How Injection Molding Actually Works

Whether you're new to the industry, explaining the process to someone outside the factory, or just want a clear refresher — here's how an injection molding machine turns plastic pellets into finished parts. No jargon overload, we promise.

The Big Picture

Injection molding is conceptually simple: melt plastic, push it into a metal mold, let it cool, open the mold, take out the part. Repeat. A typical machine does this every 15-60 seconds, thousands of times per day.

The machine has two main units:

1. The injection unit — melts the plastic and pushes it into the mold
2. The clamping unit — holds the mold shut while the plastic is being injected

Let's walk through one complete cycle.

The Cycle, Step by Step

Step 1: Mold Close

The clamping unit pushes the two halves of the mold together. On toggle machines (the most common type), a toggle mechanism driven by a hydraulic cylinder presses the halves together with enormous force — anywhere from 50 to 3000+ tons depending on the machine size.

This force is needed because the plastic will be injected under high pressure, and if the mold isn't held shut tightly, the plastic will squirt out at the parting line (that's what "flash" is).

The force is borne by four tie bars — thick steel rods that run through the corners of the platens and stretch elastically under load.

Step 2: Injection

The injection unit is essentially a heated tube (the barrel) with a rotating screw inside it.

Before this injection happens, the screw has been rotating in the previous cycle, melting plastic pellets and accumulating a "shot" of molten plastic at the front of the screw. Now, the screw stops rotating and pushes forward like a plunger, forcing the molten plastic through the nozzle and into the mold's cavity at high pressure and speed.

The plastic flows through channels in the mold (called runners) and enters the cavity through a small opening (the gate). The whole fill happens in a fraction of a second to a few seconds, depending on part size.

Step 3: Pack and Hold

After the cavity is filled, the machine maintains pressure on the plastic (called holding pressure or packing pressure). This compensates for the shrinkage that occurs as the plastic cools. Without packing, you'd get sink marks — depressions on the part surface where the plastic shrank away from the mold wall.

Step 4: Cooling

The plastic needs to solidify before you can open the mold and take it out. Cooling channels inside the mold circulate water (or oil, for hot molds) to pull heat out of the plastic.

Cooling time is typically the longest single step in the cycle — often 50-70% of total cycle time. This is why mold cooling design is so critical to productivity.

Step 5: Screw Recovery (Plasticizing)

While the part is cooling, the screw starts rotating again. Raw plastic pellets drop from the hopper into the barrel, and the screw's rotation moves them forward along the barrel's length. Heater bands wrapped around the barrel provide heat, and the frictional heat from the screw's rotation also melts the plastic.

As molten plastic accumulates at the front of the screw, the screw is pushed backward by the growing melt pool. When enough material has been plasticized for the next shot, the screw stops. The machine is now ready for the next injection.

Step 6: Mold Open

The clamping unit releases and opens the mold. The two halves separate.

Step 7: Ejection

Ejector pins built into the mold push the solidified part off the mold surface. The part falls out (sometimes into a bin, sometimes picked by a robot or conveyor). The mold is now empty and ready for the next cycle.

Step 8: Repeat

The mold closes again, and the cycle starts over.

The Key Components

Every injection molding machine has these essential components:

• Barrel and screw: Where plastic is melted and pushed — the heart of plasticizing
• Heater bands: Wrapped around the barrel to provide and control heat
• Thermocouples: Sensors that measure barrel temperature for each zone
• Injection nozzle: The tip where molten plastic exits the barrel and enters the mold
• Tie bars: Steel rods that bear the clamping force
• Toggle or hydraulic clamp: The mechanism that opens and closes the mold
• Hydraulic system: Pumps, valves, cylinders that power the machine's movements
• Control system: The computer that coordinates everything — temperatures, pressures, speeds, timings

Why Parts Wear Out

Every cycle puts stress on the machine:

• The barrel and screw experience abrasion from filled materials, thermal cycling, and corrosive resins
• Heater bands cycle on and off thousands of times, degrading the heating element
• Tie bars stretch and relax with every clamp cycle — millions of times over years
• Hydraulic pumps deliver pressurized oil for every movement, wearing internal surfaces
• Toggle pins and bushings bear cyclic loads with every open and close

That's why spare parts exist — and why regular replacement of wear items keeps your machine producing quality parts year after year. It's just the nature of the process.

Learn More

Browse our parts catalog to see the components that keep injection molding machines running. If you're new to maintaining these machines, our maintenance checklist for Haitian MA machines is a good starting point.