PET Blow Molding Machine Principles and Process

Currently, most blow molding machines employ a two-step method, requiring the conversion of plastic raw materials into preforms before the actual blowing process. The commonly used material is environmentally friendly PET (Polyethylene Terephthalate) plastic, collectively referred to as PET blow molding machines.

I. Blow Molding Principles

The blow molding process in a blow molding machine comprises two main stages:

1. Preheating: The preform is exposed to infrared high-temperature lamps, softening the undesirable parts while preserving the preform neck shape. A cooling device is utilized to cool the preform neck, which does not require heating.
2. Blow Molding: In this phase, the preheated preform is placed into a pre-prepared mold. High-pressure inflation is applied to shape the preform into the desired bottle. Fully automatic blow molding machines combine these two operations with the assistance of a robotic arm, eliminating the manual step of placing preheated preforms into molds and significantly accelerating production rates.

II. Blow Molding Process

The blow molding process involves bidirectional stretching of the PET chain, enhancing mechanical properties, tensile strength, and impact resistance while ensuring excellent air-tightness. Although stretching contributes to strength improvement, excessive stretching should be avoided, and the stretch blow ratio should be controlled, typically below 3.5–4.2 radially and 2.8–3.1 axially. The preform's wall thickness should not exceed 4.5mm.

Blow molding occurs within the temperature range between the glass transition temperature and crystallization temperature, typically controlled between 90 to 120 degrees Celsius. In this temperature range, PET exhibits high elasticity, allowing for rapid blow molding and forming transparent bottles after cooling. In a one-step process, this temperature is determined by the cooling time during injection molding (as in Aoki blow molding machines), necessitating coordination between the injection and blow molding stations.

During the blow molding process, which includes stretching, first blowing, and second blowing, precise coordination is essential, especially in the initial stretching, stretching speed, start and end times of pre-blowing, pre-blowing pressure, and pre-blowing airflow. If possible, temperature distribution throughout the preform and temperature gradients on inner and outer walls should be controlled.

During rapid blow molding and cooling, induced stresses develop in the bottle wall. For carbonated beverage bottles, this stress can withstand internal pressure, providing benefits. However, for hot-fill bottles, it is crucial to ensure the stress is fully released above the glass transition temperature.

PET bottles produced through blow molding find applications in beverages, dairy, alcoholic beverages, condiments, and personal care products. HZM Machinery is a professional automatic blow molding machine manufacturer, offering factory-direct sales, a 2-year warranty, and global distribution. With a reputable brand image, advanced system solutions, meticulous manufacturing standards, robust engineering capabilities, and acclaimed after-sales service, we have earned the trust and recognition of both domestic and international brand manufacturers.