Solutions for Various Types of Silver Streaks in PC Injection Molding

1. Hydrolysis-Induced Silver Streaks

Hydrolysis-induced silver streaks occur when moisture in the resin hydrolyzes in the barrel. Causes include:

• Insufficient pre-drying of pellets or reabsorption of moisture in the hopper due to inadequate thermal insulation.

• Moisture content exceeding 0.03%, leading to vaporization and resin degradation during melting, generating CO₂ gas trapped in the melt.

Characteristics:

• Evenly distributed along the flow direction; severe cases show dense streaks across the surface.

• Weakens part strength and causes brittleness.

Solutions:

• Thorough drying: Ensure pellet moisture content <0.03%.

• Validation method: Sandwich a pellet between glass slides, heat to 280–300°C, and press. No bubbles indicate proper drying.

• Hopper modifications:

  • Install infrared lamps (25–30W/m²) or steam jackets for thermal insulation.

  • Limit pellet residence time in the hopper to 30–60 minutes.

2. Decomposition-Induced Silver Streaks

Caused by resin overheating and degradation during processing, producing CO₂ gas. Common triggers:

• Excessive barrel temperatures.

• Dead zones in the barrel/nozzle.

• Prolonged melt residence time.

• Reduced resin molecular weight.

Characteristics:

• Irregular distribution (e.g., comet-shaped patterns).

• Accompanied by discoloration or brown spots, especially in the sprue.

Solutions:

• Reduce barrel temperatures in overheated zones.

• Eliminate dead zones via barrel/nozzle cleaning.

• Shorten cycle time or switch to a smaller injection machine.

3. Structural Silver Streaks

Result from poor part design (e.g., abrupt wall thickness changes), causing air entrapment during filling.

Characteristics:

• Fixed distribution along flow direction near thickness transitions.

• Often accompanied by sink marks or internal bubbles.

Solutions:

• Adjust injection speed:

  • For mild thickness changes: Use slow speed for stable filling.

  • For drastic changes: Use high speed + increased pressure to expel air.

• Redesign part geometry or improve mold venting.

4. Runner-Induced Silver Streaks

Caused by improper runner design or blockages:

A. Excessive Sprue Taper Angle (>10°)

• Creates air gaps during filling.

• Solution: Reduce taper angle to 4–6°; polish or replace the sprue bushing.

B. Undersized Gate

• Turbulent flow introduces air.

• Solution: Enlarge gate or modify its shape; reduce injection speed.

C. Nozzle Cold Slug Blockage

• Solution: Increase nozzle temperature and enlarge mold cold slug wells.

5. Pulsation-Induced Silver Streaks

Caused by irregular screw retraction during plastication, introducing air.

Characteristics:

• Random distribution with occasional short shots or bubbles.

Solutions:

• Reduce rear barrel temperature to prevent pellet clumping.

• Increase melt temperature for better plastication.

• Adjust back pressure (≥10% of injection pressure).

• Optimize hopper heating to avoid pellet agglomeration.

6. Gas Entrapment Silver Streaks

Occur when trapped air merges with weld lines.

Characteristics:

• Localized near weld lines.

Solutions:

• Modify gate position/design or add venting channels.

• Process adjustments:

  • Adjust injection pressure/speed.

  • Regulate mold temperature differential between stationary/moving halves.

Key Technical Terms

• Melt Flow Index (MFI): Critical for viscosity assessment.

• Heat Deflection Temperature (HDT): Determines annealing parameters.

• Back Pressure: 3–15 MPa for homogeneous melt.