Last updated: 2026-09-02 · 5 min read

Optical-Grade Resins for Injection Molding — Which Material Transmits Light Without Distortion?

Optical-grade resins for injection molding include PMMA (acrylic, 92% light transmission), PC (polycarbonate, 88–90%), COP/COC (cyclic olefin, 92–94%) and optical-grade PS (polystyrene, 88–90%) — each serves different requirements for clarity, UV resistance, temperature tolerance and chemical exposure. The selection depends on the optical path length, the operating temperature, whether the part contacts chemicals or solvents, and the required surface quality (scratch resistance, haze level).

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

  • Light transmission: COP/COC leads at 92–94% (near-glass clarity), followed by PMMA at 92%, PC at 88–90% and optical PS at 88–90%. The difference matters for thick parts (>5mm) where absorption accumulates.
  • Birefringence: PC and PS develop internal stress under injection pressure, causing birefringence (double refraction) that distorts polarized light. COP/COC has the lowest birefringence of all optical resins — the standard choice for lenses and light guides.
  • Temperature resistance: PC withstands 120–135°C continuous use; COP/COC up to 130–150°C; PMMA only 70–90°C; PS only 60–80°C. For parts near heat sources, PC or COP/COC are required.

    • 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.

    Optical Resin Comparison

    Resin Transmission Best For
    PMMA (acrylic) 92% Light guides, displays, indoor lenses (low temp)
    PC (polycarbonate) 88–90% Impact-resistant lenses, automotive lighting, safety glasses
    COP/COC (cyclic olefin) 92–94% Precision lenses, medical optics, low-birefringence applications
    Optical PS 88–90% Low-cost light diffusers, disposable optics
    Optical-grade silicone 95%+ LED encapsulation, high-temp optics (compression molded)

    Mold and Process Requirements for Optical Parts

  • Mold surface finish: SPI A1 (mirror polish, Ra < 0.012μm) for lenses and light guides; SPI A2 (Ra < 0.025μm) for non-critical optical surfaces. The mold must be polished in the direction of light travel to minimize scatter.
  • Gate location: gate vestige on an optical surface causes light scatter — valve-gated hot runners place the gate on a non-optical surface, or the gate is located at an edge where the vestige does not affect the optical path.
  • Drying and contamination: optical resins require rigorous drying (PMMA: 80°C for 4 hours; PC: 120°C for 4 hours) and a clean processing environment — any moisture or particulate causes cloudiness or inclusions in the optical surface.

    • What This Means for Your Quotation

    When the part requires optical clarity, the quotation should specify the resin grade (optical-grade, not general-purpose), the mold surface finish (SPI A1 or A2), the gate type (valve-gated hot runner for cosmetic surfaces) and the drying protocol. Optical parts cost more to mold than structural parts — the mold polish, hot runner and drying requirements add 20–40% to the per-part cost, and the quotation should document these factors.

    Frequently Asked Questions

    Can optical parts be made from regrind?

    No — regrind introduces contamination, moisture absorption history and molecular degradation that cause cloudiness, inclusions and birefringence. Optical parts must be molded from virgin resin only. The runner scrap from optical molds is discarded or used for non-optical parts, never blended back into optical production.

    How do you measure optical clarity in molded parts?

    Haze (ASTM D1003) measures the percentage of light scattered by the part — lower haze means clearer transmission. Optical-grade PMMA achieves haze below 1%; PC below 2%. Total light transmission (ASTM D1003) and refractive index (ASTM D542) are also measured on optical test plaques molded alongside production parts.

    What causes birefringence in optical molded parts?

    Birefringence is caused by frozen-in molecular orientation from injection pressure and flow. The melt freezes under pressure, locking the polymer chains in a stressed state that distorts polarized light. The mitigation is higher melt temperature (reduces orientation), lower injection speed (reduces shear), and COP/COC resin (which has inherently low birefringence regardless of processing).
    For precision lenses and light guides, the mold design should also balance fill symmetrically so that orientation is uniform across the optical path — asymmetric fill creates differential birefringence that cannot be corrected by process adjustments alone.

    Key Takeaways

    1. Optical resins: COP/COC (92–94%, lowest birefringence), PMMA (92%, best scratch resistance), PC (88–90%, highest impact strength), optical PS (88–90%, lowest cost) — selected by clarity, temperature and stress requirements.
    2. Optical molding requires SPI A1/A2 mold polish, valve-gated hot runners and rigorous drying — adding 20–40% to per-part cost vs. structural parts.
    3. The quotation should specify optical-grade resin, mold finish, gate type and drying protocol — optical parts must use virgin resin only, with no regrind in the optical production.

    Send aCatMould the optical part drawing with the clarity and temperature requirements, and the material selection specifies the optical resin grade, mold finish and gate type — with the quotation documenting the per-part cost for optical-grade production, virgin resin only.

    Sources
    Plastics Technology — optical resin selection and molding
    SpecialChem — optical polymer grades and properties
    ASTM D1003 — standard test method for haze and luminous transmittance

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