| Summary: | The overall aim of this work was to relate the molecular structure of injection-moulded PMMA with the levels of residual stress and processing conditions with levels of residual stress for extruded sheet. Experimental work was conducted on an experimental copolymer, supplied by ICI Plc, containing both methyl methacrylate and ethyl acrylate. Injection-moulded samples were manufactured varying in terms of molecular weight and percentage of ethyl acrylate. The influence of roll temperature and line speed during extrusion on levels of residual stresses and orientation were also studied in samples extracted from both the longitudinal and transverse directions of the extruded sheet. Residual stress distributions were calculated using the layer removal technique, birefringence measurements and in extruded samples, initial curvature measurements. Shrinkage measurements were also taken to identify the relative degree of orientation in the materials. Properties such as tensile and flexural modulus, together with tensile strength were measured. Overall, injection mouldings exhibited a parabolic residual stress distribution, with compressive stresses on the surface and tensile stresses in the core. Increasing molecular weight and decreasing levels of ethyl acrylate led to greater residual stresses being present in the material. Extruded material, however, contained tensile stresses on the surface and a less symmetrical distribution overall. The magnitudes of residual stress were lower than found in injection mouldings and lower in the transverse direction of the extruded sheet. Overall, lower roll temperatures and faster line speeds led to increased residual stresses, shown by initial curvature, layer removal and birefringence measurements. However, higher roll temperatures and slower line speeds led to increased orientation in the sheet as shown by shrinkage measurements. Thus, processing conditions during extrusion had opposite effects on residual stresses and orientation.
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