Injection molding machine
it melts, injects, and holds the mould shut until the part sets

Machine overview

A horizontal injection molding machine has four units. The injection unit melts and injects. The clamping unit opens, closes and locks the mould. The drive supplies force and motion, hydraulic or electric. The control runs every movement to the set values and checks whether each shot matches the last. Safety systems guard all four.

Our article on injection molding already covers the cycle and the process variables. This page is about the machine itself, for people who buy, accept, repair or troubleshoot it and need to tell whether a problem comes from the machine, the mould or the material.

1. Hopper / feed throat — holds pellets and feeds the screw; the throat is water-cooled so pellets do not soften and stick.
2. Barrel / heater bands — heats in separate zones, each read by a thermocouple.
3. Screw / check ring — melts and mixes, then drives the melt forward like a plunger.
4. Nozzle — delivers melt from the barrel into the mould's sprue bushing.
5. Injection cylinder / screw drive — pushes the screw forward to inject and rotates it during recovery.
6. Carriage cylinder — moves the injection unit forward to seat the nozzle against the mould, or back.
7. Fixed platen — carries the nozzle-side half of the mould.
8. Mould — not part of the machine, but it decides what size of machine you need.
9. Moving platen / ejector — opens and closes the mould and pushes the part out.
10. Tie bars — carry the clamp force in tension and guide the moving platen.
11. Toggle / clamp cylinder — multiplies cylinder force into clamp force and locks the mould.
12. Frame / tank / pump / controller — the machine's power source and brain.

Unit by unit

Injection unit

The injection unit decides whether the melt entering the mould is evenly hot, well mixed and the same amount every shot. Most colour problems, black specks and part-weight drift start here.

A general-purpose injection screw has three zones. Roughly the first half of its length is the feed zone, with deep flights that convey solid pellets. About a quarter is the compression (transition) zone, where the channel gets steadily shallower and squeezes the pellets into melt. The last quarter or so is the metering zone, a shallow channel that evens out melt temperature and delivers a steady output.

L/D is the flighted length divided by the diameter; general-purpose injection screws are usually around 18:1–24:1. The compression ratio compares feed-zone channel depth with metering-zone depth and is usually around 2:1–3:1 (typical reference range — confirm with the machine maker and the TDS).

The check ring wears faster than anything else in the injection unit, and it is the cause people overlook most often. Once the ring or its seat wears, some melt leaks back during injection and pack, so shots stop being equal. The number that gives it away is the cushion, the melt left ahead of the screw at the end of pack. If the cushion wanders or slowly creeps to zero, suspect the check ring before touching any other setting.

Clamping unit

The clamp has three jobs: open and close quickly, lock tightly enough that the mould does not breathe during injection, and stop in time if a part is left in the way. On a toggle machine the clamp force comes from stretching the tie bars when the links lock straight, so the rear platen must be set to the mould height every time the mould changes.

Drive & hydraulics

A hydraulic machine generates force from oil that the pump sends to the various cylinders, with valves controlling speed and pressure. An all-electric machine drives each axis with a servo motor through a ball screw or belt. Thin-wall work that needs very fast injection often uses an accumulator that stores pressurised oil and releases it during injection.

Control

The control turns set values into motion and records whether each shot followed them. The injection settings screen has almost the same structure on every machine.

• Multi-stage injection speed set by screw position, for example slow through the gate and fast to fill the cavity.
• V/P switchover (velocity to pressure) triggered by position, pressure, time, or cavity pressure if a sensor is fitted. The common guideline is to fill about 95–98 % of the part on velocity before switching.
• Holding pressure / time in several stages, held until the gate freezes.
• Back pressure and screw speed set how dense and well mixed the melt is during recovery. Many screens show back pressure as hydraulic pressure, not melt pressure; multiply by the injection unit's intensification ratio before comparing machines.
• Decompression / suck-back reduces nozzle drool; too much draws in air and causes splay.
• Cushion, the melt left ahead of the screw after pack. It must not reach zero, or pack pressure no longer reaches the part.

The two values worth watching every day are cushion and recovery (plasticizing) time. Both change before the parts go bad. If recovery time keeps growing at the same screw speed, look at feeding at the throat and at screw wear.

Safety

An injection machine combines high clamp force, high-pressure oil, hot surfaces and high voltage in one place. International safety standards for injection molding machines, such as ISO 20430, require every opening into a danger zone to be closed by a guard or an interlocked gate.

Machine types

Machines differ along two axes: the drive and the clamp design. Neither axis has a type that is best for every job.

How to size the machine

Start from the mould and the part, not from the tonnage on the nameplate. Go through each check; if one fails, the machine does not fit even if all the others pass.

