PMMA — Polymethyl Methacrylate (Plexiglas)

Polymethyl Methacrylate (Plexiglas)

PMMA (polymethyl methacrylate), or acrylic, is an amorphous thermoplastic prized for its optical clarity and UV resistance. It is the reference for optical parts, displays and signage, where clarity and weather stability are paramount.

• Mold temperature: 50-80°C
• Melt temperature: 220-260°C
• Shrinkage: 0.2-0.8%
• Density: 1.17-1.20
• Manufacturer grades: Plexiglas 8N (Röhm), Altuglas V825T (Arkema), Acryrex CM-205 (Chi Mei)

Processing

PMMA is hygroscopic: drying of about 2 to 4 h at 80 °C is recommended before molding, otherwise moisture causes bubbles and silver streaks that ruin transparency. Run the melt between 220 and 260 °C with a mold regulated between 50 and 80 °C, favoring steady injection speeds and highly polished molds to preserve the optical surface finish.

Advantages

• Exceptional optical clarity: optics, lenses, displays
• Excellent UV and weather resistance (does not yellow)
• Fine surface finish and good gloss
• Good dimensional stability (amorphous, low shrinkage)

Limitations

• Hygroscopic: drying at 80 °C required to avoid bubbles and silver streaks
• Brittle material: low impact resistance
• Scratch-sensitive without anti-scratch surface treatment
• Limited chemical resistance to solvents and alcohols

Common injection molding defects

• Silver streaks — Residual moisture: dry PMMA at 80 °C before molding
• Bubbles — Moisture flashing to vapor: improve drying and degas the material
• Brittle parts — Intrinsically brittle material and notches: soften corners and limit internal stresses

Typical applications

Optics, automotive tail lights, signage, display cases