Injection moulding works by melting plastic pellets and forcing them under high pressure into a shaped metal mould, where the material cools and solidifies into a finished part. The entire cycle, from melting to ejection, typically takes between a few seconds and a couple of minutes depending on part size and complexity. It’s the process behind nearly every mass-produced plastic item you encounter, from bottle caps to car dashboards.
The Machine: Barrel, Screw, and Clamp
An injection moulding machine has two main halves. The injection unit melts and pushes the plastic. The clamping unit holds the mould shut against enormous force.
On the injection side, a hopper sits on top of a heated barrel. Plastic pellets (called feedstock) drop from the hopper into the barrel, where a large rotating screw carries them forward. As the screw turns, the combination of friction and external heaters melts the pellets into a thick, flowing liquid. A growing pool of molten plastic collects at the front of the barrel, ready to be injected. The screw then stops rotating and drives forward like a plunger, pushing that pool into the mould at pressures typically between 10,000 and 20,000 psi.
On the clamping side, a powerful mechanism holds the two halves of the mould together so they don’t blow apart under injection pressure. Clamping force is measured in tonnes and depends on the size of the part and the type of plastic. A soft, flexible material like polyethylene might need 2 tonnes of clamping force per square inch of part area, while a rigid material like polycarbonate could require up to 8. A thin-walled container with a surface area of about 140 square inches, for example, would need a machine rated somewhere between 280 and 420 tonnes.
Inside the Mould: How Plastic Reaches the Cavity
The mould itself is a precision-machined block of metal, split into two halves that open and close like a clamshell. The empty space between those halves, called the cavity, is the negative shape of the final part. But molten plastic doesn’t flow directly from the machine nozzle into the cavity. It takes a short journey through a series of internal channels.
First, the plastic enters the sprue, a tapered channel that runs through the mould plate. The taper controls flow speed and helps prevent air bubbles from forming. From the sprue, the plastic moves into runners, which are channels that branch out to distribute material evenly. Finally, the plastic passes through gates, which are deliberately narrow openings where each runner meets a cavity. These constrictions build up pressure right before the plastic enters the cavity, helping it fill every detail of the shape. A single mould can contain multiple cavities fed by separate runners and gates, producing several identical parts in one shot.
The Six Stages of the Moulding Cycle
Every shot follows the same sequence, and understanding each stage helps explain why the process is so repeatable.
Startup. The barrel and nozzle are heated to the correct temperature for the chosen plastic. The mould is closed and its temperature stabilized by circulating water through internal cooling channels. No plastic is injected yet.
Plasticising (screw-back). The screw rotates, pulling pellets from the hopper and melting them as they travel along the barrel. Molten plastic accumulates at the front, and the growing pool pushes the screw backward. A small amount of back pressure (typically 50 to 500 psi) resists this movement, helping ensure the melt is uniform and free of air pockets.
Injection. The screw stops rotating and rams forward, pushing the molten plastic through the nozzle, sprue, runners, and gates into the mould cavity. This happens fast, often in under a second for small parts.
Holding (packing). Once the cavity is full, the machine maintains pressure on the plastic. As the material begins to cool, it shrinks slightly, and the holding pressure forces a little more plastic into the cavity to compensate. This phase continues until the gate freezes solid, sealing the cavity off from the runner system.
Cooling and plasticising. While the part continues to cool inside the mould, the screw starts rotating again to prepare the next shot of molten plastic. Cooling is the longest part of the cycle, accounting for up to 80% of total cycle time. The rest is split between mould opening, ejection, closing, and the injection itself.
Ejection. Once the part is solid enough to hold its shape, the mould opens and ejector pins push the finished part out. The mould closes, and the cycle begins again at the injection stage.
Why Cooling Takes So Long
Plastic is a poor conductor of heat. Even with water-cooled channels running through the mould, heat escapes slowly from the centre of the part outward. Thicker sections take disproportionately longer to cool than thin ones, which is why designers try to keep wall thickness as uniform as possible. Rushing the cooling stage leads to warped or dimensionally inaccurate parts, so manufacturers generally accept the wait rather than risk scrapping a batch.
The cooling system’s design has a direct impact on production speed and energy use. Channels that follow the contour of the part (called conformal cooling) extract heat more evenly and can cut cycle times significantly compared to straight-drilled channels.
Aluminium vs. Steel Moulds
The mould is usually the most expensive component in the process, and the choice of mould material comes down to how many parts you need.
Aluminium moulds start at around $1,500 and can handle 10,000 or more cycles. They’re faster to machine and work well for prototyping, bridge production, or lower-volume runs. Steel moulds cost $50,000 or more but last for millions of cycles. If you’re producing a million units or more, the higher upfront cost of steel pays for itself through a lower per-part price and far greater durability.
Common Defects and What Causes Them
Even a well-tuned process can produce imperfect parts. Three of the most frequent defects give a useful window into how sensitive the process is to pressure, temperature, and timing.
Flash is a thin film of excess plastic that squeezes out along the seam where the two mould halves meet. It happens when injection pressure is too high, the plastic is too hot, or the clamping force isn’t sufficient to keep the mould fully sealed. The fix is usually straightforward: reduce pressure, lower temperature, or increase clamp tonnage.
Short shots are the opposite problem: the cavity doesn’t fill completely, leaving the part with missing sections or voids. The root cause is insufficient material reaching the cavity, whether from too little injection pressure, too slow an injection speed, a worn check valve on the screw, or a part design with very thin walls that the plastic can’t flow through before it starts to solidify.
Sink marks are small depressions on the surface of the part, usually near thicker sections. Thicker areas cool more slowly than the surrounding material, and as they shrink inward they pull the surface down with them. Increasing holding pressure and hold time, or redesigning the part to reduce thick sections, typically eliminates them.
What Makes Injection Moulding So Widely Used
The upfront cost of tooling is significant, but the per-part cost drops dramatically at volume. Once a mould is built and the process dialled in, a machine can run around the clock with minimal human intervention, producing identical parts every few seconds. The process handles an enormous range of plastics, from flexible packaging materials to rigid engineering polymers used in aerospace. Parts come out of the mould with fine surface detail and tight dimensional tolerances, often requiring no secondary finishing at all. That combination of speed, consistency, and versatility is why injection moulding accounts for the vast majority of plastic parts manufactured worldwide.