Say Goodbye to Scrap: Root-Cause Fixes for the 4 Most Common Nylon Injection Molding Defects
Classification: Industry News
Aug 20,2026
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:
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:
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:
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
Related Blog
Sep 07,2026
Sep 10,2026
Sep 03,2026
Block C, Rongyu City Square, 200 meters north of the intersection of Zhongyuan Road and Pushang Road, Puyang City, Henan Province, China
