PMMA / Acrylic Injection Moulding: Properties, Uses & Design Guide
PMMA (polymethyl methacrylate), commonly known as acrylic or Plexiglas, is the optical-clarity standard among injection-moulding thermoplastics. With light transmittance of 92–93% — higher than polycarbonate — excellent UV stability, and a surface hardness that outperforms PC, PMMA is the preferred material for lighting components, display covers, optical diffusers, and premium transparent housings. Nordmould processes PMMA for both functional optical parts and high-aesthetic consumer applications.
What are the mechanical and thermal properties of PMMA?
PMMA is an amorphous glassy thermoplastic. Its amorphous structure delivers the optical clarity and low shrinkage that define the material, while its lack of crystallinity also means lower chemical resistance and toughness than semi-crystalline alternatives.
| 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 | 92–93% | ASTM D1003 |
| Refractive Index | 1.49 | ASTM D542 |
| Rockwell Hardness | M80–100 | ASTM D785 |
| Water Absorption (24 h) | 0.2–0.4% | ISO 62 |
PMMA's low shrinkage (0.2–0.6%) and isotropic dimensional behaviour allow toolmakers to maintain tight tolerances on optical surfaces. Its Rockwell M80–100 hardness is the highest among standard optical thermoplastics, contributing to good scratch resistance compared to PC (which is significantly softer).
Where is PMMA injection moulding used?
PMMA's defining advantages — optical clarity, UV stability, and surface hardness — dictate where it is applied. It fills the gap between PC (high impact, lower clarity) and glass (high temperature, non-formable).
Lighting and luminaires: Automotive tail lamp lenses, rear light diffusers, streetlight covers, LED secondary optics, and light guide plates for flat-panel backlit displays. PMMA's UV stability prevents yellowing in outdoor lighting applications where PC would degrade.
Display and visual merchandising: Point-of-sale display covers, museum exhibit screens, information kiosk panels, and retail price-tag holders. The combination of clarity and mouldability makes PMMA an efficient alternative to cut-and-polish glass panels.
Optical instruments: Camera lenses, microscope eyepieces, instrument windows, and optical fibre end ferrules. Optical-grade PMMA achieves precisely controlled refractive indices for lens design.
Medical and dental: Bone cement (PMMA is the basis of surgical acrylic cement), intraocular lenses, dental prostheses, blood oxygenator windows, and fluid-path components. ISO 10993-compliant grades are widely available.
Consumer goods: Watch glasses, cosmetic packaging lids, premium electronics covers, and decorative bezels where a glass-like appearance is desired at plastic weight and cost.
Architecture and signage: Illuminated sign faces, channel letters, and architectural glazing inserts. PMMA sheet is used here, but injection-moulded PMMA components (edge connectors, clips, diffuser caps) are common.
What are the moulding characteristics of PMMA?
PMMA is considered a moderately demanding injection-moulding material. Its brittleness, hygroscopicity, and sensitivity to thermal degradation require careful process control, but it rewards a well-controlled process with outstanding surface quality.
Melt temperature: 220–270°C. Degradation (yellowing, reduced molecular weight) occurs above 280°C. Localised overheating from a poorly designed hot runner or screw geometry must be avoided.
Mould temperature: 50–80°C. Higher mould temperatures improve surface quality, reduce internal stress, and improve clarity. For optical parts, 70–80°C mould temperature is recommended; lower temperatures produce haze and internal birefringence.
Injection speed: Slow to moderate. PMMA is more viscous than ABS or PC at equivalent melt temperature. Excessive injection speed causes jetting and surface splay. Gradual fill profiling reduces gate blush and optical artefacts.
Drying: Essential. PMMA absorbs moisture readily. Target moisture content: below 0.08% (general use), below 0.04% (optical applications). Drying: 4–6 hours at 80–90°C in a dehumidifying dryer. Do not use hot-air dryers for optical-quality work.
Shrinkage: 0.2–0.6%. Very low and isotropic, which is why PMMA is trusted for precision optical shapes and assemblies with tight mating tolerances. This is among the lowest shrinkage of any commodity thermoplastic.
Draft angles: Minimum 1° on smooth surfaces; 1.5–2° on textured or long draw surfaces. PMMA's high surface hardness means it releases well from polished tooling; adequate draft prevents drag scratching on optical surfaces.
Sink and warp: PMMA's low shrinkage substantially reduces warp compared with POM or PP. Sink marks are possible at thick bosses; standard rib-to-wall ratio rules (50–60%) apply. Wall uniformity is important for optical flatness.
Gate design: For optical parts, fan gates, film gates, or edge gates at non-visible faces are preferred. Pinpoint gates in the optical field leave visible gate vestige and stress halos; careful gate location is critical. Weld lines in optical PMMA are visible and should be directed out of the optical path.
Which PMMA grades and variants should you consider?
| Grade / Variant | Key Modification | Typical Use |
|---|---|---|
| Standard PMMA | Baseline clarity | Lighting, signage, display |
| Optical PMMA | Tight refractive index, ultra-low haze | Precision optics, camera lenses |
| High-impact PMMA | Rubber-toughened | Covers requiring some impact tolerance |
| UV-filtering PMMA | UV-absorbing comonomer | Protective museum glazing, UV-blocking covers |
| Medical-grade PMMA | ISO 10993, USP Class VI | Medical optics, implant bone cement |
| Heat-resistant PMMA | Higher HDT (up to 115°C) | Automotive lighting, high-ambient environments |
| Light-diffusing PMMA | Diffusing agent incorporated | LED diffuser panels, lighting covers |
High-impact PMMA (rubber-modified) improves notched Izod impact strength to 60–100 J/m — still well below PC but adequate for many lightly handled consumer parts. The trade-off is a slight increase in haze.
