What PC vs PMMA clarity means on a datasheet
PC vs PMMA clarity is the most common optical comparison in transparent plastics, and it starts with transmission numbers that look close but behave differently. Cast PMMA - acrylic - transmits about 92 to 93 percent of visible light, with optical grades reaching higher, and its haze sits below one percent. Polycarbonate transmits roughly 88 to 91 percent depending on grade and thickness, with haze nearer one to three percent that becomes visible in thick walls. So when buyers ask which resin is clearer, the direct answer is PMMA - and the useful answer is that clarity is one line in the specification, because the resin that wins on transparency loses badly on impact.
| Property | PMMA (acrylic) | PC (polycarbonate) |
|---|---|---|
| Light transmission | 92-93% | 88-91% |
| Haze | Under 1% | 1-3%, worse in thick sections |
| Notched impact (Charpy) | About 15-20 kJ/m2 | About 65-90 kJ/m2 |
| Surface hardness (Rockwell M) | 85-95 | 70-75, needs hard coat |
| Heat deflection (1.8 MPa) | 85-95 C | 124-140 C |
| Refractive index | 1.49, close to glass | 1.58 |
Why PMMA wins on optical parts
For optical parts - light guides, lenses, diffusers, display windows and instrument covers - PMMA light transmission and surface quality decide the argument. Its refractive index of about 1.49 sits close to glass, which matters for lens design, and low birefringence keeps images clean through moulded optics. PMMA also brings inherent UV resistance: the polymer absorbs UV without additives and holds colour and transmission outdoors for years, where unstabilised PC yellows and loses a few percent of transmission. Add the harder surface - PMMA resists the scuffs that permanently mark softer PC - and acrylic is the default for optics that never see impact.
When polycarbonate haze is the acceptable trade
PC's clarity deficit is the price of toughness. Notched impact strength several times higher than PMMA, a service range reaching about 120 C, and ductile rather than brittle fracture behaviour make PC the only sensible choice for machine guards, visors, protective covers and thick glazing. The workarounds are established: hard-coat the surface for scratch-sensitive cosmetic parts, and specify UV-stabilised or co-extruded UV grades for outdoor service. Where both properties are genuinely needed, impact-modified acrylic narrows the gap but does not close it.
Processing and cost notes for buyers
The two resins process differently and price differently. PMMA moulds at 200 to 240 C with mould temperatures around 60 to 80 C and typically costs less per kilogram; PC needs 260 to 300 C melt, more robust temperature control and protection against residual stress. PC is also chemically weaker against some solvents, which can stress-crack it, while PMMA tolerates dilute acids and alkalis better. Neither fact changes the optical ranking - they change the landed cost of the part that carries the optics.
Specifying the resin in an RFQ
Transparency claims are easy to make and easy to misread, so put numbers in the specification: transmission percent, haze percent, yellowness index and the wall thickness they were measured at. PC haze in particular climbs in sections thicker than about 6 mm, so a 3 mm datasheet figure does not describe a 10 mm cover. For moulded optical parts, also ask for a first-article report with a light-transmission check on a produced part - gate position, cooling and surface finish all move the result. And where the part will be cleaned, wiped or shipped loose, scratch resistance belongs in the specification alongside clarity, because a hazed surface reads as a failed part even when transmission is unchanged.
Every transparent resin selection trades transmission against toughness, so specify from the function: maximum light output and surface durability point to PMMA; impact, heat and thickness point to PC. Read the grade-level data before choosing - our datasheet reading guide covers the test methods behind the numbers - and compare material families in the plastic materials catalogue. The ABS vs PC housing guide extends the same logic to structural housings, and the company directory lists suppliers across both resin families.
