Acrylic (PMMA)
Also: PMMA, polymethyl methacrylate, Plexiglas, Plexiglass, Acrylite, Lucite, Perspex, Optix, cast acrylic, extruded acrylic, acrylic glass, acrylic sheet
Acrylic (PMMA): the clear, hard, weatherproof plastic for signs, displays and optics. Laser-cuts and polishes well but is brittle; cast and extruded differ.
Sourcing acrylic parts or stock?
- Machine shops that work with plastics · 189
- Extrusion companies that work with plastics · 121
- 3D Printing companies that work with plastics · 55
- All U.S. manufacturers working with plastics · 1,204
At a glance
| Density | 1.19 g/cm³ (0.043 lb/in³) |
|---|---|
| Tensile strength | 62-76 MPa (9-11 ksi) at break; cast typically at the upper end |
| Yield strength | n/a (brittle: it fractures at a few percent strain without a distinct yield point) |
| Hardness | Rockwell M 90-100 |
| Elongation | 2-6% at break |
| Modulus of elasticity | 2.8-3.3 GPa (0.40-0.48 Msi) tensile |
| Thermal | Tg about 105-115 °C (220-240 °F), cast higher than extruded; amorphous, no melting point; continuous service about 65-90 °C (150-195 °F); HDT about 90-100 °C (195-212 °F) at 1.82 MPa |
| Composition | Polymethyl methacrylate, (C5H8O2)n, amorphous; cast sheet is very high molecular weight, extruded sheet lower; impact-modified grades add an acrylic rubber phase |
Typical values, Clear, unmodified cast and extruded sheet, 23 °C (73 °F); cast values at the upper end of each range. Confirm against the mill certificate and the purchase spec.
- MachinabilityGood
- Cast acrylic machines and polishes very well; extruded is gummier. Use sharp, zero- or low-rake tools, air or water cooling, and zero-rake drills with backing to avoid chipping and cracking at breakthrough. Anneal before bonding or cleaning.
- WeldabilityGood
- Solvent bonds with water-thin capillary cements (methylene chloride based) or thickened cements into clear, strong joints; cast bonds more reliably than extruded, which crazes. Ultrasonic welding works on molded parts. Anneal machined parts first.
- FormabilityGood
- Line-bends cleanly with a strip heater and thermoforms well; extruded forms at a lower temperature, cast keeps better optical quality. It cannot be bent cold: it cracks.
- Corrosion resistanceGood
- Excellent outdoor weathering and UV stability; resists water, dilute acids and alkalis, and many oils. Attacked or crazed by ketones, esters, aromatic and chlorinated solvents, and by alcohols and window cleaners on stressed parts. Clean with mild soap and water.
Acrylic is polymethyl methacrylate (PMMA), an amorphous thermoplastic sold as Plexiglas, Acrylite, Lucite, Perspex and Optix. It transmits about 92% of visible light, resists scratching better than other clear plastics, and stays clear outdoors for decades without yellowing. It weighs about half as much as glass (1.19 against about 2.5 g/cm3) and resists impact many times better than glass of the same thickness, but it is brittle next to polycarbonate.
The choice that matters most is cast versus extruded. Cast acrylic is polymerized in molds or on a continuous caster, has a much higher molecular weight, and machines, polishes and engraves better (laser engraving turns frosty white); it resists solvents better and is stocked in thick sheet and blocks. Extruded acrylic is cheaper, more uniform in thickness, thermoforms at a lower temperature and laser cuts to a clean polished edge, but it is softer and gummier to machine, crazes more readily when solvent bonded or cleaned, and carries more internal stress from extrusion.
Acrylic is brittle: elongation at break is only a few percent, it cracks from sharp notches, careless drilling and over-tightened fasteners, and it cannot be bent cold. It is heat-bent along a line with a strip heater or thermoformed, cut by CO2 laser, router or saw, and joined with thin solvent cements that leave nearly invisible seams. Stressed acrylic crazes when it meets alcohol, acetone or many cleaners, so quality shops anneal after machining and before bonding.
Heat treatment
Not heat treated for properties. Anneal after machining, laser cutting or polishing and before solvent bonding: heat slowly to about 70-85 °C (160-185 °F), extruded at the lower end, hold for a few hours depending on thickness, and cool slowly. Annealing removes the stress that causes crazing.
Tempers and conditions
- Cast sheet (cell-cast and continuous-cast)
- Extruded sheet
- Impact-modified
- UV-filtering and UV-transmitting grades
- Abrasion-resistant coated sheet
- Colored, tinted, fluorescent, frosted and mirrored
- Heat-resistant aircraft glazing (MIL-PRF-5425)
- Optical molding grades
Stock forms
- Sheet (cast and extruded)
- Block and thick cast slab
- Rod and tube
- Mirror sheet
- Molding pellets
Relative cost
Tier 2 of 5. Extruded sheet is one of the cheapest clear plastics, below polycarbonate; cast sheet costs more, and thick cast block much more per pound.
