Acrylic (PMMA) injection molding aims to produce high-clarity, high-gloss parts while minimizing internal stress.
PMMA is highly transparent but brittle, and it is sensitive to stress, temperature, and moisture. Therefore, design, mold, and processing must be coordinated; neglecting any one area often causes defects that cannot be fixed later.

1. Key Material Properties of PMMA

• High optical clarity (≈92% light transmission), making surface defects highly visible.
• Brittle nature, prone to stress cracking and crazing.
• Narrow processing window; sensitive to overheating and shear.
• High internal stress sensitivity, leading to delayed cracking if not controlled.

2. Design Guidelines (DFM)

• Wall thickness: 2–4 mm and as uniform as possible.
• Avoid sharp corners; internal radii ≥ 0.5 × wall thickness.
• Draft angles: polished surfaces ≥ 1°, textured surfaces ≥ 2–3°.
• Avoid weld lines and ejector marks on visible surfaces.
• Ribs should be ≤ 50–60% of wall thickness and include fillets; avoid thick ribs.

3. Mold Design Requirements

• Mirror-polished mold surfaces (SPI A1/A2) for clear optical parts.
• Preferred gate types: fan gate, film gate, or tab gate for stable flow.
• Venting: 0.02–0.04 mm depth to prevent gas burns and bubbles.
• Uniform cooling to avoid warpage, birefringence, and internal stress.

4. Processing Parameters

• Drying: 80–90°C for 2–4 hours; target moisture ≤ 0.04%.
• Melt temperature: 220–260°C; avoid overheating to prevent yellowing.
• Mold temperature: 60–90°C (higher for optical or thick parts).
• Injection speed: smooth and continuous; avoid hesitation and jetting.
• Packing pressure: moderate; excessive packing increases internal stress.
• Cooling time: sufficient before ejection to prevent warpage and stress.

5. Post-Processing and Annealing

• Annealing relieves internal stress and improves durability.
• Recommended for thick parts, optical components, and parts exposed to chemicals.
• Typical annealing conditions: 70–80°C for 2–4 hours with slow heating and cooling.
• Proper annealing reduces cracking, improves chemical resistance, and stabilizes dimensions.

6. Common Defects and Prevention

• Bubbles or silver streaks: caused by moisture or poor venting → fix by drying and improving vents.
• Flow marks/jetting: caused by low mold temperature or unstable injection → fix by increasing mold temperature and stabilizing injection speed.
• Weld lines: caused by poor gate location or low temperature → fix by relocating gate or raising temperatures.
• Stress cracking: caused by sharp corners, high packing, or chemical exposure → fix by adding fillets, reducing packing, and annealing.
• Warping: caused by uneven wall thickness or cooling → fix by uniform design and balanced cooling.

FAQ (Acrylic Molding Design & Processing)

What is acrylic molding?

Acrylic molding is the injection molding process used to manufacture parts from PMMA (Polymethyl Methacrylate).
It is mainly used for transparent, high-gloss components like light covers, displays, and cosmetic packaging.

Why does acrylic require special design?

Acrylic is brittle and stress-sensitive, and it has very high optical clarity, which makes defects highly visible.
Therefore, part geometry, mold design, and processing must minimize stress and protect surface quality.

What is the best wall thickness for acrylic parts?

The ideal wall thickness is 2–4 mm.
Uniform wall thickness is critical because acrylic is sensitive to uneven cooling and internal stress.

What draft angle should acrylic parts have?

• Polished surfaces: ≥ 1° draft
• Textured surfaces: ≥ 2–3° draft

Insufficient draft increases ejection stress and can cause cracking.

Why are radii important in acrylic design?

Sharp corners create stress concentration points that can lead to cracking.
Internal radius should be at least 0.5× wall thickness, and external radius should match accordingly.

What gate type is best for acrylic?

Preferred gate types for PMMA include:

• Fan gate
• Film gate
• Tab gate

These gate types support stable flow and reduce visible defects.

What mold surface finish is required for clear acrylic parts?

Clear acrylic parts require mirror-polished mold surfaces.
Recommended standard: SPI A1 or A2.

What are the common acrylic molding defects?

Common defects include:

• Bubbles and silver streaks
• Flow marks and jetting
• Weld lines
• Stress cracking and crazing
• Warpage and distortion
• Surface haze or whitening

What causes acrylic parts to crack after molding?

Cracking is usually caused by internal stress, which can be triggered by:

• Sharp corners
• High packing pressure
• Low mold temperature
• Chemical exposure
• Poor annealing

Is annealing necessary for acrylic parts?

Annealing is recommended for thick, optical, or chemically exposed acrylic parts.
It relieves internal stress and improves durability.

What are the typical annealing conditions for PMMA?

• Temperature: 70–80°C
• Duration: 2–4 hours
• Cooling: slow, controlled cooling

How should PMMA be dried before molding?

PMMA must be dried to ≤ 0.04% moisture to avoid bubbles and streaks.
Typical drying conditions:

• 80–90°C for 2–4 hours

What temperature ranges are used for acrylic molding?

• Melt temperature: 220–260°C
• Mold temperature: 60–90°C (higher for optical parts)

What makes PMMA different from polycarbonate (PC)?

PMMA offers higher optical clarity and UV resistance, while PC provides much higher impact strength.
Choose PMMA for clarity and PC for toughness.

Can acrylic be used for high-impact applications?

Acrylic is not recommended for high-impact applications due to its low impact strength and brittleness.
For impact resistance, polycarbonate (PC) is usually the better choice.

What is the main goal of acrylic molding design?

The main goal is to minimize internal stress and preserve optical clarity through proper geometry, mold design, and processing.