PA6 Injection Molding Defects: 8 Common Problems and Solutions
PA6 is one of the most forgiving nylons to mold — and one of the most sensitive to moisture, mold temperature and holding time. When parts come out with sink marks, gas traps, sticking, short shots or dimensions that drift from batch to batch, the cause is almost always one of a handful of process or design factors, not bad material. This guide covers the eight defects we see most often on PA6 jobs, what actually causes each one, and the machine adjustments that fix them — in the order worth trying.
If you are working through a defect you have not seen before, the defect troubleshooting hub on our site lists the full symptom-to-cause workflow. Below is the PA6-specific version.
Contents
1. Sink Marks and Voids
Symptom: shallow depressions on the surface, usually over thick walls, ribs or bosses. Voids are the hollow cavities that form inside thick sections when the surface freezes before the core is packed.
Root cause: PA6 has high crystallization shrinkage — about 1.2–1.5% for unreinforced grades — so if the holding phase cannot feed the shrinking core, the surface collapses. Typical triggers: holding pressure too low or holding time too short, a gate that freezes early, melt or mold temperature so high that shrinkage is amplified, and thick ribs or bosses compared with the nominal wall.
Fixes in order: raise holding pressure to roughly 40–80% of injection pressure and extend holding time; enlarge the gate so pressure reaches the thick section before seal-off; keep ribs at or below 60% of the nominal wall thickness; hold mold temperature in the 60–100 °C band instead of running cold; make sure the material is dried below 0.1% moisture so trapped gas is not adding to the collapse.
2. Gas Traps and Blisters
Symptom: burned or silvery marks at the end of fill, blisters that lift the surface, or bubbles visible through thin translucent sections.
Root cause: three sources in practice — trapped air that cannot escape from a dead-end cavity, moisture flashing to steam (PA6 with more than ~0.2% moisture degrades and outgasses), and melt degradation products. Fast fill makes it worse by trapping air before it vents.
Fixes in order: add or deepen venting at the last-filled area (typical vent depth for PA6 around 0.02–0.04 mm); dry the pellets to below 0.1% moisture; slow the injection speed in the final filling stage so air has time to escape; as a last resort, move the gate or add a venting insert. More detail on moisture and drying is in our nylon drying guide.
3. Sticking and Difficult Ejection
Symptom: the part sticks on the core, deforms on ejection, or needs hammering out; surface gets scratched or stretched.
Root cause: most sticking on PA6 is a mold issue, not a material issue. PA6 shrinks more than most plastics, so the part grips the core hard; add insufficient draft, a rough or unpolished surface, high core-side temperature that delays solidification, or an overheated barrel that degrades the melt into a tacky layer, and the part will not release.
Fixes in order: increase draft angle on deep walls; drop the core-side mold temperature 5–10 °C so the skin solidifies faster; lower the front barrel and nozzle temperature 5–10 °C to reduce melt tackiness; polish and coat the core; check ejector area and add ejector pins where deformation appears.
4. Dimensional Drift
Symptom: the same cavity produces parts that measure differently between shifts, or parts that grow after shipping to a humid customer site.
Root cause: PA6 dimension is set by crystallization and by moisture. If mold temperature, cooling time or holding varies, crystallinity varies and so does shrinkage. And PA6 absorbs water in service — dimensions can grow measurably as moisture content rises, which also weakens electrical and dimensional stability.
Fixes in order: hold mold temperature steady (within about ±5 °C); keep cooling time consistent; dry material to below 0.1% before molding so the starting moisture is always the same; where tolerance demands it, anneal at 130–150 °C for 1–2 h to stabilize crystallinity; if the part must stay stable in humid use, consider how much moisture uptake the design allows.
5. Short Shots (Incomplete Fill)
Symptom: the cavity does not fill completely; corners, thin fins or the far end of the part are missing.
Root cause: insufficient shot volume or metering, melt temperature at the low end of the PA6 range, cold mold, small gates or runners, or poor venting that builds back-pressure.
Fixes in order: increase shot metering; raise melt temperature into the 250–270 °C band for the front zone; increase injection pressure and speed; enlarge gates and runners; improve venting. Check that the screw is not slipping or the check ring worn, which quietly robs shot volume.
6. Jetting and Flow Marks
Symptom: worm-like, snake-shaped trails on the surface near the gate, or rippled flow lines.
