Calculation of ejection force of hollow, thin walled, and injection moulded cones

Abstract

As far as products of open or closed hollow cones or cylinders are concerned, the moulded parts grip the mould cores after mould opening because of differential thermal contraction. To strip the parts from the mould core, most injection and compression moulds have systems to eject the moulded parts automatically. The ejection force is defined as the force needed to strip the moulded parts from the mould cores. An injection mould with changeable cavities and cores has been designed. Using different configurations of the mould cavities and cores, open (or closed), hollow, thin walled cones in various thicknesses, diameters, lengths, and cone angles were injected. The ejection force was measured using a tensile machine and a specially designed tool. The dependence of the ejection force on the size, wall thickness, cone angle, elastic modulus, Poisson's ratio of the moulded parts, and the friction coefficient between steel and plastics are considered and a new method for calculating the ejection force in an acceptably accurate way is presented. The relationship between the ejection force and plastics properties as well as mould configuration is discussed. The calculation formula of the ejection force indicates that: the ejection force is directly proportional to the thickness and length of the part and has little relationship with the radius of the part; the thermal contraction strain and Young's modulus of the plastics impose considerable influence on the ejection force; a high friction coefficient f leads to a high ejection force; and for a cone angle of 2α, moulded parts can drop down from the core automatically without ejection when tan α ≥ f.