Powder Coating Castings: Outgassing

One of the major challenges in powder coat painting of cast parts, whether it is a sand casting or die casting, is outgassing. Gases may become trapped during the casting process or formed when the molten metal is cooled. During paint application, these gases are sometimes released which results in pinholes or bubbles (outgassing) on the surface of the part. These defects can cause the parts to fail the ASTM B-117 700 hour salt spray test.

In order to overcome this obstacle, the root cause must be identified. The quality of the metal being used and the care given during the casting process is directly related to the amount of gas that is entrapped. Surface contamination may also lead to results that appear similar to those of outgassing. When solvents or mold release agents are not properly removed, they can vaporize during the powder coat painting process and create tiny pinholes in the finished coating.

Some of the methods used to eliminate outgassing are not always dependable; therefore, this phenomena continues to be a problem. However, some precautions may be taken to reduce or eliminate outgassing.

Communicating and working with the casting supplier to add chills or vents can reduce the amount of gas that becomes entrapped. Switching to higher quality materials is another effective method.  

If the ingredients of the powder are not closely controlled, an outgassing additive can be mixed in with the powder. This will delay the solidification of the powder and allow gases that are trapped in the casting to escape without causing the unsightly pinholes and bubbles on the surface of the part. Finally, having the parts sealed through a high-pressure impregnation process will also help prevent this issue.

Experienced associates, process monitoring, and continuous improvement techniques have helped Millat Industries achieve the desired cosmetic quality our customers expect.