Adhesion to Aluminum

Q. I run the finishing shop for my company, a commercial and residential window manufacturer. One of our lines is a residential, aluminum-framed window painted with white baking enamel. We chromate-treat the aluminum extrusions and then paint them with a thermosetting acrylic enamel. The painted aluminum extrusions are sheared to size and punched with holes where necessary. Our problem is poor adhesion (paint flaking) around the sheared areas and punched holes. We have been using the same pretreatment and paint for some time and have had trouble only recently. Can you help us? N.C.

A. If no changes have been made in materials, the problem is probably due to changes in your pretreatment or painting processes. Since most paint failures are due to inadequate metal preparation, I suggest you investigate your chromate pretreatment system first. Check the chemical solution concentration and operating temperature of each stage, as well as the processing times. Next, I suggest you check the film thickness, then check the temperature of the paint bake oven and verify the cure time of the paint on your parts. Taking these steps should show the cause of your paint adhesion failures.