Analyzing Silver Streak Defects in Injection

소개

Silver streaks – also known as silver lines or splay marks – are among the most common surface defects in 사출 성형 . They typically appear as shiny, silvery streaks along the flow direction, indicating gas entrapment or material degradation. The primary culprit is inadequate drying of the resin (moisture or volatile components), but many other factors can also trigger the problem.

> First‑step rule: Always run a trial with thoroughly dried virgin material. If streaks persist, systematically investigate the four areas below.

Component / System	Potential Issue	Remedy
Screw & barrel	Wear or damage creates dead zones where melt stagnates and degrades.	Inspect and replace worn screw/barrel; check non‑return valve (check ring) for proper fit.
Heating system	Thermocouple or heater band malfunction causes uncontrolled overheating.	Calibrate sensors, test heater bands, and verify PID control stability.
Screw design	Poor compression or venting introduces air into the melt.	Use a screw with proper venting (e.g., barrier screw) or adjust back pressure.

Mold geometry and cooling directly affect melt flow and gas escape.

Mold Feature	Problem	Corrective Action
Venting	Insufficient venting traps air and volatiles.	Add or enlarge vents (depth 0.02–0.04 mm for most engineering plastics).
Runner / gate / cavity	High friction resistance causes localized overheating and degradation.	Polish flow surfaces; increase gate size or change gate location.
Flow balance	Uneven filling leads to differential heating and air entrapment.	Optimize runner balancing (simulate with Moldflow).
Cooling system	Poor cooling creates hot spots; leaking water enters the cavity.	Check seals, adjust cooling channel layout.
Cold slug	Unmelted plastic particles degrade or obstruct flow.	Enlarge cold‑slug well; use a heated nozzle or sprue bush.
Runner/gate size	Too small or too long → premature solidification and gas pockets.	Increase runner diameter and gate depth; shorten runners if possible.

Process settings control melt temperature, 압력 , and residence time – all critical to silver‑streak formation.

매개변수	Fault Condition	Optimization
Temperature (barrel, nozzle)	Too high → thermal degradation; too low → poor melt homogeneity.	Set within material supplier’s range; use nozzle temperature 5–10 °C lower than front zone.
Injection pressure / speed	Excessive shear heating → degradation; too low → insufficient compaction to dissolve gases.	Use multi‑stage injection (slow–fast–slow) to balance fill and venting.
Back pressure & screw speed	Low back pressure + high speed → air entrainment.	Increase back pressure (e.g., 5–10 bar) and reduce screw rpm.
Cycle time / residence	Over‑long residence causes degradation (especially for heat‑sensitive resins).	Use machine with shot size between 20 % and 80 % of barrel capacity.
Cushion (buffer)	Too large → melt stagnates; too small → pressure loss.	Optimize cushion to 3–5 mm (depending on machine).
Mold temperature	Too low → poor flow and freeze‑off; too high → extended cooling, gas not vented.	Adjust mold temp within material spec (use water/oil circulating units).
Mold release agent	Excessive agent on cavity surface → volatilizes into silver streaks.	Clean mold with alcohol; apply minimal, even coating if needed.

4.

재료 – Related Causes

Even with perfect machine and mold, the resin itself can be the source of volatiles.

Material Factor	Issue	해결책
Moisture content	Hygroscopic resins (PA, PET, PC, etc.) absorb ambient moisture.	Dry according to supplier’s recommendation (time + temperature); use hopper dryer.
Regrind / recyclate	High regrind ratio introduces degraded material or fines.	Limit regrind to 20–30 %; screen out fines.
Additives (lubricants, stabilizers)	Excessive or poorly dispersed additives release gas.	Reduce additive level; use masterbatch with better dispersion.
Contamination	Mixing with incompatible polymers causes decomposition.	Purity raw materials; clean feeding lines and hoppers.
Volatile solvents	Residual solvents in colorants or fillers.	Pre‑dry colorants and fillers separately; use vacuum dryer if necessary.

Troubleshooting Flowchart (Recommended)

To quickly isolate the root cause, follow this logical sequence:

Dry the resin thoroughly and run a test → if solved, focus on material drying.
If not, check mold vents and cold‑slug well → improve if needed.
Then review process settings – reduce barrel temperature, increase back pressure, shorten cycle.
Finally inspect machine hardware – screw, barrel, heater bands.

Visual aid suggestion: A clear decision‑tree flowchart (Yes/No) for silver‑streak diagnosis.
배치: Near the end of the article, before the conclusion.

Summary Table – Quick Reference

카테고리	Most Common Culprit	Quick Fix
Machine	Worn screw / dead zones	Replace screw/barrel; check check ring
곰팡이	Poor venting	Enlarge vents; add vacuum assist
프로세스	Over‑high melt temperature	Reduce barrel/nozzle temp; increase back pressure
재료	Insufficient drying	Dry at proper temp/time; use dehumidifying dryer

Final Recommendation

Silver streaks are rarely caused by a single factor. A systematic, step‑by‑step approach – starting from material drying, then moving to mold, process, and machine – will resolve >90 % of cases. Keep a log of settings and changes to build your own troubleshooting database.

For heat‑sensitive materials (e.g., PVC, POM, PC), always prioritize short residence time 그리고 low shear to prevent degradation.

저자 소개

Alex Yu

아이디얼 프로 매뉴팩처링

알렉스는 Ideal Pro의 공동 창립자이지만, 주요 프로젝트를 총괄하거나 기술 팀을 이끌지 않을 때는 장거리 자전거 타기를 즐기거나, 동네에서 가장 맛있는 커피를 찾아다니거나, 다음 여행 계획을 세우는 모습을 볼 수 있습니다. 국제 무역 영업과 프로젝트 관리 분야에서 20년 이상의 경력을 쌓아온 그는 촉박한 마감 기한부터 까다로운 엔지니어링 문제까지, 그 사이의 모든 상황을 다 겪어보았습니다. 그는 훌륭한 성과는 충분한 휴식을 취한 마음과 맛있는 커피에서 나온다고 믿으며, 맡은 모든 프로젝트에 그와 같은 호기심과 세심한 주의를 기울입니다. 사무실에 있지 않을 때는 대개 야외 어딘가에서 새로운 장소를 탐험하며, 나중에 나눌 이야기를 수집하곤 합니다.