Die Variator Mandrel Forces Encountered in Blow Moulding Parison Control Systems—Computer-Aided Design

Abstract

In designing control systems for parison shape in the blow moulding process it is necessary to take into account the forces that act upon the movable mandrel inside the convergent annular die. These forces can be considered as consisting of two parts; firstly, a steady-state force acting along the axis of the mandrel whilst it is stationary and, secondly, an axial force Pertubation that arises as a consequence of controlled movements of the mandrel during the parison shape control process. The present paper deals only with steady-state axial forces and its aim is to assist the design engineer by showing him how to calculate mandrel forces and die pressure drops, using a digital computer, for dies having a form composed of a series of tapered and parallel annular flow passages. By careful design, under certain circumstances, an almost force-free arrangement can be obtained, so minimizing the power and strength requirements of the mandrel drive control system. The theoretical analyses used assume that the polymer being processed is described by a power-law equation relating shear stress and shear rate. This is typical of polymers such as high-density and low-density polyethylene.