Still More About Painting Plastics
Q. I saw your answer to the question by J.C. about ABS telegraphing in the March issue. There is a high probability you are right on the money that it is a molding issue. The temperature or pressure (or both) needs to be increased to make the plastic less subject to stress. The stress then comes from the solvents in the coating and reducing solvents. Esters cause the most stress, then aromatic hydrocarbon, then ketones. The higher the molecular weight of the solvent, the less it is considered aggressive and inclined to stress, but you also have the issue that the slower that solvent will evaporate and the longer it will be in contact with the plastic. Everything depends on how much total film is required to meet the specification and how much air the customer moves in the spray booth to dry the surface. If the coatings manufacture can not adjust the reducing solvent and eliminate the aggressive solvents, it is normally best to go with fast solvents that flash of quickly and put on a thin coating, and allow it to flash off and set prior to a second coat. It will then act as a barrier coat to protect the surface from the final application. More total film in one coat usually makes the situation worse. As a rule, I suggest they try to put on 1/3 with the first and 2/3 with the second coat in nice, smooth applications for the best appearance. B.R.
A. This subject has taken on a life of its own. Thanks for more excellent suggestions, B.R. I appreciate you taking the time to read, clarify and answer these problems.
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