Introduction: Understanding Polycarbonate (PC)

Polycarbonate (PC)¹ is renowned for its exceptional performance, offering high transparency, excellent impact toughness, creep resistance, and a wide operating temperature range. However, processing this engineering plastic requires precision.

Key Material Characteristics:

• Viscosity Sensitivity: PC melt viscosity is highly sensitive to temperature but less sensitive to shear rate.
• No Clear Melting Point: It softens over a range.
• High Melt Viscosity: Requires higher processing temperatures.
• Hydrolysis Risk: The resin is prone to hydrolysis at high temperatures if moisture is present, leading to part degradation.

Pro Tip: To improve melt flow, do not rely solely on increasing injection pressure. Instead, raise the injection temperature. However, strict moisture control is non-negotiable.

🛠️ Critical Pre-Processing: Drying & Mold Design

Before the injection process begins, specific steps must be taken to ensure quality.

1. Drying is Mandatory²: PC must be dried thoroughly to reduce moisture content to below 0.02%.
2. Moisture Protection: Keep the resin insulated during processing to prevent re-absorption of moisture from the air.
3. Mold Design: Runners and gates should be short and thick to minimize pressure loss.
4. Stress Reduction: High mold temperatures and post-processing annealing are recommended to eliminate internal stress and prevent cracking.

⚙️ The 7 Key Parameters of PC Injection Molding

Here is a detailed breakdown of the processing window for PC.

1. Injection Temperature³ Temperature selection depends on part geometry, mold structure, and requirements.

• Standard Range: 270°C – 320°C.
• Warning: Exceeding 340°C can cause decomposition. Signs include darkened color, silver streaks, dark spots, and bubbles. Mechanical properties will significantly degrade.

2. Injection Pressure⁴ Pressure affects physical properties, internal stress, and shrinkage.

• Standard Range: 80 – 120 MPa.
• High Pressure (120 – 145 MPa): Required for thin-walled, long-flow, or complex parts with small gates to overcome flow resistance.

3. Holding Pressure & Time Crucial for preventing vacuum voids and sink marks.

• Strategy: Low holding pressure with high melt temperature is often the best approach to reduce internal stress.
• Timing: Thin parts need short holding times; thick parts require longer times. This is often determined by the gate freeze-off time.

4. Injection Speed While speed has less impact on PC properties, flow control is key.

• Recommendation: Use medium to slow speeds.
• Technique: Multi-stage injection (Slow-Fast-Slow) is preferred to ensure smooth filling without trapping air.

5. Mold Temperature⁵

• Standard: 80°C – 100°C.
• Complex/Thin Parts: Can be increased to 100°C – 120°C (ensure this is below the mold steel's tempering temperature).

6. Screw Speed & Back Pressure⁶ Due to high viscosity, excessive speed can damage the machine or the material.

• Screw Speed: 30 – 60 r/min.
• Back Pressure: 10% – 15% of injection pressure.

7. Material Usage

• Release Agents: Use strictly controlled amounts.
• Recycled Material: Limit re-grind usage to max 20% and no more than 3 cycles.

Quick Reference: PC Processing Parameters

For easy reference, here is a summary of the optimal settings.

Screw Selection & Machine Requirements

Choosing the right hardware is as important as setting the parameters.

Machine Requirements:

• Shot Size: The maximum part weight (including runners) should not exceed 70-80% of the machine's nominal injection capacity.

Screw Specifications:

• Type: Single-flight, constant pitch, gradual compression screw with a non-return valve (check ring).
• L/D Ratio (Length/Diameter): 15:1 to 20:1.
• Compression Ratio: 2:1 to 3:1.

Conclusion

Mastering PC injection molding requires a balance of thermal control and pressure management. By strictly controlling moisture and adhering to the temperature and screw speed guidelines above, you can produce high-clarity, impact-resistant parts with minimal internal stress.