What are the steps of the injection molding process?

Six steps in a repeating cycle: clamping (the mold halves close and lock), injection (molten plastic fills the cavity), holding or packing (pressure compensates for shrinkage), cooling (the part solidifies), mold opening, and ejection (ejector pins push the part out). Then the cycle repeats for the next part.

Which step takes the longest?

Cooling. The plastic must solidify enough to eject without deforming, and heat leaves through the mold's cooling channels gradually, so cooling commonly accounts for 60 to 80 percent of the total cycle time. Thicker walls cool slower, which is a key reason uniform, moderate wall thickness matters.

Why is holding pressure needed after injection?

Because plastic shrinks as it cools, usually 1 to 3 percent. After the cavity is filled, the machine holds a sustained pressure to pack in a little more material and compensate for that shrinkage, which prevents sink marks and voids and keeps the part dimensionally accurate.

What ejects the finished part?

An ejection system, most commonly ejector pins, pushes the solidified part out when the mold opens. Other methods include stripper plates, air valves, or a robotic arm. Ejector pins can leave small marks, so their placement is a deliberate design decision.

Injection Molding Process Steps

Updated November 13, 2026

Every injection molded part comes from the same six-step cycle, repeated over and over: clamp, inject, hold, cool, open, eject. Understanding each step explains why molded parts look the way they do, why cooling dominates the clock, and why a few design choices decide whether a part comes out perfect or flawed. Here is the process, step by step.

One repeating cycle

Injection molding cycle: the repeating sequence a machine runs to make each part: clamp the mold, inject plastic, hold pressure, cool, open the mold, and eject the part. The time for one full cycle sets how fast parts come off the machine.

Injection molding isn't a one-time event; it's a cycle that repeats for every part. The faster the cycle, the more parts per hour and the lower the cost each, which is why so much molding engineering is really about shaving seconds off these six steps.

Step 1: Clamping

The two mold halves, one fixed and one moving, close and lock under the clamping unit. The clamp force has to exceed the force the injected plastic exerts on the cavity walls, or molten plastic squeezes out of the seam and leaves a thin unwanted edge called flash. Bigger parts and higher pressures need bigger clamping machines.

Step 2: Injection

A screw acts as a plunger and pushes molten plastic into the mold cavity at high speed and pressure (often several hundred to over a thousand bar). The plastic flows through the gates and runners and fills the cavity. Gate location matters here, because it affects how the cavity fills, where weld lines form, and where a small gate mark is left.

Step 3: Holding (packing)

Once the cavity is essentially full, the machine switches to a lower but sustained pressure. Plastic shrinks as it cools (typically 1 to 3 percent), so this holding pressure packs in a little extra material to compensate. Skimp on it and you get sink marks, voids, and parts that measure slightly small.

Step 4: Cooling

The part now solidifies inside the mold as heat flows out through cooling channels. This is the longest step, commonly 60 to 80 percent of the whole cycle, because the part must harden enough to eject without deforming. Thick sections cool slowly and unevenly, which causes warping and sink, and this is exactly why uniform, moderate wall thickness is a core molding design rule.

Step 5: Mold opening

With the part solid, the clamping unit retracts and the two mold halves separate. For the part to come free in one clean pull, its walls need a slight draft angle and no undercuts, the design rules covered in what is injection molding.

Step 6: Ejection

The ejection system pushes the finished part out of the open cavity, most often with ejector pins, sometimes with stripper plates, air, or a robot. Ejector pins can leave small witness marks, so their placement is planned to fall on surfaces that don't matter. Then the mold closes and the next cycle begins.

Why the steps matter to you

Each step maps to a design decision: clamping to part size, injection to gate placement, holding to wall thickness, cooling to uniform walls, opening to draft and undercuts, and ejection to pin placement. Designing with the cycle in mind is what makes a part both moldable and cheap. For the full process and its design rules, see what is injection molding, and for where molding fits among the processes, how products are made.

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Frequently asked questions

What are the steps of the injection molding process?: Six steps in a repeating cycle: clamping (the mold halves close and lock), injection (molten plastic fills the cavity), holding or packing (pressure compensates for shrinkage), cooling (the part solidifies), mold opening, and ejection (ejector pins push the part out). Then the cycle repeats for the next part.
Which step takes the longest?: Cooling. The plastic must solidify enough to eject without deforming, and heat leaves through the mold's cooling channels gradually, so cooling commonly accounts for 60 to 80 percent of the total cycle time. Thicker walls cool slower, which is a key reason uniform, moderate wall thickness matters.
Why is holding pressure needed after injection?: Because plastic shrinks as it cools, usually 1 to 3 percent. After the cavity is filled, the machine holds a sustained pressure to pack in a little more material and compensate for that shrinkage, which prevents sink marks and voids and keeps the part dimensionally accurate.
What ejects the finished part?: An ejection system, most commonly ejector pins, pushes the solidified part out when the mold opens. Other methods include stripper plates, air valves, or a robotic arm. Ejector pins can leave small marks, so their placement is a deliberate design decision.

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