Last updated: 2026-09-02 · 5 min read

Cold Slug vs Moisture Streak vs Air Streak: How Do You Tell Silver Marks Apart?

Silver marks on a molded part are thin, silvery-white lines or patterns on the surface — but the visual appearance hides three different root causes that each demand a different fix. Cold slugs (semi-solid melt entering the cavity), moisture vaporization (water flashing to steam at the flow front), and entrapped air (air pockets compressed at flow convergence) all produce silver marks. Diagnosing which mechanism caused the marks — and applying the correct corrective action — starts with examining where the marks appear on the part.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

Key Facts

  • Cold slugs appear as short, curved, silvery marks within 10–30 mm of the gate. The first melt in the nozzle tip cools during the cooling phase and enters the cavity as a semi-solid slug that does not merge with the main flow front. A cold-slug well at the runner end captures this material before it reaches the cavity.
  • Moisture streaks appear as random, branching, fine silver lines distributed across the part surface — not localized near the gate. Hygroscopic resins (PC, PA, PET, ABS) absorb moisture from ambient air; when the wet melt reaches the cavity, the water flashes to steam at 200–300 °C and bursts at the surface, leaving branching streaks along the flow path.
  • Air streaks appear at the end of fill and near weld lines — locations where converging flow fronts trap air. The compressed air burns or oxidizes the resin surface, producing silver or brownish marks. Air streaks are often accompanied by a faint burnt smell and may darken with increasing injection speed.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    Diagnostic Comparison

    Feature Cold Slug Moisture Streak Air Streak
    Location near gate (10–30 mm) random, across surface end of fill, weld lines
    Pattern short, curved, single fine, branching, random clustered, may brown
    Affected resins all hygroscopic (PC, PA, PET, ABS) all
    Root cause cooled melt in nozzle tip water vaporizing at melt front trapped air at convergence
    Primary fix cold-slug well, nozzle temp drying (time + temp) venting, speed reduction

    Step-by-Step Diagnosis

    1

    Map the mark locations

    Examine 5–10 consecutive parts. Mark the location of every silver streak on a printed part drawing. If the marks cluster within 30 mm of the gate, suspect cold slugs. If they are distributed randomly, suspect moisture. If they appear at the last-to-fill area or at weld lines, suspect trapped air.

    2

    Run a drying test

    Take properly dried resin (dried to the supplier’s specified moisture content — typically below 0.02 % for PC and PET, below 0.1 % for PA) and run 10 shots. If the marks disappear, moisture was the cause. If the marks persist, the cause is cold slugs or air.

    3

    Adjust nozzle temperature

    Increase the nozzle temperature by 5–10 °C. If marks near the gate disappear, cold slugs were the cause. If the higher nozzle temperature causes drooling (melt leaking between shots), reduce the temperature and install a nozzle shut-off valve instead.

    4

    Inspect and add venting

    If marks persist at the end of fill after the drying test and nozzle adjustment, inspect the vents. Vents should be 0.02–0.05 mm deep (resin-dependent) at the last-to-fill areas. Clean blocked vents, add missing vents, and reduce injection speed during the final 10–20 % of the shot to give air more time to escape.

    Common Diagnostic Mistakes

  • Assuming all silver marks are moisture — the most common error. Operators see streaks and increase drying time, which fixes the problem only if moisture was the cause.
    If the cause was cold slugs or air, the extra drying has no effect and production is delayed while the real cause goes unaddressed.
  • Confusing cold-slug marks with gate blush — both appear near the gate, but cold-slug marks are curved, comma-shaped streaks while gate blush is a uniform haze at the gate land.
    Examine the marks under 10x magnification to distinguish them.
  • Increasing vent depth without checking the flash threshold — deeper vents may eliminate air streaks but cause flash. Always check the resin supplier’s recommended maximum vent depth before modifying vents.
  • Running wet resin at high speed — high injection speed increases shear heat, which vaporizes moisture faster and makes streaks worse.
    If the resin is wet, the fix is drying, not speed adjustment.

    • Frequently Asked Questions

    Are silver marks always a cosmetic defect?

    In most cases, yes.
    Silver marks are surface defects that do not affect mechanical properties unless they are accompanied by internal voids or bubbles. For structural parts that are painted, plated, or hidden inside an assembly, silver marks may be acceptable. For visible cosmetic surfaces, they are a reject condition.

    Can non-hygroscopic resins produce moisture streaks?

    Rarely. PP, PE, and PS absorb less than 0.01 % moisture from ambient air. Silver marks on these resins are almost always cold slugs or trapped air. Surface condensation (cold pellets moved to a warm floor) can cause temporary streaks, resolved by letting pellets room temperature before opening the bag.

    How do you verify resin moisture content at the machine?

    Use a portable moisture analyzer (loss-on-drying or Karl Fischer type). Take a sample from the hopper throat (not the top of the dryer — the bottom of the hopper, where the driest resin should be). If the reading exceeds the resin supplier’s maximum (e.g., 0.02 % for PC), extend drying time or check the dryer’s desiccant condition and dew-point output.

    Key Takeaways

    1. Location is the fastest diagnostic tool. Cold slugs are near the gate, moisture streaks are random across the surface, and air streaks are at the end of fill or near weld lines. Map the marks before changing any parameter.
    2. A drying test with properly dried resin confirms or rules out moisture in 10 shots. If the marks disappear, the root cause was moisture; if they persist, investigate cold slugs and air.
    3. The most common diagnostic error is assuming all silver marks are moisture and increasing drying time without first checking the mark location. This wastes production time when the real cause is a cold-slug well or insufficient venting.

    A process technician who diagnoses silver marks by location first — then confirms with a targeted test — solves the problem in one shift. A technician who tries every parameter change in sequence (dry more, raise nozzle temp, slow injection speed, add vents) may spend three shifts and still miss the root cause. The diagnostic sequence (location, drying test, nozzle temp, venting) takes 30 minutes and eliminates the wrong fixes.

    Sources
    Plastics Technology — troubleshooting injection molding defects
    Beaumont Technologies — molding defect analysis
    Polymer Processing — drying guidelines for hygroscopic resins

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet