Injection moulding

Injection moulding (also spelled injection molding) is a manufacturing process for producing parts by injecting molten material into a closed mould. The process is widely used for shaping thermoplastic polymers, thermosetting plastics, elastomers, and certain low-melting-point metals and glasses.

Process Overview

1. Material Preparation – Granulated polymer or other feedstock is dried to remove moisture that could cause defects.
2. Melting – The material is conveyed into a heating barrel where it is melted by shear heating and external heaters.
3. Injection – A screw or plunger advances the molten material forward under high pressure (typically 500–2 000 bar) through a nozzle into the cavity of a mould.
4. Cooling and Solidification – The mould is kept at a controlled temperature; the material cools and solidifies, taking the shape of the cavity.
5. Ejection – Once solidified, the mould opens and ejector pins or plates push the part out.
6. Cycle Repeat – The mould closes and the cycle repeats, allowing rapid production of large quantities.

Equipment

• Injection Unit – Contains the hopper, barrel, heating zones, and screw/plunger.
• Mould (Tool) – Typically machined from steel or aluminium; consists of two halves (cavity and core) that form the part’s geometry.
• Clamping System – Holds the mould halves together during injection and releases them for ejection.
• Control System – Programmable logic controllers (PLCs) and human‑machine interfaces (HMIs) regulate temperature, pressure, injection speed, and timing.

Materials
Common thermoplastic resins include polypropylene (PP), polyethylene (PE), polycarbonate (PC), acrylonitrile‑butadiene‑styrene (ABS), and polystyrene (PS). Thermosetting resins such as phenolics and epoxy, as well as elastomers like silicone, can also be processed by injection moulding when appropriate mould temperatures and cure cycles are applied.

Advantages

• High production rates and repeatability.
• Ability to produce complex geometries, thin walls, and intricate details.
• Minimal waste; excess material can often be recycled.
• Consistent mechanical properties and surface finish.

Limitations

• High initial cost for tooling (mould design and fabrication).
• Longer lead time for new part designs compared to additive manufacturing.
• Part design must account for flow, cooling, and shrinkage to avoid defects such as sink marks, warpage, or short shots.

Applications
Injection moulding is employed across numerous industries, including automotive (dashboards, interior trim), consumer goods (bottles, containers, toys), medical devices (syringe components, housings), electronics (connectors, housings), and packaging.

Historical Development
The first patents for injection moulding of thermoplastic material date to the late 19th century, with the earliest commercial machines appearing in the 1930s. Advances in computer‑aided design (CAD), computer‑numeric control (CNC) machining, and process monitoring have significantly increased mould precision and cycle efficiency in the latter half of the 20th century and into the 21st century.