Q. I am a materials engineer for an auto parts manufacturer. We supply plastic components to the industry, so I have a lot of experience in molding and painting plastics. You are usually right on the money, and your response to the person having solvent attack on his ductile plastics was close. He also has a number of other options.
Polycarbonate (PC) is the most prone to attack and nylon the least. There are a number of things he should do:
He should increase both the molding temperature and pressure. This will reduce the tendency for the parts to stress. Although this will slow the process, it is better than scrapping parts.
He should also discard the first five to 10 minutes worth of parts at the daily startup. Parts that are not up to temperature will be more susceptible to stress and cause rejects as part of the process.
He should also talk to the paint manufacturer about the present solvent blend. Esters are the most aggressive solvents, followed by aromatic hydrocarbons and then ketones. The lower the molecular weight of the solvent, the more aggressive it is, but also the shorter period it is in contact with the plastic because of its faster evaporation rate. Alcohols and ethers are not aggressive at all, nor are aliphatic hydrocarbons, but these are usually not compatible with the resin in the coating system.
Changing to a water-based system is also an option (because of the above), but going through the approval process for a new system is often difficult, if not impossible.
One other thing which I assumed was well-known but may also be an issue: parts, especially PC and ABS parts, should be aged a minimum of 24 hours and preferably 48 hours at 80°F prior to painting to relieve stress. If parts need to be painted sooner, they can be stress-relieved for one hour at 176°F, two hours at 158°F, four hours at 140°F, four hours at 122°C, etc. This time of the year cold storage of parts causes a lot of issues, as the reverse is true. If parts are stored at 50°F, you need to wait for 96 hours before painting, and at 32°F you need to wait for 192 hours. Below this temperature, don’t bother to paint them. I just had this issue come up again on some test parts that were painted right after molding in one of our plants, so I thought it might be worth mentioning.
Barrier coats are also an option, but add cost and also are tough to do because of production line configurations. A polyvinyl butyral wash primer at 6-10 microns or epoxy, which is usually better with the nylon applied at 8-12 microns, are normally used. I hope this helps. B.R.
A. B.R. is referring to a question by B.U. in the January 2012 Products Finishing concerning solvent attacks on plastics that reduces the impact resistance of the painted parts. In my answer to B.U., I stated, “Although structural plastics are a little removed from my area of expertise, my guess is that the organic solvent in the paint is attacking the plastic surface.” At least I got part of the answer right. Furthermore, I know that in today’s market, most people would want to paint immediately after molding.
Q. Unfortunately, that is the tendency. In doing so, they waste paint, parts, time and money. Another bad practice is the tendency is to turn down the heat at night or store parts in an unheated warehouse in the winter. This is penny-wise and pound- foolish. B.R.
A. Thanks for reading Painting Clinic, and thanks for the excellent suggestions. I learned something about molding and painting plastics.