What are injection shoes?

Injection moulding of complete (‘all-moulded’) footwear or to mould a sole unit on its own – or as a ‘direct injection moulding’ process to both form and join soles to uppers – is used by many shoemakers around the world. The sole material types which are capable of injection moulding are thermoplastics – such as polyvinyl chloride (PVC) and thermoplastic rubber (TR), as well as polyurethane (PU), for ‘reaction injection moulding’.

In ‘direct moulding-on’, the lasted upper forms the lid of the sole mould, and the base of the mould is the cavity in which the sole forms. The mould must seal well against the upper at the ‘nip line’, in order that the sole material cannot leak out to an excessive degree.

Injection moulding uses a ram or screw-type plunger to force molten plastic material into a mould cavity. This then cools and solidifies into a shape that conforms to the contour of the mould. It is most commonly used to process thermoplastic polymers, although recent developments have also enabled the moulding of thermosetting polymers. Thermoplastics are prevalent due to characteristics which make them highly suitable for injection moulding, such as the ease with which they may be recycled, their versatility allowing them to be used in a wide variety of applications, and their ability to soften and flow upon heating.

While direct moulding is a recognised process in 21st century footwear production, the history of its development started with small steps in the world of chemistry some 169 years ago.