Porosity in castings has been an age-old problem since they were first made. Porosity can be described as a sponge-like structure, caused by factors such as oxide films, inclusions, shrinkage or gas cavitation. It can be micro, macro or totally enclosed. It is important, right up front, to state that impregnation is not intended to make good castings out of poorly cast components, ones with a gross size or quantity of imperfections. It will not increase the strength of a cracked or unsound casting. Continuous porosity extends completely through the wall of the casting. Enclosed porosity is totally contained within the wall of the casting and possibly opened up upon machining. Macro-porosity is that type that is usually visible to the naked eye and micro-porosity small, very fine imperfections that are easily corrected thru impregnation. 
  
The porosity in a casting can create several types of production "headaches". The most common is when castings are to contain or transport liquids or gases under pressure. Product leakage will occur and create function failure, whether it’s classified as micro- or macro-porosity.   It is possible that when parts are painted, both micro and macro-porosity can be impregnated with inert materials, filling the voids, and preventing entrapped contaminants from escaping. The severity of the damage to the coating will depend upon the amount of pressure forcing out the moisture, the volume, the chemical nature of the contaminant and the time interval between cleaning and painting. Gross external voids can be salvaged for cosmetic painting purposes with a two-part epoxy: that is, heated and "drawn" into the casting. They are used in a non-vacuum, localized application. 
  
What is the requirement of a good impregnant material? The ideal impregnant should be a low-viscosity liquid, without inert solvents, that will penetrate and fill the porosity and then be capable of solidification inside the pore. It must be a material that can fill the tiniest pore, when under vacuum, by capillary action without the use of pressure. Resins currently in use withstand pressures of well over 10,000 psi. They have high strength, low shrinkage and are resilient and non-brittle to withstand shock. A stable, environmentally safe material with very long pot life is a necessity since Anoplate's resin reservoir has well over $30,000 worth of impregnant. 
  
Well, now that we have determined the characteristics of what is needed, what is it that we do use? The early material, sodium silicate (water glass), does not have all of the above features. The next generation was the styrene modified polyester resin which did not meet all of the "good requirements". The third generation impregnant used at Anoplate, is the RC-80 Series produced by IMPCO of Providence, Rhode Island. This is a heat cured acrylic blend. There are both heat cured and anaerobic (polymerizes in the absence of air) acrylic resins. In testing done at Anoplate, over fifteen years ago, there appeared to be just a slightly higher "bleed out" when using anaerobic resins. Since many of the parts we do for the military, are very costly, we went with the heat cured product.  
  
We have discussed the various impregnant materials but now need to talk about the four different methods of application. In all methods the parts must be free of grease and oils. Vapor degreasing and/or alkaline cleaning, followed by an oven bake cycle to eliminate moisture is preferred. 
  
The DRY VACUUM-PRESSURE method is the one used at Anoplate. 
a) Within an empty autoclave, parts are introduced, a vacuum is drawn, and the air in the pores is removed, without an impregnating liquid present to impede the process. 
b) The liquid impregnant is introduced while the parts are still under vacuum. 
c) A pressure cycle forces the impregnant deep into the pore structure for more positive sealing.

The WET VACUUM-PRESSURE and WET VACUUM methods only differ in the application of pressure. 
a) Within an autoclave, CONTAINING THE IMPREGNANT LIQUID RESIN, parts are introduced and the air above the liquid evacuated with subsequent air removed from the immersed castings. 
b) A pressure cycle may or may not be applied.

The INTERNAL impregnation method is accomplished by placing the impregnating liquid inside the casting and applying hydraulic pressure, after capping all holes and ports. 
  
After the above impregnating cycles, the part is rinsed with water, blind holes "squirted out" and washed in an appropriate detergent, then heat cured. There are special modifications of this cure for specific resins, especially the anaerobic acrylics.

Certain methods are less costly and easier to maintain. These less expensive systems are not as efficient at sealing fine leaks in high-pressure castings. At Anoplate, using the largest autoclave on the East Coast, we strive for the highest success rate for our customers. Our 4 ft diameter, 10ft deep vessel is available to perform an added value service for our customers, coast to coast.

To discuss how vacuum impregnation can be applied to your products or if you wish to discuss any metal finishing project or application, please feel free to contact us at 315-471-6143 or by e-mail at sales@anoplate.com.