How Does Injection Molding Work? Process & Materials Guide

The Injection Molding Cycle, Step by Step

A single molding cycle is the time between one mold closing and the next mold closing. Depending on part size, wall thickness, and material, a cycle can run anywhere from about 10 seconds for a thin-wall packaging item to well over 120 seconds for a thick technical component, based on benchmarking data published across the molding industry in 2025 and 2026. Each cycle moves through five distinct phases, and understanding what happens in each one makes it much easier to diagnose problems later.

1. Clamping

The two halves of the mold are closed and locked together by the clamping unit. The clamping force must be high enough to keep the mold shut against the pressure of the incoming molten plastic; undersized clamp tonnage is one of the most common causes of flash at the parting line. Clamp force is generally estimated by multiplying the projected area of the part and runner system by a cavity pressure factor, which typically falls somewhere between 2 and 8 tons per square inch depending on the material and wall thickness involved.

2. Injection (Fill)

A rotating screw pushes molten resin through the nozzle, through the sprue, runners, and gates, and into the cavity. Fill time is usually very short, typically 0.5 to 5 seconds, since most of the cavity volume needs to be filled before the melt front begins to cool and freeze. Modern machines use multi-stage velocity profiles during fill, starting slow to avoid jetting near the gate, accelerating through the body of the part, and slowing again as the melt front approaches the last areas to fill, which reduces air entrapment and visible flow marks.

3. Packing and Holding

Once the cavity is nominally full, the machine switches to a lower holding pressure to pack additional material into the mold as the part begins to shrink. This stage typically lasts 1 to 15 seconds depending on wall thickness, and it is the primary control point for preventing sink marks and voids. Packing continues until the gate freezes off; once the gate solidifies, no more material can enter the cavity, so any packing pressure applied after that point has no further effect on the part.

4. Cooling

This is usually the longest portion of the cycle. The part must cool below its ejection temperature throughout its thickest cross-section before the mold can safely open. Cooling time is driven mainly by wall thickness and the material's thermal diffusivity, and it commonly accounts for well over half of total cycle time on thicker parts. Screw recovery, which plasticizes the resin for the next shot, typically overlaps with cooling rather than adding extra time to the cycle, so the longer of the two durations is what actually governs total cycle length.

5. Mold Opening and Ejection

The mold opens, ejector pins push the solidified part free, and the cycle resets. Ejection itself is fast, generally 0.5 to 2 seconds, though parts with undercuts, threaded inserts, or side-action slides can add several seconds here. On automated lines, a robot arm often removes the part directly from the mold during this window to keep the cell running without an operator standing at the press.

Cycle time is not just a technical curiosity; it is directly tied to cost. Because a molding machine represents a large fixed hourly cost, every second added to a cycle spreads that cost across fewer parts. On a 4-cavity mold running a 30-second cycle, output works out to roughly 480 parts per hour; shaving the cycle down to 25 seconds lifts that to around 576 parts per hour, a meaningful production gain without any additional capital investment. This is why experienced process engineers treat cycle time reduction as an ongoing optimization exercise rather than a one-time setup task.