SURFACE 'MARBLING' IN MINERAL FILLED NYLON: ORIGINS AND SOLUTIONS

摘要

The surface appearance of an injection moulded thermoplastic article is often critical, especially when it cannot be painted. Sometimes, when high levels of mineral filler are used in the resin, unattractive surface markings appear around the gate. This is known as marbling or gate-marking. In this study we show that the rough marks appear because the polymer skin which forms at the surface of the cold mould is stretched and oriented by the shear forces from the moving polymer melt. We have linked the phenomenon to a non-isothermal flow model. The occurrence of marks is seen to correlate with a critical shear force which, in turn, is influenced mainly by melt viscosity and shear rate. We give evidence that this critical force depends on temperature, mineral loading, resin melt flow rate and injection speed. Good surface finish is associated with slow rates of skin formation (low thermal diffusivity), low shear forces (melt viscosity and/or shear rate) and good skin integrity (high tensile strength).