Causes and Improvement Solutions for Silver Streak Defects on Transparent Plastic Parts

Transparent optical plastic parts such as PC, PMMA, PET and AS frequently suffer from silver streaks and cloud-like white marks during injection molding. These lines extend along the plastic flow direction and appear silvery white under light transmission. In mild cases, the products fail appearance inspection; in severe cases, internal microcracks will reduce light transmittance and structural strength. Silver streaks are essentially tiny voids inside the melt that form silvery lines due to light refraction. The main triggers include excessive moisture of raw materials, thermal degradation, excessive shear, trapped air and internal stress. This article comprehensively sorts out all root causes and practical improvement solutions.

1. Visual Identification Methods of Silver Streaks

Different types of silver streaks can be distinguished visually for quick on-site troubleshooting. Silver streaks caused by moisture are scattered all over the product with the most obvious whitening at the sprue, without fixed orientation. Silver streaks induced by high-temperature shear concentrate around the gate and present thin straight lines. Silver streaks from trapped cavity air mostly appear at product ends and rib dead corners, accompanied by black marks in serious conditions. Stress-induced silver streaks do not show right after ejection; they turn white and form streaks after storage or bending under force. Silver streaks generated by mixed materials and carbonized impurities exist as dotted miscellaneous lines with irregular defective batches.

2. Root Causes of Silver Streak Defects

(1) Silver Streak Induced by Raw Material Issues

Raw material problems account for most batch silver streak failures. Transparent plastics have strong water absorption, especially PC, which requires a moisture content below 0.02% for production. Insufficient drying temperature, inadequate drying time or poor sealing of the feeding barrel will leave internal moisture in materials. The moisture vaporizes under high temperature to form bubbles, which stretch into filament silver streaks during melt filling. Materials stored for too long or mixed with excessive regrind will break molecular chains under heat and shear, releasing small-molecule gas that forms silver streaks when mixed with melt. Mixing different transparent grades, residual carbonized materials in hoppers and barrels, and dust contamination will cause cracking at material interfaces to form white lines. Excessive lubricants or external release agents precipitate under high temperature and attach to the melt surface, damaging the bonding force of surface molecules and resulting in widespread foggy silver streaks on plastic parts.

(2) Silver Streaks Caused by Unbalanced Injection Molding Parameters

Improper injection parameters directly aggravate silver streak defects. Excessively high temperatures of barrel and nozzle lead to thermal decomposition and gas generation of plastics; overly low temperatures reduce melt fluidity, causing huge tensile stress on the surface during filling and tearing to form stress silver streaks. Full-speed injection or high injection speed at the gate generates intense shear heating at narrow gates, forming dense circular silver streaks around the gate. Too low backpressure fails to fully mix materials and discharge internal moisture and gas, while excessive backpressure increases screw shear and accelerates material degradation and gas production. Insufficient holding pressure and short holding time create internal shrinkage micropores that evolve into internal silver streaks. Low mold temperature instantly freezes the melt surface, creating tensile stress between the surface and core layers due to inconsistent shrinkage. Excessive ejection force and uneven ejector pin layout concentrate stress at ejection positions and induce silver streaks. Unvented cavity air is compressed and heated to burn the plastic surface, forming flaky blackened silver streaks locally.

(3) Silver Streaks Resulting from Mold Design and Processing Defects

Mold defects continuously trigger silver streaks that cannot be completely eliminated by parameter adjustment alone. Small gate cross-sections or sharp-tipped pin gates cause excessive melt shear rate and persistent silver streaks at gate outlets. Right-angle runners and abrupt cross-section changes create turbulent flow that entraps air, leading to scattered silver streaks all over the part. Missing vent slots at parting lines, product ends, insert joints and deep ribs, or vent depth exceeding 0.03mm leads to ineffective ventilation and burn marks from trapped air. Transparent part molds with ordinary polishing instead of mirror finish increase flow friction, causing fine silver streaks from surface scratches on molded products.

(4) Silver Streaks Derived from Product Structure and Post-Processing

Unreasonable structural design and post-processing also produce silver streaks. Large wall thickness differences create internal shrinkage voids in thick sections that appear as silver streaks under light transmission, with concentrated stress at thickness transition zones generating filament marks. Sharp corners without rounded transitions raise flow resistance and local stress concentration. Parts without stress relief annealing release internal stress after storage or assembly and gradually develop silver streaks. Forcible extrusion and forced bending during assembly also pull the material to form secondary silver streaks.

3. Targeted Improvement Solutions

(1) Raw Material Control Solutions

Dry materials strictly in accordance with material standards: PC at 120°C for over 4 hours, PMMA at 80°C for 3–4 hours. Store dried materials in sealed heat preservation containers to prevent secondary moisture absorption. Limit the proportion of regrind; aged and yellowed recycled materials are prohibited. Thoroughly clean barrels and hoppers before material switching, store different grades separately, reduce external release agent usage and select substrates with internal lubrication preferentially.

(2) Injection Molding Parameter Optimization Solutions

Slightly lower barrel and nozzle temperatures within the lower-middle range of material recommended values. Adopt segmented injection and reduce speed at the gate section to cut shear heat. Adjust backpressure to a reasonable range to ensure sufficient mixing and degassing without over-shearing. Raise mold temperature to narrow internal and external temperature differences and lower molding internal stress. Appropriately increase holding pressure and extend holding time to fill shrinkage micropores. Match low injection speed with adequate ventilation to reduce trapped air. Adjust ejection speed and add ejector pins to evenly distribute ejection force.

(3) Mold Modification & Optimization Solutions

Enlarge gate cross-sections and adopt fan or submarine gates to reduce shear. Add rounded transitions to all runners to eliminate abrupt cross-sections. Open 0.02–0.03mm deep vent slots at melt ends, rib positions and parting lines with extended vent length. Apply mirror polishing to cavities and runners to reduce flow friction. Control product wall thickness difference within twice the thin wall thickness and add R rounded corners to sharp edges.

(4) Post-Processing Control Solutions

Add an annealing procedure after molding to slowly release internal stress of transparent parts. Avoid forced pressing and heavy bending during assembly to reduce stress-induced silver streaks from external force. Maintain a dry and dust-free production environment to cut impurity silver streaks caused by dust mixed into raw materials.

4. Quick On-Site Troubleshooting Process

First inspect raw material drying status and test moisture content to eliminate moisture-related silver streaks. Next lower injection speed and barrel temperature to distinguish shear degradation silver streaks. Check whether mold vent slots are blocked to resolve trapped-air streaks. Finally verify mold structure, product wall thickness and annealing process to handle silver streaks derived from internal stress.