How Does an Injection Molding Machine Work?

The Basic Principle

An injection molding machine melts plastic pellets, injects the melt under high pressure into a closed mold, holds it under pressure while it solidifies, then opens the mold and ejects the finished part. The cycle repeats automatically, often every few seconds for thin-wall packaging and every 30 to 90 seconds for thick technical parts.

Every machine, whether small or very large, is built from the same main units. Understanding what each unit does, and how the cycle time is divided between them, is the basis for choosing a machine, setting a process and finding the cause of defects.

Main Units of an Injection Molding Machine

Injection unit

Hopper, barrel with heater bands, reciprocating screw, check ring and nozzle. It melts the material and injects a measured shot into the mold at controlled speed and pressure.

Clamping unit

Fixed platen, moving platen, tie bars and a toggle or hydraulic clamp mechanism. It opens and closes the mold and holds it shut against injection pressure, and includes the ejector system that pushes parts off the core.

Hydraulic or drive system

Pump, motor, valves, cylinders and oil cooling on hydraulic and servo-hydraulic machines, or servo motors and ball screws on electric axes. It powers every movement of the machine.

Control system

The controller sets and monitors temperatures, speeds, pressures, positions and times, sequences the cycle, records process data and manages safety interlocks and interfaces to robots and auxiliaries.

Mold

Cavities and cores that shape the part, a runner or hot runner system to distribute the melt, cooling channels to remove heat, and an ejection system. The mold is specific to the part; the machine is the platform that runs it.

The Molding Cycle Step by Step

Each cycle follows the same sequence. Times vary with part design and material, but the proportions below are typical for a part with walls of 1.5 to 3 mm. Cooling is almost always the longest phase, usually 50 to 70 percent of the cycle, which is why mold cooling design and wall thickness have the largest effect on output.

Plasticizing for the next shot happens in parallel with cooling, so it does not add to cycle time as long as the screw finishes recovery before the mold opens.

For thin-wall packaging the whole cycle can be under 5 seconds, with filling in 0.1 to 0.3 seconds.

Plasticizing: How the Screw Melts the Material

A reciprocating screw has three zones. In the feed zone, deep flights convey pellets from the hopper and start to warm them. In the compression or transition zone, the flight depth decreases, compressing the material against the heated barrel wall and melting it mainly by shear. In the metering zone, shallow flights homogenize the melt and deliver it to the front of the screw at uniform temperature.

Most of the melting energy comes from shear generated by screw rotation, not from the heater bands. The heaters set and stabilize the barrel temperature profile, which typically rises from the feed zone to the nozzle.

As the melt collects in front of the screw, it pushes the screw backward. Back pressure is the hydraulic resistance applied to this movement. Moderate back pressure improves mixing, removes trapped air and gives a more consistent shot. Too much increases shear heat and recovery time, which can degrade sensitive materials. A check ring at the screw tip prevents melt from flowing back during injection, so the shot weight stays repeatable.

Typical Process Parameters

The values below are typical starting ranges for common materials. Every grade, part and mold needs its own optimized settings, so use them as a starting point and confirm with the material supplier's data sheet.

Injection speed

Set by wall thickness and flow length; thin walls need very fast fill to avoid premature freezing.

Switchover point

The position at which the machine switches from fill speed to holding pressure, usually when the cavity is about 95 to 98 percent full.

Holding pressure and time

Typically 30 to 70 percent of peak injection pressure, held until the gate freezes, which can be confirmed with a gate-freeze weight study.

Cushion

A small amount of melt left in front of the screw at the end of hold, often 3 to 10 mm, shows that the shot is complete and consistent.

How a Servo-Hydraulic Drive Works

In a conventional hydraulic machine, an induction motor runs the pump at constant speed, and valves throttle the flow that the cycle does not need. The excess energy turns into heat in the oil. During cooling, when the machine is mostly waiting, the pump still runs.

A servo-hydraulic machine drives the pump with a servo motor controlled by the machine controller. Motor speed and torque follow the flow and pressure demand of each phase of the cycle: high speed during fast movements and injection, low speed with high torque during holding, and near standstill during cooling. This reduces energy use and oil temperature, lowers noise and gives precise, repeatable pressure and speed control.

HWAMDA's M8SII series and the machines based on it use servo-hydraulic drives, combining the force and robustness of hydraulics with efficient, responsive control across 900 to 33,000 kN.

Auxiliary Equipment in the Molding Cell

HWAMDA supplies these items with machine projects so the cell can be planned, matched and commissioned as one system.

Resin dryer

Removes moisture from hygroscopic materials such as PET, PA and PC before molding to prevent splay and loss of properties.

Hopper and vacuum loaders

Convey material from bags, octabins or central storage to the hopper automatically.

Chiller

Supplies cooled water to the mold and to the machine's oil cooler.

Mold temperature controller

Holds the mold at a stable temperature, heating or cooling as needed for consistent part quality.

Robot

Removes parts and runners, places them on conveyors or stackers, and handles labels or inserts in IML and insert molding.

Granulator

Regrinds runners and rejects for controlled reuse where the product allows.

Common Defects and Their Usual Causes