Acrylic sheet, decoded
Acrylic sheet (PMMA) is a lightweight transparent thermoplastic: density 1.18-1.20 g/cm3, roughly half that of glass, and up to 92% visible light transmission at 3 mm. This reference decodes the standard property figures and lays out the ordered procedure for cutting, bonding and choosing between cast and extruded stock.
| Property | Value | Condition or note |
|---|---|---|
| Density | 1.18-1.20 g/cm3 | About half the density of glass |
| Visible light transmission | Up to 92% | At 3 mm sheet thickness |
| Surface reflection | About 4% | From each surface |
| Glass transition temperature | 105 °C | Atactic PMMA; commercial 85-165 °C |
| Ignition temperature | 460 °C | Burns to CO2, H2O and CO |
| Refractive index | 1.4905 | At 589 nm (sodium D-line) |
| Coefficient of thermal expansion | (5-10) × 10^-5 /°C | Linear, per °C |
Every number on an acrylic datasheet is conditional: on thickness, wavelength, grade or heating rate.
Density of PMMA is 1.18 to 1.20 g/cm3, roughly half that of glass, which is why a large acrylic window weighs about half a glass pane of the same size.
Light transmission reaches up to 92% of visible light at 3 mm thickness, with about 4% reflected from each surface; the two reflections together account for most of the remaining 8%.
The refractive index is 1.4905 at 589 nm, the sodium D-line, so optical calculations for lenses and light guides use that single reference wavelength.
PMMA softens near 105 °C and ignites at 460 °C.
The glass transition temperature of atactic PMMA is 105 °C; commercial grades range from 85 to 165 °C, so a datasheet figure outside that band signals a modified or filled compound.
Below the glass transition the sheet behaves as a rigid glassy solid; above it the modulus drops sharply and the sheet can be thermo-formed.
PMMA ignites at 460 °C and burns to carbon dioxide, water and carbon monoxide — no halogenated products, which matters for fire-safety classification.
Score, snap or saw, then let the edge rest before any bonding.
For sheet up to about 6 mm, score repeatedly with a acrylic knife and snap over a straight edge; thicker stock is sawn with a fine-tooth blade and the protective film left on to prevent scoring.
Laser cutting produces a polished edge but leaves a heat-affected zone; laser-cut PMMA must be annealed before chemical bonding, otherwise solvent wicks into micro-cracks.
Unmodified PMMA is brittle under impact and scratches more easily than glass, so edges handled repeatedly should be flame-polished or fitted with a protective profile.
The sequence below is the order of operations, not a chronology.
Each plate isolates one figure with its unit and test condition.
Standard material property data for poly(methyl methacrylate) sheet.