Numerical Prediction of Epoxy Shrinkage Related Problems in Ignition Coils

While assembling ignition coils for gasoline engines, Epoxy in liquid form is poured into a plastic cassette and is allowed to cure from a high temperature. This assembly sequence is very common in many ignition coil products as epoxy resins are used to protect, insulate, and conceal circuitry, components, and devices against environmental and severe operating conditions. This curing process causes shrinkage of the epoxy and subsequently warpage of the cassette. For optimal performance of the coil and also to have proper assembly with other components, it is essential to retain the flatness and planarity of the cassette and minimize warpage. Prediction of the warpage and auto-correction at design to nullify the effect of warpage could result in significant savings in terms of product quality. Since the ignition coil assembly has numerous materials having dissimilar properties, calculation of thermally induced stress in various components is very challenging.

This paper describes about numerical modeling of the epoxy shrinkage process and thereby prediction of cassette failure due to thermal shock. The paper presents a systematic methodology to conduct failure analysis and develop high fidelity simulation models. Epoxy stresses are considered as it undergoes traditional cure shrinkage. The predicted results are compared with experimental data conducted on the test bench. The comments on the fidelity of these models and the applicability of models to develop alternate design concepts are discussed.