HEE&HATO Engineering Team | Plastics Processing Equipment | Published: August 6, 2026

An injection moulding machine has four subsystems: the injection unit that melts and injects resin, the clamping unit that holds the mould shut, the drive unit that supplies force, and the control system. Roughly a dozen named components sit inside those four, and about eight of them appear as numbers on a spec sheet.

Two ways to answer “what parts does it have”

The standard answer is a list: hopper, barrel, screw, heater bands, nozzle, platens, tie bars, clamping mechanism, ejector, control panel. That list is correct and it is what almost every page on this topic gives you.

It is also not the answer a buyer needs, because none of those words appear on a quotation as words. They appear as numbers: distance between tie bars, minimum and maximum mould height, platen dimensions, opening stroke, screw diameter, injection rate, ejector force. Each of those numbers is a physical part expressed as a constraint.

So this page runs the list twice. First as components and what they do. Then as the same components translated into the specification lines that decide whether a given machine can run your mould.

The injection unit, part by part

Hopper and feed throat. The hopper holds dried granules; the feed throat below it is water-cooled to stop heat travelling up from the barrel and softening pellets before they reach the screw. A bridged feed throat is a classic cause of shot weight scatter that gets blamed on the screw.

Barrel. A thick-walled cylinder, usually nitrided or bimetallic-lined. Nitriding depth and hardness determine service life against abrasive and filled compounds. On our own extrusion equipment, for reference on what these figures look like, nitrided surfaces are specified at 0.5–0.8 mm depth with hardness in the HV900–1050 range.

Screw. The single most consequential part in the machine. Three zones (feed, compression, metering), characterised by diameter, length-to-diameter ratio and compression ratio. Common alloy is 38CrMoAlA, nitrided or bimetallic depending on the resin.

Check ring, or non-return valve. A three-piece assembly at the screw tip that lets melt pass forward during recovery and seals during injection. It is the most under-specified wear part on the machine and the most frequent hidden cause of shot-to-shot weight variation. When a process that was stable starts drifting, this is where to look before touching a setpoint.

Heater bands and thermocouples. Zone count matters more than total wattage. More zones means finer control over the melt profile; it also means more thermocouples to fail, and a failed thermocouple reads as a temperature problem that is actually a wiring problem.

Nozzle. The interface between barrel and sprue bush. Open, shut-off or valve types, with a radius and orifice diameter that must match the mould’s sprue bush. A mismatch here leaks melt at the interface and is discovered on the first shot, not on the quotation.

Injection cylinders and screw drive. On a servo-hydraulic machine, hydraulic cylinders push the screw forward and a hydraulic motor or servo motor rotates it. Two cylinders arranged symmetrically avoid the side load on the screw that a single off-centre cylinder produces.

The clamping unit, where the money and the constraints are

Platens. Three: fixed (nozzle side), moving, and the rear or tail platen that carries the toggle. Platen deflection under clamp load is what produces flash in outer cavities of a multi-cavity mould, so platen design is a real engineering variable rather than a plate thickness figure. Finite element design targets the deflection distribution across the face.

Tie bars. Four hardened steel bars carrying the clamp load in tension. The distance between them is the number that decides whether your mould physically fits, and it is checked before anything else on the spec sheet.

Clamping mechanism. Three families in general use:

Compared with the two-platen designs common at very high tonnage, a toggle clamp gives faster dry cycle and lower cost at mid tonnage, with the trade-off that clamp force depends on correct mould height setting rather than on a directly measured hydraulic pressure.

Mould height adjustment. The mechanism that positions the tail platen so the toggle links straighten fully at the moment the mould faces meet. Manual versions use a hand crank or motorised nut; automatic versions set it from the controller. This part is invisible on most parts lists and it determines whether the tonnage on your screen is real.

