Are Splay Marks Moisture or Shear — and How Do You Tell?

Last updated: 2026-09-01 · Reading time: about 4 min

Splay is the melt leaving silver in its wake: silvery streaks radiating along the flow path, most visible on glossy and dark faces. The streaks are gas trails at the skin — and the two gases that put them there are steam from wet resin and volatiles from over-sheared, degrading material; the fix differs completely, so the first job is telling the two apart. The good news: each suspect answers to a different test, and both tests are quick.

Key facts

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  • Splay is a skin phenomenon: gas bubbles at the flow front get stretched into the frozen surface as silver lines — the gas is the message, and its origin is the diagnosis.
  • Moisture splay is democratic: it appears along the whole flow path, tracks the drying record, and fades when the resin is dried to spec — the quickest test in molding is simply running dried material.
  • Shear splay is territorial: it concentrates near the gate and high-shear regions, worsens with speed and screw aggression, and often brings discoloration or a burnt smell as accomplices.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

    Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

    What draws the silver lines?

    Gas at the flow front is the artist; the suspects are a short list. Moisture first: hygroscopic resins — polyamide, polyester, polycarbonate, TPU — hold water that flashes to steam in the barrel, and the steam rides the front until the skin freezes it into streaks; the drying page covers the whole chain from hopper to part.
    Volatiles second: over-sheared or over-heated resin sheds its own decomposition gases, and those paint the same silver — this is degradation showing early, with the burn-mark page describing where it heads next. Foreign films third: release agent sprayed too generously, oil migrating from the machine, condensation on a cold hopper — each introduces a vapor layer the front exposes.
    What splay is not: it is not the same defect as the broader family of flow marks — banded gloss changes record how the front traveled, while splay specifically records gas in the skin. Keeping the two separate matters because their cures differ: profiles treat travel, drying and thermal discipline treat gas.

    How do you tell the two apart?

    Run the two-question test. Question one — position: silver spread evenly along the whole flow path points at moisture; silver hugging the gate and the first inches of travel points at shear.
    Question two — response: dry the resin to datasheet spec and run — splay that fades was steam; splay that persists survives the dryer and belongs to the shear family, confirmed by easing screw speed and back pressure until the silver answers. The tests stack in your favor because each is reversible and costs a run, not a tool change.
    When both fail — splay that neither drying nor gentler shear clears — look at the third tier: a contaminated hopper, condensation on cold material, or an additive the base resin refuses. Whatever the verdict, the evidence goes on record: the pattern photographed, the drying log cited, the speed steps noted — the trial record that closes the case belongs beside the acceptance file, so the next occurrence starts with history instead of from zero.

    Which splay checks belong in the record?

  • Pattern photographed and mapped — full-path or gate-hugging, on the same part orientation every time.
  • Drying log verified before testing — temperature, time and dew point against the datasheet, not from memory.
  • Dried comparison run made — the fastest discriminator: steam splay fades, shear splay stays.
  • Shear inputs eased and compared — screw speed, back pressure and first-stage velocity stepped down while watching the silver.
  • Foreign sources eliminated — release agent, hopper condensation and machine oil ruled out by a clean run.
  • The verdict documented — cause, test and cure recorded with the process file, so the fix outlives the shift.

    • Frequently asked questions

    Does splay mean the part is weak?

    Not by itself — a silver streak is gas trapped at the skin, and the wall beneath usually carries full strength.
    But splay caused by shear degradation is an early warning: the same heat history that made the gas is cutting chains in the bulk, and the brittleness arrives later if the cause stays. That is why the moisture-versus-shear verdict matters beyond appearance — steam splay is cosmetic, degradation splay is a countdown.

    Why did the splay appear only after a resin change?

    Because drying requirements are grade-specific: the new grade may demand longer drying, higher temperature or a lower dew point than the last one, and a hopper set for the old resin under-dries the new. Moisture content that was fine yesterday is steam today. The datasheet’s drying line is part of the material spec, not a suggestion — re-check it at every change, and the mystery disappears before the first shot.

    Can higher mold temperature fix splay?

    It can hide it: a warmer tool lets the skin freeze later, giving gas more time to escape before the surface sets — some streaks fade as a result. But it is a mask, not a cure, and it costs cycle time forever. Treat warmer steel as a diagnostic aid at most; the cure is removing the gas — drying for steam, gentler shear for degradation — and the silver leaves with the gas.

    Key takeaways

    1. Splay is gas at the skin: steam from wet resin or volatiles from over-sheared material — two causes, two very different cures.
    2. Tell them apart by position and response: full-path silver fades with drying; gate-hugging silver answers to gentler shear.
    3. Shear splay is an early warning of degradation — treat it as a countdown, not a blemish.

    If silver is streaking your parts, send a photo with the drying log and current settings. You will get the two-question test run in order and the cause named — by a named tooling engineer, before the next batch runs.

    Sources
    Plastics Industry Association — industry resources on injection molding practice
    MatWeb — material property database, reference for resin drying requirements
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization

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