Say Goodbye to Scrap: Root-Cause Fixes for the 4 Most Common Nylon Injection Molding Defects

Time of issue:2026-08-25 18:14

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Silver streaks, internal bubbles, sink marks and burn marks — four defects that quietly turn nylon (PA6/PA66) injection molding runs into scrap piles and missed deadlines. None of them are unavoidable. Each one has a clear root cause and a clear fix. This guide walks through all four, with the process parameters and mold details that eliminate them for good.

1. Silver Streaks & Bubbles — Moisture and Trapped Air

Silver streaks and bubbles are almost always caused by moisture in the resin or air trapped in the melt. Nylon is hygroscopic: it absorbs water quickly, and when that water vaporizes inside the barrel it produces silver marks on the surface and bubbles inside the part.

The fixes:

Action	Parameter
Pre-dry the resin thoroughly	80–100°C for 4–6 hours; control moisture content to ≤0.2%
Use staged, slow injection	Reduce front-stage injection speed to avoid gas entrapment
Raise back pressure	Increase by 2–5 MPa to push trapped gas out of the barrel
Add mold venting	0.01–0.03 mm deep, 5–10 mm wide vents at cavity ends and weld lines
Control barrel temperature	Prevent overheating, which decomposes the resin and generates gas

Drying discipline alone eliminates most moisture-related rejects. If streaks persist after correct drying, the problem is venting — open the mold and check the vent channels first.

2. Sink Marks — Insufficient Packing and Uneven Cooling

Sink marks appear when the melt shrinks during cooling and there is not enough material packed in to compensate. Thick-wall sections are the classic hotspot.

The fixes:

Action	Parameter
Raise packing pressure	Increase holding pressure and extend holding time
Optimize mold temperature	PA6: 60–80°C; PA66: 65–95°C — keeps the melt flowing until the cavity is full
Enlarge the gate / shorten gate sealing time	On thick-wall parts, ensures pressure transfers to the shrinking section
Extend cooling time	Lets the part cool evenly and reduces inner/outer shrinkage differences

The mold temperature window matters: run PA6 and PA66 in their respective ranges, not "somewhere warm." Correct packing plus correct mold temperature eliminates sink marks on 95% of geometries without touching the part design.

3. Burn Marks — Trapped Gas and Localized Overheating

Burn marks (blackened, carbonized traces near edges or at flow ends) come from poor venting and localized overheating. High-speed melt friction plus trapped air creates a pressure spike that scorches the surface.

The fixes:

Action	Detail
Lower front barrel temperature and injection speed	Reduces friction-generated heat that carbonizes the melt
Clean the mold vents thoroughly	Remove carbon deposits and debris so gas can escape
Calibrate clamping force	Prevents air lock and pressure build-up in the cavity
Optimize the gate position	Move it away from dead corners where gas accumulates

When a burn mark appears, the first question is not "how do I polish it off" — it is "where is the air trapped?" Fix the venting and the burn mark disappears at the source.

4. Prevention System: Control All Three Dimensions

Defects are symptoms. The real cure is a control system across three dimensions — material, process and mold:

Material : Store nylon resin in sealed, moisture-proof packaging; dry before every run, not just the first one.
Process : Standardize parameters — drying, temperatures, pressures, injection profile — and record them per mold so every repeat run starts from a proven baseline.
Mold : Schedule regular maintenance: clean vents, check gates, verify clamping and cooling channels.

Run all three consistently and the four defects above drop to near zero. Production stays stable, delivery dates stay honest, and your scrap bin stays empty.

Partnering with a Supplier Who Understands Process Stability

Consistent parts start with consistent material. DOWREN supplies PA6 and PA66 grades with controlled moisture and batch-stable quality, backed by our production base, QC system and technical team. Capacity assurance protects your schedules, and every shipment carries a COA. If a defect is costing you money, send us your part parameters — our technical team will help you match the right grade and the right process window within 24 hours.

Get a quote · Request samples · Download datasheets

📧 dowren001@dowren.com | ☎️ +86 18137458807 | WhatsApp: +86 18137458807

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