CASE STUDY No. 9506
KEY WORDS PAINTING OPERATIONS, MASKING
Delphi Interior & Lighting Systems (GM)
2915 Pendleton Avenue
Anderson, IN 46016
Contact: Stephen E. Doll, Senior Manufacturing Project Engineer. 317-641-5615. Fax: 317-641-5345
Summary
A reusable magnetic paint mask has eliminated disposable paper masking, reduced tooling expense, reduced paint waste, reduced reject rate, increased productivity.
Action
This General Motors plant molds and paints tail light assemblies and various plastic trim for automobiles. An example is the 50-inch-long piece of acrylic spanning the back end of an Oldsmobile model. The entire piece is painted except for two small rectangular areas that serve as lenses for the back-up lights, which are mounted immediately behind the acrylic cover. Edges of painted areas must be crisp and straight; areas to remain paint-free must be covered, or "masked."
Paper masks had been used in the past, but paper can be difficult to apply quickly and accurately. In addition, paper masks cannot be reused and generate large amounts of solid waste. The next solution was to clamp the parts to be painted into a frame, masking with precisely shaped pieces of metal plate that swung into position on arched rods connected to the frame. To prevent atomized paint from creeping into unpainted areas, the metal plate had to be held tightly against the plastic part, requiring more than one rod for adequate pressure. The masking apparatus created a barrier for the sprayer to work around without leaving any visible variations. A significant amount of paint landed on the masking assembly instead of the product. But the real problem was quality--25% to 30% of tail light lenses for one car line were rejected because of painting defects. A team of manufacturing engineers determined that most defects were caused by misalignment of masks caused by normal variations in the molded plastic parts, magnified by the width of parts and their curved surfaces.
The team came up with a better idea. The elaborate metal framework was replaced by an epoxy rest pad. Embedded in the pad below the lens area to be masked are GM MagnaQuench magnets. After the part is positioned on the pad, individual masks made of a reusable piece of stainless steel are placed manually over the lens area to be masked. The magnets are strong enough to pull the thin piece of steel tightly against the lens, creating a sharp paint edge. The reject rate dropped to 3% or less. And because there is less masking tooling, less paint is required per part--1.8 ounces vs. 3.2 ounces under the former method.
Payback
GM personnel were able to fabricate a model of the magnetic paint mask system from materials already available in the plant; thus, there was virtually no out-of-pocket cost to demonstrate the method. The cost of tooling to incorporate this method into production was in the range of $5,000 to $7000, and the payback period 6 to 9 months.
Additional Waste Prevented
Because the new method virtually eliminates masking tooling (i.e., rods to hold
masks against material to be painted), parts can be placed closer together for painting,
increasing production. In addition, some paint formerly lost to the sides of work now
lands on adjacent masked parts, where it belongs. And overall paint-related VOC emissions
dropped.