The co-injection moulding process was studied. The experimental work from several tools, such a square plaque plate moulding has led to an understanding of the mechanism of dual injection mould filling. Emphasis has been focussed the relationships between the rheological property of the polymers and the relevant moulding parameters. The skin-core formations, which cot-relate to these relationships, were also studied. Many factors were introduced for understanding the effect on the skin-core adhesion where the two polymers are incompatible. In this case a compatibiliser was found to be one of the most important factors. In the presence of compatibiliser, the chemical reaction between active functional groups of skill and compatibiliser in the core occurred. Suitable conditions were necessary to produce good bonding between skin and core. The greater thickness of skin layer and greater . simultaneous injection times led to more probability for the skin and core active functional groups to react with each other before the skin became no-flow layer. Methods available to achieve these thicknesses and simultaneous injection times were possible by controlling the moulding parameters, such as melt temperature, tool temperature, injection speeds, and lengths of simultaneous phase-, these parameters could affect the skin-core thickness formations and their adhesion to different degrees.