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

As far as products of open or closed hollow cones or cylinders are concerne d, the moulded parts grip the mould cores after mould opening because of di fferential thermal contraction. To strip the parts from the mould core, mos t injection and compression moulds have systems to eject the moulded parts automatically. The ejection force is defined as the force needed to strip t he moulded parts from the mould cores. An injection mould with changeable c avities and cores has been designed. Using different configurations of the mould cavities and cores, open (of closed), hollow, thin walled cones in va rious thicknesses, diameters, lengths, and cone angles were injected. The e jection force was measured using a tensile machine and a specially designed tool. The dependence of the ejection force on the size, wall thickness, co ne angle, elastic modulus, Poisson's ratio of the moulded parts, and the fr iction coefficient between steel and plastics are considered and a new meth od for calculating the ejection force in an acceptably accurate way is pres ented. 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 t he radius of the part; the thermal contraction strain and Young's modulus o f 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 2 alpha, moulded parts can drop down from the core automatically withou t ejection when tan alpha greater than or equal to f.