Common points of injection molding of transparent plastic products

I. Raw Material Preparation

1. Raw Material Selection

• Polycarbonate (PC): Offers great optical clarity, high impact strength, and good heat resistance. Ideal for optical lenses, high - end electronics' transparent casings, and car lamp covers. But its poor processing fluidity demands precise process control in injection molding.

• Polymethyl Methacrylate (PMMA): Also known as acrylic, has excellent light transmittance like glass and good weather resistance. Used for display racks, light boxes, etc. However, its low surface hardness makes it easy to scratch.

• Polyethylene Terephthalate (PET): Widely used in food and drug transparent packaging for its chemical stability, transparency, and reasonable cost. But its crystallization affects injection molding, requiring proper temperature control.

• Other Transparent Plastics: Polystyrene (PS) is cheap and transparent, suitable for simple items with low mechanical requirements. Cyclic olefin copolymer (COC) is for high - precision optical lenses, though costly.

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2. Raw Material Drying

• Drying Equipment Selection: Dehumidifying dryers are best for large - scale production with strict moisture control. Hot air circulation ovens suit small - to - medium batches or less - demanding drying.

• Drying Parameter Setting: For PC, use a dehumidifying dryer at 110 - 130°C for 4 - 8 hours, keeping humidity below 10% to get moisture below 0.02%. For PMMA, dry at 70 - 90°C for 3 - 6 hours.

II. Selection and Debugging of Injection Molding Machines

1. Injection Molding Machine Selection

• Screw Structure: Choose a gradual - change screw. Its length - diameter ratio should be 20:1 - 25:1 for better plasticization, especially for high - viscosity plastics.

• Injection System Precision: The injection system must control injection volume within ±1% accuracy to avoid product defects.

• Clamping System Stability: Sufficient clamping force is needed to prevent flash, and smooth, precise opening and closing actions are required.

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2. Parameter Adjustment

• Injection Speed: Keep it slow to avoid turbulence. Use multi - stage speeds for complex products. For simple ones, 10 - 30cm³/s is okay.

• Packing Pressure Speed: Set it at 1 - 5cm³/s for stable melt replenishment.

• Injection Pressure: Varies by product shape, size, and wall thickness. Thin - walled products need 100 - 150MPa or more; thick - walled ones, 50 - 100MPa. Determine through trial moldings.

• Packing Pressure: Usually 60% - 80% of injection pressure for PC, 50% - 70% for PMMA, and 70% - 90% for PET. Adjust packing time (5 - 20s for PC, 3 - 10s for PMMA, 8 - 20s for PET) by trial moldings.

• Barrel Temperature: For PC, 260 - 280°C (front), 240 - 260°C (middle), 220 - 240°C (rear). For PMMA, 200 - 230°C (front), 180 - 200°C (middle), 160 - 180°C (rear). For PET, 270 - 290°C (front), 250 - 270°C (middle), 230 - 250°C (rear).

• Nozzle Temperature: Slightly lower than barrel front. 250 - 270°C for PC, 190 - 210°C for PMMA, 260 - 280°C for PET.

III. Key Points of Mold Design and Manufacturing

1. Venting Design

• Venting Grooves: Depth 0.02 - 0.05mm, width 3 - 8mm. Place at areas where air accumulates, like cavity corners.

• Permeable Steel Inserts: With 15% - 30% porosity. Install according to product needs.

• Other Venting Methods: Use vacuum pumps or parting - surface gaps, with the latter often combined with other methods.

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2. Optimization of the Cooling System

• Layout of Cooling Channels: Design based on product shape and wall thickness. Use zonal cooling for irregular products.

• Size of Cooling Channels: Diameter 8 - 12mm, 10 - 20mm from cavity surface.

• Coolant Selection and Control: Water is cheap but needs anti - rust. Oil has good rust and lubrication. Control coolant at 5 - 30°C.

3. Ejection Mechanism Design

• Ejection Method Selection: Choose push - plate (for simple products), lifter (for products with undercuts), or ejector pin (flexible but mind the marks).

• Draft Angle Setting: 1 - 3°, 3 - 5° for high - precision or soft products.

• Surface Treatment of Ejection Parts: Polish to Ra0.8μm or less, consider hard - chrome plating.

IV. Injection Molding Process Control

1. Multi - stage Injection Control

• Usually in 3 - 5 stages. Start slow (3 - 5cm³/s, 30% - 40% of final pressure), then adjust according to cavity filling.

2. Melt Flow Control

• Monitor through machine parameters and mold viewing windows. Adjust speed and pressure for laminar flow.

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3. Packing Pressure Curve Optimization

• Use a decreasing pressure curve. Determine packing time by product thickness and trial moldings.

4. Packing Process Monitoring

• Monitor pressure and time. Use pressure sensors in the mold.

5. Cooling Time Determination

• Calculate roughly and adjust by trial moldings. Consider product deformation and quality.

6. Cooling Uniformity Assurance

• Optimize cooling system and control coolant flow and temperature.

V. Product Post - treatment

1. Annealing Treatment

• Eliminate internal stress. For PC, heat at 120 - 130°C for 1 - 2 hours and cool slowly. Adjust by product thickness and material.

2. Surface Treatment

• Polish for better clarity. Consider coating for high - end optical products.