Investigating the bond strength in micro insert moulding

Abstract

Micro insert moulding (µinsert moulding) is a cost efficient manufacturing process capable of fabricating multi material micro parts. It is a combination of micro replication (µreplication moulding) and micro assembly (µassembly) and involves moulding a polymer over a micro 'insert'. Three out of four works reviewed on µinsert moulding considered the bond strength as a quality criterion. A critical appraisal of works relevant to the bond strength in µinsert moulding reflected that the effects of all the relevant design considerations on the bond strength were not known. Amongst these was the aspect ratio of the insert embedded in the polymer as well as the shrinkage of the polymer. These were to be investigated experimentally along with a new 'bond enhancing' treatment for the insert as well a new polymer material. A discussion on the theoretical effects of the physical mechanisms relevant to the bond strength in µinsert moulding helped derive the relevant experimental factors. A sand blasting treatment was selected as the new surface treatment. This was applied to carbon steel wire and the effects of this treatment on the roughness of the insert were characterized using image metrology software. A mould was successfully designed and fabricated in order to carry out experiments. Simulations of cavity filling were done subject to the used process parameters. The new material tested was ethylene-vinyl-acetate (EVA). Acrylonitrile-butadiene styrene (ABS) and POM (polyoxymethylene) were also investigated. The sand blasting treatment indicated statistically to increase the bond strength for all materials. Cross sections of the polymer/insert interface were taken in order to observe the interlocking. It was visually evident that the interlocking for sand blasted inserts was superior to that achieved by untreated inserts. A high embedded insert aspect ratio indicated statistically to result in poor bond strength. This was noted from the effects of the embedded insert length. This was attributed to the possibility of polymer hesitation and was observed under a microscope. The effects of the shrinkage of the polymer on the bond strength were discussed analytically.