Insert Moulding is a specialised injection moulding process where a pre-formed component, typically made from metal, ceramic or another plastic, is placed into the mould cavity before molten plastic is injected around it. The Plastic then solidifies and bonds to the insert, forming a single, integrated part.

Examples above include cistern components, parts for delayed action float valves, and a control panel for an electric hoist.

How does the process work?

1. The insert (e.g. metal bushing, threaded insert, or electrical contact) is cleaned to remove oil, dust, oxidation or other contaminants. Sometimes it is preheated to improve bonding with the molten plastic.

2. The insert is manually (or automatically with the use of a robot) positioned in a specially designed cavity of the mould. Locating features such as pins, recesses, magnetic or vacuum assists, hold it in place.

3. The mould closes, and molten thermoplastic is injected around the insert under high pressure. In the examples pictured, thermoplastics used are ABS, Polycarbonate and Acetyl. The Plastic flows around the insert, filling the cavity and encapsulating the insert partially or fully.

4. The mould and plastic cool, allowing the. part to solidify and bond tightly to the insert. Proper cooling is crucial to avoid warpage or gaps between the insert and the plastic. The examples above contain inserts made from brass, stainless steel and powder coated steel but many options are available. 

5. Once cooled, the mould opens and the composite part is ejected. Any excess plastic (flash) may be trimmed, and the parts undergo inspection or post-processing.