On Defect Generation and the Appearance of Injection-Moulded Polymers
This thesis addresses the generation of some important manufacturing defects associated with intrinsic limitations of the injection moulding process. These include bulk-surface structural discontinuities, particularly air traps and welds, bulk-surface compositional inhomogeneities and gloss and colour variations. These defects can limit the performance of the mouldings during service and they can affect their appearance quality.
A novel modelling approach (the distance model) has been used to simulate the advancing flow front during mould filling and to predict the location of welds and air traps in polystyrene mouldings. The simulations using the distance model indicated that the movement of the advancing flow front and the potential location of welds and air traps could be predicted with good accuracy. The distance model was also found to be particularly useful for showing the repositioning of welds by changing the wall thickness and gating scheme for a relatively simple mould cavity with different wall thicknesses and a central core. The molecular composition at the surface and in the bulk of these mouldings was studied using time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The results of the TOF-SIMS analysis indicated not only the segregation of additives (paraffin waxes) at the surface of the mouldings during processing but also that segregation of additives at the butting-weld interface for polystyrene is not the main cause of the structural weakness of these defects. A relatively new process methodology has then been applied to study the effects of processing on the appearance characteristics (gloss, colour and surface topography) of mouldings made of different pigmented polypropylenes. The effects of mould texture and pigment content have also been investigated. Differences in visual appearance at the surface of mouldings were associated with the degree of replication of the mould surface topography. The gloss and colour measurements indicated, for example, that the gloss of polypropylene can change significantly during processing and that the filling rate and the topography of the mould surface play a significant role. Correlations were also observed between the colour coordinates and the gloss. Thermal ageing effects and processing effects on the tensile and impact properties and on the gloss and surface topography are also discussed.