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PMMA

Polimetil Metacrilato (Acrílico)

PMMA·Acrylics·Amorphous

PMMA (Polymethyl Methacrylate — better known by its brand names Plexiglas, Lucite, Altuglas, Acrylic) is the transparent plastic par excellence: 92% light transmission (more than glass), native UV resistance that does not yellow over the years, and the best polishable surface in the polymer market. It is the material of museum display cases, illuminated signage, fish tanks and large aquariums, automotive headlights, dentures and dental prostheses, optical lenses, Fresnel panels, LED lamps and thousands of applications where crystal clarity is priority #1.

But it has a serious weakness: it is brittle to impact (only 1/10 the resistance of PC, practically like tempered glass), and sensitive to stress cracking in the presence of alcohols, solvents and common cleaners. Here we have compiled the reference ranges from the PDS, plus the questions that come up over and over on the shop floor: why you need to dry it even though it absorbs little, when PMMA vs PC vs PETG makes sense, how to avoid stress whitening and crazing, what grades to choose for optics vs signage, and how to polish or chemically bond it without cracking.

Share your experience in the comments — ranges vary by manufacturer and grade (cell-cast, extruded, injection), and collective discussion is what gets us out of trouble on the floor.

The ranges shown in these data tables were compiled by the MVPS team from various parameter sheets and literature, integrating the lower and upper limits for each material type.

This information must be carefully reviewed when developing injection molding processes. Final ranges and processing tolerances are the responsibility of the engineer in charge.

These ranges are not recommended for developing specific process tolerances. MVPS always recommends requesting and consulting the supplier's data sheet.

General Properties

Chemical StructureAmorphous
Specific Gravity (Density)1.2:1
L/D Ratio18 – 24
Compression Ratio2 – 3
Tonnage Factor4.63 – 6.18kN/cm²
Thermal Diffusivity0.1155mm²/s
Max Shear Rate40,0001/s
Shrinkage0.2 – 0.8%
Regrind30%
Heat Deflection (HDT) @ 1.82 MPa71°C
Glass Transition (Tg) @ 10°C/min85°C
Vicat Softening @ 50N104°C

Drying

Drying Temperature71 – 79°C
Drying Time3 – 5h
Recommended Moisture0.02%
Recommended Dryer TypeAir
Dew Point-40°C

Temperatures

Melt218 – 279°C
Nozzle199 – 249°C
Front221 – 252°C
Middle210 – 241°C
Rear199 – 229°C
Demolding66 – 91°C
Mold (Cooling)49 – 79°C
Feed Throat35 – 79°C

Processing

Back Pressure6.9 – 13.8bar
Screw Speed40 – 70RPM
Injection SpeedLow – Medium
Barrel Occupancy20 – 60%
Injection Pressure700 – 1,500Pbar
Holding Pressure175 – 1,200Pbar
Cushion3.2 – 6.4mm

Mold

Runner Diameter4.06 – 12.2mm
Gate Diameter0.76 – 2.03mm
Gate Area0.46 – 3.24mm²
Wall Thickness1.02 – 5.08mm

Venting

Depth (Vent Depth)0.0102 – 0.0203mm
Land (Vent Land)0.508 – 1.02mm
Width (Vent / Clearance)3.05 – 6.1mm
Relief (Relief Channel)0.1016 – 0.2032mm

