Development of the skin layer in injection moulding: phenomenological model

Abstract

This work explores the conditions affecting the development of the skin layer in injection moulding. The thermomechanical environment is varied by systematic changes on the operative processing parameters for three moulding geometries: an axi-symmetric tensile bar, a lateral gated disc and a two-point central gated box. The thermomechanical environments are evaluated by computer simulations, by the local bulk temperature and shear stress level. The calculation of dimensionless numbers (Cameron, Brinkman and Nahme) allows the interpretation of the main physical phenomena occurring in the distinct moulding geometries. The relationships between the imposed thermomechanical environment and the thickness of the skin layer are established. For all the mouldings the skin thickness decreases with the increase in the thermal and shear stress levels, although with distinct contributions for the different moulding geometries. The development of the skin layer is interpreted in the light of a phenomenological model involving two time variables: the time allowed for relaxation of the highly oriented melt until the crystallization temperature is reached and the relaxation time of the material. The influence of the operative processing parameters on both time variables are discussed and used to interpret the experimental variations of the thickness of skin layer.