The high flow material requires a special polymeric plasticizer to eliminate long term crazing of the rigid substrate. "Typically, we blend both monomeric and polymeric type plasticizers in our [Alcryn] TPE products," says Jeff Armour, Business Manager at Ferro Specialty Polymers Group. "A specialty polymeric system was used in this material to eliminate surface migration and thus minimize potential long term crazing."
The previous thermoset rubber material had a higher Shore durometer (60 Shore A) that did not seal properly and felt too hard. In the industry there were no 50 Shore A materials that existed for a tw0-shot insert overmold to ABS+PC and have a tight water seal and GM was in a no build situation according to Ferro, which was able to develop a new material that worked within a record 11 months when traditionally it takes 24 months or longer to develop and approve. The material's bond strength to ABS+PC alloy was characterized by cohesive failure, with peel strength in excess of 25 lbs. /linear (pli). The newly developed material allows incorporation of 20% regrind compared to no regrind for the previous material. Its softness also presents an image of quality. It is color-matched to Anthracite 598F and maintain a PGH gloss.
In term of the critical CLE or shrinkage parameter, Armour notes that while similar CLEs between the elastomer and substrate are indeed preferred when over-molding to minimize potential bond failure, "It is very difficult to match the CLE between a soft 50 A Shore material to a rigid 45 D Shore material." The difference can be reduced, however, by optimizing processing parameters (temperatures, pressures, hold times) and eliminating internal stresses induced into the part during manufacturing.
In addition, the soft material will shrink when exposed to elevated temperatures in the end use environment. "Knowing that a soft TPE will shrink over time, it is imperative that a cohesive bond is achieved between the two materials to prevent failure," says Armour.