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Polycarbonate (PC) is known for its high transparency, excellent impact resistance, creep resistance, weather resistance, and high-temperature stability. However, it also has some unique processing characteristics that require careful attention during molding. The melt viscosity of PC is less sensitive to shear rate but highly sensitive to temperature. It does not have a clear melting point, and the melt tends to be very viscous. Additionally, PC is prone to hydrolysis at high temperatures, which can lead to cracking in the final product.
To improve the flowability of the melt without increasing injection pressure, it's more effective to raise the injection temperature. The mold design should feature short and thick flow paths and gates to minimize pressure loss, while maintaining a relatively high injection pressure. Before processing, it's essential to thoroughly dry the resin, keeping the moisture content below 0.02%. During processing, PC must be insulated to prevent moisture reabsorption.
In addition to proper mold design, the product design must be reasonable, and the molding process needs to be carefully controlled. Increasing the mold temperature and performing post-processing can help reduce internal stress. Process parameters should be adjusted based on the specific conditions of each product.
Let’s now discuss the molding process in detail:
1. **Injection Temperature**: It should be set according to the product’s shape, size, and mold structure. Typically, the temperature range is between 270°C and 320°C. If the temperature exceeds 340°C, PC may decompose, leading to discoloration, silver streaks, dark spots, bubbles, and a significant drop in mechanical properties.
2. **Injection Pressure**: This has a significant impact on the physical properties, internal stress, and surface appearance of the product. Too low or too high pressure can cause defects. Generally, it should be maintained between 80–120 MPa. For thin-walled, long-flow, or complex parts with small gates, higher pressure (120–145 MPa) is recommended to ensure proper filling and a smooth surface.
3. **Holding Pressure and Holding Time**: These parameters greatly influence internal stress. Too low a holding pressure may result in voids or sink marks, while too high a pressure can create high internal stress around the gate. In practice, this is often managed by using higher material temperatures and lower pressures. The holding time depends on the product thickness, gate size, and mold temperature. Thin parts usually don’t need long holding times, whereas thicker parts require longer ones.
4. **Injection Speed**: PC is generally processed at medium or slow speeds, especially for thin walls, small gates, or long flow paths. Multi-stage injection (slow-fast-slow) is preferred for better control and quality.
5. **Mold Temperature**: It is typically kept between 80–100°C. For complex or thinner parts with higher requirements, it can be increased to 100–120°C, but must not exceed the mold’s heat distortion temperature.
6. **Screw Speed and Back Pressure**: Due to PC’s high melt viscosity, screw speed should be moderate—ideally between 30–60 rpm. Back pressure should be set at 10–15% of the injection pressure to ensure good plasticization and venting.
7. **Release Agents and Recycled Materials**: The use of release agents should be strictly controlled. Recycled materials should not be used more than three times, and their usage should be limited to about 20%.
For PC production, the maximum injection volume (including runners and gates) should not exceed 70–80% of the machine’s nominal capacity. The screw should be a single-flight, forward-moving type with a check ring. A compression screw with an L/D ratio of 15–20 and a C/R of 2–3 is ideal.
By following these guidelines, manufacturers can achieve high-quality PC products with minimal defects and optimal performance.
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Editor: Hardware Business Network Information Center