There's a reason you won't find a photo of any products using "soft-on-soft" overmolding technology in this story. It's not that polymer maker Kraton Polymers (Houston, TX.), compounder AlphaGary (Leominster, MA), or even injection molder Slatebond (Wiltshire, UK) didn't want to cooperate. Rather their customer–whose idea it was to use a two-shot molding of different durometer materials for the handle of his high-line product–wouldn't budge. Worried that his Asian competitors might produce a cheap imitation of the product, he has kept even close-up cross-sections from being published. But the aforementioned suppliers–who are certain the technique has numerous automotive applications–walked us through the process, and its possibilities.
In its current handle application, the lower layer–a 60-durometer thermoplastic elastomer (TPE)–is encircled by a 30-durometer TPE in a high-pressure co-injection process that is free from deformation. "This gives a nice cushion grip, and a solvent resistant, durable inner layer," says Derek Fraser, AlphaGary's global business manager. "With conventional materials," he continues, "the inner layer would be very rigid and need a four millimeter-thick layer on top to get the same feel." In the soft-on-soft process, the upper layer is just one millimeter thick, which helps keep costs in check, and the strength of the part unaffected by limitations on its overall diameter. "Automotive applications that come to mind are things like soft-touch knobs, pulls and handles," says Fraser, though a graduated-layer instrument panel is another possibility. "Thermoplastics have some issues with scuff resistance," he says, "but these can be overcome through the addition of slip additives, the removal of fillers, and choosing a proper graining and finish for the instrument panel."
"The polymers are unique to Kraton," says Rolf Schrauwen, European sales manager for Kraton Polymers, "and are part of our Kraton G family of SEBS [styrene-ethylene-butylene-styrene] polymers, though the flow properties have been improved." Since both the soft upper layer and medium-hard inner layer are from the same polymer grouping, recyclability is enhanced. "Despite the two-shot process, these parts are still fully recyclable," says Fraser. "They can be reground together, and used to make other things." Even more interesting is the product's ability to be molded in almost any color–including none at all–or onto a specific area of a larger part made of compatible (i.e. easy to adhere to) material.
"We are working with European auto suppliers on silver components for the aftermarket," says Fraser, though this trend may go mainstream if interiors like the Mini Cooper's become popular. Fraser even holds out the possibility of adding a metallic soft-touch finish to a specific area on a larger part, like a door panel. "Designers now have the option of adding a graduated hardness section to a door panel–a door pull or airbag door, for example–rather than a thick layer of soft material over something hard. And," he continues, "they can do it in color."
The colors extend beyond black and metallic sliver, however. Says Kraton's Schrauwen, "Whatever color you wish, from transparent to any color you can imagine, can be made, which is quite exceptional for this class of soft materials." AlphaGary is working with medical suppliers to replace crystal PVC with transparent, soft-touch compounds of Kraton's polymers. "Even at 1/8-in. thick," says Fraser, "you can read through it." Automotive applications for this material are underway too. Just don't expect to see this material in entry-level vehicles. "As the polymer becomes more common," says Fraser, "the cost will come down." For non-clear applications, the difference is well under 20%. The clear product, however, is about 30% more expensive than crystal PVC when the density difference is included in the cost equation. "That's why, in the beginning, we expect it to be used in upper level vehicles," says Fraser. Picture that, if you can.