Glass-fiber-reinforced nylon (PA + GF) delivers dramatically higher strength, hardness, fatigue resistance, dimensional stability and creep resistance than unreinforced PA. But those gains depend on one fragile link: how evenly the fibers disperse in the PA matrix and how strongly they bond to it. In real production, that link shows up as a familiar set of surface defects. Here is how to fix all seven of them — starting from the process, not from polishing the parts.

The Process Foundation: Three Stages That Decide Quality

Before any defect fix makes sense, the process itself must be under control:

1. Pre-molding preparation — confirm raw material performance and dry it properly. Residual moisture vaporizes into silver streaks, bubbles and blisters, and at high temperature and pressure it catalyzes PA crosslinking and degradation. Use hot-air, vacuum or infrared drying.
2. Injection process — dosing (fixed-volume feeding for stable plasticizing) → plasticizing → injection (fill / pack / hold) → cooling after gate freeze → ejection.
3. Post-treatment — heat treatment, moisture conditioning and secondary processing to stabilize and improve part properties.

Get these three stages right and the defects below shrink dramatically.

Defect 1: Glass Fiber Exposure

Fibers flow worse than the PA melt. During filling the melt advances from the core outward, so the fast-flowing resin runs ahead while fibers stay pinned at the cavity surface — leaving visible fibers on the part.

Fixes:

• Increase injection speed — the speed difference between fibers and resin becomes relatively small at high speed
• Raise mold temperature — reduces fiber/mold contact resistance, thickens the melt core and thins the skin layer
• Lower metering-zone temperature and reduce melt volume — limits fiber/resin separation

Defect 2: Burn Marks 

Air in the cavity plus volatiles from the melt get compressed at high injection speed, generating heat that carbonizes the part locally.

Fixes: improve mold venting · lower molding temperature · reduce injection speed and pressure · control moisture content in the resin.

Defect 3: Fiber Clumping / "Flower Marks" 

Clumps of glass fiber appear on the surface. The material solution matters more than the machine settings here:

Fixes:

• Compound with a twin-screw extruder for uniform fiber dispersion
• Use PA-specific reinforcing glass fiber
• Add coupling agent or graft-type compatibilizer (~3%)
• Add ~0.6% silicone-based additive — surface quality improves significantly

Defect 4: Sink Marks 

Caused by insufficient packing, wrong gate position, undersized runners/gates, or uneven wall thickness.

Fixes: increase feed · reposition or balance the gate · enlarge runner and gate · modify the mold or raise injection pressure for uneven wall sections.

Defect 5: Silver Streaks 

Metallic-looking silver marks appear on surfaces, especially at raised flow areas and weld lines.

Fixes: use multi-stage injection (slow-to-fast) instead of low single-speed · raise mold temperature · raise barrel temperature and screw back pressure · reduce screw speed and use a longer barrel to shorten stroke and stabilize melt temperature.

Defect 6: Flow Ripples / Waviness 

Fixes: enlarge a small gate that causes jetting or reduce injection pressure · rework narrow, rough, kinked runners · design uniform cross-sections and remove bosses/ribs where possible · adjust the cooling system or raise mold temperature · raise barrel and nozzle temperatures · increase injection pressure and speed settings.

Defect 7: Flash / Overflow 

Thin flash along the parting line — usually because clamping force cannot seal the mold, aggravated by high molding temperature and high injection speed.

Fixes: increase clamping force · use multi-stage injection (fast-to-slow) · switch to holding pressure earlier · lower barrel temperature · lower mold wall temperature · lower holding pressure.

The Bottom Line

Defects in GF-reinforced PA are rarely random. They follow process logic: moisture, venting, speeds, temperatures and clamping. Master those levers and you can cut scrap, protect margins and ship parts that pass the first time.

DOWREN supplies glass-fiber-reinforced PA6 and PA66 grades with controlled fiber dispersion and batch-stable quality, backed by our production base, QC system and technical team. Capacity assurance protects your schedules; every shipment carries a COA. Send us your part parameters — our technical team will help you match the right grade and process window within 24 hours.

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