Causes And Elimination Methods Of Silver Streaks On Injection‑Molded Parts

Silver streak is a frequent appearance defect for injection‑molded articles, manifested as silvery hair‑like stripes extending along melt filling direction. In severe cases, large‑area whitening appears. It not only ruins surface quality, but also lowers strength of assembly‑mating surfaces. Silver streaks are mostly formed when gas, water vapor and low‑molecular volatile substances are wrapped inside melt and stretched during filling. Defect sources cover raw‑material, equipment, mold and process links, requiring troubleshooting on multiple inducements and corresponding improvement solutions.

1. Excessive moisture and volatile substances in raw materials

Water adsorbed inside plastic pellets serves as the most common trigger for silver streaks. Inadequately dried raw‑materials produce water vapor under heating. Water vapor is stretched during high‑speed melt filling and forms silver marks on part surface. Some raw materials contain low‑molecular additives and residual monomers. Re‑grind material after multiple high‑temperature degradation releases large quantity of volatile gas and also induces silver streaks. Different polymers own different critical moisture content. Hygroscopic materials such as PA, PC and PET must follow strict drying procedures. Non‑hygroscopic PP and PE shall also be protected from storage‑environment moisture. Operators shall verify actual heating temperature of dryer and avoid display‑value deviation from real barrel temperature. Regrind addition ratio shall be controlled. Heavily‑aged degraded re‑grind shall not be fed into production. Raw‑material bags shall be consumed quickly after opening to reduce moisture absorption from ambient air.

2. Thermal degradation at barrel and nozzle sections

Over‑high barrel zone temperature, excessive screw back pressure or fast screw rotation speed brings shear heat and causes thermal degradation of plastics. Cracked material releases massive gas, and bubbles inside melt form silver streaks on molded‑part surface during filling. Over‑heated nozzle leads to continuous material decomposition inside nozzle cavity, with defects concentrating around gate position. Dead corners inside barrel where melt dwells for long time also produce volatile gas from carbonizing decomposition. Barrel and nozzle temperature shall be moderately lowered to stay away from thermal‑degradation range. Excessive screw back pressure and screw rotating speed shall be reduced to cut shear heat generation. When machine stands‑by for long hours, barrel temperature shall be decreased or melt shall be purged to avoid high‑temperature retention. Nozzle and check‑ring shall be disassembled and cleaned regularly to eliminate material‑stagnant dead corners.

3. Poor mold venting performance

Blocked vent slots, insufficient vent‑depth or unreasonable vent positions prevent air inside cavity from escaping during filling. Air is encapsulated by high‑speed melt, compressed and heated, then forms silver streaks on part surface. This type of silver marks often emerges at far‑gate end and thin‑wall corner areas. After parting surface and insert gaps are clogged by adhesive residue and carbon deposit, original vent passages lose effectiveness. Even molds passing early trial may generate batch silver‑streak defects after a period of mass production. Additional vent slots shall be set at melt flow terminal and weld‑line positions. Carbon and glue dirt on parting surface, inserts and ejector pins shall be cleaned to restore smooth venting. Gate position can be optimized to avoid gas trapped in mold dead corners. Ejector‑pin and slide‑block clearances can be utilized for auxiliary venting for deep‑cavity and thin‑wall molded‑parts.

4. Unreasonable injection molding process parameters

Excessively fast injection speed entrains air during rapid cavity filling. Gas cannot exhaust timely and is sealed under part surface to create silver streaks. Insufficient packing pressure fails to compact melt, and tiny inner bubbles are stretched and exposed on outer surface. Improper metering setting and small cushion volume let gas‑entrapped melt enter cavity directly. Injection speed shall be decreased and multi‑stage filling rate shall be adopted. Low speed shall be applied when melt passes gate, then injection speed rises to complete filling. Sufficient cushion volume shall be secured. Packing pressure and packing‑switching point shall be optimized to prevent abrupt pressure change at filling end. Nozzle and sprue‑bush fitting condition shall be inspected. Poor fit will suck external air through gaps. Nozzle center‑line shall be calibrated to eliminate air‑intake clearance.

5. Air intake issues from mold surface and surrounding gaps

Mold‑cavity residual release agent, anti‑rust oil and grease will vaporize upon contact with hot melt and produce silver streaks. Excessive release‑agent spray especially near gate position causes recurring defects. Clearance on mold sealing positions inhales outside air during mold opening‑closing cycles. Cavity and runner shall be thoroughly wiped clean before production. Release‑agent usage shall be minimized, and mold‑structure demolding shall be relied on preferentially. If spray is necessary, dosage shall be strictly controlled. Mold‑plate clamping condition shall be checked to remove abnormal gaps and extra air‑intake sources.

Silver‑streak improvement cannot merely depend on single‑side parameter adjustment. Some products suffer combined influence of moisture absorption, poor venting and material degradation. Operators shall verify factors one by one. Drying effect of raw‑materials shall be checked first, followed by venting status inspection, then optimization on temperature and injection parameters. Removing water vapor, decomposition gas and cavity‑trapped air helps eliminate defects and stabilize molded‑part appearance quality。