Electric-car headlights, toothbrushes and buttons on a dashboard have something in common — a manufacturing process called multimaterial or multicolor molding.
The process is fairly simple to explain. Several types of resin, or colors, are combined to make a single part. Typically, one of two injection molding methods are used — a press with two or more injection units that work either in sequence or simultaneously to fill the mold, or a process called overmolding that first molds a ``preform'' part, then encapsulates the preform with another material.
The mold, always a key factor in making a plastic part, becomes even more important in multicomponent or multicolor molding. The process can require elaborate rotating molds, or robots that move parts around.
Product designers have embraced two-component molding for common consumer products. For proof, take a close look at the toothbrush, a once-boring product category now sparkling with color and wild soft-touch designs. The process also fits with the strong manufacturing trend to eliminate assembly and handling costs by doing more in the primary machine.
But multicomponent molding is not ideal for every product, according to officials at plastic molders and a machinery supplier. A molder needs high volumes — the toothbrush is a good example — to pay for the higher cost of the molds and multiple-shot injection presses.
``You need to make a lot of parts for a long time,'' said Robert Hare, North America general manager at Ferromatik Milacron, the unit of Milacron Inc. that markets high-speed Ferromatik machines built in Germany.
``In North America it's a very rapidly growing market segment,'' Hare said. ``It's been very commonplace in Europe for many, many years. Probably the thing that had held it back in the United States is the conservative U.S. attitude. It does get rather capital intensive. The machine is more expensive and so is the tooling.''
Rob Banning, a partner of Trimax Ltd., a design firm specializing in two-component parts using thermoplastic elastomers, said a processor needs orders for about 500,000 parts a year before two-shot molding makes sense.
Even so, more custom molders are adding multimaterial and multicolor capability. Most start out with one or two major commitments from large customers, then use the service to try to pick up more business.
Miami Lakes, Fla.-based Security Plastics Inc. now runs two-shot automotive parts on three presses at its plant in McAllen, Texas — Security's first two-shot molding ever. One of the company's longtime customers, an automotive supplier, approached Security about molding two-shot buttons to supply its plant in Mexico.
Mary Ellen Thomas, marketing communications manager, said Security now molds 11 steering-wheel-mounted buttons for stereo and cruise control.
She declined to identify the customer.
Security Plastics uses an overmolding process, with two shots of ABS, to create parts with two different colors, she said. The company now is installing a new Nissei injection press in McAllen to do overmolding.
Another company, Makuta Technics, has made a livelihood out of molding automotive buttons. The Columbus, Ind., molder specializes in micromolding.
``Multimaterial injection molding, when you mention the subject, there are a wide number of ways to do this. What makes it unique is we are making these parts on injection molding machines with clamping forces of 15 tons,'' said President Stuart Kaplan. The company runs engineering materials on 16 tiny Sumitomo injection presses. Six of the machines are two-shot models.
Makuta builds its own molds, which Kaplan said is a major advantage when molding small parts.
A typical two-component mold has three parts. The A and B parts are mounted on the stationary platen. A is connected to the first injection unit, running white polycarbonate (which spells out a function on the dashboard). B is lined up with the second injection unit, running black ABS. Component C is mounted on the moving platen, lined up to close with A on the first shot.
After the first shot, the clamp opens and a sprue picker removes the sprue and runner. Then C shuttles horizontally to line up and close with mold part B. When the clamp opens again, a robot removes the parts, gets rid of the new sprue and runners and deposits the finished button on a conveyor.
Kaplan said the company's switches go on a number of cars, including Saturn and Toyota models made in North America. Makuta is looking for more work. Right now, the company is helping to develop a medical component that requires two-material expertise, he said.
Another automotive component drew Canadian mold maker Waltec Engineering Inc. into the multicolor arena. Waltec, a 100-employee toolmaker in Wallaceburg, Ontario, cut its teeth on an exotic project: a taillight mold for an electric car made by Bombardier Recreational Products of Montreal. Automotive lamps were one of the original products to use multicolor molding technology.
Waltec builds the molds in Flora, Ill., for its customer, North American Lighting Inc. of Farmington Hills, Mich.
``This is our first multicolor mold,'' said Mike Meloche, Waltec's sales manager. ``A lot of people make molds, but there are not a lot of people that make multimaterial molds. We're hoping to expand this business.''
Since its introduction as a concept vehicle several years ago, the battery-powered Bombardier NV has evolved from a golf-cart- type car for retirement communities to a niche vehicle for community policing programs. Now the company is exploring a faster model for road use.
Waltec's mold injects three colors of acrylic — black, red and clear — at the same time.
``We had to build two molds: one mold shot two different colors, and the other mold shot a third color around that,'' Meloche said.
North American Lighting makes the taillights on a 1,300-ton Meiki press.
Soft-grip mania also is driving multimaterial molding, with a flurry of new screwdrivers, can openers and toothbrushes that fuse hard plastics with rubber-like thermoplastic elastomers. St. Louis-based Trimax helps firms design two-component TPE parts.
A decade ago, design using TPEs was easy. There were only a few types of TPEs around, Banning said.
``Today there's actually greater than 100 types of TPEs that can be married with different types of plastics,'' he said. ``There's never been more options available.''
TPEs once were restricted mainly to under-the-hood automotive uses, but the wider variety has helped the material take off in everyday products. Because of the high cost of two-shot molding, insert molding remains the dominant way to mold parts that use both hard plastic and a TPE, he said. But two-shot molding eliminates some problems with insert molding, such as components cooling at different rates once they leave the mold.
Even though soft-touch products have grown, Banning said there is room for more design improvements. Some are simple, such as using bumps and other surface texturing and gating through the back of a part to get rid of a gate blemish. Both techniques would cut down the required amount of TPEs, which are fairly expensive, he said.
Mold technology continues to evolve. Ferromatik Milacron and German mold maker Foboha GmbH developed an alternative to a rotating platen. Traditionally, one way to do two-component molding is to mold one component, then rotate the platen like a turntable for the second shot. Both injection units are on the same side of the machine.
With the new system, the injection units are on opposite sides. In the middle is a center platen that spins around 180 degrees to face each injection unit alternately.
Hare of Ferromatik Milacron said the main advantage of the central rotating platen is that it uses the entire mold face for each shot, instead of just half the mold using the rotating method.
Hare said Ferromatik Milacron, based in Cincinnati, has sold a few of the new systems in North America, including one that makes automotive lenses. He declined to identify the buyers.