Two-component injection moulding of thermoset rubber in combination with thermoplastics by thermally separated mould cavities and rapid heat cycling

Abstract

Two-component injection moulding is a manufacturing process used to combine polymers with different properties within a single product. The process is often used to combine thermoplastics of different colours or to combine thermoplastic elastomers with thermoplastics to create hard and soft areas. In this study, a two-component injection moulding process is proposed for combining thermoplastics with thermoset rubbers. This poses technological challenges since rubbers require a heated mould (160-200 C-degrees) for the rubber to vulcanise whereas thermoplastics need a relatively cold mould (20-100 C-degrees) for the polymer to solidify. The mould used for this study is equipped with thermally separated mould cavities and allows to reverse the injection sequence of the two materials. It was found that the optimal sequence is to inject the thermoplastic first, followed by rubber, and that the mould temperature at the interface during the vulcanisation of the rubber is a critical process parameter. Too low mould temperatures at the interface result in long vulcanisation times and poor adhesion, whereas higher temperatures at the interface both decrease the vulcanisation time and increase the adhesion strength. However, when the temperature is too high, the adhesion strength decreases again due to gas bubbles at the interface released during the vulcanisation process.