Key Points Of PC Injection Molding For Polycarbonate
Índice
Polycarbonate (PC) is a high-performance non-crystalline engineering plastic with a density of approximately 1.20-1.22 g/cm³, making it moderately heavy. One of its most prominent characteristics is its extremely low molding shrinkage, ranging from 0.5% to 0.7%, with uniform shrinkage. This gives PC injection molding products excellent dimensional stability and precision, making them ideal for manufacturing precision parts.
PC also boasts excellent transparency (up to 90% light transmittance), extremely high impact strength, good heat resistance (heat distortion temperature approximately 130-135℃), and flame retardancy. However, PC has high melt viscosity, moderate fluidity, and ester groups in its molecular chain make it highly hygroscopic. Therefore, it must be deeply dried before processing (to a moisture content of <0.02%), otherwise hydrolytic degradation will occur, leading to silver streaks and reduced strength in the product. These combined characteristics necessitate the use of relatively high processing temperatures (melt temperature 300-320℃) and relatively high mold temperatures (typically 80-120℃) to ensure successful molding and obtain low-stress, high-performance products.
PC is an engineering plastic with excellent performance, but its processing characteristics require strict process requirements. Mastering its key points is the key to producing high-quality products.
Material Properties For Polycarbonate (PC)
Understanding the material properties of PC is a prerequisite for developing the correct process:
Non-crystalline material
PC does not form a crystalline structure during solidification. This means:
- Low and uniform shrinkage: typically 0.5%-0.7%, resulting in good dimensional stability and less shrinkage (compared to crystalline materials like PE/PP).
- Excellent transparency: Pure PC is highly transparent.
- No distinct melting point: Only a glass transition temperature (Tg, approximately 145-150℃), gradually softening above Tg.
High viscosity, shear sensitive
The melt viscosity is very high, with moderate fluidity, requiring high injection pressure and speed for filling.
However, its viscosity is more sensitive to shear rate (injection speed) than to temperature. Increasing the injection speed effectively reduces viscosity and improves fluidity, more effectively than simply increasing the temperature.
Extremely hygroscopic
This is the most common root cause of problems in PC processing. The ester groups on the PC molecular chain are extremely sensitive to moisture.
- Moisture content requirement: Must be dried to below 0.02% (usually <0.015%), as even trace amounts of moisture can lead to hydrolytic degradation.
- Degradation symptoms: Products exhibit silver streaks (silver threads), bubbles, surface turbidity, and a decrease in molecular weight, resulting in severe deterioration of mechanical properties (especially impact toughness).
- High glass transition temperature (Tg): Up to approximately 150℃, which determines its high molding and mold temperature processing characteristics.
- High melt strength: Suitable for blow molding and sheet extrusion.
Sensitive to notches, prone to residual internal stress: Improper product design or processing can easily lead to stress concentration, causing cracking. High mold temperature is the most critical means of eliminating internal stress.
Detailed Explanation Of Key Points In PC Injection Molding Process
Pretreatment: Drying (crucial!)
Equipment: A dehumidifying dryer must be used; ordinary hot air dryers are ineffective.
Drying Conditions:
- Temperature: 120±5℃. Too low a temperature results in slow and incomplete drying; too high a temperature may cause particle agglomeration or slight pre-plasticization.
- Time: 3-4 hours or more (depending on hopper capacity and initial moisture content of the raw material). Sufficient residence time must be ensured.
- Dew Point: The dew point of the dried air should be ≤-30℃.
- Precautions: The dried material should be used immediately or stored briefly in an insulated hopper (set to 80-100℃).
If the machine is stopped for more than 15-20 minutes, the material in the hopper must be emptied or cooled to prevent prolonged heating and degradation.
A simple judgment can be made using the “air injection method”: Immerse the injected molten material strip in cold water, remove it immediately, and observe. If it is soft, transparent, and free of bubbles, the drying is satisfactory; if it is whitish, bubbly, and easily broken, the drying is insufficient.
PC Injection Molding Temperature
Barrel Temperature:
- Principle: Use a “higher at the front, lower at the back” distribution. PC has some stability against overheating, but prolonged exposure to excessively high temperatures will still cause thermal degradation.
- Rear Zone (Feeding Section): 250-280℃. Primarily for preheating and initial plasticizing; temperature should not be too high to prevent premature melting and agglomeration of particles, affecting feed.
- Middle Zone (Compression Section): 280-310℃. Main plasticizing zone; temperature setting is crucial.
- Front Zone (Metering Section): 300-320℃. Ensures uniform melt temperature, preparing for injection.
Nozzle Temperature: Slightly lower than the front zone, approximately 300-310℃, to prevent drooling. A lock-up (spring-return) nozzle can be used.
- Melt Temperature: Actual measured value should be between 300-320℃. This is the most critical reference temperature.
- Excessively high (>330℃): Melt color darkens, black spots and silver streaks appear, strength decreases.
