How plastic injection moulding works: step-by-step guide
A clear, jargon-free explanation of the injection moulding process, from raw plastic granules to finished component. Covers materials, tooling, the machine cycle and quality control.
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The process works by melting thermoplastic granules and injecting the molten material under high pressure into a precision steel mould tool. The material cools and solidifies in the shape of the mould cavity, and the finished component is then ejected. This cycle can be completed in seconds, making injection moulding ideal for high-volume production.
Molten thermoplastic material is injected under high pressure into a precision-engineered steel mould tool, where it cools and solidifies into the exact shape of the cavity. The mould opens, the part is ejected, and the cycle begins again - often in under 30 seconds.
At Broanmain, we have been refining our injection moulding processes for over seven decades. Our Operations Director trained at one of only a handful of specialist plastics engineering institutes in the world.
The injection moulding process
From raw pellets to finished component in five precisely controlled stages.
Material Preparation
Thermoplastic pellets are dried to remove moisture (2-8 hours depending on polymer), then loaded into the machine hopper. Moisture causes defects like sink marks, voids and surface blemishes.
Melting & Injection
A heated barrel with a reciprocating screw melts the pellets into a homogeneous molten mass. The screw then acts as a plunger, injecting material into the mould cavity at 500-1,500 bar pressure.
Cooling & Solidification
The mould is held at a precisely controlled temperature via water-based cooling channels. Cooling accounts for 60-80% of total cycle time. Temperature control is critical for dimensional accuracy.
Ejection
Once cooled, the mould opens and ejector pins push the component out. Parts may land on a conveyor, into a collection box, or be picked by a robot arm. The cycle then repeats.
Quality Inspection
Components are inspected at defined intervals during production - dimensions, surface finish, colour, weight and function. Our ISO 9001 system defines every inspection point.
How long does it take?
From first concept to parts in your hands - typical timelines for different scenarios.
What can be injection moulded?
Almost any plastic component can be injection moulded if designed correctly for the process.
Complex Geometries
Ribs, bosses, snap-fits, threads, textures and undercuts - all achievable with correct tool design.
Tight Tolerances
Dimensional accuracy of ±0.1mm or better for precision engineering applications.
Excellent Finishes
Mirror-polish, textured, matte or branded surfaces directly from the mould - no secondary finishing required.
High Volume
From 100 to 100,000+ components. Unit cost drops significantly as volumes increase.
Insert Moulding
Metal inserts, threaded bushes and labels moulded directly into the component during the process.
Overmoulding
Combine multiple materials or colours in a single component for grip, sealing or aesthetics.
Injection moulding vs other processes
vs Compression Moulding
Injection moulding is faster and more automated for thermoplastics. Compression moulding is the right choice for thermoset materials requiring extreme heat resistance, electrical insulation or fire retardancy.
vs CNC Machining
Injection moulding produces near-net-shape components with no material removal - far more cost-effective for volumes above a few hundred parts. Machining is better for prototypes and very small quantities.
vs 3D Printing
3D printing excels for prototyping and tiny batches. For production quantities, injection moulding is significantly faster and cheaper per part, with better surface finish and mechanical properties.