porosity and leaking. Impregnation costs are small fractions of the costs of remelting, recasting, re-machining and part overruns. Impregnation allows the manufacturer to save time, money, energy and insure quality by salvaging parts which would otherwise have to be rejected. The elimination of scrap and rework substantially increases productivity. In addition, 100% impregnation of metal parts sometimes eliminates the need for expensive leak testing, and often results in a dramatic reduction of field rejects in products such as transmission cases, air-conditioners, pumps and other metal parts.

Impregnation of powdered metal parts provides the added benefit of prolonged tool life (up to 100 times) because IMPCO resins serve as lubricants as well as supporting the individual powered metal particles. Lubricity eliminates the chatter effect during the machining process of unimpregnated powdered metal parts.

Because of the proven effectiveness and economies of impregnation, many engineers specify its use for all types of metal parts that must contain liquids or gases under pressure. It is now common for impregnation processes to be incorporated directly into production schedules to insure quality, rather than to be used strictly as a salvage operation.

MACRO- & MICRO-POROSITY

There are two general classifications of porosity found in metal parts: macro-porosity in the form of large flaws in the part which may be visible to the naked eye; and micro-porosity in the form of very small, almost invisible voids. In powdered metal parts, the structure of the metal results in a condition similar to macro-porosity in castings having low density, and micro-porosity in high density castings.

Porosity can be found as “continuous, blind or totally enclosed” (see diagram below). Continuous porosity stretches completely through the wall thickness of a metal part causing a leakage path. Blind porosity is connected only to one side of the part wall. Totally enclosed porosity is totally isolated within the wall thickness of a part. When castings are machined, both blind and totally enclosed porosity are often “opened up” becoming continuous porosity and causing leaks.

Modern “Impregnation Technology” permanently seals porosity leaks caused by either micro- or macro-porosity.

IMPREGNATION METHODS

There are four common methods of impregnation consisting of dry vacuum-pressure, internal pressure, wet vacuum-pressure and wet vacuum only.

The dry vacuum-pressure which IMPCO pioneered is accomplished as follows:

1.within an autoclave a vacuum is drawn, the air in the pores is exhausted without an impregnating liquid present to impede the evacuation (Figure 1)

2.the liquid impregnant is introduced while the parts are still under vacuum (Figure 2)

3.a pressure cycle, up to 80-90 psi of shop air pressure (or up to six atmospheres) forces the impregnant deep into the porous cavities of the part for more positive sealing (Figure 3). 

After impregnation the surface of the part is then rinsed in plain water, leaving no evidence or film of the impregnating material on the part surface. Machined surfaces or tolerances are not affected. The liquid material in the pores is cured by the application of heat.

Internal impregnation is accomplished by placing the impregnant inside the casting and applying hydraulic pressure. This procedure is utilized in extremely large castings, forcing the liquid impregnant through the leak paths in the casting wall.

Wet vacuum-pressure and wet vacuum only differ in the application of pressure. They both introduce parts into an impregnant bath and evacuate the air above the bath and subsequently from the porosity of the parts through the surrounding liquid impregnant. Pressure, either atmospheric or shop air is then applied to aid in penetration of sealant.

After internal or wet vacuum impregnation, parts must be washed and heated to solidify the resin.