This work investigates and identifies the mechanisms that are at work in the creation of instabilities during co-injection moulding. Two aspects are investigated, neither of which has been previously reported. One seeks to eliminate the instabilities, the other to control them to produce mechanical interlocking of incompatible polymeric materials. Complex rheological and thermodynamic interactions take place during the co-injection moulding of materials of different generic families, which need to be understood before successful multi material mouldings can be achieved. Moulding trials on miscible, compatibilised and immiscible polymer systems were carried out to determine processing parameter effects.\ud \ud Analysis of tensile behaviour identified differences between injection moulding and co-injection moulding samples which are indicative of different heating and cooling regimes in the systems. Scanning electron microscopy analysis also assisted explanation of these effects. A previously unobserved bulk weakness in compatibilised systems was found. Surface profilometry was used to measure the size of disturbances at the wave fronts. The extent to which the interfacial instability occurs and to the material systems to which it applies was found. Instabilities were found or induced in all material systems investigated, including those where skin and core materials are the same. Mechanisms of instability at the melt front interface were determined and were found to be the result of stratification of elastic properties. Processing conditions were found to minimize instability by minimizing differences in elasticity at the interface. A novel process route using controlled instabilities was also proposed and investigated for the use of immiscible material systems. By controlling the moulding parameters, the potential problem of instability was used to provide a solution to bonding immiscible materials in co-injection mouldings without the use of compatibiliser
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