Injection moulding is a manufacturing process that produces plastic parts by melting raw material and forcing it into a shaped mould under high pressure. It’s the method behind nearly every plastic object you encounter daily, from bottle caps and phone cases to car dashboards and medical devices. The global market for injection moulded plastics was valued at roughly $362 billion in 2025, making it one of the most widely used manufacturing techniques on the planet.
How the Process Works
The basic cycle has six steps: clamping, injection, dwelling, cooling, mould opening, and part removal. It starts with plastic pellets fed from a storage container called a hopper into a heated barrel. Inside the barrel, a large rotating screw pushes the pellets forward while heaters along the barrel’s length melt them into a uniform liquid. Once enough molten plastic has accumulated, the screw drives it through a narrow nozzle and into the mould cavity at high pressure.
Before injection begins, a clamping unit presses the two halves of the mould together with enough force to keep them sealed against the incoming pressure. After the cavity fills, the machine holds pressure briefly (the “dwelling” phase) to pack extra material in and compensate for shrinkage as the plastic cools. The part then sits in the mould until it solidifies. Finally, the mould opens and ejector pins push the finished piece out. The whole cycle can take anywhere from a few seconds for a thin, simple part to over a minute for something thick and complex.
Key Machine Components
An injection moulding machine has two main halves: the injection unit and the clamping unit. The injection unit handles everything from melting to delivery. Its core component is the reciprocating screw, a helical shaft that rotates inside the barrel to simultaneously move, heat, and mix the plastic. Heaters are placed along the barrel to keep the temperature precise and consistent. At the barrel’s tip, the nozzle channels the molten material into the mould.
The clamping unit’s job is simpler but just as critical: hold the mould shut during injection and open it afterward. It operates on hydraulic, mechanical, or electric power, depending on the machine. The mould itself consists of two precision-machined halves, the “core” and the “cavity,” which together define the shape of the final part. A hydraulic unit powers the machine’s movements, providing the force for both clamping and injection.
Materials Used in Injection Moulding
The vast majority of injection moulded parts use thermoplastics, polymers that can be melted, shaped, and remelted without degrading. The most common choices include polypropylene, ABS, polycarbonate, nylon, and polyethylene, each selected based on the part’s requirements for strength, flexibility, heat resistance, or appearance.
One important material property is shrinkage: every plastic contracts as it cools in the mould, and the amount varies significantly by type. Polycarbonate shrinks relatively little (around 0.5% to 0.7%), while high-density polyethylene can shrink 2.5% to 3.5%. Acetal (commonly known by brand names like Delrin) shrinks even more, up to 3.5%. Engineers account for this when designing the mould, making cavities slightly oversized so the finished part hits the correct dimensions. Choosing the wrong material or failing to account for its shrinkage rate leads to parts that don’t fit.
For parts that need extra rigidity or dimensional stability, manufacturers often use filled materials. Adding glass fibres to nylon, for instance, cuts its shrinkage roughly in half and dramatically increases stiffness. Talc-filled polypropylene and carbon-filled polyethylene serve similar roles in other applications.
Mould Types and Tooling Costs
The mould is typically the single largest upfront expense in injection moulding. Two main material options exist: aluminium and steel. Aluminium moulds start around $1,500 and can handle 10,000 or more cycles before wearing out. Steel moulds cost $50,000 or more but last for millions of cycles. The choice comes down to volume. If you need tens of thousands of parts or fewer, aluminium often makes financial sense. For runs of a million or more, steel’s durability spreads that high upfront cost across so many parts that the per-unit price drops well below what aluminium can achieve.
Mould design also affects part quality and cycle time. Features like uniform wall thickness, proper draft angles (slight tapers that help the part release cleanly), and well-placed cooling channels all influence how fast you can run cycles and how consistent the finished parts will be.
When Injection Moulding Makes Financial Sense
Injection moulding’s economics are defined by a simple trade-off: high tooling costs upfront, extremely low per-part costs at volume. For a typical mid-complexity part in ABS or polypropylene, the break-even point compared to 3D printing or CNC machining typically falls between 2,500 and 5,000 units. Below that range, additive manufacturing or machining can be cheaper because there’s no mould to pay for.
The gap widens dramatically at scale. A 3D-printed part might cost around $2.50 each at 100 units and barely drops to $2.30 at 10,000, because the printing time per part stays roughly constant. Injection moulding, by contrast, gets cheaper with every unit produced as the tooling cost spreads thinner. Past 100,000 parts, per-unit costs can approach $0.20 with tooling fully amortized. That cost structure is why injection moulding dominates high-volume consumer products.
Precision and Tolerances
Injection moulding can produce parts with surprisingly tight dimensional accuracy, though the achievable precision depends on part size, material, and mould quality. For small parts in the 1 to 3 mm range, the tightest tolerance group allows dimensions within ±0.007 mm. That level of precision is reserved for tool-controlled dimensions on high-end moulds, but even standard tolerance grades achieve ±0.05 mm for small features.
Larger parts naturally have looser tolerances. A part in the 800 to 1,000 mm range might hold ±0.70 mm at a mid-range tolerance grade. Parts exceeding 1,000 mm require special agreements between manufacturer and customer because controlling shrinkage and warpage at that scale becomes significantly harder. For context, most consumer plastic parts fall comfortably within standard tolerance capabilities without requiring the tightest (and most expensive) grades.
Common Defects and How They’re Prevented
Even well-tuned processes produce defects when something falls out of balance. The most frequent issues include:
- Sink marks: Small depressions on the surface caused when the outer skin of the plastic solidifies before the interior. As the inner material cools and contracts, it pulls the outer layer inward. Thick sections and transitions from thick to thin walls are the usual culprits. Keeping wall thickness uniform across the part is the most effective prevention, along with adjusting injection pressure and cooling time.
- Flash: Thin excess material that seeps out along the mould’s parting line. This happens when clamping force is insufficient or the mould surfaces don’t seal perfectly. Worn or damaged moulds are a common cause.
- Short shots: Parts that aren’t completely filled, leaving missing sections. Insufficient injection pressure, too-low melt temperature, or blocked flow paths can all produce short shots.
Material selection plays a role in defect prevention too. Switching to a filled resin (glass-filled nylon or talc-filled polypropylene, for example) helps the molten plastic maintain its structure as it solidifies, reducing the internal shrinkage that causes sink marks. Part designers can also add ribs for reinforcement or core out thick sections to eliminate the uneven cooling that triggers most surface defects.
Where Injection Moulding Is Used
The process shows up in virtually every industry that needs plastic parts at volume. Automotive manufacturers use it for interior panels, bumper components, and under-hood parts. Medical device companies rely on it for syringes, housings, and disposable instruments where consistency across millions of units is non-negotiable. Consumer electronics, packaging, toys, and household goods all depend on injection moulding as their primary production method.
The industry is projected to reach $481 billion by 2033, growing at about 4% annually. That steady growth reflects both increasing global demand for plastic components and the process’s fundamental advantage: once you’ve invested in a mould, you can produce identical parts faster and cheaper than any competing method at scale.