Coinjection is a polymer injection technology in which different polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s are injected into the same mold

Mold

Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

.

Reasons for using coinjection are:

1. reducing cost by using a cheaper fill material as the non-visible core of a product
2. combining desired properties of polymers, such as colour (the differently coloured parts in car tail lights), feel (soft-touch toothbrush) or mechanical properties.

Despite these advantages, coinjection requires machinery that is more expensive and difficult to maintain than standard single injection machines. The coinjection process also has difficulty with complex geometric shapes.