Ejector. Hydraulic or electric, defined by stroke, force and pattern. The pattern is the bolt hole layout on the moving platen that your mould’s ejector plate must match. EUROMAP, the European Plastics and Rubber Machinery Association, publishes interface recommendations covering machine dimensions; when buying a machine that must accept existing moulds, ask which EUROMAP sheet the platen bolt pattern follows and compare it against the moulds you already own.

Safety gates and guards. Interlocked mechanical, electrical and, on hydraulic machines, hydraulic safety devices. For EU-bound equipment these fall under the Machinery Directive 2006/42/EC, with ISO 12100 as the reference for risk assessment and risk reduction.

The drive and control components

Power unit. On a fixed-pump machine, a constant-speed motor and pump. On a servo-hydraulic machine, a servo motor driving the pump with closed-loop pressure and flow feedback. On an all-electric machine, servo motors and ball screws replace the hydraulic actuators.

Oil tank, filtration and cooler. Present on any machine with a hydraulic circuit, including all-electric machines fitted with an auxiliary power pack for mould-mounted core pulls. Oil temperature control belongs here, and it matters more than most buyers expect, because oil viscosity changes injection speed even when no setpoint has moved.

Proportional and directional valves. The valve resolution sets the ceiling on how finely injection speed and pressure can be commanded.

PLC and HMI. Controller with recipe storage, alarm handling and, increasingly, production data output. Ask what the machine can export and in what format, because retrofitting this later is expensive.

Components translated into spec-sheet lines

This is the table to keep open while comparing quotations.

The screw diameter trade-off, and why the biggest option is usually wrong

Most machine series offer the same frame with two or three screw diameters. Buyers commonly take the largest one on the assumption that more capacity is better. It is a decision with a hidden cost.

A larger screw gives more shot volume per stroke. It also gives lower maximum injection pressure, because the same hydraulic or drive force is spread across a larger screw cross-section. Thin-wall packaging parts need pressure and speed more than volume, so the largest screw on the list is often exactly the wrong choice. Thick-wall parts with generous walls need volume more than peak pressure, so it is the right one.

A second penalty: an oversized screw running small shots leaves material sitting in the barrel across many cycles. Residence time rises, and heat-sensitive resins degrade. The symptom is discolouration or brittleness that no temperature adjustment fixes, because the cause is time, not temperature.

The related misconception on mould height: buyers check maximum mould height and skip the minimum. The minimum is the more common blocker, particularly with small moulds on a large-frame machine. A mould below minimum height needs spacer plates, and every s

How to read a parts list on a quotation

Seven checks, in the order that eliminates machines fastest.

• Tie-bar clearance against your widest mould, including hose and connector clearance, not just the steel.
• Minimum and maximum mould height against your thinnest and thickest moulds, adding for insulation plates and manifolds.
• Opening stroke against your deepest part plus sprue plus any robot or take-out clearance.
• Screw option against part weight and wall section, using the trade-off above rather than defaulting to the largest.
• Check ring and screw material specification. If the quotation says only “nitrided”, ask for depth and hardness figures. Vague answers here predict vague answers about spare parts later.
• Ejector pattern and force, compared against the platen drilling on the moulds you already own.
• Spare parts list and lead time for the wear items: check ring assembly, heater bands, thermocouples, seals, filters. A machine with a two-week wait on a check ring is a different machine from one with the same spec and stock parts.

Item 7 is the one that gets skipped and the one that determines what the machine costs you in year three.

How these components are configured on the IJT-SV330

The IJT-SV330 Standard Servo IMM makes a usable reference for the clamping and injection components described above.

Five-point inward-moving double toggle. The linkage multiplies force mechanically, which allows a smaller clamp cylinder for a given tonnage and therefore less oil moved and less heat generated per cycle. The inward-moving geometry directs clamp load toward the mould centre, which is what limits the platen bending behind cavity-to-cavity weight differences.

FEM-designed platens. Deflection distribution across the platen face is the design target rather than raw plate thickness.

