Injection moulding plastic substrates

Self-adhesive silicone rubbers do not require a primer and, therefore, the plastic substrate can be injection moulded on the same machine as the silicone rubber. 

A silicone rubber can be bonded to a solid substrate if it is clamped into or onto it. A very good example is the baby soother. The silicone nipple is injection moulded by a common LR process and then post cured. After that the nipples are clamped into a plastic construction or even overmoulded by thermoplastic. 

The most commonly used group of plastics in this class are polyurethanes which are resistant to most solvents, with the exception of n-methyl pyrollidone which swells the substrate but improves adhesion. RIM (reaction injection moulded) polyurethane finds use in the automotive sector for bumpers, spoilers and external trims. Polyurethanes are affected by heat because of their cellular structure, often craters or pimples are formed in the coating as a result of the substrate s porosity. Hence components are often manually prepared with primer filler, pore filler and several de-nib stages prior to the application of a topcoat. In recent years this high labour requirement has resulted in the replacement of polyurethane by alternative plastics. 

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