The overmolding and two shot process both provide the industrial designer unique design benefits not easily achieved with the normal injection molding processes, like combining different material compositions with different colors, different hardness and different tactile feeling using elastomer to elastomer combinations or elastomer to plastic combinations. This simple process enables the attachment of one material onto another without the use of fasteners, or the application of adhesives. This paper covers a small portion of the process study Star Thermoplastics undertook to understand the injection molding variables that affect the bond strength in the overmolding process. Important information is provided that will enable injection molders to optimize the bond strengths in the overmolding process. When the molder insert or outsert molds, the designer usually employs undercuts or physical interlocks to improve separation force between the parts. However, in overmolding the two different resins are just molded together with both surfaces in direct contact with each other using the principles of chemical adhesion. Bonds between the two different materials can range from no adhesion to those too difficult to peel away without destroying the parts. Of the many combinations of substrates to overmolding materials, this paper is limited to the study of TPE elastomer overmolding onto polar substrates like polycarbonate.
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