Acrylic vs Polycarbonate: How to Choose for Machined Parts
PMMA or PC — one is clearer, harder, and cheaper; the other shrugs off 30× the impact. A practical, numbers-first comparison for engineers specifying transparent plastic parts.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 12 min read
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Quick Answer
In the acrylic vs polycarbonate decision, impact strength is the dividing line: polycarbonate absorbs roughly 30× the impact energy of acrylic (12–16 vs ~0.4 ft·lb/in notched Izod), so it owns machine guards, safety shields, and anything that can be struck. Acrylic (PMMA) wins nearly everything else — 92% light transmission vs 86–89%, a harder, more scratch-resistant surface (Rockwell M90+ vs ~M70), inherent UV stability, and a price 30–50% lower. Acrylic is stiffer and machines to a finer finish but chips at edges; PC is tougher and heat-resistant to ~140 °C but scratches and yellows without coatings. Pick PC when parts get hit, acrylic when they get looked at.
Two Clear Plastics, Two Very Different Personalities
Acrylic — polymethyl methacrylate (PMMA), sold as Plexiglas, Perspex, or Lucite — and polycarbonate (PC), sold as Lexan or Makrolon, are the two default transparent engineering plastics. Both machine cleanly on standard CNC equipment and both show up constantly in our machined plastics work, yet they fail in opposite ways: acrylic cracks where PC would bounce, and PC scratches and yellows where acrylic stays water-clear for a decade.
The chemistry drives everything. PMMA is a rigid, glassy polymer — hard, stiff, optically superb, and brittle under sudden load. PC’s carbonate linkages let its chains flex and absorb energy, making it one of the toughest thermoplastics ever commercialized, at the cost of a softer surface and UV sensitivity. Neither is “better” in the abstract — the datasheets tell a clean story once you line the numbers up. For polymers beyond these two, see our CNC machining materials guide.
Acrylic vs Polycarbonate: Property Data Side by Side
Values below are typical ranges for standard cast/extruded acrylic and general-purpose polycarbonate — the grades shops actually stock. Specialty grades shift individual rows but rarely flip the ranking.
| Property | Acrylic (PMMA) | Polycarbonate (PC) |
|---|---|---|
| Density | 1.18–1.19 g/cm³ | 1.20–1.22 g/cm³ |
| Tensile strength | 65–80 MPa | 55–75 MPa |
| Notched Izod impact | ~0.4 ft·lb/in (≈16–21 J/m) | 12–16 ft·lb/in (≈640–850 J/m) — roughly 30× acrylic |
| Light transmission (clear) | 92% | 86–89% |
| Heat deflection @ 264 psi | 75–100 °C | 125–140 °C |
| Glass transition temperature | ~105 °C | ~147 °C |
| Surface hardness | Rockwell M90–M105 (hardest common clear sheet) | ~Rockwell M70 — scratches more easily |
| UV resistance | Excellent — inherently stable, resists yellowing | Poor unprotected — yellows; needs UV-coated grades outdoors |
| Typical sheet cost | Baseline | 30–50% more than acrylic |
| Machined surface finish | Ra 0.4–0.8 µm achievable; polishable to optical clarity | Ra 1.6–3.2 µm typical; stringy chips limit finish |
Read the table as two profiles: acrylic is the optical and cosmetic material — clearer, harder, cheaper, weatherproof — while polycarbonate is the impact and temperature material. The only rows PC wins are the ones that keep people safe; every appearance or cost row goes to acrylic.
Strength, Clarity, Scratch, UV, and Heat in Service
Impact and toughness. The 30× Izod gap is the difference between a guard that cracks the first time a part flies off a fixture and one that lasts years: polycarbonate bends, whitens at the impact point, and keeps working; acrylic fractures. That is why machine guards, face shields, and safety glazing are polycarbonate almost by definition. Acrylic still beats glass roughly 17×, but it is the wrong material anywhere a strike is plausible.
Tensile strength and stiffness. Quietly, acrylic is the stronger material under steady load: 65–80 MPa tensile strength against PC’s 55–75 MPa, with a higher modulus, so an acrylic bracket deflects less. Acrylic also resists creep far better — a loaded PC feature slowly sags where acrylic holds dimension. For structural parts that will never be struck, acrylic is the better engineering choice, not just the cheaper one.