Automatic mould height adjustment. Removes the manual calibration step described above, which matters most in plants running frequent changeovers where the error accumulates unnoticed.

Dual-pillar support and dual-cylinder injection balance. Symmetrical injection force avoids screw side load, addressing a root cause of shot weight variation and accelerated barrel wear rather than compensating for it in software.

Imported hydraulic pump and control valve. Valve resolution sets the achievable precision of injection speed and pressure control.

Clamping tonnage, shot volume, platen size, tie-bar spacing and stroke on this series are configured per application, so those figures belong on a project datasheet rather than in an article. The IJT line covers servo-hydraulic and all-electric systems and is built for thin-wall packaging, multi-cavity production and hot-runner moulds. Machines are designed to meet CE and ISO requirements.

Pricing depends on model, tonnage or output, automation level and final configuration. Share your material, part geometry and capacity targets — our engineers reply within 24–48 hours with a recommended configuration and lead time.

FAQ

Q: What are the main parts of an injection moulding machine?

A: Four subsystems. The injection unit (hopper, feed throat, barrel, screw, check ring, heater bands, nozzle). The clamping unit (three platens, four tie bars, clamping mechanism, mould height adjustment, ejector, safety guards). The drive unit (motor, pump and valves, or servo motors and ball screws, plus oil tank, filtration and cooler). The control system (PLC, HMI, sensors).

Q: Which part wears out first?

A: In most plants, the check ring, followed by the screw tip and barrel liner. Wear rate depends on the resin: glass-filled and mineral-filled compounds are abrasive, and PVC is chemically aggressive to standard steels. Heater bands and thermocouples fail often but are cheap and quick to change, so they cost downtime rather than money.

Q: What is the difference between platen size and distance between tie bars?

A: Platen size is the full face of the plate, including the area outside the tie bars. Distance between tie bars is the clear opening the mould has to pass through. A mould can be smaller than the platen and still not fit between the tie bars, which is why the tie-bar figure is checked first.

Q: Why do machines have four tie bars?

A: Four bars distribute clamp load symmetrically around the mould centre while leaving a clear opening for the mould. Tie-bar strain is also proportional to clamp force, so some machines measure tonnage from tie-bar elongation, which turns the bars into a sensor as well as a structural part.

Q: Does an all-electric machine still have these parts?

A: Nearly all of them. Platens, tie bars, toggle, mould height adjustment, screw, barrel, check ring, heater bands and nozzle are the same components. What changes is the drive: servo motors and ball screws replace hydraulic cylinders, and the oil tank, pump and valves disappear from the main axes, though an auxiliary power pack often reappears if the mould has hydraulic core pulls.

Q: How do I know which screw diameter to order?

A: Work from part weight plus runner weight, and from wall section. Thin walls and short cycles favour the smaller screw for pressure and speed; thick walls and larger shots favour the larger one for volume. Sizing so the shot sits in the middle band of barrel capacity avoids both excessive residence time and running out of stroke.

Q: Are machine parts interchangeable between manufacturers?

A: Structural parts are not. Screws and barrels often are, since geometries are widely replicated and can be produced to match an existing machine’s dimensions. Heater bands, thermocouples and seals are largely commodity items. Toggle links, platens and controllers are machine-specific and should be sourced from the builder.

What to ask for before the quotation arrives

Send the mould drawing, not just the part drawing. Almost every constraint in the table above is a mould dimension rather than a part dimension, and a supplier working from a part drawing alone is guessing at your tie-bar clearance and mould height.

If you have no mould yet, send the part drawing with projected area, wall section, expected cavity count and annual volume, and ask for the tie-bar clearance and mould height range to be stated explicitly in the reply rather than left to the general catalogue.

One area this page did not cover: the auxiliary equipment that sits around the machine and often decides real output more than the press does. Dryers, hopper loaders, mould temperature controllers, chillers, robots and granulators are separate purchases with their own specification traps, and mismatched auxiliaries are a common reason a correctly specified machine underperforms.