THE LOW-PROFILE MECHANISM IN SHEET MOLDING COMPOUND

During crosslinking, unsaturated polyester shrinks 7-10 percent, leadi ng to rough surfaces, warpage, cracks, and sink marks in some of the e arly sheet molding compounds. When thermoplastics, called low-profile additives, are added to the compound, molded parts are produced with z ero shrinkage and smooth surfaces. Many mechanisms have been proposed to explain the low-profile action, but these are mainly qualitative, a nd the best additives have been developed by trial and error. A criter ion is proposed to account for the shrinkage compensation and improved surface quality.