Injection molding defects are symptoms, not diagnoses. A sink, short shot or burn mark can be linked to part design, resin condition, tooling, machine capability or process settings. Start by locating the defect and comparing it with the part, material and process evidence; there is no single setting change that fixes every part.
This guide explains six common defects, what to check first and how to frame a corrective action. Processing windows and acceptable appearance or strength limits depend on the resin grade, mold, machine and part specification, so confirm proposed changes with the responsible process engineer and material supplier.
How to investigate an injection molding defect
- Describe the symptom. Record photos, defect location, cavity number, affected lot and when it appeared. Separate a surface issue from incomplete fill, dimensional distortion or an internal void.
- Check the approved baseline. Compare actual cycle and process records with the validated run, including fill behavior, transfer point, cushion, hold profile, cooling, alarms and relevant melt or mold temperatures.
- Check the material and tool. Confirm the resin grade and lot, supplier handling and drying instructions where applicable, and look for contamination, blocked vents, gate or runner restrictions, wear, damage or cooling imbalance.
- Choose a cause-specific trial. Change a documented factor or run a planned experiment; avoid changing several settings without recording them. Recheck dimensions, appearance and required performance after the trial.
- Update the control record. Keep the accepted process window and inspection criteria with the part and mold revision so the correction can be repeated.
Injection molding defects troubleshooting chart
| Defect | Common contributors | Evidence to check first | Possible direction |
|---|---|---|---|
| Sink marks or voids | Thick sections, localized shrinkage, early gate freeze or insufficient packing | Location relative to ribs/bosses and gate; part weight or cavity-pressure trend | Review part thickness, gate/runner and packing profile |
| Warpage | Differential shrinkage, cooling, material or flow orientation | Where and when distortion appears; wall thickness, cooling balance and material lot | Review geometry, cooling, gate/fill pattern and material data |
| Flash | Parting-line or shutoff problems, overpacking, clamp capacity, process or venting | Parting line, ejectors and vents; fill/pack record and clamp condition | Find the leak path and correct the tool or process cause |
| Short shot | Flow restriction, trapped air, insufficient material, pressure or temperature | Where filling stops; shot size, fill trace, material feed and venting | Correct the identified restriction, vent, shot or fill condition |
| Weld line | Flow fronts meeting around holes, inserts, gates or thickness changes | Line location relative to the fill pattern, appearance and load path | Assess gate/fill changes, venting and part requirements |
| Burn marks | Trapped gas compression or resin degradation from heat, residence time or shear | Last-to-fill areas, vents, flow restrictions and material/process history | Resolve venting or degradation source within the resin window |
These are investigation paths, not universal recipes. A change that improves one symptom can create another; validate the complete part against its drawing and acceptance criteria.
Six common injection molding defects and what to check
1. Sink marks and internal voids
A sink mark is a surface depression; a void is an internal cavity. Both can result when material in a thicker region shrinks without enough compensation during cooling. They often appear opposite ribs, bosses or other heavy sections, but the gate, runner and packing conditions also matter. Autodesk Moldflow describes localized shrinkage, early gate freeze and insufficient packing among the possible contributors in its sink-mark and void troubleshooting guide.
Check whether the mark or void repeats in the same geometry or cavity, then review the section design, gate path and available packing evidence. Corrective work may involve reducing heavy sections, changing the gate or runner, or tuning the packing profile. Do not raise holding pressure or time blindly: the gate may already be frozen, and excessive packing can create flash or other quality problems.
2. Warpage
Warpage is dimensional distortion associated with uneven shrinkage and internal stress. Cooling differences, thickness transitions, flow orientation, material variation and inconsistent process conditions can all contribute. The Autodesk Moldflow warpage guide lists non-uniform cooling and inconsistent shrinkage among the causes.
Measure the part at a defined time and condition, and compare the affected regions with wall thickness, cooling circuits, gate location and material history. Depending on the evidence, review geometry, cooling balance, gate/fill pattern or material choice. “Make the wall uniform” may be a useful design review, but it is not a complete process diagnosis for an existing tool.
