Processing Troubleshooting · Updated June 29, 2026 · Juhai R&D Center

13 Common Defects in Injection Molding GF-Nylon Parts & Solutions

Glass-fiber reinforced nylon (PA6/PA66 GF) is one of the most cost-effective structural engineering plastics — but its high melt viscosity, anisotropic shrinkage and abrasive fibers make it one of the most defect-prone materials to mold. This troubleshooting manual covers the 13 defects you will actually see on a GF-nylon molding line, each with root cause, parameter-fix checklist and material-formulation adjustment.

Table of Contents

1. 1. Splay Marks (Silver Streaks)

2. 2. Floating Fiber (Surface Fiber Show)

3. 3. Warpage

4. 4. Weld-Line Weakness

5. 5. Voids & Bubbles

6. 6. Burning / Diesel / Discoloration

7. 7. Jetting (Snake-Flow)

8. 8. Black Spots

9. 9. Excessive Shrinkage

10. 10. Dimension Mismatch

11. 11. Short Shot

12. 12. Flash

13. 13. Sink Marks

14. Master Troubleshooting Quick-Reference Table

15. FAQ

16. Summary & Next Steps

1. Splay Marks (Silver Streaks)

Appearance: Fan-shaped silver streaks radiating from the gate, visible on glossy surfaces.

Root cause: Moisture in the melt flashing to steam at ~250°C and erupting onto the mold surface. Secondary causes: trapped air in the feed throat; degraded material in the barrel.

Parameter fix:

• Dry resin at 80°C × 4 h in a dehumidifying dryer to ≤0.08% moisture (Karl Fischer check before each shift);

• Increase back pressure 3–5 bar to compress trapped air;

• Reduce screw speed at end of recovery to avoid air entrainment;

• Drop barrel temperature 5–10°C if material is degrading;

• Purge 3–5 shots if splay appears mid-run.

Formulation fix: Switch to a pre-dried Juhai GF grade with low moisture pickup; for humid-climate molding, ask for a low-moisture PA6 grade variant.

2. Floating Fiber (Surface Fiber Show)

Appearance: Visible individual glass fibers poking through the surface, giving a matte / fuzzy texture, especially on cosmetic parts.

Root cause: Glass fibers pushed to the mold surface because the polymer skin is too thin / freezes too early.

Parameter fix (in order of effectiveness):

• Raise MOLD temperature to 80–110°C — the most effective single fix;

• Increase injection velocity to bury fibers below the skin before freeze;

• Increase back pressure 5–10 bar to improve fiber wetting;

• Increase hold pressure to pack fibers into the matrix;

• Move gate closer to the cosmetic face.

Formulation fix: Use a "CF" cosmetic-grade additive package (Juhai offers this on PA6 BFE01); or switch to ETF02 PA6/PA66 copolymer which wets fibers better; or drop to a lower GF%.

3. Warpage

Appearance: Part bows, twists or cups out of plane after ejection.

Root cause: Anisotropic shrinkage — flow-direction shrinkage (~0.35% at 30% GF) is ~2.7× smaller than cross-direction (~0.95%); the differential internal stress bends the part.

Parameter fix:

• Position the gate so melt flows along the part's long axis — aligns fibers with the long direction;

• Raise mold temperature to 80–90°C to relieve stress and slow cooling;

• Reduce packing pressure in the cross-flow direction;

• Increase cycle time 5–10 s to allow uniform cooling;

• Use variotherm (hot/cold) mold for flat cosmetic plaques.

Formulation fix: Cap GF at 30% (higher GF worsens anisotropy); consider a mineral-filled PA6 (talc / wollastonite) which isotropizes shrinkage, or a PA6/PA66 copolymer. See our GF content selection guide for the anisotropy curve.

4. Weld-Line Weakness

Appearance: A visible line where two flow fronts meet; part cracks or snaps cleanly along the line under load.

Root cause: Glass fibers cannot cross the weld — the weld is resin-only and weak.

Parameter fix:

• Raise melt temperature 5–15°C and mold temperature 10–20°C to delay freeze and let fibers migrate;

• Increase injection velocity through the weld;

• Move the gate closer to the weld, or add an overflow well to dump the cold front;

• Increase hold pressure on the weld;

• Re-design to eliminate multi-gate flow fronts where possible.

Formulation fix: Use a coupling-agent grade (all Juhai GF grades include one); consider lower GF% where weld strength is critical (lower GF lets fibers rotate across the weld).

5. Voids & Bubbles

Appearance: Internal or surface voids; bubbles visible in thick sections or under transparent surfaces.

Root cause: Trapped gas (air or steam) or volumetric shrinkage in thick sections not packed out.

Parameter fix:

• Dry resin properly (moisture steam causes internal bubbles);

• Increase hold pressure and extend hold time until gate freezes;

• Reduce melt temperature 5–10°C to reduce volumetric shrinkage;

• Increase back pressure to compress gas;

• Re-design thick sections to uniform wall where possible.

Formulation fix: For thick-section parts, use a low-shrink or mineral-filled grade; avoid GF% above 35% on thick parts (volatility and shrink).

6. Burning / Diesel / Discoloration

Appearance: Brown / yellow / black discoloration at end-of-fill or in ribs; dieseling sound during fill.

Root cause: Trapped air in the cavity ignites or oxidizes the resin (auto-ignition of compressed air / gas).

