PMMA Injection Molding Guide

PMMA (Polymethyl Methacrylate) is the most transparent plastic material available, often used as a glass substitute. However, it is more brittle than PC (Polycarbonate) and ABS. To improve its impact resistance, PMMA is often modified through:

• Copolymerization with styrene and butadiene.

• Blending with other plastics like PC.

A common example:

• 372 PMMA: Copolymer of methyl methacrylate and styrene.

• 373 PMMA: 372 PMMA modified with ~50% nitrile rubber for better impact strength.

Material Behavior and Thermal Properties:

• Amorphous polymer

• Melt temperature: >200 °C

• Decomposition temperature: Up to 270 °C

• Viscosity: Higher than PS and ABS; highly sensitive to temperature.

Moisture Control:

• PMMA absorbs moisture: 0.3–0.4%

• Required moisture content for injection molding: <0.1% (Ideal: ~0.04%)

• Effects of high moisture: bubbles, gas lines, poor transparency

Drying Conditions:

• Temperature: 80–90 °C

• Time: ≥3 hours

• Drying hopper recommended

Recycled Material Usage:

• Regrind material use: Up to 30% typically (can reach 100% in special cases)

• Ensure no contamination to maintain transparency and mechanical integrity

Injection Molding Machine Requirements:

• No special machine required, but:

• Deeper grooves and larger nozzle diameter recommended due to higher melt viscosity

• Use screw with larger L/D ratio for higher strength requirements

• Always use drying hopper for feeding PMMA

Mold and Gate Design:

Mold Temperature:

• 60–80 °C

Sprue:

• Match internal taper: 5°–7°

• For wall thickness ≥4 mm: use 7°

• Sprue diameter: 8–10 mm

• Max length: ≤50 mm

Runner System:

• Wall thickness <4 mm: runner diameter 6–8 mm

• Wall thickness ≥4 mm: runner diameter 8–12 mm

Gate Types:

• Fan/tab gate depth: 0.7–0.9 × t (t = wall thickness)

• Pin gate diameter: 0.8–2 mm (lower viscosity → smaller size)

Vent Design:

• Depth: 0.05–0.07 mm

• Width: 6 mm

Draft Angle:

• Stripping slope: 30′–1°

• Cavity portion: 35′–1° to 30′

Injection Parameters:

Melt Temperature:

• Range: 210–270 °C

• Use air shot method for accurate measurement

Injection Speed:

• Prefer multi-stage injection (e.g., slow–fast–slow)

• Use slow injection for thick parts to avoid internal stress

Dwell Time:

• At 260 °C: ≤10 minutes

• At 270 °C: ≤8 minutes
  (Avoid longer residence to prevent material degradation)