‌Transparent Plastics: Properties and Injection Molding Considerations‌

Driven by advantages like light weight, toughness, moldability, and low cost, plastics are increasingly replacing glass in industrial and consumer products—particularly in optical instruments and packaging. However, achieving high transparency, wear resistance, and impact strength demands rigorous control over material composition, processing, equipment, and mold design.

I. Critical Properties for Transparency

• High light transmittance (>90%)

• Strength and wear resistance

• Impact toughness

• Thermal/chemical stability

• Low water absorption

II. Universal Processing Precautions

‌Surface defects (e.g., streaks, voids, clouding) are unacceptable. Key measures:‌

A. Material Handling & Drying

• Hermetic storage/transport to prevent contamination

• Mandatory drying:

  • PMMA: 2–4 hrs @ 80–90°C

  • PC: 3–4 hrs @ 120°C

  • PET: 4–6 hrs @ 140–160°C

  
  

• Use dehumidifying hoppers with filtered air

B. Machine & Screw Hygiene

• Purge barrels/screws with dedicated cleaners or virgin PE/PS resin

• Reduce temperatures during pauses:

  • PC/PMMA: Barrel ≤160°C

  • PC: Hopper ≤100°C

  
  

C. Mold Design Guidelines

1. Uniform wall thickness (≥1mm for non-PET) + adequate draft angles

2. Smooth transitions (no sharp corners)

3. Wide/short runners + cold slug wells

4. Surface roughness ≤0.8 μm

5. Sufficient venting (0.02–0.04mm depth)

E. Post-Processing

• Avoid mold releases

• Regrind limit: ≤20%

• Stress relief annealing:

  • PMMA: 70–80°C × 4 hrs (air-circulation oven)

  • PC: 110–135°C in glycerin/mineral oil

  
  

• PET: Requires biaxial orientation

III. Material-Specific Processing

PMMA

• ‌Viscosity‌: High

• ‌Melt Temp‌: 160–240°C

• ‌Pressure‌: 80–120 MPa

• ‌Critical‌: Raise mold temp ≥60°C to reduce brittleness

PC

• ‌Melt Temp‌: 280–320°C (narrow window)

• ‌Pressure‌: 100–140 MPa

• ‌Critical‌: Minimize holding time; mold temp ≥100°C

PET

• ‌Melt Temp‌: 260–280°C

• ‌Pressure‌: 30–80 MPa

• ‌Critical‌: Anti-drool nozzle essential; mold temp 120–140°C