Typical applications
- Signs, displays and point-of-purchase fixtures
- Windows, skylights, glazing and sneeze guards
- Aquarium and tank windows (thick cast panels)
- Lenses, light guides, diffusers and lighting covers
- Microfluidic chips, cuvettes and clear lab fixtures
- Machine covers and inspection windows where impact is low
- Laser-cut enclosures, models and awards
Process compatibility
How acrylic takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| Laser Cutting | ExcellentExcellent | The best laser-cutting plastic: a CO2 laser leaves a clear, flame-polished edge. Extruded cuts cleanest; cast engraves frosty white. Anneal before solvent bonding. Can acrylic be laser cut? |
| CNC Machining | GoodGood | Machines well with sharp tools to polishable surfaces; it is brittle, so support thin sections and avoid heavy clamping. |
| CNC Milling | GoodGood | Routed and milled from sheet; single- or O-flute cutters and good chip evacuation keep edges from melting and chipping. Edges can be flame or buff polished. |
| CNC Turning | GoodGood | Cast rod turns to a polishable finish for knobs, lenses and light pipes; diamond turning gives optical surfaces. |
| Waterjet Cutting | FairFair | Cuts thick cast slab without heat, but the pierce can crack brittle sheet and the edge is frosted; pre-drill pierce holes and polish afterwards. |
| Injection Molding | ExcellentExcellent | Molds optical parts such as lenses, light pipes and covers. Dry the resin and control molded-in stress, which causes crazing later. |
| Press Brake Forming | Not recommendedNot recommended | Acrylic cracks when bent cold. Heat a line with a strip heater and bend it in a jig, or thermoform. |
| SLA 3D Printing | FairFair | Clear SLA resins give acrylic-like transparent prototypes after polishing; they are not PMMA and are weaker and less UV-stable. |
| FDM 3D Printing | PoorPoor | PMMA filament exists but prints poorly and is not clear. Laser cut or machine acrylic, or use clear SLA resin for prototypes. |
| Bead Blasting | GoodGood | Frosts clear sheet evenly for diffusers, privacy panels and matte displays. |
| Vibratory Finishing | PoorPoor | Scuffs and hazes clear surfaces; use it only on non-cosmetic parts. |
Compare acrylic
Design and sourcing tips
- Say cast or extruded on the drawing: cast for machined, polished, engraved or bonded parts and thick sections; extruded for laser-cut or thermoformed sheet parts at lower cost.
- Cast sheet thickness can vary by roughly ±10-15% of nominal, extruded much less. Do not dimension critical fits off the as-supplied thickness of cast sheet.
- Keep holes at least two sheet thicknesses from edges, oversize them for thermal expansion of about 70-80 um/m·K, and use washers: over-tightened screws crack acrylic.
- Specify annealing for parts that will be solvent bonded, cleaned with alcohol or loaded; unannealed laser-cut and machined edges craze.
- Choose polycarbonate for guards and anything likely to be struck; acrylic is for clarity, scratch resistance and outdoor stability.
- Call out the edge finish (saw, machined, flame-polished or buffed); clear edges are extra operations.
Frequently asked questions
- What is the difference between cast and extruded acrylic?
- Cast acrylic is polymerized in molds or between belts and has a much higher molecular weight; it machines, polishes, engraves and solvent bonds better, resists chemicals better and comes in thick sheet. Extruded acrylic is pushed through a die: cheaper, more uniform in thickness, easier to thermoform and laser cut to a clear edge, but softer to machine and quicker to craze. Laser-cut signs are often extruded; machined and bonded parts are usually cast.
- Is acrylic stronger than polycarbonate?
- In stiffness and hardness, slightly; in impact, no. Acrylic has a similar tensile strength and a higher modulus, and it resists scratching better, but its notched impact strength is only about 20-30 J/m against 640-850 J/m for polycarbonate. Use polycarbonate where the part may be hit, and acrylic where clarity and weathering matter.
- Can acrylic be laser cut?
- Yes. Acrylic is the classic CO2 laser material: it vaporizes cleanly and leaves a smooth, flame-polished edge. Extruded sheet gives the clearest cut edge; cast sheet gives a crisp frosted engraving. Laser-cut edges carry stress, so anneal parts before solvent bonding or cleaning with alcohol.
- Why does acrylic craze or crack?
- Crazing is a network of fine surface cracks that forms when stressed acrylic meets a chemical such as alcohol, acetone, many glass cleaners or solvent cement. The stress comes from machining, laser cutting, bending or tight fasteners. Annealing after fabrication, mild soap and water for cleaning, and cast rather than extruded sheet for bonded parts prevent most of it.
- Is Plexiglas the same as acrylic?
- Yes. Plexiglas, Acrylite, Lucite, Perspex and Optix are brand names for acrylic (PMMA) sheet, and "plexiglass" with a double s is a common name for the generic material. Grades differ between cast and extruded and between producers, so specify cast or extruded and any optical, UV or impact requirement.
Processes that commonly use it
Related materials
Where to verify these values
- Plexiglas, Acrylite and Lucite sheet data sheets and fabrication guides
- ASTM D4802 and D788
- ASTM D638, D256 and D1003 (haze and luminous transmittance) test methods
