Optical Grade Resin Selection for Injection Molding — How Do You Choose Between PC, PMMA, COP and COC for Transparent Parts, and What Are the Key Differences in Clarity, Heat Resistance and Cost?
Last updated: 2026-09-03 · Reading time: about 6 min
Optical resin selection is a four-way trade among PC, PMMA, COP and COC, and each wins one axis: PC takes impact and heat, PMMA takes surface clarity and scratch, and the cyclic olefins take near-zero birefringence, chemical resistance and low water absorption. The costly decision is whether your part actually needs a cyclic olefin — a specialty resin at a multiple of PC or PMMA — or whether PC or PMMA already meets the requirement.
See also related article.
Key facts
<>
Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.
Which of the four optical resins fits the requirement — and when do you jump to a cyclic olefin?
PC and PMMA cover most commercial optics. The cyclic olefins earn their cost only when a specific requirement — birefringence, chemical or fluid contact, heat above PMMA’s ceiling, or dimensional stability in humidity — is one the commodity resins cannot meet.
| Resin | What it wins | Where it is chosen | Where it falls short |
|---|---|---|---|
| PC — polycarbonate | Impact and heat | Headlamp lens, safety glazing, machine-guard window | Stress birefringence; soft surface needs a hard coat; poor solvent resistance |
| PMMA — acrylic | Surface clarity and scratch resistance | Display window, signage, light-guide face, cosmetic lens | Brittle, low impact; low heat ceiling; attacked by ketones |
| COP — cyclic olefin polymer | Heat plus near-zero birefringence | Precision and polarized optics, LED reflector, solder-temperature electronics | Specialty cost; narrow processing window; lower melt strength |
| COC — cyclic olefin copolymer | Chemical resistance plus low water absorption | Fluid-contact and analytical optics, cuvette and reservoir windows | Specialty cost; lower heat than COP; limited supplier base |
Read the table as a decision, not a ranking. Start from the requirement that actually disqualifies a resin: if the part sees impact, PC stays in; if the surface must stay clear and resist scratching, PMMA stays in; only if the part needs near-zero birefringence under polarized light, aggressive chemical or fluid contact, or heat past PMMA’s ceiling does a cyclic olefin earn its multiple. Absent one of those, a commodity optical resin does the job.
COP or COC — which cyclic olefin, and what does the switch change in the tool and process?
Once a cyclic olefin is justified, two decisions follow: COP or COC, and how the tool and process must change — because a mold cut for PC or PMMA is not automatically a drop-in for a cyclic olefin.
- Pick COP when heat drives the choice. The homopolymer holds a higher glass-transition and heat-deflection temperature, so it stays dimensionally stable near SMD LEDs and electronics that see solder-temperature excursions.
- Pick COC when fluid or humidity drives it. The copolymer takes up the least water of the four, so it holds dimension and clarity in fluid contact; it runs lower-Tg and a little more flexible than COP.
- Re-qualify the tool for the switch. A cyclic olefin shrinks differently, has lower melt strength and a narrower processing window than PC or PMMA — so gate, wall, venting and cooling get checked against the new resin, not assumed.
- Lock the grade and supply early. Cyclic olefins come from a small set of specialty producers, so lead time and lot availability are tighter; fix the exact grade and qualify sample resin before the tool is cut.
COP versus COC. A cyclic olefin polymer (COP) is built from a single cyclic-olefin monomer — a homopolymer with a higher glass-transition temperature and better heat. A cyclic olefin copolymer (COC) adds an alpha-olefin comonomer, which lowers Tg, adds a little flexibility and cuts water absorption. Both are amorphous and isotropic, so both give near-zero birefringence; the split is heat versus fluid stability, not clarity.
“The reflex on a clear part is to for PC because it is tough and available — and on a display window to for PMMA. Both are usually right. What we push back on is specifying a cyclic olefin out of habit, because it multiplies the resin cost and tightens the supply. We ask which requirement actually disqualifies PC and PMMA: birefringence under polarized light, fluid contact, or heat. If none does, the commodity optical resin wins.”
— Project Engineer, aCatMould
Frequently asked questions
When is a cyclic olefin actually worth its cost over PMMA or PC?
When one requirement rules out both commodity resins: near-zero birefringence under polarized light, aggressive chemical or continuous fluid contact, heat above PMMA’s ceiling, or dimensional stability in humidity.
Any single one of these is the trigger.
If the part is a display window or a decorative clear lens with no polarized-light path and no fluid or heat load, PMMA or PC meets it at a fraction of the resin cost — there the cyclic olefin buys nothing you can see on the part.
For a fluid-contact optical part, is COP or COC the better pick — and who owns the compliance?
COC, for the water absorption. It takes up the least moisture of the four, so it holds dimension and clarity when it sees fluid or humidity; COP is the pick when heat, not fluid, is the driver.
On compliance: we do not certify the end product — the regulatory pathway and any biocompatibility or fluid-contact qualification for your assembly and your market is yours. We mold to the exact grade and file you specify, and we document the resin lot we ran.
Can I reuse the mold I already cut for PC if I switch to a cyclic olefin?
Usually not as a drop-in. A cyclic olefin shrinks differently from PC, has lower melt strength and a narrower processing window, so the gate, wall thickness, venting and cooling all have to be re-checked against the new resin.
Sometimes an insert change or a gate and venting revision is enough; sometimes the cavity needs re-cutting. The honest answer comes from reviewing the existing tool against the cyclic olefin’s shrinkage and window before you commit — not after the first short shot.
Key takeaways
- Select from the requirement, not the habit: PC wins impact and heat, PMMA wins surface clarity and scratch, and COP/COC win near-zero birefringence, chemical resistance and low water absorption.
- Jump to a cyclic olefin only when a specific requirement disqualifies both PC and PMMA — polarized-light birefringence, fluid or chemical contact, or heat past PMMA’s ceiling — because it multiplies resin cost and tightens supply.
- Split COP (heat, higher Tg) from COC (fluid, lowest water absorption), and re-qualify the tool for the switch — different shrinkage, lower melt strength, narrower window — with the grade and supply locked before the tool is cut.
Optical resin selection is a decision tree, not a preference: name the requirement that actually disqualifies a resin, and the four sort themselves. PC and PMMA carry most commercial optics; the cyclic olefins are reserved for the birefringence, fluid and heat loads those two cannot meet. At aCatMould we run that decision against your drawing before the deposit, and if a cyclic olefin is justified we re-qualify the tool for its shrinkage and processing window rather than assume the old mold still fits.
Sources
Covestro — polycarbonate optical technical data
Prospector — plastics database (optical and cyclic olefin grades)
MatWeb — material property database (transmission, haze, Tg by grade)
Plastics Technology — optical molding and resin selection
Society of Plastics Engineers — optical part design guidance
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet