The moulding cycle is five stages that repeat without interruption, and almost every defect a buyer ever sees can be traced to one of them. Knowing which stage owns which symptom turns a frustrating conversation with a moulder into a short one, because the fix for a short shot lives in a different stage from the fix for a warped part.
Clamping and injection
The two halves of the tool close and are held shut by a force large enough to resist the pressure inside. The screw then drives melt through the sprue, runners and gates into the cavity, filling it in a fraction of a second. Too little clamp force and the tool separates and flashes; too fast a fill and the melt jets, traps air or burns at the last place to fill.
Packing and holding
Once the cavity is full, pressure is held so more melt can be pushed in as the plastic cools and shrinks. This is where dimensions are actually made. Insufficient packing shows as sink marks over thick sections and parts that measure small; excessive packing shows as flash, stuck parts and internal stress that appears later as cracking or warp.
Cooling and ejection
The tool holds the part until it is rigid enough to handle, and this is the longest part of the cycle. Cooling is governed by the thickest section, which is why wall thickness sets part price. Ejection then pushes the part off the core with pins, sleeves or a stripper plate, and insufficient draft or an over packed part shows up here as drag marks or ejector pin witness.
Reading a defect back to its stage
Short shots and burn marks point at filling. Sink marks and dimensional drift point at packing. Warp usually points at uneven cooling or an uneven wall. Flash points at clamp force, parting line damage or over packing. Silver streaks usually point further upstream still, at wet material that was not dried before it reached the barrel.
Questions people ask about injection molding process
What is injection molding?
A manufacturing process in which molten polymer is injected into a closed metal tool under pressure, held while it cools, and then ejected as a finished part.
How long does one cycle take?
From a few seconds to over a minute. Cooling dominates and is driven by the thickest wall in the part, so thinner and more uniform walls mean faster cycles and cheaper parts.
Why do my parts have sink marks?
Almost always a thick section, a rib that is too thick relative to the wall, or insufficient packing pressure. The design fix outlasts the process fix.
Is the moulding process the same everywhere?
The physics is identical. The spelling changes, the standards a supplier works to change, and the resin grades available locally change, which is why a tool moved between regions often needs its process re developed.