Root cause: the melt shoots through an undersized gate in a jet before it folds against the cavity wall — classic when injection speed is high and the gate is small. Cold material layers freeze on top and leave the trail visible.
Fixes in order: slow the first-stage injection speed so the melt advances as a front; enlarge the gate or switch to a fan or film gate; raise mold temperature to delay freezing of the trail.
7. Yellowing and Degradation
Symptom: yellow, brown or brittle material at the gate or nozzle area, sometimes with black specks.
Root cause: thermal-oxidative degradation from melt temperature too high or residence time too long, hydrolytic degradation from moisture above ~0.1–0.2%, or excessive shear from high back pressure and screw speed.
Fixes in order: keep melt temperature within 230–270 °C and avoid long dwell on small machines; reduce back pressure and screw speed; dry pellets to below 0.1%; purge the barrel before restart; if the color shifts mid-run, shorten cycle or reduce barrel temperature before suspecting the grade.
8. Flash
Symptom: thin fins of material escaping at the parting line, around pins or vents.
Root cause: PA6 is a low-viscosity melt when hot, so any gap becomes a leak path: holding pressure too high, mold not clamped hard enough, or a worn parting line. High mold temperature also keeps the melt too fluid.
Fixes in order: reduce holding pressure and speed near the end of fill; increase clamping force; check and re-cut the parting line; confirm the mold is vented instead of leaking through gaps.
9. Reference Process Parameters
| Parameter | Typical range (PA6) | Note |
|---|---|---|
| Drying | 80–90 °C, 4–6 h, dehumidifying dryer | Target moisture below 0.1%; above ~0.2% causes silver streaks and degradation |
| Barrel temperature | Rear 220–240 / mid 240–260 / front 250–270 °C | Glass-filled grades run 5–10 °C hotter |
| Mold temperature | 60–100 °C | Structural GF parts 70–90 °C; thick parts 100–120 °C for even crystallization |
| Injection pressure | 80–140 MPa | Start mid-range, adjust for short shots or flash |
| Holding pressure | 40–80% of injection pressure | Longer holding time fixes sink marks before higher pressure does |
| Injection speed | Medium to high, profiled | Slow start avoids jetting, slow final stage avoids gas traps |
| Back pressure | 0.3–1.0 MPa | Higher back pressure improves mixing but can degrade the melt |
| Mold shrinkage | Unreinforced 1.2–1.5%; GF30 flow 0.3–0.5% / transverse 0.9–1.1% | Anisotropy drives warpage on glass-filled parts |
10. Quick Troubleshooting Chart
| Defect | Main causes | First fix to try |
|---|---|---|
| Sink marks / voids | Low holding, early gate freeze, thick sections | Raise holding pressure, extend holding time |
| Gas traps / blisters | Poor venting, wet material, fast final fill | Deepen vents, dry below 0.1%, slow final stage |
| Sticking / ejection | Low draft, hot core, overheated barrel | More draft, cool core side 5–10 °C |
| Dimensional drift | Unstable mold temp, moisture uptake | Stabilize mold temp, dry consistently |
| Short shots | Low metering, cold melt, poor venting | Increase metering, raise melt temperature |
| Jetting / flow marks | Fast first stage, small gate | Slow first stage, enlarge gate |
| Yellowing / degradation | Overheated melt, long dwell, wet pellets | Lower melt temperature, dry below 0.1% |
| Flash | High holding, worn parting line, low clamp | Reduce holding pressure, check parting line |
11. Common Mistakes
Chasing a defect with one parameter at a time and never drying the material first — wet PA6 produces streaks, voids and degradation that look like unrelated defects.
Running the mold cold to shorten cycle; low mold temperature on PA6 leaves more residual stress and less stable dimensions.
Judging melt temperature by the barrel gauge; a small machine with long dwell can degrade PA6 while the gauge looks normal.
Treating sink marks with higher injection pressure instead of longer holding time — injection pressure stops mattering after gate seal.
Blaming the material for flash when the parting line is simply worn; check the mold before changing grades.
12. FAQ
Summary
Most PA6 defects trace back to three things: moisture, crystallization, and holding. Dry the material below 0.1%, run the mold temperature in the 60–100 °C band and keep it steady, and fix sink marks with holding time before pressure — then most of the list above stops appearing. When a defect is new to you, work from symptom to cause instead of changing grades first.
