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An SMC Paint Manual has been compiled from information provided by members of the SMC Automotive Alliance. This manual reviews issues related to the painting of exterior automotive SMC panels. It discusses options available to the design engineer and materials engineer; and describes recommended design practices to optimize the compatibility of SMC body panels with existing automotive painting facilities.

Because of federal regulations defining vehicle protection and damage resistance, major changes in design, appearance and materials selection have taken place since 1973 for passenger car bumpers. These bumper protection standards have changed continually over the past 17 years, leading to a wide range of different bumper system designs, utilizing a variety of different plastic and elastomeric materials. This change in bumper system design likely will continue over the next 10 years because of increased emphasis on weight reduction, styling, aerodynamics and cost effectiveness. Truck bumpers (including vans and utility vehicles) have not been regulated for vehicle damage protection and in most cases have not changed significantly in design over the past 20 years. Over the next 10 years it is expected that most segments of the light truck market (particularly vans) will begin to experience bumper performance specifications.

Gas-assisted injection molding, “GAIN”, is a thermoplastic molding process which provides stress-free large parts with a class “A” surface and vitually no sink marks. “GAIN” is a low-pressure molding process, compared to conventional injection molding. In this process, a controlled volume of inert gas is injected into the plastic after it enters the mold. The gas does not mix with the plastic, but remains in the middle of the thicker sections of the molding. By controlling the gas pressure, the quantity of plastic injected into the mold (short shot), and the rate of gas flow, a predetermined network of hollow interconnecting channels is formed within the molded part. The gas pressure remains constant in the network of hollow channels throughout the molding. This compensates for the tendency of the plastic to shrink at the thicker areas of molding, preventing warpage and reducing stress. The gas pressure is relieved just prior to opening the mold.