Clear Parts: PC vs PMMA — Which Clear Resin Fits?

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

Polycarbonate and acrylic (PMMA) are the two workhorse clear resins, and they trade places on a simple axis: PC is the tough one that survives impact and heat; PMMA is the hard, optically clean one that resists weathering and scratches better. Choose PC for anything that can be hit, flexed or heated; choose PMMA for lenses, light pipes and covers where optical clarity and outdoor stability lead the requirement list.

Key facts

<>

  • PMMA offers higher light transmission and lower haze, with a harder surface that resists everyday scratching.
  • PC absorbs impact many times better and tolerates higher service temperatures — at the cost of a softer surface and slight yellowing over long outdoor exposure.
  • Both are amorphous and both dry before molding, but PC’s drying discipline is stricter; wet PC hydrolyzes and loses toughness.

    • 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.

    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.

    What does each clear resin do better?

    The two resins fail in opposite directions. PMMA is rigid, dimensionally calm and optically honest — transmission stays high, haze stays low, and the surface takes and keeps a polish — but it is brittle: a PMMA cover that takes a knock cracks, often cleanly. PC is the opposite temperament — it dents, it flexes, it survives drop after drop — but its surface is softer, so it scratches where PMMA would shrug, and unstabilized grades age in sunlight.
    Because clear parts show everything, the choice is usually visible on the first prototype: scratch marks all over a PC lens, or a cracked PMMA housing after a drop test. Each tells you the other resin was waiting.

    How do they compare side by side?

    Clear-part property PMMA (acrylic) PC (polycarbonate)
    Light transmission Highest; low haze for optics. Very good; slightly less clean optically.
    Impact resistance Brittle; cracks under sharp knocks. Excellent; survives drops and flex.
    Scratch resistance Hard surface, resists daily wear. Softer; needs hard coating for scratch zones.
    Heat tolerance Modest service temperatures. Substantially higher continuous range.
    Outdoor aging Excellent weatherability. Needs UV-stabilized grades outdoors.

    Which one survives your environment?

  • Indoor lens or light guide, rarely touched — PMMA, for optics and surface life.
  • Machine cover or guard that gets knocked — PC, for survival; hard coat if scratched looks matter.
  • Outdoor signage or glazing — PMMA for weathering, or UV-stabilized PC where impact risk is real.
  • Near heat sources — PC, with the grade datasheet confirming the service temperature.
  • Either with coatings — hard-coated PC and coated PMMA both exist; specify the coating as part of the part, and test the combination at T1.

    • One caution spans both resins: clear parts show every flaw, so neither choice survives an undefined cosmetic standard.
    Agree on the viewing conditions, the limit samples and the defect vocabulary before the tool is cut, and add polarizer inspection for PC if stress whitening matters to the application. The resin decision and the acceptance standard are one decision made twice — and making them together at DFM is what keeps the T1 conversation short.

    Frequently asked questions

    Which clear resin looks better on a lens?

    PMMA, typically — higher transmission, lower haze and a harder polish keep it looking new longer in untouched applications. PC matches it when hard-coated, which is why coated PC dominates wear-touch lenses.

    Can I mold PC and PMMA parts on the same tool?

    Not usually without review — they shrink differently and process at different temperatures, so a tool compensated for one resin measures differently in the other. Change the resin after tooling and the parts change with it.

    Do both resins need drying?

    Yes. PMMA absorbs less and dries faster, but both are hygroscopic enough to splay when wet — and wet PC also loses toughness chemically. Dry to the datasheet and check moisture at the hopper.

    Key takeaways

    1. PMMA leads on optics, surface hardness and weathering; PC leads on impact and heat.
    2. Each resin’s failure mode points to the other: scratched PC wants PMMA, cracked PMMA wants PC — coatings can bridge.
    3. Both dry before molding, both show every flaw on clear parts, and both deserve an agreed cosmetic standard at T1.

    If your program involves a clear cover, lens or light guide, send us the 3D model, the optical requirements and the abuse it must survive before the deposit. You will get the resin recommendation with its failure modes named, the cosmetic standard proposal, and the T1 evidence list — from a named tooling engineer, before any steel is ordered.

    Sources
    MatWeb — material property database for optical PMMA and polycarbonate grades
    ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization

    Sources: Plastics Technology · Plastics Technology Online · Prospector — Resin Datasheets · MatWeb — Material Property Database