Defects visible on the surface of injection moulded articles include dark spots, gloss differences, or fogging areas, and surface wrinkling or what is known as orange peel. Usually these defects occur near the gate or behind the sharp corners of the gate area. Starting from the two aspects of the mould and the moulding process, we can identify the causes of these defects.
Dark spots appear near the gates, like dim dizziness. It is particularly noticeable in the production of high viscosity, low flow materials such as PC, PMMA or ABS. This visible defect may occur on the surface of the product as the cooled surface layer resin is carried away by the centrally flowing resin.
It is often assumed that such defects occur frequently in the filling and holding pressure stages. In fact, dark spots appear near the gate and usually occur at the beginning of the injection cycle. Tests have shown that the occurrence of surface slip is actually due to the injection speed, more precisely the flow rate at the front end of the melt stream.
Dark spots around the gate and dark spots after sharp corners are formed are due to the initial injection speed being so high that the cooled surface is displaced by internal fluids. Gradually increasing the injection speed and step injection can overcome this defect.