Swirl Marks In The Paint

Q. We have a rather large plastic part. After painting it shows swirls that appear to be made by a brush. These marks are visible in certain areas underneath the paint. The swirls are positive and can be felt using one’s finger. It’s actually like a lattice structure in layout. We apply a primer and a high-gloss urethane topcoat.
This problem only pops up once in a while, and no one can identify the cause. Can you make any suggestions as to what might be causing this problem? Could there be a residue from the wash left on the surface or something in the molding of the part? W.M.

Since we need to use an independent testing laboratory, can you recommend one? R.A.

 

A. Recent comments by R.B. published in the November Painting Clinic may help you. Your problem could be caused by his number 5, molded-in stresses. I’ve included the important issues mentioned by R.B. below:
“...Some huge points to look at when you have these issues:

1. Operators using hand lotion on their hands then handling the parts...
2. A change in mold releases used in the plant, or operators using excessive amounts...
3. Tool surfaces not being cleaned of rust preventative) before production beginning...
4. Many resins have internal mold releases in them...
5. Molded-in stresses are the last quiet generator of scrap. While molding interior parts for a European car manufacturer, we had some strange paint adhesion issues. We sprayed the parts with a mold cleaner and the high-stress area became evident when the surface began to look like wind-blown sand dunes. We increased the mold temperature to remove the hidden stress, then made sure that this test was performed every four hours during production.”

Thank you again R.B. for your comments. They were helpful to me and hopefully will be to W.M.