3. Flash
Flash is excess material at a parting line, ejector pin or another mold interface. Possible contributors include a poorly fitting or obstructed shutoff, insufficient clamp force, overpacking, process conditions and vent design. Autodesk’s flashing troubleshooting guidance treats these as distinct causes that need to be checked.
Inspect where the flash starts and whether it is isolated to a cavity or interface. Check for debris or tool damage, then compare fill/pack conditions and clamp capability with the approved setup. Correct the identified source; increasing clamp force without checking overpacking or a damaged shutoff can mask the cause and raise tool loading.
4. Short shots
A short shot means the cavity is not completely filled. It is different from a fully formed part that is underpacked. Flow restrictions, a long or hesitant flow path, trapped air, insufficient shot volume, machine limitations, or material and mold temperatures outside the grade’s processing range may contribute. See Autodesk’s short-shot troubleshooting guide.
Note where the flow stops and compare fill traces, shot size, material feed, machine response and vent locations. Only then decide whether to address a restriction, gate/runner, vent, material selection or fill conditions. Higher speed, pressure or temperature is not a universal correction and must stay within machine, mold and resin limits.
5. Weld or knit lines
A weld or knit line forms where flow fronts meet, for example around a hole or insert or between multiple gates. The line may be visible, and strength depends on the material, fillers, meeting conditions and part loading. Autodesk notes that flow fronts can meet because of gates, holes, inserts or thickness-related flow effects in its weld- and meld-line guidance.
Map the line against the fill pattern and identify whether it lies on a cosmetic surface or a load-bearing area. If the requirement is not met, a design review may consider gate position, fill balance, venting or validated processing conditions. Do not assume that a higher melt temperature or a hot runner will remove every line; sometimes the practical goal is to move it to a less sensitive location and verify the part.
6. Burn marks
Dark streaks or spots can result from trapped air compressing and heating, or from resin degradation associated with excessive temperature, residence time or shear. Autodesk’s burn-mark troubleshooting guide distinguishes these possible mechanisms.
Check whether the defect is at a last-to-fill location, and inspect venting and flow restrictions. Also review material handling, stoppages/residence time, temperature and screw or runner conditions against the resin supplier’s guidance. Slowing injection or lowering temperature may help in some cases but can worsen filling or other defects, so use evidence and a controlled trial.
What to send a molding supplier
For a production issue or a new OEM part, provide the 3D model and 2D drawing with critical dimensions and tolerances, resin grade and color, target volume, defect photos and affected cavity/lot, plus the required appearance, strength and inspection criteria. Include the current mold and process revision when available. This gives a supplier enough context to review the part, material, tool and process as one system.
For project scope, see DAYIN’s custom plastic injection molding services or contact the team with the part requirements.
FAQ
What are the most common injection molding defects?
Common examples include sink marks, warpage, flash, short shots, weld or knit lines, and burn marks. A part can have more than one issue, and the same symptom can have different causes on different tools or materials.
Are sink marks and voids the same defect?
No. Both can relate to localized shrinkage in thicker sections, but a sink mark is a visible depression and a void is an internal cavity. A surface-only inspection may not reveal an internal void.
Are short shots and underpacking the same?
No. A short shot is incomplete cavity filling. Underpacking can occur even when the part appears fully filled, if the cavity was not adequately packed to compensate for shrinkage.
Should I raise injection pressure to fix a defect?
Not without identifying the cause. Pressure changes may help a specific fill or packing issue, but can worsen flash, stress or other quality outcomes. Check the material and machine limits and verify the complete part after a controlled change.
What information helps a supplier assess a molding defect?
Share part drawings or CAD, resin grade, defect photos, cavity and lot information, relevant process records, expected volume, and measurable acceptance criteria. Those details help distinguish a part-design, material, tool or process issue.
Related injection molding resources
- Plastic injection molding: process, materials and OEM sourcing
- How to choose a resin for injection molding
- How to choose plastic injection molding companies