What are PMMA's advantages and limitations?
Advantages:
- Highest optical transmission (92–93%) and lowest haze of standard moulding plastics
- Excellent UV stability: does not yellow under outdoor sunlight exposure — superior to standard PC
- Higher surface hardness than PC: more scratch-resistant without hard coating
- Very low shrinkage (0.2–0.6%): precise optical dimensions achievable
- Good biocompatibility: ISO 10993 grades widely available for medical applications
- Accepts high-gloss polish from tooling directly, producing optical-grade surface finishes
Limitations:
- Low impact resistance in standard grades: PMMA is brittle; avoid sharp corners and dynamic impact applications
- Poor chemical resistance: solvents (alcohols, ketones, esters, aromatic hydrocarbons) attack and craze PMMA
- Hygroscopic: requires rigorous drying before processing
- Lower heat resistance than PC: HDT of 90–105°C, lower than PC's 130–145°C
- More difficult to process than ABS: viscous, injection-speed-sensitive, prone to gate blush in optical applications
- Cannot be electroplated or metallised without specialised primer systems
When should you choose PMMA over alternative materials?
PMMA vs PC: Choose PMMA when maximum optical transmission, UV stability, or scratch resistance without hard-coat are the priority. Choose PC when impact resistance is critical — PC is roughly 30–40× tougher than standard PMMA. For applications that need both, a PC core with PMMA cap or co-moulding is possible.
PMMA vs PS (GPPS): Choose PMMA when UV stability, surface hardness, or weatherability are important. PS is cheaper and easier to process, but brittle and UV-unstable. For interior display applications with no UV exposure and minimal mechanical loads, PS can substitute.
PMMA vs glass: Choose PMMA when weight reduction, complex three-dimensional form, or injection-mouldable integration features (clips, threads, snap-fits) are needed. Glass has better temperature resistance, chemical inertness, and scratch hardness, but cannot be injection moulded. PMMA is roughly half the weight at comparable thickness and can be formed into any mouldable shape.
PMMA vs SAN: SAN (styrene-acrylonitrile) is stiffer and slightly harder than PS but less clear than PMMA. It suits rigid transparent packaging where PMMA's optical quality is not required.
Is PMMA recyclable?
PMMA carries resin code 7 and has limited municipal recycling infrastructure. However, PMMA is one of the more practically recyclable transparent plastics at an industrial level. Post-industrial PMMA regrind blended with virgin material is used in non-optical applications (diffusers, structural covers) where optical grade is not required. Chemical (depolymerisation) recycling of PMMA back to MMA monomer is technically established and commercially operated at scale in Japan and the UK, producing virgin-equivalent resin. Nordmould can discuss recycled-content and recyclability options for PMMA parts during the design phase.
Frequently asked questions
Is PMMA injection moulding optically clearer than polycarbonate?
PMMA has a light transmittance of 92–93% and very low haze, slightly better than PC (88–90%). Its refractive index of 1.49 is closer to optical glass than PC. For applications requiring maximum optical transmission — light guides, display covers, optical diffusers — PMMA is typically the first choice.
Is PMMA (acrylic) brittle in injection moulding?
Standard PMMA is notch-sensitive and has low impact strength compared to PC — roughly 15–25 J/m notched Izod versus 600–900 J/m for PC. High-impact PMMA grades incorporate rubber modifiers that improve toughness by 3–5×, at a modest reduction in optical clarity. Nordmould recommends reviewing impact requirements before selecting PMMA.
Does PMMA need to be dried before injection moulding?
Yes — PMMA is hygroscopic. Moisture content above 0.08% causes silver streaks, bubbles, and reduced surface quality. Standard drying is 4–6 hours at 80–90°C in a dehumidifying dryer. Optical grades may require longer drying to achieve streak-free clarity.
What is the UV resistance of PMMA compared to PC?
PMMA has excellent inherent UV resistance — it does not yellow outdoors even after years of exposure, unlike standard PC. This makes PMMA the preferred material for outdoor light covers, signage, solar panels, and any application where long-term colour and transmission stability under sunlight are required.
What surface finishes can be achieved with PMMA injection moulding?
PMMA accepts high-gloss, mirror-polish finishes better than most thermoplastics. It can replicate ultra-polished (diamond-turned) steel tooling surfaces directly, producing optical-quality surfaces without post-processing. Matt and textured finishes are also achievable. Nordmould uses polished hardened steel tooling for optical PMMA parts.
Can PMMA be used for medical or food-contact applications?
Yes — PMMA is biocompatible (ISO 10993) and is widely used in medical optics (bone cement, intraocular lenses, blood oxygenator windows). Food-contact grades compliant with EU regulations are available for display and packaging applications. Nordmould can confirm grade suitability during DFM review.
How does PMMA compare to glass as a transparent material?
PMMA is roughly half the weight of glass (density 1.18–1.20 g/cm³ vs 2.5 g/cm³), can be injection moulded into complex shapes, and offers comparable or better optical transmission. It is more scratch-resistant than PC but less than mineral glass. PMMA cannot match glass in temperature resistance or chemical inertness.
Send your STEP file to Nordmould for a free DFM review — our team will assess optical path, gate placement, and tooling requirements for your PMMA part before any tooling commitment.