What Causes Delamination in Molded Parts?

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

Delamination is the part coming apart in layers: thin skins lifting, flaking or peeling where the material should be one continuous piece. It is an interface failure — a weak plane where two layers of melt met but never fused, usually because something stood between them: moisture flashing to steam, excess release agent, incompatible carrier or foreign resin left in the system. Find what sits on the interface and the layers stop coming apart.

Key facts

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  • Molded material fuses because it is one clean, hot, mobile melt; a layer splits where the melt was two things at once — separated by moisture, contamination or an incompatible blend partner.
  • Moisture is the leading suspect: water held in the resin flashes to steam during plastication and leaves a vapor-weakened plane inside the wall — the classic pre-delamination crack that later peels.
  • Delamination repeats in place: the same face, the same depth, shot after shot — because its cause is systemic (drying, contamination, material) and not random molding noise.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. See also related guide.

    Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

    What separates the layers?

    Three families cover most cases. Moisture first: resin dried below its datasheet requirement carries water into the melt, and that water expands to steam inside the barrel — the steam does not dissolve, it forms its own micro-layer at the flow front, and the part is born with weak planes waiting to peel, as the drying page details.
    Contamination second: anything foreign in the system — a different resin left in the barrel from the last job, oil migrating from the machine, an over-eager hand with release spray, regrind carrying labels or foreign color — introduces a substance the melt cannot fuse with, as the regrind rules warn.
    Incompatibility third: the material itself may contain its own rival — a masterbatch carrier the base resin does not accept, or two grades blended that were never meant to meet. All three write the same symptom, which is why the cure begins with identifying which family owns the part, not with adjusting pressures.

    How do you find the family — and fix it?

    Read the part, then run the quick experiments in order. Surface skin that peels in flakes points at the flow front — moisture or front contamination; internal splits found in sectioning point at whatever sat in the melt longest.
    Then test by removal: dry the resin to specification and run again — if the flaking fades, moisture owned it, and the fix is a drying regime with a record, not a hope. If it persists, purge the system thoroughly and drop every additive for one run — release agent, colorant, regrind — running natural resin clean: a part that comes out whole convicts the additive or the contamination, not the base resin.
    Where the dry-and-purge cycle cannot clear it, the blend itself is questioned against the datasheet: carriers and grades have compatibility limits the supplier documents. The timeline matters too — a defect present from the first shots points at material and conditioning; a defect that appeared after months points at something that changed — a new drum, a new colorant lot, a service visit that left a film. Each answer narrows the family before anyone spends money.

    Which delamination checks belong in the record?

  • Symptom photographed and mapped — which face, which depth, how the layers part: peel, flake or internal split.
  • Drying records verified — moisture at press time within datasheet limits, with the log to prove it.
  • Release agent audited — how much, where, and whether the part needs it at all with proper draft.
  • Regrind lineage checked — ratio, source cleanliness and storage, per the regrind page.
  • Additives questioned one at a time — colorant and its carrier confirmed compatible with the base grade.
  • The clean run documented — the natural-resin purge test and its result written down, so the family is on record.

    • Frequently asked questions

    Is delamination always a moisture problem?

    It is the most frequent single cause and the first to rule out, because the test is fast: dry to spec and compare.
    But it is not the only one — contamination from a previous resin, oil in the system, an incompatible colorant carrier or an off-spec blend can all open the same weak plane. Moisture earns its reputation by being common and invisible; the other families get missed precisely because moisture takes the blame by default.

    Can the barrel itself cause layering?

    Yes — the barrel is where previous jobs, purge compounds and lubricants wait to surprise the next part. A resin swap without a full purge leaves old material sheared into the new melt; machine oil drawn past seals enters at the feed throat; even over-lubricated regrind carries its history. A suspect barrel answers to a thorough purge with natural resin — and to checking where the peel sits relative to the first shots after every changeover.

    Why does it look like fish scaling on the surface?

    Fish scaling is the surface skin parting from the bulk in small, overlapping flakes — the visible end of a weak plane sitting just under the skin. The plane forms at or near the flow front, where steam pockets or contaminant films have the least material above them; under packing pressure it flattens invisibly, and only after the part cools and flexes does the skin lift.
    That is why the defect so often shows up in handling or assembly, not at the press — the weak plane was manufactured in, and daylight finished the job.

    Key takeaways

    1. Delamination is an interface failure — moisture, contamination or an incompatible blend stands between layers that should have fused.
    2. Diagnose by removal: dry to spec, purge clean, strip additives one run at a time — the family reveals itself.
    3. Appearance timing tells history: defects from shot one implicate material and conditioning; late arrivals implicate a change.

    If parts are peeling, flaking or splitting in layers, send a photo with how the part failed and what changed recently. You will get the suspect list ranked and the removal tests in order — by a named tooling engineer, before more material is scrapped.

    Sources
    Plastics Industry Association — industry resources on injection molding practice
    MatWeb — material property database, reference for resin drying and compatibility data
    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