Optical clarity and scratch resistance. At 92% light transmission, cast acrylic beats window glass and PC’s 86–89%, with lower haze and no bluish cast in thick sections. Its Rockwell M90–M105 surface is the hardest of any common clear sheet plastic; PC at ~M70 picks up scratches from routine cleaning. PC can be hard-coated to close the gap, but coatings add cost — for display and cosmetic parts, acrylic is unmatched at its price.
UV, yellowing, and heat. Acrylic is inherently UV-stable — outdoor signs stay clear for ten-plus years — while bare PC yellows and embrittles in sunlight and needs a UV-stabilized grade outdoors. Heat runs the other way: acrylic’s heat deflection tops out around 75–100 °C, while PC holds shape to 125–140 °C with a glass transition near 147 °C. Near motors, lamps, or sterilization cycles, PC is the only transparent option of the two.
Machining Acrylic vs Polycarbonate: Chips, Chipping, and Stress Cracking
Both materials machine on standard 3-axis CNC equipment with carbide tooling and excellent achievable tolerances — but they punish different mistakes. Acrylic fails by brittleness; polycarbonate fails by heat. That single sentence prevents most transparent-part scrap.
Acrylic produces short, clean chips and takes a brilliant finish — Ra 0.4–0.8 µm is realistic, and edges can be flame- or vapor-polished back to full clarity. The hazards are edge chipping and breakout: use sharp single-flute cutters, climb mill thin walls, and never force a dull tool. Specify cast acrylic for machining — it chips less than extruded sheet. Polycarbonate is almost impossible to crack with a cutter, but its toughness makes long, stringy chips that wrap the tool, and friction quickly smears the surface (Ra 1.6–3.2 µm typical). Sharp tools, high feed, lower RPM, and strong air blast keep PC cutting clean.
Both share one trap: stress cracking (crazing) from chemicals. Many liquid coolants and solvents craze acrylic and polycarbonate, so most shops machine both dry with air blast. Acrylic is also notch-sensitive at tapped holes — over-torqued fasteners are a classic killer — while PC absorbs moisture and may need pre-drying before tight-tolerance work. This is routine process control, the same discipline we apply to acetal and nylon in Delrin vs nylon.
Cost and Application Selection: Which Material Wins Where
On raw stock, acrylic runs 30–50% cheaper than polycarbonate, and PC often machines a touch slower because of chip control and heat management. The cost decision is simple: if the part does not need PC’s impact or temperature performance, acrylic wins on price every time. The application map below shows how the two split real-world work.
| Application | Better Choice | Why |
|---|---|---|
| Machine guards & safety shields | Polycarbonate | Impact strength is the whole point — PC survives flying debris and dropped tools |
| Sight glasses & machine windows | Polycarbonate (or acrylic if unimpact-loaded) | PC for abuse-prone stations; acrylic where clarity and scratch resistance matter more |
| Signs, displays & retail fixtures | Acrylic | Higher clarity, harder surface, lower cost, and no yellowing outdoors |
| Optical lenses & light pipes | Acrylic | 92% transmission and better polishability; optical grades widely available |
| Outdoor enclosures & glazing | Acrylic (or UV-coated PC) | Bare PC yellows in sunlight; acrylic stays clear for years |
| High-heat lamp covers & near-motor guards | Polycarbonate | HDT to ~140 °C vs acrylic softening near 100 °C |
| Prototype covers & low-volume brackets | Acrylic | Cheaper stock, faster machining, cleaner edges |
Two edge cases: safety eyewear uses polycarbonate despite its optical inferiority because impact protection is non-negotiable. Bullet-resistant glazing laminates the two — acrylic layers for clarity and hardness bonded to PC layers for impact absorption. At a boundary case, the answer is often a coated or laminated construction rather than a single material.
Specifying Acrylic or Polycarbonate on Your Drawing
Three callouts prevent most transparent-part problems. First, name the grade: “cast acrylic” for machined cosmetic parts, “UV-stabilized polycarbonate” for anything outdoors, “hard-coated PC” for abrasion-prone guards. Second, state the finish expectation — “edges vapor polished” on an acrylic display cover, “as-machined Ra 3.2 acceptable” on a PC guard. Third, flag the failure you fear: impact, scratch, heat, or yellowing. That single line of intent lets the shop catch a wrong material choice before it becomes scrap.
