Impregnation Solves An Old Problem
Since metal casting was first discovered, porosity has been a problem. Porosity is a sponge-like area in an otherwise sound metal part. It can be caused by internal shrinkage, gas cavitation, oxide films, inclusions, or combinations thereof.
Though porosity can be found in any type of metal casting or part, it 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.
Over the years, various methods to fill porous openings in parts designed to contain liquids or gases under pressure have been tried. One of the first materials used was 'water-glass', or sodium silicate. It had limited success. Other materials such as tung oil, linseed oil, pitch gum, were also used with little success.
Shortly after World War II, thermosetting plastics 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.
Impregnation of metal castings and powdered metal parts means sealing the leaks resulting from porosity. The impregnating material is introduced as a liquid into the voids, or porosity, within the wall of the part, typically using vacuum and pressure. The material is then solidified, filling the porous openings and making the part pressure tight.
There are two general classifications of porosity found in metal parts: macro-porosity in the form of large flaws that may be visible to the naked eye; and micro-porosity in the form of very small, almost invisible, voids. In powder metal parts, the structure of the metal results in a condition similar to macro-porosity in low density castings, and micro-porosity in high density castings.
Porosity can be found as continuous, blind, or totally enclosed. 'Continuous' porosity extends 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 completely 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 allowing leakage.
Modern impregnation technology permanently seals porosity leaks caused either by micro or macro-porosity on all ferrous and nonferrous metals, including die castings, sand castings, investment castings, pressure castings, powder metal parts, as well as forgings or weldments. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metal, and alloys of these metals, can be impregnated. Nonmetallic materials, such as wood, plastic, and ceramics, can also be impregnated
If castings have blind or continuous porosity, impregnation before painting or plating improves and protects the final surface finish from bleedout and blistering.
That is also true with powder metal parts. Impregnation not only seals powder metal parts for pressure applications, but also improves plating or finishing because bleedout and spotting due to entrapment of plating solutions in the pores are eliminated.
When porosity in a metal part causes leakage, 'bad' parts are often sorted out by testing and inspection. The 'good' parts 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 break the thin membrane that keeps the blind porosity from being continuous, resulting in 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.
Another benefit of impregnation is the lubricity from the impregnant helps to extend the life of tools used to machine powder metal parts.
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 a small fraction of the costs of remelting, recasting, re-machining, and part overruns. Impregnation allows the manufacturer to save time, money, and energy, and ensures quality by salvaging parts that would otherwise have to be rejected. The elimination of scrap and rework substantially increases productivity.
Also, 100 percent impregnation of 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 powder metal parts provides the added benefit of prolonged tool life--up to 100 times--because, besides supporting the individual powdered metal particles, impregnant resins act as lubricants. Lubricity reduces the chatter present during the machining of unimpregnated powdered metal parts.
Impregnation will not distort, discolor, or damage castings in any way. Because impregnation is within the wall thickness of the casting, there is no film on the surface of the part to change any dimensional machined tolerances. There are some metals that may be slightly discolored by washing and curing operations. However, the materials used are usually designed to protect against discoloration or corrosion and allow for plating or anodizing.
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 ensure quality, rather than to be used strictly as a salvage operation.
It should be pointed out, impregnation is within the wall of the part; it is not a surface treatment and will not, therefore, improve surface flaws.
Also, impregnation will not increase the strength of a cracked or unsound casting. Under pressure, the cracks could open. Therefore, unless some other means such as welding or epoxy repair is introduced, a cracked casting cannot be reclaimed by impregnation alone.
There are four common methods of impregnation: dry vacuum-pressure, internal pressure, wet vacuum-pressure, and wet vacuum only.
1. The dry vacuum-pressure method, which IMPCO pioneered, is accomplished as follows:
- Within an autoclave, a vacuum is drawn, and the air in the pores is evacuated without an impregnating liquid present to impede the process.
- The liquid impregnant is introduced while the parts are still under vacuum.
