CHARLOTTE, N.C. (March 30, 3:40 p.m. EST) — In-mold assembly and in-mold decorating are attractive to U.S. injection molders looking for a way to compete globally, according to two speakers at the Society of the Plastics Industry Inc.'s Structural Plastics Division conference.
Faced with losing work to China and other low-cost countries, what can domestic molders do? “We can re-engineer. We can start working on how can we become more efficient,” said Hermann Plank, product manager for technology solutions at Tech Group Inc. of Scottsdale, Ariz.
Plank said consumer products and health care are the main end markets for in-mold assembly. He said designers have to be aware of the technology, so they design products that can be manufactured that way.
In his SPD presentation March 22, Plank gave a few examples from the molding floor at Tech Group. Traditionally, a baby-bottle nipple and screw-on ring and cap are molded in three separate parts, then assembled. “We have molded the nipple and the baby ring together in a two-shot process,” he said.
Tech Group uses a spin–stack process, with two injection units feeding a mold that rotates 90 degrees. At one station, one component is molded, then at the next station it can cool, be trimmed or undergo another secondary operation. The third station does overmolding. The finished part is ejected at the fourth station.
“The basis of this technology is productivity improvement,” Plank said.
In another example, Tech Group insert molds a medical needle in one station, then applies a silicone coating, puts a cap on the needle and does final packaging at other stations.
Using in-mold painting, Tech Group molds complete lenses for cell-phone viewing windows. The lens is molded, covered with a scratch-resistant coating, then removed from the mold. Because the part still is warm and remains inside the mold, there is no need to reheat the part when applying the coating, and no chance for overspray.
Advanced molding technology also reduces cost, by cutting the number of molds and injection presses required to make a part, and slashing cycle times. Plank cited an ink-cartridge job now in its third generation that uses in-mold assembly and overmolding.
Larry Doyle, marketing manager of machinery maker Demag Plastics Group in Strongsville, Ohio, focused on in-mold decorating through a single injection unit.
Doyle showed a video from a February open house at PGAM Advanced Technologies AG in Auburn Hills, Mich., which featured a 350-ton DPG injection press making an automotive part by molding a plastic backing together with a cloth exterior. A robot picks up the fabric and lays it into the moving half of the mold, then comes back to remove the molded part off the stationary side. Finished parts are ejected off the stationary side, reducing the chance of damage to the textile portion.
Textile molding requires lower injection pressures and very even pressure, to fill out the part smoothly and keep the cloth from moving around. Sequential valve gating reduces pressure on parts with long flow lengths and removes knit lines, which can show through on the fabric.
Other examples of in-mold decorating include molding stamped-foil decorations onto cellular phones and putting special films that glow on the instrument panel.
Some car interiors now sport an actual wood grain, back-molded with plastic, for a more realistic look of wood.