In the June issue, in a question titled “Can We Strengthen ABS Parts?”, K.W. reported that one of his customers had complained of cracking of an ABS plastic part. His question was related to the structural strength of coatings. It was proposed that he coat the plastic part with a paint of the same color to increase its strength, prevent cracking and still maintain the same appearance.
In my answer I said, “You can paint ABS plastic parts the same color as the substrate to maintain their appearance and to prevent minor surface crazing, but no paint will increase the strength of an ABS plastic part to prevent structural cracking.”
I got responses from B.R. and C.S., who both agreed that the molder’s process parameters were contributing to the cracking problem. 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, “You are correct that paint will not improve the crack resistance of ABS. Different kinds of paints degrade plastic impact resistance to one degree or another. The question doesn’t address when the cracking occurs. It could be that some environmental exposure like sunlight or chemical resistance is contributing to the cracking. If so, painting would help by protecting the surface.
“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 or an ABS/PC blend, or go to PC.”
Thanks for the input, B.R. and C.S. Responses such as yours are always welcome at the Painting Clinic.