Applying Gel Coats

Q. My company produces molded plastic parts for industry. One of our customers wants us to supply him with parts in several different colors. Is it possible to apply paint into the SMC molding process which satisfies 0.4 mm paint thickness and a minimum surface hardness of 4H?

If it’s not possible to apply paint into SMC molding, can we paint after molding respecting the 0.4 mm paint thickness with 4H minimum surface hardness? L.A.

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A. Unless a person is directly involved in the plastics industry, using letters, like SMC, to describe processes and materials can be confusing. I think you are asking me if a 0.4 mm (0.015748 inches or 15.8 mils) thick coating can be applied in a sheet molding process. Although I haven’t done it, the answer is probably yes provided the materials are compatible. A coating of this thickness is usually referred to as a gel coat. Paint is applied in the 1–3 mil range. If, on the other hand, SMC means something else to you, molds can usually be coated before the molding compounds are introduced provided the materials are compatible.

An early example of the gel coating process was the production of Chevrolet Corvette automobile body parts. However, in that case, the molding compound was applied to the mold by the hand lay-up procedure using polyester resin and glass fibers to form the structure.

It is important to remember that molded parts can always be painted if the parts and paints are compatible. Naturally, the parts must be pretreated. This involves cleaning to remove any mold release residue and depending on the substrate may also include other surface treatment steps.