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Machine Design

Soft grip overmold takes the abuse

A new engineered thermoplastic elastomer (TPE) technology lets designers put an extremely durable soft-touch grip over rigid substrates. The Estagrip TPE System from BFGoodrich Performance Materials, Cleveland, offers a number of formulations spanning both softtouch overmold and rigid, dimensionally stable substrate materials. Designers can tailor overmold designs to take the harsh abuse applications such as handles on hand and power tools often see.

Estagrip TPEs have nearly three times the tear strength, 350 to 450 pli (pounds/lineal inch), of other softtouch elastomeric materials such as styrene ethylbutylene styrene (SEBS) and thermoplastic vulcanates (TPV). Tear strengths for typical TPVs and SEBSs range from 150 to 250 and 80 to 150 pli, respectively. The material also maintains a highly resilient "soft" feel in extremely cold temperatures. The soft Estagrip TPEs adhere well to a variety of rigid substrates including reinforced polypropylene, PC, polyamides, and other rigid Estagrip substrates, without mechanical interlocks or adhesives.

The Estagrip TPE overmolding technology is available in hardnesses from 60 to 80 Shore A and is easily colored. The more rigid Estagrip TPE substrates range from 25 to 1,000-ksi flex modulus which spans the mechanical performance range of polyolefin, polycarbonate, and "wet" nylon. Parts can be overmolded by two-step or two-shot injection molding, insert molding, or simultaneously by coinjection molding.

High tech letter opener

The letter opener and cross-section help illustrate some innovative design and soft-touch overmolding tricks. BFGoodrich's Application Engineering Manager, Jerry Blayne, designed the mold to help demonstrate the overmolding process. The "knife" substrate molded first from Estagrip RS 650 exhibits little, if any, flash along the edge. This, according to Blayne, is not as easy as it might appear as the knife edge serves as the mold parting line. Blayne incorporated inventive heating, cooling, and venting techniques along with carefully machined and polished mold surfaces to help eliminate flash.

Another trick Blayne used for the design is raised lettering molded as part of the substrate body. The lettering becomes part of the shutoff around which the soft Estagrip ST 80 A TPE forms. The lettering, however, does not function as a mechanical interlock. The EstaGrip TPE system bonds with enough cohesive strength that during tensile tests the materials stress before adhesion failure at the overmold/substrate interface.

Blayne's lettering design illustrates an alternative way of adding logos to soft-grip overmolds without additional stamping or printing processes. The cutaway of the lettering also illustrates Estagrip's ability to easily flow around and into tiny features and crevasses which is said to be a problem with some competing overmold materials. During overmolding, the lettering is slightly compressed so that the overmold and substrate surfaces finish flush, with no sharp edges.

Information for this article provided by BFGoodrich Performance Materials, 9911 Brecksville Rd., Cleveland, OH 44141, (216)447-5000

TAGS: Materials
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