Last updated: 2026-09-02 · 5 min read

How Do T1 Parts Compare to Your Prototype — and What Differences Are Normal?

T1 parts are the first articles from the production tool in production resin, and they should match your prototype’s geometry and design intent — but expect honest differences in shrinkage behavior, surface finish, gate vestige, weld lines and dimensional stability, because a prototype made by machining or printing never carried the material, the process or the tool’s thermal reality. The comparison that matters is not “is T1 identical to the prototype” but “does T1 meet the drawing, and can every difference be traced to a known cause.”

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.

Key Facts

  • A prototype validates geometry and fit; T1 validates the whole system — material in production grade, steel at production temperature, process at candidate settings. Only T1 can answer questions about shrink, warp, sink and weld-line strength, because only T1 has the real physics.
  • Normal T1 differences cluster in predictable places: dimensions driven by shrinkage may sit slightly off nominal until the process is tuned; surface texture differs wherever the prototype was hand-finished; gate and ejection marks exist that no prototype ever showed.
  • A red flag is a difference with no mechanism behind it: a feature out of tolerance where the fill analysis says the mold cuts it right, a wall thickness that doesn’t match the steel, a missing detail. Those point to the tool, not the process, and they stop the trial clock.

    • 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. The related guide covers adjacent topics.

    Where Real Parts Differ From Prototypes

    Aspect Prototype T1 Production Part
    Material Machined billet, printed substitute, or similar-but-not-same resin The specified production grade and color, with its real shrinkage
    Dimensions Exactly what the model said Shrink-compensated steel plus process variation; tuned at T1/T2
    Surface Tool marks from machining or print layers Replicated mold texture plus gate and ejection vestige
    Weld lines None — no flow involved Real convergence lines where fronts meet
    Strength and creep Not representative of molded, oriented material True production properties, testable and certifiable to your spec

    Differences That Are Normal at T1

  • Dimensions a few hundredths of a millimeter from nominal, moving consistently with shrinkage: the tool was cut to the moldflow shrink, and the real resin gets the final word. Small steel corrections at T2 are the designed-in adjustment mechanism, not a defect.
  • Gate vestige and ejector marks on faces a prototype never had — present by design, reviewed against the cosmetic boundary drawing that marked where they are allowed.
  • Sink over thick sections at the trial’s initial holding settings, improving as the process is tuned during the trial itself.
  • Slight gloss or texture variation where the part is hard to fill at first-pass settings — often gone by T2 after process or vent adjustments.
  • Warp direction matching the simulation’s prediction: predicted warp showing up at T1 is the model working, verified against the measurement report.

    • What Should Already Match at T1

    Features the steel cuts directly must match the drawing within toolmaking tolerance from the first shot: wall thickness in non-shrink-sensitive areas, boss and rib geometry, hole positions, snap-fit hooks, insert bores. The mold document and the full-size CMM report on the steel give you the reference before molding even starts — the T1 part is checked against that same data, so a mismatch is traceable to a specific electrode or insert, not argued about.

    Functionally, T1 should already demonstrate the assembly story: parts mate with their counterparts, snaps engage, screws thread, inserts hold. What T1 does not yet prove is stability — that dimensions hold across cavities, lots and shifts. That proof arrives with the process window work and the T2 production-rate verification.

    Frequently Asked Questions

    Should I compare T1 dimensions straight to the drawing?

    Yes — and read the report with the shrink mechanism in mind. Features cut directly in steel should sit at nominal; features free to shrink ride their shrinkage and are tuned at T2. The T1 CMM report separates these groups, so each measurement is judged against the right expectation rather than a blanket pass-or-fail.

    The prototype fit my assembly perfectly. Why does T1 need adjustment?

    Because the prototype carried no shrinkage. It was at nominal by construction; the molded part is at nominal only after the tool’s shrink compensation is verified against the real resin. T1 is where that verification happens — the same check the prototype could never perform.

    Can I skip the prototype if I’m getting a production tool anyway?

    For simple parts, a rigorous DFM and fill analysis can substitute. For new assemblies, complex mechanisms or anything where fit drives downstream design, a prototype remains the least expensive place to discover the assembly doesn’t work — weeks before the steel is cut, at a fraction of a correction’s cost.

    Key Takeaways

    1. T1 compares to the drawing and the mold’s CMM report, not to the prototype: shrinkage, vestige, weld lines and finish are supposed to be new information.
    2. Normal T1 differences have known mechanisms — shrink tuning, gate and ejector marks, first-pass sink — and each is scheduled for T2 refinement by design.
    3. Steel-cut features matching nominal and functional assembly at T1 are the pass criteria; differences without mechanisms are tool questions, not process questions.

    Send your prototype and the drawing together at DFM, and aCatMould will flag exactly which dimensions will move with shrinkage and which are steel-locked — so at T1, every difference on the report has a name, an owner and a scheduled fix, and nothing on the table is a surprise.

    Sources
    MoldMaking Technology — T1 sampling and qualification practice
    Plastics Technology — shrinkage and trial tuning
    Injection Molding Magazine — prototype-to-production comparison

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