Abstract
Exact knowledge of anisotropy in thermal and mechanical properties of moulded articles is important since it is directly related to undesired phenomena like warpage. In this study careful measurements of the coefficients of thermal expansion, elastic moduli, Poisson constants and linear compressibilities showed that the anisotropy of these properties was about 10%, for injection moulded polypropylene plates. A simple composite model was proposed which could predict all thermal and mechanical properties mentioned above from data of the amorphous and crystalline phases. The degree of crystallization, skin thickness and average orientation of the fibrillar precursors are the main input parameters. It turned out that although individual values were slightly over or underpredicted, the amount of anisotropy was usually predicted correctly. The only prediction which disagreed with the measurements was that of the coefficient of thermal expansion, which was measured to be larger in flow direction than in width direction.
The changes of shrinkage and shrinkage anisotropy along the flowpath were studied experimentally for different processing conditions. The most remarkable observations were that variations in injection velocity only affected width shrinkage, while melt and mould temperatures specifically acted on length shrinkage. Holding pressure, on the other hand, affected both length and width shrinkage with the same amount.
© 1998, Carl Hanser Verlag, Munich
