Q. We have a part molded in ABS that has eight bosses, 7 mm in diameter, molded into it with stainless steel inserts located in the bosses. The bosses are 1 mm thick and the stainless steel insert diameter is 5 mm. When we paint the part, we find cracks on the bosses. Can you guide us as to which paint and process is suitable for painting ABS?— J.A.
A. The solvents in the paint are probably attacking the ABS. One solution would be to apply a water-reducible barrier coat to the ABS parts before painting with polyurethane. That would protect the substrate from solvent attack.
I also received the following solutions to the problem from readers, which appeared in the September issue:
B.R. said, “ABS will be much more likely to crack if not molded at the correct temperature and pressure. The cracks do not show up in the molding process, but later when parts are processed or stressed (often this is when they are painted). They need to slow the process down a little, increase the pressure or time under pressure, and/or increase the temperature at which the part is molded. Molders process parts at as low a temperature as they can, and at as low a pressure as they can, for as short a period of time as possible. Often the problems do not show up until the solvents in the painting process stress the plastic. The cracks are a sign their process is not under control.”
C.S. said, “Other solutions might be to change the gas venting in the mold. Gas trapping can contribute to cracking or poorer impact resistance. Designing the mold itself with ribs might be a solution. Finally they might look at changing materials to a glass-filled ABS, an ABS/PC blend, or go to PC.”
To solve the problem I suggest you bring the aforementioned comments to the attention of your ABS supplier, apply a barrier coat, have your supplier use a solvent-resistant molding compound, or find a new supplier.