Design of experiments for the qualification of EVA expansion characteristics

Abstract

Ethylene vinyl acetate (EVA) is a highly utilised material for the manufacture of shoe soles and mid-soles due to its comfort, durability and UV stability. It is also used extensively for exercise mats, wall insulation and sports protection padding. The material can be manufactured into sheets by compression moulding or injection-moulding processes. This paper is concerned with research to gain knowledge of the behaviour of EVA during the injection moulding of shoe soles and shoe mid-soles. Though there is significant experience with the general use of EVA during the injection-moulding process this knowledge has been gained through years of experience and moulds are designed by a combination of the producer's knowledge and trial and error. With the increase in demand for customisation of mass-produced products a method for automating the design of moulds for expanding EVA products is highly desirable. When mixed with a suitable blowing agent, EVA has the remarkable ability to expand between 30% and 90% compared to the injection mould size, depending on the amount of blowing agent added. However, EVA does not expand uniformly and currently there is no precise method of predicting the expansion of components with variable thickness. This paper presents a methodology for determining the expansion characteristics of EVA coupled with results relating to the initial findings for material expansion to enable the design of a method for automatically generating mould cavity geometry.