1. Clamp force Clamp force ≈ projected area of all cavities plus runner × average cavity pressure. Textbooks and handbooks commonly start from about 2–8 tons per square inch (about 0.3–1.2 t/cm²). Thick walls, short flow and easy-flowing resin sit at the low end; thin walls, long flow and viscous resin at the high end. Then add about 10–20 % margin. Example: 400 cm² total projected area × 0.4 t/cm² = 160 t; with 15 % margin about 185 t, so choose a machine with at least that clamp force.
2. Shot size Shot weight including runner should be about 20–80 % of barrel capacity. Too low and the melt sits long enough to degrade; too high and it cannot melt in time and leaves no room for a cushion. Shot capacity on spec sheets is often given in cm³ or grams of PS; convert it with the density of the resin you actually run. The working shot stroke should be about 1–3 screw diameters.
3. Residence time Rough estimate = (barrel shot capacity ÷ shot weight) × cycle time. Example: a 500 g barrel, 100 g shot and 30 s cycle gives about 150 s. The real figure is longer because melt also sits in the screw channel. Heat-sensitive materials such as PVC, POM, FR grades and some masterbatches must be checked against the limit in the TDS.
4. Mould size versus machine The mould must pass between the tie bars (check both horizontal and vertical spacing). Mould thickness must fall between the minimum and maximum mould height, and the opening stroke should be at least about twice the part depth, plus sprue length and room for a hand or robot to pick the part.
5. Screw diameter and injection pressure One injection unit is often offered with 2–3 screw sizes. Maximum injection pressure = hydraulic pressure × (piston area ÷ screw cross-section), so it falls with the square of the screw diameter. A small screw gives high pressure but a small shot; a large screw gives a big shot at lower pressure. Thin-wall parts: choose the smallest screw whose shot is still big enough. Heavy, thick-wall shots: the larger screw.
6. Screw design for the resin A general-purpose screw handles PP, PE, PS and ABS. Rigid PVC, glass-filled grades, PET and some engineering plastics need a specific flight geometry or material. Tell the machine maker exactly which resins and masterbatches you will run.

Numbers in this section are typical reference ranges — confirm with the machine maker and the TDS.

Maintenance checkpoints

Before working on heaters or hydraulics or reaching into the mould area, always isolate and lock out energy (lock-out / tag-out), release accumulator pressure and let hot parts cool.

The oil temperature is a typical reference range — confirm with the machine maker and the TDS. Follow the machine manual for actual intervals.

Masterbatch and additives on this machine

On an injection machine, masterbatch has one way in: the feed throat, together with the base resin. There are two ways to do it.

• Pre-blending is simple and cheap, but pellets of different size or density can separate during conveying and in the hopper.
• A dosing unit at the throat: volumetric units turn a feed screw for a set time; gravimetric units weigh and correct themselves, holding the ratio more steadily and recording what was actually used. On an injection machine the doser must run only while the screw is rotating for recovery. If it doses continuously, masterbatch piles up in the throat and goes in as a lump in some shots.

A general-purpose screw has limited mixing. Residence time is short, and the metering zone's main job is to even out temperature. If you change colours often or run several masterbatches, ask about a screw with a mixing section, such as a fluted (Maddock-type) or pin mixer, when you buy the machine or replace the screw.

Back pressure directly affects colour dispersion. Raising it works the melt harder before it collects ahead of the screw, and streaks usually decrease. Too high, and shear heats the melt, recovery takes longer and heat-sensitive additives such as some flame retardants can degrade. Change it in small steps and watch recovery time. The same goes for screw speed: think in screw surface speed (m/s), because a larger screw at the same rpm shears the melt harder.

Purging at colour change uses the next job's resin or a purge compound, which may work mechanically or chemically. ACS does not sell purge compounds. If purging takes unusually long, inspect the check ring, screw tip and nozzle before switching purge brands.

Straight talk: if streaks or black specks come from a worn screw, a worn check ring, a dead pocket at the screw tip or a failed heater zone, changing masterbatch will not help. Fix the machine first. Once the machine is sound, the formulation matters: a carrier compatible with the base resin and no harder to melt, and pigments well dispersed at the compounding stage.

The additive most directly tied to the injection machine is the nucleating agent in PP, which lets parts become rigid enough to eject sooner and affects shrinkage. How much depends on the grade and the mould, so measure it on the real machine; see slow cycles and warped parts. UV stabilisers and flame retardants link to the machine through residence time and temperature; see plastics cracking in sunlight and flame-retardant PP V-0. The cost that matters is cost per good part, not price per kilogram. Try the cost-in-use calculator.

Further reading: process and defects

The cycle, the process variables and reference temperatures for common resins are in the article on injection molding. Defects that often trace back to the machine are covered in detail here:

• Short shot — not enough shot, cushion at zero, leaking check ring
• Flash — too little clamp force, platens out of parallel, uneven force across the corners
• Sink marks — not enough pack because the cushion ran out or the check ring leaks
• Splay — too much suck-back, long residence time, a heater zone off target
• Black specks — material hanging up and burning at a worn screw, screw tip or nozzle
• Colour streaks — too little screw mixing, low back pressure, doser not synchronised with screw rotation
• Burn marks — injection speed set so high that trapped air is compressed and heats up (often with blocked mould vents), or a heater zone off target overheating the melt