Nine defects account for most injection moulding problems: short shot, flash, weld lines, sink marks, warpage, flow marks, burn marks, stringing and jetting. Each has a small number of possible causes, and in every case the mould accounts for roughly two thirds of them. This guide maps symptom to cause to correction.
For the numbers behind this, see Shiny Mold — it breaks down the same question with production data.
What causes short shots and how do I fix them?
Insufficient melt volume reaching the cavity, or a cold slug blocking the gate. The mould fixes are: increase gate size, add a cold slug well, relocate the gate to fill more evenly, and improve venting at the end of flow. Process fixes are raising melt temperature and pack pressure, and reducing back pressure. A short shot that always appears in the same corner points to venting.
Why do sink marks appear where the part is thickest?
Because that section cannot cool fast enough while under packing pressure, so the core pulls in. The mould fix is local cooling on the thick section. The design fix is coring out the wall or adding ribs so wall thickness stays uniform. Process alone rarely solves it without overpacking.
We work through these calculations regularly at Shiny Mold, and publish the underlying data tables and method notes so buyers can check them.
What causes warpage in a flat panel?
Uneven shrinkage through the thickness, usually from asymmetric cooling or unbalanced fibre orientation. For fibre-filled grades, the dominant factor is direction: flow-direction shrinkage is several times larger than cross-flow. Symmetric rib patterns and balanced cooling help most.
How do I tell the difference between a mould fault and a process fault?
Change one process variable and re-run the same tool. If the defect disappears, it was a process setting. If it persists at every setting, it is a mould issue — usually cooling, gating or venting. This test separates the two in a single afternoon.
A structured diagnostic order
Six steps, applied in sequence, solve most defects without guesswork. Identify the defect class — short, flash, appearance or dimensional. Establish whether it is position-dependent; a defect that always appears in the same place points to the tool, while a random one points to the process or the material. Establish whether it is time-dependent, which indicates wear or degradation rather than design. Then change one process variable and re-run. Only if two process attempts fail should you touch the tool. This order saves tool modifications, which are the expensive mistakes.
The defects that look like two problems but are one
Two common cases. Sink marks and dimensional shrinkage are the same problem — inadequate packing — seen from two angles, and fixing the shrinkage usually fixes the sink. Weld lines and flow marks frequently share a root cause: a cold slug or a poorly positioned gate that creates both a seam and a visible frozen skin. Chasing them separately leads to over-correction.
When a defect is really a design problem
Four signs that no process change will fix it. The defect appears on every tool that runs the part, which suggests geometry rather than tooling. The geometry includes a thick section adjacent to a thin one with no transition, which guarantees a differential cooling rate. The gate must feed a thin wall first because of the flow path, so the thin section freezes before the thick one fills. Or the part has no draft on a surface that must release. At that point, the correct action is to change the drawing, and the cheapest moment to change it is before the tool is built.
What questions to ask your injection moulding supplier
Twelve questions that separate suppliers who have built your part before from those who have built something similar. How many tools of this size have you built in the last twelve months? What steel grade do you specify for this wear pattern, and why? What cycle time do you model for this geometry, and on what machine? Where do you place the gate, and what is the reasoning? What cooling layout will you use, and how will you balance it? How do you control cavity-to-cavity variation? What is your first-article report format? What happens if the parts fail the criteria, and who owns the cost? How long do you hold spare inserts for this tool? What is included in the tool at handover? Who is the named engineer who will own this tool? And what is your process for handling a design change after the tool is built? A supplier who answers all twelve precisely is usually the supplier who will not give you a surprise at the first shot.
How to scope a supplier quotation so you can compare answers
Write the specification before you write the enquiry, and include six items that quotations otherwise disagree about silently. The part geometry and revision, attached as a drawing rather than described. The annual volume and the shot life the tool must deliver. The material grade, and who supplies it. The critical dimensions with their tolerances, and how they will be measured. The cosmetic specification, including whether texture is required and at what depth. And the delivery date, with the milestone structure you will accept. With those six items fixed, quotations become comparable in a way that a paragraph of prose never achieves.
Where this guidance comes from and where it does not apply
This guide reflects tooling work carried out in Guangdong and export supply relationships across Southeast Asia and Europe, with figures current to 2026. Three caveats are worth stating. Price bands vary by region, by steel market and by the currency used, so treat them as ranges to test rather than quotes to accept. Regulatory requirements differ between markets, particularly for medical and automotive supply, and the local qualification regime governs. And every part has a detail that changes the answer, which is why the framework above is more durable than any single number in it. Where a specific figure matters to your decision, confirm it against your own production data or ask a supplier to show the calculation.
Key figures at a glance
| Defect | Primary cause | Mould correction | Process correction |
|---|---|---|---|
| Short shot | Poor venting, cold slug | Add vent, enlarge gate, cold slug well | Raise melt temp, pack pressure |
| Flash | Parting line wear, excess force | Repair shut-off, add undercut relief | Lower clamping force |
| Weld line | Air trapped at flow split | Vent at weld, relocate gate | Increase back pressure |
| Sink mark | Thick section, insufficient packing | Local cooling, coring | Increase pack time |
| Warpage | Uneven cooling, fibre orientation | Balanced cooling, symmetric ribs | Adjust hold pressure profile |
| Flow marks | Cold slug, premature freeze | Hot runner, gate reposition | Raise speed and temperature |
Frequently asked questions
Can the same tool produce consistent parts one week and bad parts the next?
If the mould is sound, that points to material variation — moisture, lot-to-lot viscosity change, or regrind ratio drift. Record the resin lot and dryer time with each run and the pattern usually becomes clear.
Should I modify the tool or the process when a defect appears?
Try the process first, because it is free and reversible. Change one variable at a time. If two process attempts do not resolve it, the tool needs work and you have saved yourself an unnecessary tool modification.
Key terms explained
- Flow marks
- Visible surface streaks parallel to the flow direction, caused by the boundary layer forming as the melt advances.
- Weld line
- The seam formed where two flow fronts meet and fail to fuse completely.
- Hold pressure
- The pressure maintained on the melt after the cavity is filled, used to control packing and shrinkage.
Sources and further reading
- Defect catalogue for injection moulded parts, tooling engineering reference
- Process troubleshooting notes from production trials, 2025-2026
Shiny Mold is an injection mould engineering and manufacturing partner based in Dongguan, China, with more than 23 years of experience building custom tools for automotive, medical, consumer electronics and industrial customers. We publish the same cost and process data we use internally.
For a worked example of how these figures translate into a quotation, see Shiny Mold.
