PMMA / Acrylic Injection Moulding: Properties & Design Guide
PMMA (polymethyl methacrylate), widely known as acrylic, is the benchmark for optical clarity in injection moulding. At 92% light transmittance and with inherent UV stability, it outlasts polycarbonate outdoors without coatings. The material turns up in LED lenses, display covers, automotive tail-lamp clusters, and anywhere a glass-like aesthetic needs to be moulded into a complex 3D form.
Mechanical and Thermal Properties
PMMA is a rigid, amorphous thermoplastic. Its glass-like transparency comes from the absence of crystalline structure, though that same structure makes it brittle—it will fracture under sharp impact rather than deform.
| Property | Typical Value | Test Standard |
|---|---|---|
| Tensile Strength | 55–80 MPa | ISO 527 |
| Flexural Modulus | 2,700–3,300 MPa | ISO 178 |
| Izod Impact (notched, 23°C) | 15–25 J/m | ISO 180 |
| Heat Deflection Temp (0.45 MPa) | 90–105°C | ISO 75 |
| Vicat Softening Point | 100–115°C | ISO 306 |
| Density | 1.17–1.20 g/cm³ | ISO 1183 |
| Mould Shrinkage | 0.2–0.6% | ISO 294-4 |
| Light Transmittance | ~92% (3 mm) | ASTM D1003 |
| Refractive Index | 1.49 | ASTM D542 |
| Rockwell Hardness | M80–100 | ASTM D785 |
Acrylic's very low shrinkage (0.2–0.6%) and isotropic cooling behaviour allow tight dimensional control on optical surfaces and mating geometries. On this measure it sits alongside PC and well ahead of semi-crystalline materials.
Typical Applications
LED Optics and Lighting: Automotive tail lamps, light pipes, and secondary optics for street lighting. PMMA is preferred here because it doesn't yellow under UV exposure—a chronic problem with uncoated polycarbonate.
Consumer Electronics: Display windows for appliances, smartwatch covers, and premium bezels. Its surface hardness (Rockwell M80–100) gives better scratch resistance than PC, which matters for any part regularly touched.
Medical Hardware: Blood oxygenator windows and fluid containers where visual monitoring is critical. ISO 10993-compliant grades are available; grade and documentation should be confirmed at the quoting stage.
Point of Sale and Display: Premium display cases and cosmetic packaging. Acrylic gives a glass-like appearance at roughly half the weight and a fraction of the machining cost.
Moulding and Design Guidelines
PMMA is more viscous than ABS at equivalent temperatures and is particularly sensitive to moisture-related defects. Gate blush, splay, and jetting are the three most common issues.
- Melt Temperature (220–270°C): Overheating above 280°C causes yellowing and gas bubbles. Unlike PC, PMMA doesn't tolerate long residence times at high temperatures.
- Mould Temperature (50–80°C): For optical parts, run toward the high end (70–80°C) to reduce internal stress and improve surface replication. Cold moulds freeze in stress and produce hazy parts.
- Injection Speed: Moderate to slow, with a ramped profile. PMMA is prone to jetting—where the material shoots into the cavity as a fast stream rather than filling progressively. A gradual fill front gives the cleanest result.
- Drying is mandatory: Dry to below 0.08% moisture (0.04% for optics) in a dehumidifying dryer at 80–90°C for 4–6 hours. Even minor moisture produces silver streaks that cannot be polished out.
- Draft Angles: Minimum 1° on polished surfaces. PMMA is rigid and low-shrinkage, so ejection force is higher than with softer materials like PP—adequate draft prevents drag marks on the polished cavity wall.
- Gate Placement: For clear parts, edge or fan gates keep vestige and stress marks out of the viewing area. Avoid pinpoint gates on optical surfaces.
Material Grades
| Grade / Variant | Key Feature | Typical Use |
|---|---|---|
| Standard PMMA | High clarity | General signage, covers |
| Optical Grade | Low haze, tight RI | Camera lenses, light guides |
| High-Impact | Rubber-modified | Consumer housings |
| Heat-Resistant | Higher HDT (up to 115°C) | Automotive lighting |
| Light-Diffusing | Scattering agents | LED panels, backlit signs |
Rubber-toughened PMMA can push notched Izod impact strength to around 100 J/m, but expect a small drop in light transmittance—usually 2–4 percentage points depending on the grade.
Advantages and Limitations
Advantages:
- Highest light transmission of any commodity moulded plastic (~92%).
- Superior UV resistance—no yellowing without coatings.
- Better scratch resistance than PC (Rockwell M80–100 vs M70 for PC).
- Very low, isotropic shrinkage for precise optical and dimensional work.
Limitations:
- Brittle: PMMA will fracture under impact rather than deform. Not suitable for drop-tested assemblies.
- Chemical sensitivity: Attacked by many common solvents—alcohols, ketones, esters. Check compatibility before specifying assembly adhesives or cleaning agents.
- Low heat resistance: Parts soften above 90–105°C (HDT). Not suitable for environments near dishwashers or engine bays.
- Viscous flow: Harder to fill thin walls than ABS or PP. Gate design and fill analysis are important for walls below 1.5 mm.
Choosing PMMA vs Polycarbonate
The decision usually comes down to impact resistance vs optical quality and outdoor durability.
- Choose PMMA if you need the clearest possible part, inherent UV stability, or better surface hardness.
- Choose PC if your part needs to survive being dropped, struck, or exposed to temperatures above 100°C.
Sustainability
PMMA carries resin code 7. At an industrial level, it is relatively easy to recycle via depolymerisation, which converts it back into the original monomer for high-quality reuse—an advantage over most other code-7 materials. For non-optical parts, post-industrial regrind can often be incorporated to reduce material waste.
Nordmould offers tooling from €3,000 and low-volume production starting at 100 pieces. Send us your STEP file for a free DFM review—we'll verify wall thicknesses and gate locations to confirm your acrylic parts are production-ready.