The quality of extruded sheet for use in subsequent thermoforming operations may be defined in terms of its resistance to sag, low and consistent shrinkage, tensile properties and overall consistency of these properties with time. The sheet extrusion process has many variables such as melt temperature and output rate that may influence the properties of the sheet and it is the objective of this work to identify the critical variables in the process. To achieve this objective a series of experiments were conducted using a statistical design of experiments methodology. This investigated the effects of MFI, Melt Temperature, Chill Roll Temperature and Line Speed on the properties of the extruded polypropylene sheet which are of importance in the future thermoforming process. The sheet properties investigated included tensile properties, crystallinity, sheet sag and shrinkage. The main findings of the study were that melt temperature had the main effect on the shrinkage in the machine direction, while the chill roll temperature and line speed had the main effects on the tensile properties. Sheet sag was found to be greatest at the lower MFI value, which correlated to the lower levels of crystallinity of the as made sheet.
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