REDUCING BLISTERING IN SMC MOLDING.

Quite often compression molding produces some blistered parts. The blisters usually form when the mold opens, although some may arise during overheating in paint baking and other post-finishing. Although often mechanically sound, blistered parts are visually unappealing. Consequently, they either must be repaired or discarded. Recently, the blistering problem has become more important because of rapid growth in compression molding of automotive panels, where low defect ratios are vital to smooth operation in subsequent assembly line operations. In most of these applications, a pliable, unsaturated polyester-based sheet molding compound (SMC) is used because of its generally excellent physical properties and moldability. This paper presents test results which identify both the cause of blister formation and a number of effective methods for minimizing its occurrence.