- Too low (<280℃): poor plasticization, large barrel torque, poor flowability, insufficient filling of products, rough surface, and high internal stress.
PC Injection Molding Mold Temperature
This is the most critical parameter for controlling the quality and internal stress of PC products!
Range: 80-120℃. For ordinary products, a temperature no lower than 90℃ is recommended; for high-transparency, high-gloss, high-requirement, or complex/thick-walled products, a high mold temperature of 100-120℃ should be used.
The effects of a high mold temperature:
- Significantly reduces molding internal stress: Allows molecular chains to relax slowly within the mold cavity, reducing frozen orientation.
- Improves melt flowability: Slows down the cooling rate of the mold cavity surface, which is beneficial for thin-walled or long-flow filling.
- Enhances surface quality: Resultes in high gloss and good reproducibility of the product surface, avoiding defects such as “floating fibers” and ripples.
- Prevents premature solidification: Avoids obvious weld lines and low strength.
For transparent products, it is a necessary condition to ensure high transparency.
Uneven mold temperature can lead to product bending, warping, and uneven shrinkage.
Injection Pressure and Speed
Injection Speed:
- Medium to high speed injection is recommended. Utilizing PC’s shear sensitivity, rapid mold filling effectively reduces viscosity, maintains a good melt front temperature, and facilitates filling and a smooth surface.
- For thin-walled parts: High-speed injection is mandatory.
- Precautions: Excessive speed may cause jetting. In this case, adjust the gate design (e.g., use a fan gate or submarine gate) or employ a “slow-fast-slow” multi-stage speed control, first slowly passing through the gate and then rapidly filling.
Injection Pressure/Holding Pressure:
- Use the lowest possible injection pressure while ensuring full filling. The typical range is 80-120 MPa.
- Holding Pressure and Time: Moderate. Due to PC’s low shrinkage, holding pressure should not be too high or too long, otherwise it will cause overfilling, difficulty in demolding, and increased internal stress. Holding pressure should be approximately 50-70% of the injection pressure.
- Holding Pressure Switching Point: It is recommended to use position switching rather than pressure switching for greater precision and stability.
Back Pressure and Screw Speed
- Back pressure: Moderate, typically 5-15 Bar. Appropriate back pressure promotes melt compaction, venting, and uniform plasticization; however, excessive back pressure generates excessive shear heat, potentially leading to overheating and degradation.
- Screw speed: Low to medium speed, recommended at 40-70 rpm. High-speed rotation generates excessive shear heat, also posing a risk of degradation.
Cooling Time
Due to the high mold temperature, PC requires a relatively long cooling time.
- The cooling time depends on the wall thickness and mold temperature. The principle is to ensure the product is fully cured and ejected without deformation.
- Insufficient cooling time can lead to ejection deformation, sticking to the mold, or later warping.
Common Defects And Countermeasures For PC Injection Molding
- Silver streaks (silver lines): The primary suspicion is insufficient drying. Second, excessively high material temperature or excessively low mold temperature can cause resin decomposition and gas production.
- Bubbles: Internal bubbles are mostly caused by insufficient holding pressure or cooling shrinkage; surface bubbles may be related to drying or material temperature.
- Cracks/Stress whitening: Caused by excessive internal stress. Increasing the mold temperature is the most effective solution. Also check:
- Whether the product has sharp corners, notches, or other stress concentration designs.
- Whether ejection is balanced and whether there is forced demolding.
- Whether the holding pressure is too high and whether cooling is too rapid.
- Insufficient filling: Increase material and mold temperature, increase injection speed, and improve venting.
- Flash: Reduce injection pressure/speed, check if the clamping force is sufficient, and check for mold wear.
- Obvious weld lines: Increase mold and material temperature, increase injection speed, improve venting, and adjust the gate position.
FAQs
1. What drying conditions are required for PC material before PC injection molding?
A dehumidifying dryer is mandatory (hot air dryers are ineffective). Dry at 120±5℃ for over 3-4 hours, with air dew point ≤-30℃, until moisture content <0.02% (preferably <0.015%).
2. What is the recommended barrel and melt temperature range for PC injection molding?
Barrel temperature (front low to back high): 250-280℃ (rear), 280-310℃ (middle), 300-320℃ (front). Nozzle temperature: 300-310℃. Melt temperature: 300-320℃ (avoid >330℃ or <280℃).
3. Why is mold temperature crucial in PC injection molding, and what is the range?
It determines internal stress and product quality. Range: 80-120℃; ≥90℃ for general products, 100-120℃ for high-transparency, complex or thick-walled products.
4. What are the key requirements for injection speed and pressure in PC injection molding?
Speed: Medium-high (high for thin-walled parts); use “slow-fast-slow” to avoid jetting. Pressure: 80-120MPa (low as possible); packing pressure is 50-70% of injection pressure.
5. What causes silver streaks in PC injection molding products, and how to solve it?
Main cause: Insufficient drying. Solutions: Improve drying conditions; adjust material temperature (avoid overheating) and mold temperature (increase appropriately).
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