How to Produce Satisfactory PET Plastic Bottles Through Injection and Blow Molding

To achieve optimal production results, attention must be given to each stage of the injection and blow molding processes.

What is Injection Molding?

Injection molding is a manufacturing technique where PET resin pellets are first heated to a molten state (approximately 250-280°C). The molten PET is then injected into a mold using an injection screw, which shapes the resin according to the mold's design. The PET cools and solidifies quickly, taking on the desired shape. Once cooled, the molded product is ejected and inspected for quality and adherence to specifications.

How to Produce High-Quality PET Preforms

Selecting the Right Injection Molding Machine

For optimal PET injection molding, choose a screw with a check ring for enhanced wear resistance and a length-to-diameter ratio of 15 to 20:1, and a compression ratio of 3:1 for optimal plasticization. For glass fiber-reinforced PET, use wear-resistant materials or lined barrels. Opt for a short, smooth-bore, large-diameter hydraulic shut-off nozzle with effective insulation and temperature control. Clean the barrel with low-pressure polypropylene (PP) before molding to ensure stability.

Controlling Key Parameters

Barrel Temperature: Maintain between 240°C and 280°C, with a higher range (250°C to 290°C) for glass fiber-reinforced PET. The nozzle temperature should be 10-20°C lower than the barrel temperature.

Mold Temperature: Mold temperature directly affects the cooling rate and crystallinity of the material, which in turn impacts the performance of the final product. It is typically set between 100°C and 140°C. Lower temperatures suit thin-walled components, while higher temperatures are needed for thicker parts.

Injection Pressure: Moderate pressures (80 to 140 MPa) are generally used. For glass fiber-reinforced PET, pressures should range from 90 to 150 MPa. Adjust based on PET viscosity, filler types, gate size, part shape, and mold temperature.

What is Blow Molding?

In blow molding, molten PET is first injected into a preform mold to create a preform. This preform is then heated to a pliable state and placed into a blow mold. High-pressure air is used to expand the preform to match the mold’s interior shape, forming bottles or other containers. The PET cools and solidifies quickly in the mold, and the final product is ejected and inspected for quality.

How to Produce Ideal PET Bottles Through Blow Molding

Key Factors in Blow Molding PET Bottles

Preform Quality: Ensure preforms are pure, transparent, free of impurities, and have consistent length and uniform orientation. Store preforms for at least 24 hours before use, and reheated preforms should be used within 48 hours, not exceeding six months in total storage time.

Heating: Adjust temperature and time based on preform thickness. Avoid overheating or underheating to prevent defects. Use cooling plates and fans for even cooling, and maintain equipment for stability and precision.

Pre-Blowing: This initial shaping process is crucial. Ensure correct pre-blowing position, pressure, and airflow to produce a spindle-shaped bottle. Adjust pre-blowing pressure based on bottle size and production capacity, with larger bottles generally requiring lower pressure.

Mold Temperature: Maintain consistent mold temperatures for stable product quality. Bottle body temperature should be higher than the base temperature. For cold bottles, control the base temperature around 5-8°C, while hot bottles require a higher base temperature.

Overall Recommendations

High-quality PET resin is essential for both injection and blow molding processes. Opt for PET resins with high molecular weight and intrinsic viscosity (0.78-0.85 cm³/g) can achieve clarity and mechanical strength. Avoid low-quality materials or substandard recycled PET (rPET) to prevent defects such as bubbles, impurities, or deformation. For reinforced PET resins, especially those with glass fibers, precise control of temperature and pressure is crucial to avoid fiber breakage and ensure optimal performance.

Wankai New Materials Co., Ltd. is one of the world’s largest PET resin manufacturer, producing a wide range of bottle-grade PET resins. Choosing Wankai’s high-quality PET resins will enhance your production outcomes significantly.