Rubber to metal bond failure during the manufacture of concentric bushes

Abstract

The research is concerned with bond failures found during the manufacture of rubber-to-metal bonded bushes, the bond being formed using a polymeric adhesive intermedium and completed concurrently with the vulcanisation of the bulk rubber. Failure occurs when stresses developed as the composite parts cool after moulding act on the comparatively weak "hot" bond. Three techniques are utilized for measuring the shrinkage stress i) a finite element method which shows the distribution of the direct stresses, ii) an empirical calculation which indicates the average direct stress acting on the bond, iii) an experimental method serving as a confirmation of the theoretical approach. The main factors influencing the hot bond strength immediately after moulding are identified and include:- Rubber hardness Rubber moisture content Adhesive type (reactivity) Heat input during vulcanisation These are quantified using a specifically developed test-piece. Bush cooling characteristics are studied to allow a direct comparison of shrinkage stress with bond strength and to predict the theoretical likelihood of bond failure. The research shows that, under ideal conditions, only in a few products is the shrinkage stress sufficient to cause bond rupture. Bond failure rates are greater than the theory suggests and the main causes are identified as being due to lack of process and management control on the shop-floor. Recommendations are made as to methods required to obtain better control. A new adhesive was evaluated and introduced into the company giving potential for a 40% reduction in oond failure levels with a cost saving of £50,000 p.a.