The effects of wall friction on the ejection of pressed ceramic parts

In this paper, the stress required to eject a pressed ceramic part from a rigid die is investigated with reference to the frictional forces that occur between the powder compact and the die walls. Results obtained from the ejection of an agglomerated alumina powder compact from a cylindrical hardened stainless steel die are reported. Different types of ejection profile are found for alumina compacted in either a zinc stearate lubricated or unlubricated die. Unlubricated compacts are shown to produce a 'stick-slip' effect during ejection, the magnitude of the fluctuations depending upon the die wall frictional forces. The influence of the interfacial friction forces upon the ejection stress is investigated by studying the effect of the compact aspect ratio, the applied compaction stress and the ejection speed for lubricated and unlubricated die walls. The maximum ejection stress is found to be directly related to these frictional forces, and increases with an increase in the aspect ratio and the compaction stress, and in the absence of lubrication. The variation in the compact-die wall friction coefficient during ejection with the state of the die wall lubrication and the aspect ratio is investigated by measuring the stress transmission generated in the die compaction process, and calculating the subsequent residual radial stresses normal to the die wall. The value of the friction coefficient is shown to be dependent upon these normal stresses. (C) 1998 Elsevier Science S.A. All rights reserved.