Mold Trial Shot Review — How Do You Evaluate the First Shots from a New Mold, and What Defects Must Be Fixed Before the Mold Is Accepted?

Last updated: 2026-09-03 · Reading time: about 6 min

A trial shot review is the systematic inspection of the first 20–50 shots from a new or reworked mold, to decide whether it is production-ready. Review in four stages — stabilize the process, label every sample, visual-inspect to numeric limits, then CMM the critical dimensions — and triage each defect as critical (fix and re-trial), major (fix if the face is cosmetic) or minor (deviate and monitor).
See also related article.

Key facts

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  • Do not judge the first shots. Run 20–30 to let the mold steady-state temperature, confirm the part weight holds within ±1%, then collect shots 30–50 as the review sample — each labeled with shot, mold, date and cavity.
  • Inspect to numbers, not eyes: sink ≤0.1 mm cosmetic, flash ≤0.05 mm (≤0.02 mm precision), flatness 0.1–0.5 mm/100 mm, critical dimensions within ±0.05 mm on a CMM across 5 samples.
  • Triage before you accept: critical = will not function or fit, fix and re-trial; major = works but shows, fix if cosmetic; minor = hidden in assembly, deviate and monitor. Accept at zero critical and zero cosmetic major.

    • 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.

    How do you review the first shots — and how many do you need?

    The review is a sequence, not a glance. Each stage produces the evidence the next one needs.

    1. Stabilize before you judge. The first 10–20 shots are not representative — the mold is still climbing to steady-state temperature. Run 20–30 shots at the target parameters, confirm the part weight holds within ±1% from shot 20 to shot 30, then collect shots 30–50 as the review sample.
    2. Label every sample for traceability. Mark the shot number, mold ID, date and cavity number (C1, C2…) so each part traces to the shot and the parameters that made it. Log melt and mold temperature, injection and packing pressure, cooling and cycle time every 10 shots, and have the technician and quality engineer sign the sheet.
    3. Visual-inspect all samples. Under 500–1,000 lux at 45°, check each part for sink, weld lines, flow marks, burns, flash and short shots — and measure each against its numeric limit rather than calling it by eye.
    4. CMM the critical dimensions on 5 samples. Measure shots 30, 35, 40, 45 and 50 so you see process consistency, not one lucky part. Record nominal, tolerance and measured value for every critical dimension, flag anything out, and check flatness while the part is on the table.
    What you check Acceptance limit How measured
    Sink mark ≤0.1 mm cosmetic, ≤0.2 mm non-cosmetic Feeler or depth gauge
    Flash at parting line ≤0.05 mm standard, ≤0.02 mm precision Micrometer
    Weld line 50–80% of bulk strength; not on a Class-A face Visual, plus a pull test if structural
    Flatness / warpage 0.1–0.5 mm per 100 mm, by function Surface plate or CMM
    Critical dimensions Within ±0.05 mm or the drawing callout CMM across 5 samples
    Short shot None — the cavity fills completely Visual

    How do you classify the defects — and which must be fixed before acceptance?

    Not every defect stops the mold. Sort what you found into three severities, and let the severity decide the action.

    Severity What it means Action Examples
    Critical The part will not function or will not fit Fix the mold and re-trial at T2 Dimension out of tolerance, short shot, out-of-spec flash, warpage past flatness
    Major The part works but a visible defect shows Fix if it sits on a cosmetic face; otherwise deviate Sink on a cosmetic surface, weld line in view, flow mark on Class-A, burn at end of fill
    Minor The defect is not visible in assembly Document in a deviation report and monitor in production Ejector-pin mark on a hidden face, slight color variation, gate vestige within tolerance

    The acceptance line. The mold passes when every critical dimension is in tolerance, there are no short shots, flash is within spec, warpage is inside the flatness callout, and no major defect sits on a cosmetic face. Agree the critical / major / minor definitions with the customer before T1, so triage is a shared decision, not an argument afterward.

    “The trial shot is where the mold tells you what the drawing could not. We never judge the first few shots — we stabilize, label, inspect to a number and CMM across five parts, so we are reading the process and not one lucky sample. Then we triage: critical gets fixed before the mold ships, cosmetic major gets fixed, minor gets written down. Acceptance rests on that evidence.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • What is the difference between a mold trial and a production trial?

    A mold trial (T1, T2, T3) validates the tool on a small run — 20–50 shots, measured and inspected to find and fix mold defects.
    It runs on a trial setup with the parameters tuned for the mold.

    A production trial validates the process on the machine that will run it — 500–5,000 shots to prove the dimensions hold and the process is capable (Cpk ≥ 1.33). It happens after the mold is accepted, and it is what a customer’s capability study is built on.

    What if the production resin is not available — can you trial with a substitute?

    You can, with limits. The substitute must be the same resin family (ABS for ABS, PC for PC), with an MFI within 30% and a shrinkage within 0.2% of the production grade — different families shrink differently and the dimensions will not match.

    A substitute-resin trial is valid for the mold-function check — gate, cooling, ejection — but not for dimensions. Re-run the dimensional check with the production resin at the next trial, because only the real grade gives the real shrinkage.

    How do you track mold modifications between trials?

    Keep a change log shared with the customer: one line per modification with its number, date, description, root cause and result. It shows what is done and what is pending, so nobody re-litigates a fix at the next trial.

    Verify each change by measuring the T2 parts against the T1 parts on the same dimensions — a before-and-after that proves the fix worked and closes the log line. The report carries the modified dimensions, so acceptance rests on data, not memory.

    Key takeaways

    1. Never judge the first shots — stabilize 20–30, confirm the weight holds ±1%, then review shots 30–50, each labeled to its shot, mold, cavity and parameters.
    2. Inspect to numbers, not eyes: sink ≤0.1 mm cosmetic, flash ≤0.05 mm (≤0.02 mm precision), critical dimensions within ±0.05 mm on a CMM across 5 samples.
    3. Triage every defect — critical fixed and re-trialed, major fixed if cosmetic, minor deviated and monitored — and accept only at zero critical and zero cosmetic major.

    The trial shot is where the mold tells you the truth the drawing could not. Stabilize the process, inspect the parts to numeric limits, CMM the critical dimensions, and triage every defect before you accept. Fix the critical and the cosmetic major, deviate the minor, and the mold moves from steel to production on evidence — not on hope.

    Sources
    ISO 20457 — Plastics moulded parts: tolerances and acceptance conditions
    Zeiss — industrial metrology and CMM dimensional inspection
    Mitutoyo — CMM and hand measurement instruments
    Plastics Technology — mold trial and process stabilization
    MoldMaking Technology — mold trial and acceptance practice

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