Polycarbonate (PC) vs Acrylic (PMMA) Injection Molding: Optical, Impact & Cost Guide

When an injection-molded part must be water-clear—such as an automotive headlamp lens, medical fluid cuvette, LED light pipe, optical sensor window, or rugged display cover—90% of resin selections narrow down to a head-to-head choice between Polycarbonate (PC) (Lexan, Makrolon) and Polymethyl Methacrylate (PMMA / Acrylic) (Plexiglas, Acrylite, Altuglas).

On a CAD rendering, both look identical. In service and at the molding press, they behave as polar opposites:

• Polycarbonate (PC) is a virtually unbreakable, high-heat (125–135 °C HDT), naturally flame-retardant (UL 94 V-2 to V-0) structural armor that scratches easily (HB pencil hardness), yellows in unstabilized outdoor UV exposure, and costs 30%–50% more per kilogram.
• Acrylic (PMMA) delivers superior optical transmission (92% vs 89%), crystal-clear non-yellowing 10-year outdoor UV stability, and high scratch resistance (3H–4H pencil hardness) at a significantly lower resin price, but shatters brittlely under impact (1/30th the notched impact energy of PC) and softens above 85–95 °C.

Use this quantitative comparison matrix alongside our material selection guide to select the right optical polymer before sizing gates and specifying mold steel.

1. Head-to-Head Engineering Comparison: Polycarbonate (PC) vs Acrylic (PMMA)

Property / Engineering Metric	Polycarbonate (PC — Unfilled Optical)	Standard Acrylic (PMMA — General Optical)	Impact-Modified Acrylic (IM-PMMA)	Winner & Engineering Implication
Light Transmission (@ 3.0 mm, ASTM D1003)	88% – 90% (slight blue/straw tint in thick sections)	92.0% – 92.5% (matches optical crown glass)	89% – 91% (haze increases below 0 °C)	PMMA: Crucial for long LED light guides (> 100 mm path length) where PC attenuates light and shifts colour temperature.
Refractive Index ($n_D$, ASTM D542) & Abbe Number	1.586 (Abbe 30 — higher chromatic dispersion)	1.491 (Abbe 58 — very low dispersion)	1.49	Context-Dependent: PC’s higher refractive index bends light more in thin corrective lenses; PMMA’s high Abbe number prevents rainbow edge fringing.
Notched Izod Impact (23 °C, ASTM D256)	650 – 850 J/m (ductile yield; ~30× PMMA)	16 – 25 J/m (brittle fracture)	65 – 120 J/m (4–5× standard PMMA)	PC by an order of magnitude: Specify PC whenever drop tests, rock strikes, or vandal resistance are mandatory.
Surface Scratch Hardness (Pencil / Rockwell M)	2B – HB Pencil / R118 (M70) (scratches when wiped dry)	3H – 4H Pencil / M92 – M100 (hard, mar-resistant)	H – 2H Pencil / M65 – M80	PMMA: Bare PMMA survives consumer wiping without secondary hard-coating; exposed PC windows usually require a $0.15–$0.60 polysiloxane hard-coat.
Heat Deflection Temp (HDT @ 1.8 MPa, ASTM D648)	125 – 135 °C ($T_g \approx 147^\circ\text{C}$)	85 – 98 °C ($T_g \approx 105^\circ\text{C}$)	78 – 88 °C	PC: Required near high-power LEDs, power supplies, under-hood sensors, or hot-fill/dishwasher cycles.
Outdoor UV Weatherability (10-Year Sunlight)	Poor (Unmodified) — yellows ($\Delta YI > 10$) and loses surface impact unless UV-stabilized (f1) or coated	Best-in-class (Inherent) — aliphatic structure is transparent to UV; < 1–2% transmission loss over 10 years	Very Good (butyl-acrylate modifier remains UV-stable)	PMMA: Standard choice for outdoor taillights, architectural glazing, and solar lenses without UV coating cost.
UL 94 Flammability (@ 1.5–3.0 mm)	UL 94 V-2 (unmodified); V-0 / 5VA (FR grades)	UL 94 HB (burns cleanly with zero smoke, but does not self-extinguish)	UL 94 HB	PC: Mandatory whenever an enclosure or lens must pass UL 94 V-2, V-0, or 5VA vertical burn compliance.
Chemical & Alcohol Wipe Resistance	Vulnerable to acetone, esters, ammonia, and threadlockers; stresses crack in 70% IPA if molded cold	Resists mild alkalis, aliphatic hydrocarbons, and dilute cleaners; crazes in high-concentration alcohol/acetone	Similar to standard PMMA	Tie (Both require low-stress hot-mold processing); for harsh hospital disinfectants (CaviWipes), use copolyester (Tritan) or PSU.
Mold Shrinkage (mm/mm)	0.5% – 0.7% (isotropic)	0.3% – 0.6% (isotropic)	0.4% – 0.7%	Nearly identical (~0.5%): A prototype tool cut for 0.005–0.006 mm/mm shrinkage can often sample both PC and PMMA.
Relative Raw Resin Price (per kg)	1.35x – 1.65x	1.00x (Baseline)	1.15x – 1.30x	PMMA saves 25%–40% on raw resin cost (and even more if it eliminates a secondary scratch hard-coat on PC).

