Last updated: 2026-09-02 · 5 min read

What Causes Gloss Variation Between Cavities — and How Do You Fix It?

Gloss that differs from cavity to cavity traces back to the cavity surfaces and their environment — polish drift, temperature differences, venting condition, and fill imbalance — and it is fixed by measuring each cavity separately, not by turning process knobs blindly. A multi-cavity mold is four or eight separate surfaces sharing one machine; gloss variation is the visible symptom that those surfaces are no longer identical.

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

  • Gloss prints the steel. On a high-gloss resin like ABS or PC/ABS, a cavity-surface roughness difference of a few hundredths of a micrometer is visible on the part. Cavities polished as a batch drift apart — different hand pressure, stroke direction, or paste batch — and the difference shows up as a gloss map across the mold.
  • Cavity temperature prints too. A cavity running colder than its neighbors freezes a micro-rough surface texture into the part and reads duller; a hotter cavity reads glossier. Uneven cooling circuits and unevenly worn channels create exactly this pattern.
  • Gas and shear leave local signatures. A blocked or worn vent on one cavity produces a faint dull patch near the vent position; a fill imbalance changes the shear history per cavity and shifts gloss on flow-sensitive resins. The pattern on the parts tells you which mechanism you are chasing.

    • 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.

    Diagnose by Cavity Position, Not by Guesswork

    1

    Build the gloss map

    Mold parts, mark them by cavity position, and measure gloss (a gloss meter at a stated angle, or a standardized visual board under fixed lighting) per position. Record the values in the same layout as the mold. The map answers the first question: is the variation random, or does it follow a direction — one side of the mold, one half of the manifold, one cooling circuit?

    2

    Measure the mold, not just the parts

    At mold open, check each cavity surface temperature with a contact probe or calibrated IR gun. Values differing more than a few degrees between identical cavities point at the cooling circuit. Then inspect each cavity under magnification against a baseline record — polish condition, deposits, and any local texture near the vents.

    3

    Check the circuits and the vents

    Verify coolant flow per circuit with a flow meter; a partially blocked channel lowers its cavity’s temperature and its gloss together. Clean or re-cut the vent on the dull position if the dull patch sits where the vent sits — the signature is that local.

    4

    Confirm the fill balance

    Run a short-shot study and compare fill progression per cavity. If one cavity fills later or with different shear history, its gloss shifts on flow-sensitive resins. Correct the runner balance before touching the surface — polish cannot fix a hydraulic imbalance.

    Fixes That Match the Cause

    Diagnosis Fix Verify With
    Polish drift between cavities Re-polish all cavities to one stated roughness spec, one operator, same consumable batch Surface roughness record per cavity + repeat gloss map
    Cavity temperature spread Restore circuit flow, descale or rework channels, equalize controller zones Flow meter readings + cavity temperature check
    Vent condition Clean or re-cut vent to stated depth on the affected cavity Dull patch gone at the same part location
    Fill imbalance Re-balance runner or gate sizes; then re-check gloss Short-shot study before and after
    Surface coating loss or wear Re-coat or re-finish the affected cavity to the baseline spec Gloss map vs the T1 baseline record

    How to Keep the Match After the Repair

  • Write the gloss specification down: measurement angle, instrument, part locations, and the acceptance range — or sign a boundary sample that carries the agreed look.
    A specification you cannot point at is an argument you will repeat.
  • Keep a per-cavity baseline record from T1: surface roughness, gloss readings, and cavity temperature.
    Future drift gets measured against that baseline instead of against memory.
  • Polish all cavities together when any of them needs it — re-polishing one cavity restarts the drift you just eliminated.
  • Log the consumables: the same paste grade and cloth per session, and the same stroke pattern, so the process is repeatable next quarter.

    • Frequently Asked Questions

    Can process settings alone fix gloss variation?

    Partly.
    Raising mold temperature and melt temperature reduces the gloss sensitivity to surface differences, and balancing fill helps where shear is the cause. But if the cavities themselves differ in roughness or temperature, process changes only shrink the visibility of the mismatch — they cannot make four different surfaces identical.

    Why did one cavity go dull after months of running?

    Progressive causes show up per cavity: wear on a polished face from an abrasive resin, loss of a surface coating, deposit buildup from degraded resin or additives, or a cooling channel slowly scaling shut. The gloss map plus a magnified look at that cavity separates wear from deposit — one needs re-finishing, the other a cleaning.

    What gloss tolerance should the drawing carry?

    A measured one: gloss units at a stated angle and named part locations, with an acceptance range — or a signed boundary sample for subjective looks. Where the part must match a neighboring painted or molded component, gloss belongs in the cosmetic acceptance criteria discussed before T1, not in a visual argument after production starts.

    Key Takeaways

    1. Map gloss per cavity position first — the pattern of the variation points to polish, temperature, venting, or fill balance as the cause.
    2. Fix the mechanism, not the symptom: re-polish to a stated roughness spec, restore circuit flow, clean vents, and re-balance the runner before any polish work on a fill problem.
    3. Keep a per-cavity baseline from T1 and a written gloss specification, so future drift is measured against a record instead of relitigated from memory.

    Gloss variation is a per-cavity bookkeeping problem wearing a cosmetic costume. aCatMould keeps the per-cavity gloss and temperature baseline with the mold record, so when a position drifts after a year of running, the repair targets that cavity against documented numbers — and the whole set goes back to matching in one maintenance window, not in a series of trial-and-error shipments.

    Sources
    MoldMaking Technology — polishing and surface measurement
    Plastics Technology — gloss and appearance control
    BYK Instruments — gloss measurement practice

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