A Breakthrough For Powder-primed Automotive Parts
By Staff | June 01, 2006
An OEM-led research partnership recently announced new developments in sheet molding compound (SMC) that allow SMC parts to be "powder-primed" without sacrificing Class A surface quality. Participants include General Motors Research and Development (Warren, Mich.), supplier Meridian Automotive Systems (Allen Park, Mich.), resin supplier AOC Resins (Collierville, Tenn.) and coatings supplier Red Spot Paint & Varnish (Evansville, Ind.).
Source: Meridian Automotive Systems
Powder-primed Automotive Parts
Powder priming, also known as electrostatic priming, refers to the process of applying a positive electrical charge to a body panel, attracting negatively charged paint particles to it. The technique uses less solvent and reduces overspray waste, an environmental and economic plus. However, traditional SMC parts can absorb moisture after molding. When powder primed and then sent through high-temperature E-coat and painting cycles, the moisture off-gasses through the coating, forming an unacceptable leather-like surface appearance (see comparison photos).
The partnership helped develop a solution to this problem. Jeff Robbins, director of research and development for SMC supplier Meridian, says that GM research concluded that the failure mechanism was a combination of air and moisture released when a common low-profile additive (LPA) was used in SMC. "After defining the problem," Robbins says, "GM's principal researcher and SMC authority Hamid Kia challenged us in the supply chain to work together."
Source: Meridian Automotive System
Meridian provided tooling and molding equipment for a series of production trials. AOC formulated a special, low-moisture-content version of its SMC resin, trademarked Atryl TCA, which incorporates new LPAs that still reduce shrinkage, as originally intended. To achieve even better surface quality, GM's Kia encouraged automotive plastics coating supplier Red Spot to develop a new SMC primer. The conductive primer, applied by Meridian before parts are shipped, forms a low-permeability barrier that restricts the release of any moisture from the SMC into the powder primer.
Full-size test parts molded with different SMC formulations were sent to GM assembly plants for standard powder primer application and cure. The trials validated laboratory data and ensured that SMC parts could be painted like sheet metal parts. The material has been approved by GM but as yet, has not been assigned to a particular production model.
