Powder coat, paint, chromate conversion, or anodizing, are common finishes applied to die cast parts to improve their performance or appearance. In some die casting applications, components must also be pressure tight to hold pressurized fluid or gases. Companies use vacuum impregnation to meet these requirements by sealing the internal leak paths without impacting any other features of the casting.
A common question asked about vacuum impregnation is “Should vacuum impregnation be done before or after finishing?”
Should Porosity Be Sealed Before or After Finishing?
The general rule is that vacuum impregnation should be done before any surface finishes. This will seal the porosity and eliminate any failure mode that could develop from outgassing, chemical compatibility or bleed out of pretreatments. Below are examples of failure modes that can occur if impregnation is done after the finish is applied.
The painted part will be exposed to sealant, and certain minerals and alkalis in the water. The sealant can react with paint and degrade the quality of adhesion of the paint with the part surface. During impregnation the painted part is heating to 195 F in hot water. The water or residual minerals in the water may leave water spots, or in the worst case, alter the color hue in color or degrade the finish (Image 1).
Image 1: The water or residual minerals in the water may leave water spots, or in the worst case, alter the color hue in color or degrade the finish
Many chemical finishes require aggressive liquid pre-treatments for the finish to ‘bond’ to the casting surfaces. These pretreatments may penetrate the near surface porosity and remain in the pore even after the finish is completed. During the impregnation process, the vacuum will pull these chemicals from the porosity and into the finish may lead to corrosion and a defect known as “blooming” .
If a chromate finish is applied to a part prior to impregnation, the heat required to cure the vacuum impregnation sealant (195 F) will degrade the quality of the coating. This will lead to a premature failure of the coating and be the cause of part oxidation.
Finally, regardless of the type of finish, the part can be damaged or scratched from handling and processing (Image 2).
Image 2: If not properly fixtured, the parts can shift during the impregnation process and become damaged or scratched.
Vacuum Impregnation Eliminates Powder Coating Out Gassing
Vacuum impregnation not only seals the porosity but may prevent cosmetic defects in powder coating. If not sealed, the pores would otherwise hold air. This air may expand and out gas during the curing stage. The air escapes through the powder, causing holes or bubbles, called pin holes, in the finish (Image 3). These pinholes are not only unattractive, they also allow in moisture and corrosion to damage the part. Vacuum impregnation prevents this issue from happening by removing the air and filling the porosity with sealant.
Image 3: If not sealed, the pores would otherwise hold air. This air may expand and out gas during the curing stage, causing pin holes.
What if the Parts are Machined?
If parts are machined after finishing, then the parts should be sealed after machining. This is because a machine tool may expose or open porosity when it cuts into the part's surface.
The new inter-connected porosity will create a leak path. The leak path will cause fluids and gases to leak from the casting, causing it to be non-conforming, and in many cases unusable. Unfortunately, this occurs precisely at the wrong time, since the non-conforming part has already been manufactured. All the value has been added to the non-conforming part.
Vacuum impregnation is the most effective way to seal casting porosity, but it must be performed at the correct stage of the production process. Performing vacuum impregnation prior to finishing will ensure that all leak paths are sealed while safeguarding and enhancing the quality of the final finish.