Design details matter more here than in metals: generous internal radii reduce acrylic’s notch sensitivity, avoid over-torqued fasteners in PMMA, and keep walls at 2–3 mm minimum. Our engineers review every transparent-part RFQ for these issues and machine both materials in-house with material certifications — upload your model on the instant quote page for a DFM-checked price within 1–2 business days.
FAQ: Acrylic vs Polycarbonate
Is polycarbonate stronger than acrylic?
It depends on which kind of strength you mean. Polycarbonate wins decisively on impact strength: notched Izod values run 12–16 ft·lb/in versus roughly 0.4 ft·lb/in for acrylic, so PC takes about 30 times the impact energy before cracking. Acrylic actually wins on tensile strength (65–80 MPa versus 55–75 MPa) and stiffness, and it has a much harder surface. In practice: if the part can be hit, dropped, or struck by debris, choose polycarbonate; if it carries a steady load and needs to stay scratch-free, acrylic is the stronger pick in everyday service.
Which is clearer, acrylic or polycarbonate?
Acrylic is clearer. Cast acrylic transmits about 92% of visible light — slightly better than standard float glass — while clear polycarbonate transmits roughly 86–89%. Acrylic also has lower haze and no bluish cast in thick sections, which is why it dominates lenses, light pipes, display cases, and museum glazing. For any part where optical quality is the primary requirement, acrylic is the default choice; PC is only chosen optically when impact safety is also required, as in safety eyewear and protective visors.
Does polycarbonate yellow over time?
Yes, unprotected polycarbonate yellows and becomes brittle under UV exposure, typically within a few years outdoors, because UV breaks down the carbonate linkages at the surface. Manufacturers address this with co-extruded UV-resistant cap layers or hard coatings — most outdoor-rated PC sheet carries one. Acrylic is inherently UV-stable and resists yellowing for a decade or more in direct sunlight, which is why it is the standard for outdoor signage and skylights. If your machined part lives outdoors, specify acrylic or a UV-coated polycarbonate grade — and say so on the drawing.
Which is easier to CNC machine, acrylic or polycarbonate?
Both machine well, but differently. Acrylic cuts crisply with sharp tools and reaches excellent finishes (Ra 0.4–0.8 µm), but it is brittle: aggressive feeds, dull cutters, or thin features cause edge chipping and corner breakout, and it can craze from coolant or solvent contact. Polycarbonate is far more forgiving of rough handling — it will not crack — but its toughness produces long, stringy chips that need good evacuation, it softens quickly under frictional heat, and finished surfaces are duller (Ra 1.6–3.2 µm). For tight cosmetic work acrylic is easier; for robust parts, PC.
Which is cheaper, acrylic or polycarbonate?
Acrylic is consistently cheaper. Sheet and rod stock typically runs 30–50% less than equivalent polycarbonate, though cast acrylic — the preferred machining grade — narrows the gap slightly. The raw-material gap also matters less on small parts where cycle time dominates cost, but PC tends to machine a bit slower because of chip control and heat management. Unless the application genuinely needs PC’s impact strength or temperature resistance, acrylic delivers the lower total part cost in most transparent-part projects.
When should I choose acrylic over polycarbonate — or the reverse?
Choose acrylic when optical clarity, scratch resistance, UV stability, and cost lead the requirements: signage, display covers, lenses, light pipes, windows that nobody will hit, and outdoor parts. Choose polycarbonate when impact, toughness, or heat lead: machine guards, safety shields, protective visors, parts near motors or lamps, and anything that might be dropped or struck. If both apply — say a clear outdoor safety guard — use UV-coated polycarbonate. When in doubt, tell your machine shop the failure you fear most and let them confirm the material choice.
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Transparent Parts, Done Right the First Time
IsoCNC machines cast acrylic and polycarbonate daily — guards, windows, lenses, covers, and displays — with dry machining to prevent crazing and polishing options for optical finishes. Send your CAD and an engineer will confirm the material choice and return a DFM-reviewed quote, typically within 1–2 business days.