TABLE OF CONTENTS In selecting material combinations for multi-material moulding applications, consideration must be given to the combination of properties required in the final product. In many cases, there must be a certain level of adhesion between skin and core in order to maintain mechanical integrity. This can be achieved in two ways:
The materials are compatible and offer some degree of bonding at the interface.
A method must be found to mechanically interlock them. In the case of over-moulding and multishot this can be done with clever usage of material properties and tool design.
Not all parts require adhesion. In fact in some cases the requirement may be the exact opposite. If joints are to be produced, it is necessary that the mouldings can move freely at the interface. Examples include over-moulding to produce what will be the moving arms and legs on dolls and other similar toys or to produce ball and socket joint mouldings. In cases like these, materials must be selected for their immisciblity to ensure smooth regions of movement. Where adhesion is required, good interfacial bond strength is a pre-requisite, otherwise the properties may come from a significantly reduced section thickness. For good adhesion, a certain amount of interdiffusion is required between the melts. This can be achieved when there is a high compatibility, or solubility between the melts.
Tables of compatible and incompatible material combinations are readily available from both machine and materials suppliers, such as the one shown in Table 10.1. However caution...
