Moulding defects: short shots, sink, flash and warpage

Read the defect signature first

A moulding defect is evidence. The location, the size, the direction and the timing of the defect point to a cause. A short shot at the end of flow is a fill problem. A sink on the face opposite a rib is a shrinkage problem. A flash line along the parting line is a clamp or tool fit problem. A warp that closes one side and opens the other is a differential shrinkage problem.

The first response is often to change the material or the machine. That is usually the wrong order. The material is a fixed input once the grade is approved, and the machine is only one part of the system. Start with the defect, then check the process window, the tool condition and the part design in that order.

Record the shot number, cavity, location, process settings and a photograph. A defect in one cavity points to the runner or tool. A defect on every shot at the same place points to design or tool. A defect that comes and goes points to process or material.

Short shots: the flow stopped early

A short shot is an incomplete fill. The plastic freezes before it reaches the end of the cavity, so a corner, a rib tip or a thin section is missing. The cause can be low injection pressure, low melt temperature, a cold slug, a blocked gate, poor venting or a section that is too thin to fill.

Check the fill pattern first. If the short shot is at the end of flow, the pressure or the temperature may be too low, or the vent may be trapping air. If it is at a rib tip, the rib may be too thin or too far from the gate. If it is behind an insert, the insert may be blocking the flow or the melt may be losing heat to the metal.

The fix is not always more pressure, which can flash the tool or overpack another area. A gate change, a vent, a wall change or a hotter melt is often better. A flow study before the tool is cut is cheaper than trial and error.

Sink: the surface pulled in

A sink mark is a local depression on a surface, usually opposite a thick section, a rib root or a boss. The thick section cools more slowly than the surrounding wall, and as it shrinks it pulls the surface in. The defect is a shrinkage problem, not a fill problem.

Check the section first. A rib base at 75% or more of the wall, a solid boss or a thick corner will sink even with a good process. The fix is to thin the section, core the boss or move the feature away from the visible face. More pack pressure only helps if the gate is open and the section is not too thick.

Sink can appear as a shadow rather than a deep depression. A faint read-through line over a rib is still a sink, and on a high-gloss part it is a defect even if the dimension is correct. Texture hides it but does not remove the shrinkage.

Flash: material outside the cavity

Flash is excess plastic at the parting line, around an insert, at a vent or at a side action. It means the melt has found a gap. The cause can be insufficient clamp force, a worn or damaged parting line, an insert that is not seated, a vent that is too deep or a melt that is too fluid.

Check the tool first. Flash in one area but not another suggests a fit problem, not a clamp problem. Flash after a tool has run for a while suggests wear. Flash around an insert suggests it is not seated or the shut-off is worn.

The process fix is more clamp force or less injection pressure, but more clamp can damage the tool. The design fix is a better shut-off, a shallower vent or a supported insert. A flash line on a sealing face is a leak path, so it needs a tool fix, not trimming.

Warpage: the part changes shape after ejection

Warpage is a difference in shrinkage across the part. One region shrinks more than another, or the flow direction shrinks differently from the cross-flow direction, and the part bends as it cools. The result can be a twisted rail, a bowed panel or a cover that will not sit flat.

The causes are uneven wall thickness, uneven cooling, a poor gate, fibre orientation and ejection stress. A part with a thick side and a thin side will warp even with a stable process. A glass-filled part warps more in the flow direction, and the gate decides which way that is.

The first check is the wall and the cooling. A uniform wall and a balanced cooling layout are more effective than a process tweak. The second check is the gate. A gate that fills the part from one end creates a long flow path and a strong orientation; a gate in the centre or a multi-gate layout can balance the shrinkage.

Measure warp after a defined cooling time. A part that is flat on the bench straight after ejection may bow after a day, so agree the condition and the fixture before deciding whether it is acceptable.

The fixes interact

A process change that fixes one defect can create another. More pack pressure reduces sink but increases flash and can increase warp. A hotter melt improves fill but lengthens the cycle and can increase sink. A faster injection fills a thin section but can trap air or create a weld line.

Change one variable at a time and record the result. A design of experiments is not necessary for every trial, but a simple record of the setting, the change and the defect is. That record is what turns a trial into knowledge.

The tool and the design set the window. If it is narrow, the process will be sensitive and the defect will return. A lasting fix is usually a design or tool change that widens the window, not a set point that works on the day.

A defect guide for the trial report

The table below is a starting point for the trial report. It pairs each defect with the first check, the likely process cause and the tool or design fix. The order matters: check the tool and the design before changing the process, because a process fix cannot correct a geometry that is wrong.

Agree the acceptance before the trial

A trial produces parts, but it also needs a decision. Agree the defect acceptance before the samples are measured. A sink that is visible on a class A surface is not acceptable, while a sink on a hidden face may be. A flash line on a sealing face is a leak risk, while a flash line on a non-sealing edge may be trimmed.

The acceptance should reference the drawing, the surface standard and the function. For a sealing face, add a leak test. For a snap fit, add an assembly and retention test. For a visible part, add a colour and gloss check under a defined light.

Send the trial report with the process settings, defect photographs, measurement results and proposed correction. That pack is what the toolmaker and moulder need to close the issue, and it supports the first article inspection.