We are a die caster who has some of our castings powder coated. We are experiencing an adhesion problem with some of the castings we supply to a certain customer. The castings are all run with the same process parameters, but some of the castings do not pass a crosshatch test and some from the same batch do. We have obtained samples of this problem, and I am hoping that you can provide us with the name of a lab that could answer what may be on these castings (if anything) or what process is adversely affecting the adhesion. P. Z.
Analytical laboratory testing, in these cases, can be very expensive for the perceived benefits. However, a simple test can be performed at your powder coater to determine the same result. Take a part ready to be coated and run water over the surface. If the water sheds from the part in a sheet, then the part is clean of organic soils. If the water sheet “breaks," the surface has organic soils on the surface. This is called the “water-break-free” test and is well documented and used frequently in our industry. A surface that actually allows the water to bead on the surface is the worse case failure of this test.
The next test is called “white towel” testing. Simply wipe the part using a white rag and inspect the rag’s surface for discoloration wiped from the surface. I normally use my handkerchief (to the chagrin of my wife who must try to wash the stain out of my handkerchief). This discoloration shows that there are inorganic soils on the part’s surface. This test is also well documented and frequently used in our industry to determine the presence of inorganic soils on the part surface to be coated.
Only substrate surfaces that are completely free of both organic and inorganic soils will allow for good powder coating adhesion. Often, die-cast parts can have mold release or other organic soils on the surface that cause adhesion problems. Depending upon how you apply the mold release agent (manually or automatically), the amount of release agent can vary from part-to-part, even in the same batch. This is the most common cause of poor adhesion on die cast parts. Do the water-break-free test to verify the problem and ask the custom coater to discuss how to remove this soil with its pretreatment chemical supplier. A small chemical modification in the pretreatment system may solve your problems without expensive analytical laboratory testing.
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