Parameter fix:

• Reduce injection velocity at end of fill;

• Add or enlarge vents at the burn location (0.01–0.02 mm depth);

• Drop melt temperature 5–10°C;

• Reduce hold pressure if burning is at end of fill;

• Check screw residence time (long hold — degradation).

Formulation fix: Use a heat-stabilized Juhai grade (all Juhai PA6/PA66 GF grades include heat stabilizer); for sensitive applications request a low-discoloration grade.

7. Jetting (Snake-Flow)

Appearance: Worm-like snake of melt shoots straight from gate before the rest of the cavity fills; visible as a serpentine mark.

Root cause: Melt enters the cavity too fast through a small / poorly-positioned gate without contacting a wall to fan out.

Parameter fix:

• Use a slower injection velocity profile (slow-fast-slow) at the start of fill;

• Move gate to contact a wall or core pin immediately;

• Enlarge the gate or use a fan / tab gate;

• Increase melt temperature 5°C to improve flow;

• Add a flow disrupter / tab opposite the gate.

Formulation fix: Use a lower-viscosity grade (e.g. copolymer ETF02) for hard-to-fix gates.

8. Black Spots

Appearance: Small dark specks scattered in the part surface or interior.

Root cause: Carbonized (degraded) material deposits flaking off into the melt — from long residence time, dead spots, contamination, or carbonized fibers at the check ring.

Parameter fix:

• Reduce barrel residence time (≤6 min PA66, ≤5 min PA6);

• Drop barrel temperature 10°C and verify thermocouples;

• Purge with unfilled PA6 natural at 25–10°C above normal every 30 min;

• Inspect and clean nozzle, check ring and hot runner;

• Schedule regular screw pulls (every 2–3 weeks for high-GF production).

Formulation fix: Use a heat-stabilized grade with anti-oxidation package; avoid switching between high-temp engineering plastics (PBT/PEEK) and PA6 without full purge.

9. Excessive Shrinkage

Appearance: Part smaller than drawing dimension; especially in thick sections.

Root cause: Insufficient packing; low hold pressure or short hold time; melt too hot; mold too cold.

Parameter fix:

• Increase hold pressure (up to 80% of injection pressure);

• Extend hold time until gate freezes (weigh parts to confirm);

• Reduce melt temperature 5–10°C;

• Increase cooling time;

• Increase gate size if it freezes too early.

Formulation fix: Switch to higher GF% (lower shrinkage); use mineral-filled or low-shrink grade; for tight-tolerance parts, request lot-specific shrinkage data on the CoC.

10. Dimension Mismatch

Appearance: Part dimensions out of drawing tolerance after cooling.

Root cause: Combination of shrinkage, warpage, and tolerance stack on tooling.

Parameter fix:

• Run an SPC study to identify flow vs cross-direction error;

• Adjust hold pressure to tune flow-direction size;

• Adjust mold temperature to tune cross-direction size;

• Adjust cooling time for post-ejection shrinkage;

• Verify part is measured at the spec temperature (24 h conditioned, 23°C).

Formulation fix: For PP-to-PA6 or PA66-to-PA6 substitution, expect ~0.02–0.05 mm per 100 mm size shift; request a sample lot and CMM check before volume.

11. Short Shot

Appearance: Cavity not completely filled; part missing material at end of fill or in thin ribs.

Root cause: Insufficient flow length, freeze-off, venting, or low pressure.

Parameter fix:

• Increase injection velocity and pressure;

• Raise melt temperature 5–10°C;

• Raise mold temperature 10°C;

• Enlarge or add vents at end of fill;

• Re-design to add gates closer to the end of fill or thicken ribs.

Formulation fix: Use a lower-viscosity / lower-GF% grade; for long-flow parts, consider a high-flow PA6 variant.

12. Flash

Appearance: Thin film of plastic escaping the parting line, slides, or ejector pins.

Root cause: Injection pressure exceeds clamp force, or tooling clearance is too large.

Parameter fix:

• Reduce hold pressure;

• Reduce injection velocity at end of fill;

• Reduce melt and mold temperature;

• Increase clamp force if available;

• Re-shim or rework parting line; replace worn ejector pin bushings.

Formulation fix: Use a higher-viscosity (higher GF%) grade for thin-wall flash-prone tools.

13. Sink Marks

Appearance: Localized depressions on the part surface opposite thick sections, ribs or bosses.

Root cause: Volumetric shrinkage in thick sections pulls the surface inward.

Parameter fix:

• Increase hold pressure and extend hold time;

• Reduce melt temperature 5–10°C;

• Increase cooling time;

• Reduce rib / boss thickness to 50–60% of adjoining wall;

• Add a small local thickening (dimple) opposite the sink.

Formulation fix: Higher GF% reduces sink (glass does not shrink); use mineral-filled or low-shrink grade; for unavoidable thick sections, consider a structural-foam variant.

Master Troubleshooting Quick-Reference Table

FAQ

Summary & Next Steps

In one sentence: 80% of GF-nylon molding defects trace back to four root causes — moisture, mold temperature, anisotropic shrinkage and residence time. Get drying right (80°C × 4 h), run a hot mold (80–110°C), design gates with the long axis, and purge regularly — and most of these 13 defects disappear. For persistent issues, our R&D team reviews your tool and process and recommends a formulation or grade change with sample lots within 5 working days.