Lighter SMC for auto body panel applications

Continental Structural Plastics (CSP, Troy, Mich.) reports that it has achieved a substantial reduction in the density of sheet molding compound (SMC) parts, using tough new 16-micron hollow glass microspheres from 3M (St. Paul, Minn.).

Continental Structural Plastics (CSP, Troy, Mich.), a supplier of molded plastic and composite components to automotive OEMs and other industrial manufacturers, is actively pursuing weight reduction as part of the automotive industry’s quest for improved fuel economy. According to the company’s general manager of materials and R&D Probir Guha, CSP has achieved substantial reduction in the density of sheet molding compound (SMC) parts, using hollow glass microspheres, without affecting manufacturing processes or compromising product quality.

“Our recent technical advancements are based on incorporating 3M [St. Paul, Minn.] high-strength Glass Bubbles in the SMC resin matrix, which reduces density without compromising mechanical properties,” Guha explains. ”3M Glass Bubbles are generally superior in performance to other fillers due to their inherent strength and lower density, and their spherical form improves resin flow and results in potentially higher filler loading.”

While the company has achieved good results with 3M’s 40-micron microspheres as a filler in the past, the latest 3M iM30K Glass Bubbles — with a nominal size of just 16 microns, a crush strength rating of 30,000 psi and true density of 0.60 g/cc — not only reduce SMC’s density, but increases its surface hardness and abrasion resistance, limits its thermal expansion and improves the dimensional stability of finished SMC parts. “We have found that the flow properties of smaller 3M Glass Bubbles actually enhance the SMC process by improving resin flow and increasing SMC shelf life,” said Mike Siwajek, 3M’s manager of materials development. “Even after a period of months in inventory, resin formulated with the new smaller Bubbles continues to flow well and fill out molds dependably. We can achieve a weight reduction of approximately 25 percent for a given part with 3M iM30K Glass Bubbles, compared to calcium carbonate.”

In addition, Siwajek says that part shrinkage in the tool is alleviated through the use of the 16-micron microspheres, and that CSP can achieve the required dimensional control and meet the very high tolerance standards and Class A requirements for parts such as automobile hoods. The company recently demonstrated that a large sport utility vehicle hood’s weight could be reduced from 40.5 lb to just 29 lb (18.37 kg to 13.15 kg) with acceptable structural properties and finish.