Last updated: 2026-09-05 · 5 min read

PC vs. PMMA for Optical Parts — How Do Polycarbonate and Acrylic Compare for Light Transmission, Scratch Resistance and Injection Molding Process Requirements?

Polycarbonate (PC) and PMMA (acrylic) are the two dominant resins for optical parts. PC transmits 88–91% of visible light and withstands impact 250× greater than PMMA. PMMA transmits 92–93% (closer to optical glass), resists scratching 5–10× better, and does not yellow under UV as quickly. Trade-off: PC wins on impact; PMMA wins on clarity, scratch and UV stability. Choice depends on which property matters most.

Key Facts

<>

  • Six-parameter comparison: light transmission (PC 88–91% vs PMMA 92–93%), impact (PC 600–900 J/m vs PMMA 15–25 J/m), scratch hardness (PC HB–B vs PMMA H–2H), UV yellowing after 1000 hrs QUV (PC ΔYI 5–15 vs PMMA ΔYI 1–3), melt temp (PC 280–320°C vs PMMA 210–250°C), cost ($4–7/kg vs $3–5/kg).
  • PC requires drying below 0.02% moisture, mold temp 80–120°C, and is prone to stress birefringence. PMMA dries to 0.05%, mold temp 40–70°C, lower residual stress.
  • PC/PMMA co-extrusion combines PC impact with PMMA UV and scratch resistance — mold cost 30–50% higher, justified for high-volume programs requiring both.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    How Do PC and PMMA Compare Across Six Key Parameters?

    Parameter Polycarbonate (PC) PMMA (Acrylic) Winner
    Light transmission 88–91% 92–93% PMMA (closer to glass)
    Impact resistance (Izod, notched) 600–900 J/m 15–25 J/m PC (250× stronger)
    Surface hardness (pencil) HB–B H–2H PMMA (5–10× harder)
    UV yellowing (1000 hrs QUV) ΔYI 5–15 (uncoated) ΔYI 1–3 PMMA (inherently UV stable)
    Melt temperature 280–320°C 210–250°C PMMA (easier to process)
    Material cost (per kg) $4–7/kg $3–5/kg PMMA (15–30% cheaper)

    If the part must survive impact (safety goggles, machine guards, headlamp lenses), PC is the only choice. If it must maintain clarity over years of outdoor exposure (light covers, display windows, optical lenses), PMMA is better — unless impact risk requires PC with a hard coat.

    What Are the Injection Molding Process Differences?

    1. PC requires drying below 0.02% moisture. PC is hygroscopic — even 0.05% moisture causes hydrolysis during injection, producing splay, bubbles and reduced molecular weight (lowering impact strength). Dry at 120°C for 3–4 hours with desiccant dryer.
      PMMA requires drying below 0.05% — less stringent but still necessary for optical clarity.
    2. PC requires higher mold temperature (80–120°C). Below 60°C, PC solidifies too quickly, trapping internal stresses that cause cracking under load or chemical exposure. Heated mold with oil or steam channels is required for optical PC parts.
      PMMA molds at 40–70°C with lower residual stress.
    3. PC is prone to stress birefringence. When molded under high injection pressure or low mold temperature, polymer chains align in the flow direction, creating birefringence (double refraction) that distorts polarized light. Unacceptable for lenses and light guides. Fix: reduce injection speed, increase mold temperature, anneal after molding.
      PMMA has inherently lower birefringence.
    4. PMMA is brittle but bondable with solvent cement. Unlike PC (which requires adhesive or ultrasonic welding), PMMA parts can be chemically bonded using solvent cement. The solvent dissolves the surface layer, and two parts fuse into one with nearly base material strength. Ideal for multi-part optical assemblies.

    PC demands stricter processing but delivers unmatched impact resistance. PMMA processes more easily and delivers superior optical performance for non-impact applications.

    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.

    PC transmits 88–91% light with 250× the impact resistance of PMMA. PMMA transmits 92–93% light with 5–10× the scratch resistance. PC yellows under UV (ΔYI 5–15); PMMA is inherently UV stable (ΔYI 1–3). The material cost difference is small ($1–2/kg) — selection should be driven by performance, not cost.

  • 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.

    • Frequently Asked Questions

    Can you use PC for outdoor optical parts without yellowing?

    Standard PC yellows under UV — after 1,000 hours QUV testing, yellowing index (ΔYI) increases by 5–15, visible as a yellow tint. UV-stabilized PC grades reduce yellowing to ΔYI 3–8, but this is still higher than PMMA (ΔYI 1–3).

    For outdoor optical parts that must maintain clarity over 5+ years, PMMA is the better choice. If impact resistance requires PC, the part must be coated with a UV-blocking hard coat or co-extruded with a UV-stable cap layer.

    What is the cost difference between PC and PMMA for an optical lens?

    Material cost difference is small — PC costs $4–7/kg, PMMA $3–5/kg. For a 10g lens, the material difference is $0.01–0.02 per part. Mold cost is similar for both — both require polished cavity (SPI A1) and heated mold.

    However, the PC mold requires higher temperature system (oil heating at 80–120°C), adding $2,000–5,000 for the heating unit. Total cost difference for a 100,000-piece program is $1,000–3,000 — small enough that material selection should be driven by performance, not cost.

    When would you choose PC/PMMA co-extrusion?

    PC/PMMA co-extrusion combines PC impact resistance with PMMA UV stability and scratch resistance — the PC core provides structural strength, the PMMA skin provides the optical surface. Used for automotive headlamp lenses, outdoor light covers and safety visors where both impact and long-term clarity are required.

    Mold cost is 30–50% higher than single-material mold (co-extrusion nozzle and flow control system), but per-part cost is lower than coating a PC part with hard coat.

    Key Takeaways

    <>

  • PC: 88–91% light, 250× impact of PMMA. PMMA: 92–93% light, 5–10× scratch resistance.
  • PC for impact-prone parts (goggles, guards). PMMA for outdoor clarity (covers, lenses).
  • PC: dry below 0.02%, mold 80–120°C, birefringence risk.
    PMMA: dry 0.05%, mold 40–70°C.
  • PC/PMMA co-extrusion: both advantages, mold cost +30–50%, justified for high volume.

    • If you are selecting between PC and PMMA for an optical part, send the drawing with the light transmission requirement, impact environment and expected service life. The DFM review recommends the correct material and any post-molding treatment needed.

    Sources: SABIC — Polycarbonate Datasheets (Lexan) · Evonik Deglas — PMMA Optical Grade Data (Deglas) · ASTM International — Optical Plastics Testing Standards · Society of Plastics Engineers — Optical Injection Molding Papers · Plastics Today — PC vs PMMA Material Selection Guide

    — Arthur, Tooling Engineer at aCatMould