Start by deciding what kind of problem it is
Before adjusting anything, establish which of three categories the defect belongs to. Is it a mould problem, a material problem, or a process problem? Getting this wrong wastes time: operators adjusting pressures for hours on a defect that is actually a blocked vent or undried resin is a common and expensive pattern.
Two habits make diagnosis much faster: change one parameter at a time and record the result, and keep a short-shot sample from the tool. A deliberately under-filled part shows the fill pattern and reveals where the melt fronts meet, which explains a great many downstream symptoms.
Short shot
The cavity does not fill completely, usually leaving thin walls or features furthest from the gate incomplete.
Likely causes
- Insufficient shot size, injection pressure or injection speed
- Melt or mould temperature too low, so the polymer freezes before filling
- Restricted flow: undersized gate or runner, or a wall section too thin to fill
- Trapped air with nowhere to escape: a venting problem rather than a pressure problem
- A worn non-return valve on the screw allowing back-flow
Where to look first
Check shot size and cushion before anything else. If there is no cushion at the end of holding, the machine has run out of material and everything downstream is unreliable. If pressure and temperature are already high and the part still will not fill, suspect venting or a flow restriction rather than pushing harder.
Sink marks and voids
Sink marks are depressions on the surface, typically opposite a rib, boss or thick section. Voids are the same phenomenon occurring internally instead: the skin holds its shape and the shrinkage forms a cavity inside.
Likely causes
- A local section that is too thick, so material continues shrinking after the gate has frozen
- Insufficient holding pressure or holding time
- Gate freezing too early, cutting off the feed before shrinkage is compensated
- Melt or mould temperature too high, increasing total shrinkage
Where to look first
Extend holding time and check whether the defect improves. If it does, the feed was being cut short. If it does not change at all, the gate has probably already frozen and the real fix is a larger gate, a relocated gate, or a design change to reduce the thick section. Sink marks are frequently a design problem wearing a process disguise.
Warpage
The part distorts after ejection: bowing, twisting or pulling out of flat.
Likely causes
- Uneven cooling between the two mould halves or across the part
- Differential shrinkage between thick and thin sections
- In glass-filled materials, fibre orientation causing directional shrinkage
- Ejection while the part is still too hot to hold its shape
- Residual stress from excessive injection speed or packing
Where to look first
Measure the actual temperature of both mould halves rather than trusting the controller setpoints. A meaningful difference between fixed and moving halves is one of the most common causes and one of the easiest to miss. After that, look at cooling time and whether the part is being ejected too early.
Flash
A thin film of plastic escaping at the parting line, around ejector pins, or at slide faces.
Likely causes
- Clamping force insufficient for the projected area of the part
- Injection or holding pressure too high
- Melt temperature too high, lowering viscosity
- Damaged, worn or contaminated parting line surfaces
- Mould not seating correctly, or platens out of parallel
Where to look first
Check whether the flash is uniform around the parting line or local to one area. Uniform flash suggests a clamping or pressure issue; flash in one spot suggests mechanical damage or debris on the shut-off face. Cleaning and inspecting the parting line takes minutes and rules out a whole branch of the investigation.
Weld lines and knit lines
A visible line where two melt fronts meet: around a hole, behind a core pin, or where flow splits and rejoins. These are always weaker than the surrounding material to some degree.
Likely causes
- Unavoidable geometry: any through-hole creates one
- Melt fronts too cool when they meet, so they do not fuse properly
- Trapped air at the meeting point preventing contact
- Gate position causing fronts to meet in a structurally poor location
Where to look first
Raising melt and mould temperature and increasing injection speed helps the fronts fuse. But if a weld line falls in a load-bearing area, no process setting will make it as strong as unbroken material; the durable fix is moving the gate so the line forms somewhere harmless. This is a strong argument for agreeing gate position during design review rather than after the tool is cut.
Splay, silver streaks and surface blemishes
Silvery or brown streaks radiating from the gate, usually following the flow direction.
Likely causes
- Moisture in the resin: by far the most common cause with PA6, PA66, PC and PBT
- Melt temperature high enough to degrade the polymer
- Excessive residence time in the barrel
- Contamination or regrind
Where to look first
Check the dryer before touching the machine. Verify the dew point and the actual material temperature at the hopper throat rather than assuming the dryer is working. Hygroscopic polymers need genuine drying, not simply warm air.
Burn marks
Dark or charred marks, usually at the last area to fill or at the end of a rib.
This is almost always trapped air being compressed and igniting (the diesel effect) rather than the material being too hot. The fix is venting at the point where the burn appears, or reducing injection speed at the end of fill so the air has time to escape.
A quick reference
| Defect | Most common cause | Check first |
|---|---|---|
| Short shot | Insufficient material or blocked venting | Shot size and cushion |
| Sink marks | Thick section or early gate freeze | Holding time and pressure |
| Voids | Shrinkage in a heavy section | Wall thickness and holding |
| Warpage | Uneven cooling | Actual temperature of both mould halves |
| Flash | Clamping force or parting line condition | Whether flash is uniform or local |
| Weld line | Melt fronts meeting too cool | Melt temperature and gate position |
| Splay | Moisture in the resin | Dryer dew point and material temperature |
| Burn marks | Trapped air compressing | Venting at the affected area |
Defects are cheaper to design out than to process out. Uniform walls, adequate draft, sensible rib proportions, a considered gate position and proper material drying eliminate the majority of the problems above before a tool is ever cut. Where a defect is genuinely inherent to the geometry, it is better to know that at design review than at first sampling.
Systematic dimensional and visual checking is what turns this from firefighting into control. Our approach to first-article inspection, in-process verification and pre-shipment audit is set out on the quality page.
Struggling with a component that will not run consistently?
Share the part drawing and a description of the defect. We can review whether the cause is more likely to sit in the tool, the material or the process.
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