- A pressure cycle, up to 90 psi of shop air pressure forces the impregnant deep into the porous cavities of the part for more positive sealing.
2. Internal impregnation is accomplished by placing the impregnant inside the casting and applying hydraulic pressure. This procedure is used in extremely large castings, forcing the liquid impregnant through the leak paths in the casting wall.
3. 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 sealant penetration.
After impregnation, the surface of the part is rinsed in plain water, leaving no evidence of the impregnating material on the part surface. Machined surfaces and tolerances are not affected. The liquid material in the pores is cured by the application of heat.
The United States Department of Defense has established various military specifications outlining the requirements for impregnating processes and impregnants. To meet the standards for pressure-tight castings, the impregnant must be capable of penetrating and filling the porosity, and then solidifying completely within the porosity.
The impregnant should be a low viscosity liquid, containing no inert solvents, no filterable solid materials in suspension, and create no gaseous or liquid by-products during curing.
These properties allow the impregnant to penetrate the tiniest openings and deepest recesses of porosity by capillary action. Capillary forces are important because it is sometimes not possible to push-in the impregnant using hydraulic pressure alone. An impregnant should also be stable, have a long pot life, be easy to handle, and test without introducing unacceptable health and safety hazards in the work environment.
There are several classes of impregnants, each with different characteristics and uses. IMPCO Inc. has engineered impregnation sealants for these classes and to meet customers' exacting requirements.
These sealants provide a permanent seal that will last as long as the casting. They are compatible with ferrous and non-ferrous materials and require no special test or modifications for different alloys.
1. Polyester Resins
RC-80 Series - Specially formulated polyester of 100 percent solids, designed for use with all ferrous and non-ferrous alloy components. The 'Quick-Break' capability of RC-80 Series responds to the need to filter solids in rinse water and control contaminants in waste water.
PowRseal - A one-part formula fully initiated at time of manufacture. Customers no longer must inventory hazardous catalyst, mix and blend additives like stabilizers, and accelerators. Nor is an activator stage needed.
IPM H/T - Impregnation of powdered metal porosity with this formula can provide longer tooling life. This one-part formula, when combined with the correct process, offers complete control of bleedout. Superior rinsing repels resin build-up on baskets and ensures excellent plating or coating adhesion to surfaces.
2. Sodium Silicate
X900 Series - Is an inorganic sealant used where high temperature is encountered.
These two part materials are used when macro-porosity locations are identified or predicted. It is used in a non-vacuum, localized application.
Parts impregnated with IMPCO resins will withstand operational temperatures from -80°F up to 450°F. In some applications, these resins have withstood temperatures greater than 2000°F. The resins are formulated for high-strength, and are resilient and non-brittle to withstand physical shock. The polyester resins have withstood pressures in excess of 10,000 psi. They are highly resistant to fuels, oils, alcohols, glycols, solvent salts, mild acids freons, and other chemicals.
IMPCO is a unique company with over 45 years of experience in impregnation technology.
The company can provide all necessary materials, equipment, and service for a manufacturer to establish in-house impregnating operations. Complete systems are custom engineered to fit customers' production requirements. There is no limit to the size of castings that can be impregnated because special techniques are used to impregnate castings too large to fit into a vacuum-pressure vessel.
Their equipment features automatic control panels, modular design to ensure minimum floor space, and layout flexibility for efficient production, and fully automated operation according to specific requirements. On-going technical assistance ensures equipment purchasers of the highest quality in-house impregnation operations.
But that's not all. Besides IMPCO's impregnation facilities at Imtech of California in Santa Fe Springs, and IMPCO's headquarters in Providence, RI, there are many impregnation job shops throughout the country featuring IMPCO's process and sealants. All these facilities offer fast turn-around, quality service, and a very competitive price. Combined, Imtech and IMPCO form the largest, non-captive impregnation service in the United States. They offer complete cleaning, degreasing, and impregnation services to a wide variety of manufacturers.
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