2. Application Decision Matrix: When to Specify PC vs PMMA vs Impact-Modified PMMA

End-Use Component	Recommended Resin	Engineering Justification
Automotive Headlamp Outer Lens	UV-Stabilized PC + Polysiloxane Hard-Coat	Must survive 120 km/h highway gravel impact (> 650 J/m) and 115 °C engine-bay/halogen heat; the secondary hard-coat supplies the missing scratch and UV barrier.
Automotive Rear Taillight & Signal Lens	Standard or Impact-Modified PMMA	Rear lenses face minimal stone-chip energy; PMMA provides superior ruby/amber optical brilliance, 10-year non-yellowing UV life, and needs zero secondary hard-coating.
Long Edge-Lit LED Light Pipe (> 50 mm length)	Optical-Grade PMMA	Over a 100 mm internal reflection path, PC absorbs blue wavelengths and turns a 5000K white LED yellow at the far end; PMMA transmits 92% neutrally across the visible spectrum.
Safety Glasses, Machine Guards & Riot Shields	Optical PC	ANSI Z87.1 high-velocity impact mandates ductile energy absorption; PMMA shatters into sharp shards on impact.
Consumer Display Window / Appliance Touch Bezel	PMMA or Impact-Modified PMMA	Resists fingernail and cleaning-cloth scratches (3H–4H) out of the mold without paying for coated polycarbonate.
Electrical Meter Cover / EV Charger Window	Flame-Retardant UV-PC (UL 94 V-0 / 5VA, f1)	Must simultaneously pass UL 94 V-0 / 5VA flammability, IK08/IK10 vandal impact, and outdoor UV exposure.

3. Tooling & Injection Molding Differences for Transparent Optical Parts

Molding an optical lens or transparent housing requires stricter steel, gating, and thermal discipline than molding an opaque enclosure:

Molding / Tooling Parameter	Polycarbonate (PC)	Acrylic (PMMA)
Cavity & Core Tool Steel	ESR 420SS (Stavax / 1.2083 at 50–52 HRC) polished to SPI A-1 or A-2 (Ra < 0.012–0.025 µm)	ESR 420SS (Stavax / 1.2083 at 50–52 HRC) polished to SPI A-1 or A-2
Desiccant Drying Before Molding	120 °C for 3–4 hours to ≤ 0.02% moisture (hydrolysis destroys impact strength)	80–90 °C for 3–4 hours to ≤ 0.04%–0.05% moisture (moisture causes visual bubbles and splay)
Melt Temperature Window	280–320 °C (535–610 °F — high viscosity)	220–250 °C (430–480 °F — lower temperature, shear-sensitive)
Mold Temperature Window	85–115 °C (185–240 °F — mandatory for stress relaxation)	60–85 °C (140–185 °F)
Gate Sizing & Geometry	Wide tab or fan gate (75%–90% of wall thickness) aimed against a core wall to prevent jetting	Wide tab or fan gate (75%–90% of wall thickness) with generous radiused transition to prevent shear blush

Can You Sample Both PC and PMMA in the Same Injection Mold?

Yes—with one important draft and gate caveat. Because both amorphous resins shrink ~0.5% (0.005 mm/mm), dimensional changes between a PC shot and a PMMA shot from the same cavity are typically under 0.001–0.002 mm/mm. However:

• Because PMMA is brittle (~2–4% elongation at break vs 100%+ for PC), a PMMA part will crack or chip during ejection if any rib or boss has < 1.0°–1.5° draft or if an undercut relies on flexing off the core.
• Never transition a barrel directly from 310 °C Polycarbonate down to 230 °C Acrylic without purging thoroughly with cast acrylic purge compound; un-purged PC specks create black carbon inclusions, while overheating PMMA at 300 °C depolymerizes it into boiling methyl-methacrylate monomer gas bubbles.

Buyer FAQs

Is polycarbonate or acrylic clearer in injection molding?

Acrylic (PMMA) is optically clearer than polycarbonate, transmitting 92.0% to 92.5% of visible light across a 3.0 mm wall with zero colour tint and low chromatic dispersion (Abbe number 58), whereas polycarbonate (PC) transmits 88% to 90% of light (Abbe number 30) and exhibits a faint blue or straw cast over long optical paths. For long edge-lit LED light pipes (> 50 mm) and precision collimating lenses, PMMA is the superior optical material.

How much stronger is polycarbonate than acrylic?

Unfilled polycarbonate has roughly 30× the Notched Izod impact strength of standard acrylic (650–850 J/m for PC vs 16–25 J/m for standard PMMA) and yields ductilely without shattering under severe hammer or rock strikes. Even when compared against impact-modified acrylic (IM-PMMA at 65–120 J/m), polycarbonate retains 6× to 10× higher impact toughness along with a 40 °C higher heat deflection temperature (130 °C vs 90 °C).

Which is better for outdoor use: polycarbonate or acrylic?

For parts that do not face heavy impact or vandalism, acrylic (PMMA) is far superior outdoors because its molecular structure is naturally transparent to UV radiation, holding > 90% light transmission for 10+ years without yellowing or surface chalking. Unmodified polycarbonate yellows rapidly in sunlight; if polycarbonate must be used outdoors for impact resistance, you must specify a UV-stabilized grade (UL f1 rated) and/or a secondary UV-blocking hard-coat.

Can you use the same injection mold for both polycarbonate and acrylic?

Yes, because both amorphous polymers share an isotropic mold shrinkage rate of ~0.5% (0.004–0.007 mm/mm), a mold designed with ≥ 1.5°–2.0° of draft, generous corner radii (R ≥ 0.5 mm), wide fan/tab gates (75%–90% of wall thickness), and an SPI A-1/A-2 polished 420SS cavity can mold both PC and PMMA within standard engineering tolerances.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Optical transmittance, haze, and impact values vary by grade and wall thickness; confirm optical and flammability specifications against the manufacturer’s datasheet.