This manufacturing process is often used to produce plastic components, but for rubbers, the temperatures are switched. A warmed rubber is injected into a hot tool, as the force required to inject uncured rubber is much greater than what's required to push molten plastic into a chilled mould. Otherwise the equipment and principles remain similar.
Uncured rubber feedstock is fed into the screw of the injection machine (normally in the form of long coiled strip). The screw rotates and is forced upwards to generate a precise volume (or shot) of rubber at the front of the screw. Once the tool is closed in the press, the screw is pushed downwards to inject the rubber through the sprues and gates, and into the cavity in the tool. The tool always remains in contact with the heated platens of the press and therefore retains the necessary heat for the curing reaction. The temperature of the shot prior to injection is tightly controlled to allow the rubber to flow easily, without the vulcanisation process progressing to the point where the material is too elastic to properly fill the cavity. Alongside automated de-moulding of the parts (via brushes or robots) this reduces the press open time. The entire moulding cycle can be very fast; potentially under 60 seconds with small cross section parts.
Elastomer materials need to be specifically formulated for injection moulding to give them the required flow characteristics. The tooling is also significantly more complex compared to compression or transfer tools, especially with multi-cavity tools and cold runner blocks to reduce the waste in the sprue and runner system. Set-up times somewhat longer.
In terms of machinery, injection presses are also more complex and costly than hydraulic compression presses, but the injection moulding process can be highly automated. This, in conjunction with fast cycle times can result in very attractive manufacturing costs for parts produced in high volumes. As with the transfer process, the flow of the rubber into the cavity can be designed to reduce the risk of air trapping, and give consistent material properties in the finished part.