Injection Moulding Process: 6 Steps, Key Parameters and Machine Selection

The injection moulding process is usually described as complicated. It is not; it is unforgiving. Four variables decide whether a cavity fills cleanly — melt temperature, injection speed, hold pressure and cooling time — and the machine's only real job is to repeat them identically, shot after shot, shift after shift. Screw geometry, servo response and control resolution all exist to make that repetition possible.

A 300-tonne press running a 2.5 mm polypropylene housing will spend roughly half of every cycle doing nothing but waiting for the part to solidify. That single fact drives most of what follows: how the process window is set, which machine type suits which part, and why two suppliers quoting the same tonnage can deliver very different unit costs.

The Injection Moulding Process in Six Steps

The cycle is a fixed mechanical sequence, and every stage has a measurable duration. Clamping builds tonnage until the mould halves are locked. The injection unit then pushes melt into the cavity at a controlled speed, and holding pressure takes over at switchover, staying on until the gate freezes. Cooling follows — the stage most people underestimate — and for a 2–3 mm wall part it typically consumes 40–60% of the whole cycle, a range repeated across general moulding handbooks. The platens then open and the ejector system pushes the part clear before the sequence restarts.

Read that distribution as a budget. If cooling holds the majority share, the fastest payback usually comes from mould cooling layout and part design, not from a bigger pump. Buying tonnage to solve a cooling problem is one of the most common purchasing mistakes in the industry.

Inside the Machine: Four Systems That Repeat the Cycle

A machine is only as consistent as its weakest sub-system, and the sub-systems interact. A fast injection unit paired with a slow clamp gives no cycle benefit at all.

Injection unit

The screw melts, meters and injects. Screw diameter and length-to-diameter ratio set the shot size and the shear history of the melt; a screw oversized for the job extends residence time and can degrade heat-sensitive resins.

Clamping unit

Toggle, hydro-mechanical or two-platen mechanisms generate clamp force and hold the mould closed against cavity pressure. Platen rigidity and tie-bar spacing matter as much as tonnage once moulds become large or deep-cored.

Mould and tooling

Gate position, cooling channel layout, draft angles, ejection and venting are decided at the tooling stage. A well-designed mould on a mid-range machine will outrun a poor mould on a premium one.

Drive and control

Fixed pump, variable pump, servo-hydraulic, hybrid or all-electric: the drive determines how much energy is wasted between movements and how precisely pressure and speed can be profiled. The controller then logs the profile so it can be repeated.

Process Parameters: Where the Window Is Won or Lost

Most defects are set before the first production shot, because one parameter was changed to hide a different problem. Document the window, then change one variable at a time.

• Melt temperature: measured with a purge probe, not read from the barrel display. Too low gives short shots and weld lines; too high gives silver streaks, flashing and longer cooling.
• Injection speed profile: fast fill suits thin walls, staged fill suits parts with thick-to-thin transitions. A common starting point is 95% cavity fill at first stage, then a low-pressure second stage for protection.
• Hold pressure and gate freeze: typically 50–80% of injection pressure, held until part weight stops changing. This is the single most effective lever against sink marks and warpage.
• Cooling time and mould temperature: cooling time is usually set to roughly 20–30 seconds per millimetre of wall as a starting estimate, then trimmed by part inspection. Uneven mould temperature on the two halves produces differential shrinkage and warp.
• Back pressure and screw recovery: enough back pressure to give a dense, consistent shot, not so much that melt temperature climbs during recovery and extends the next cooling phase.

Matching the Machine Type to the Part

Machine selection should follow the part, not the other way around. The useful questions are: how thick is the wall, how many cavities, how tight is the tolerance, and how many hours per year will the tool run? Four configurations cover most of that ground.

Servo-hydraulic machines are the general-purpose workhorse. The pump only delivers what each movement demands, so energy use tracks the actual cycle rather than the nameplate. For crates, buckets, housings and general industrial parts, a well-matched servo platform is usually the lowest total-cost starting point.

HXM258-I Energy-Saving Servo Injection Molding MachineA 258-tonne energy-saving servo machine for small-scale plastic production, offering a compact, flexible and cost-effective option for diverse SME molding needs.View Product →

High-speed machines exist for a different reason: thin walls cannot wait. Filling has to finish before the melt front freezes, which means high injection rates, accumulator support and a clamp rigid enough to resist the resulting force. This is why thin-wall packaging is a machine-class decision, not a parameter tweak. HIGHSUN builds this configuration as the HXH series, with models from 280 to 450 tonnes.

HXH388 High-Speed Thin-Wall Injection Molding MachineA 388-tonne high-speed machine for thin-walled parts, combining rapid plasticizing, precise injection and automated removal to shorten cycles in large-scale production.View Product →

Large, deep parts push platen size and mould weight rather than shot size. A two-platen design removes the rear toggle, shortens the machine and makes mould changes faster on heavy tooling; the HXZ series spans 800 to 3,300 tonnes and is the usual choice once moulds exceed what a toggle machine handles comfortably.

HXZ1400 Two-Platen Injection Molding MachineA 1,400-tonne two-platen machine for large, high-precision parts such as automotive components and appliance casings, with long mold-opening stroke and lower energy consumption.View Product →

For PET preforms, PVC fittings or two-colour components, the barrel, screw, check valve and temperature strategy are all application-specific. Running PVC on a general-purpose screw, for example, invites dead spots and material decomposition during a stop.

Defects Are Signals, Not Bad Luck

Every common defect points at a specific stage of the cycle. Chasing it with the wrong parameter is what turns a two-hour setup into a two-week trial.

Keep a process log for every tool: shot size, switchover position, hold pressure, cooling time and the resulting part weight. Weight is the cheapest in-line quality indicator you have, and a drifting part weight will predict a defect before the eye does.

Cycle Time, Energy and the Real Unit Cost

Unit cost is not machine price divided by output. It is cycle time, scrap rate, energy per kilogram and downtime added together. A machine that is 5% slower but holds tolerance for a full shift usually wins that calculation.

Energy is the easiest of these to measure and the hardest to guess. Servo and all-electric drives consume power on demand, so the saving is largest on long, cooling-dominated cycles where a fixed pump would otherwise circulate oil for nothing. Measure with a meter over a real shift rather than relying on catalogue values.

Large parts change the arithmetic again. Bigger automotive parts and appliance housings carry long cooling phases and expensive tooling, so short machine downtime and fast mould change matter more than a marginal injection-speed gain. On that scale, platen utilisation and preventive maintenance schedule become cost items in their own right.

Finally, keep the process documented and transferable. The injection moulding process that only exists in one operator's memory is a production risk, not a competitive advantage.

Frequently Asked Questions