Frequently asked questions

PMMA is an amorphous thermoplastic synthesized by polymerization of methyl methacrylate (MMA). Result: chain with pendant ester groups that give it 92% optical transparency (more than glass) and native UV resistance (ester groups are UV stable). Density ~1.18 g/cm³. It is the plastic with the best clarity/UV ratio of all, which makes it irreplaceable in optical applications exposed to sunlight. It exists in three formats: cell-cast (sheet, highest molecular weight, best mechanical), extruded (industrial sheet, medium weight), injection (low molecular weight, optimized for flow).
Key tradeoff: PMMA wins on optical clarity (92% vs 88% of PC), UV resistance (does not yellow, vs PC that does), scratch resistance (harder surface), cost (30–50% cheaper), mirror polishable with abrasive compounds. PC wins on impact resistance (10–30× more, practically unbreakable vs PMMA that breaks like glass), thermal tolerance (140°C vs 80°C), thermoforming ease. Rule of thumb: if the risk is impact breaking → PC. If the risk is yellowing under sun or scratching → PMMA. Safety lenses or helmets → PC. Display cases, outdoor signage, headlight optical lenses → PMMA. Small aquariums → PMMA (clearer); large (>500 L) → thick cell-cast PMMA or PC.
PMMA: clarity #1 (92%), UV-resistant #1, scratch #1, impact last (1× reference, brittle). PC: impact #1 (30× PMMA), high HDT, 88% transparency, yellows without protection. PETG: 88–90% clarity, medium impact (~5× PMMA), excellent chemical resistance, easy to thermoform, intermediate cost. For premium outdoor signage → PMMA. For anti-vandal glazing → PC. For transparent packaging and economical commercial display cases → PETG.
PMMA absorbs 0.3–0.4% at equilibrium — low compared to PA6 (3%) but high for an optical transparent part. At 230–270°C injection, that moisture generates silver streaks (silver radial lines from the gate) and internal microbubbles that ruin optical transparency. The PDS requires ≤0.05% before injection. Drying: desiccant 80–90°C for 3–4 h, dew point ≤ –30°C. In humid climates you may need 4–5 h. Never use hot-air (non-desiccant) dryer in tropical climate — it doesn't lower moisture enough.
Stress whitening: whitish areas appearing where the part is deformed or struck — microfibrils scattering light. Crazing: a network of surface microcracks that slowly appear under stress + chemical contact (IPA, ethanol, gasoline, generic window cleaners). PMMA is very sensitive. Solutions: (1) post-mold annealing at 80°C for 2–4 h releases residual stress (critical for parts that are machined or polished afterward), (2) avoid contact with isopropanol and ammonia-based cleaners — use neutral soapy water to clean, (3) design without sharp angles (radii ≥0.5 mm), (4) eject gently — wide well-polished pins.
The PDS marks 41–79°C. Hotter (70–80°C) = better optical detail reproduction, less residual stress, better gloss, but longer cycle. Cooler (40–55°C) = short cycle, but more frozen stress and worse surface. For critical optical parts (lenses, headlights, prisms): always 75–85°C and consider hot-cold mold cycling (variotherm) for mirror finishes. For non-critical signage and display cases: 50–65°C ideal balance. Never below 40°C — you generate stress that causes crazing weeks later.
Very low shrinkage: 0.2–0.8%, isotropic (same in all directions) — typical of amorphous polymers. This is key for optical parts: low + uniform shrinkage = parts that reproduce mold detail without distortion. That's why headlights and optical lenses are almost always made of PMMA or PC, never of semi-crystalline materials (which have high directional shrinkage). PMMA also has a refractive index of 1.49 that is very consistent batch to batch — fundamental for optics.
PMMA wins: 50% lighter (density 1.18 vs 2.5), 10–20× more impact resistant, easy to cut/machine/thermoform, better native UV transmission (actually blocks UV-B/C but transmits visible light better than glass), doesn't shatter into dangerous fragments. Glass wins: scratch resistance (glass Mohs 5.5–6 vs PMMA 2–3), chemical resistance (PMMA hates alcohols), thermal tolerance (glass 200°C continuous vs PMMA 80°C), cost at large formats. Applications where PMMA replaces glass: signage, large aquariums, building balconies, helicopter windshields (yes, military Plexiglas), industrial machine cabins.
Chemical bonding (cementing): PMMA is 'welded' with dichloromethane or MMA monomer: the solvent surface-dissolves both pieces and on evaporation they're left polymerized as one — invisible transparent joint. For large parts (aquariums, display cases) this is the standard method. Adhesive bonding: cyanoacrylate + activator gives fast joints. Polishing: progressive sanding 600 → 800 → 1200 → 2000 grit, then polishing compound (ceramic or cerium oxide) with cotton wheel. Result: optical mirror surface. Thermoforming: 150–170°C, easy — it's the basis of curved illuminated signage.
Food-safe: yes, specific PMMA grades are approved by FDA and EFSA for food contact. Used in food counters, bakery display cases, sneeze guards, mixing bowls, displays for grains/candy. Ask supplier for lot-specific certificate. Biocompatible: PMMA is the classic medical plastic for implants and prostheses — dentures, dental fillings, intraocular lenses in cataract surgery, bone cements in orthopedics, facial cosmetic prostheses. Has 80+ years of documented medical use. Important: NOT biodegradable, do not use in one-use applications without specific certification.

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