Principle and process of PET bottle blowing process

Currently, most of the bottle blowing processes are two-step methods, that is, the plastic is first made into a bottle embryo, and then blown. The main bottle blowing process can be divided into two parts: preheating and bottle blowing.

01 Preheating

The bottle embryo is irradiated with an infrared high-temperature lamp to heat and soften the embryo body of the bottle embryo. In order to maintain the shape of the bottle mouth, the bottle embryo mouth does not need to be heated, so a certain cooling device is required to cool it.

02 Bottle blowing

The molding stage is to place the preheated bottle embryo into the prepared blow mold, inflate it with high pressure, and blow the bottle embryo into the required bottle.

The fully automatic bottle blowing machine on the market can combine the two operations of bottle blowing through the operation of the robot, eliminating the process of manually putting the preheated bottle embryo into the blow mold, thereby improving production efficiency.

The process of bottle blowing is a bidirectional stretching process. During this process, the PET chain is bidirectionally extended, oriented and arranged, thereby increasing the mechanical properties of the bottle wall, improving the tensile, tensile and impact strength, and having good air tightness.

Although stretching helps to improve strength, it cannot be overstretched. The stretching and blowing ratio should be controlled well:

The radial direction should not exceed 3.5 to 4.2 times, and the axial direction should not exceed 2.8 to 3.1 times. The wall thickness of the bottle embryo should not exceed 4.3mm.

Bottle blowing is carried out between the glass transition temperature and the crystallization temperature, which is generally controlled between 90 and 110 degrees. In this range, PET exhibits a high elastic state, and becomes a transparent bottle after rapid blow molding and cooling and shaping.

The blow molding process includes: stretching → primary blowing → secondary blowing.

The time of the three actions is very short, but they must be well coordinated, especially the first two steps determine the overall distribution of the material and the quality of the bottle blowing.

Therefore, it is necessary to set parameters, such as the start time of stretching, stretching speed, start and end time of pre-blowing, pre-blowing pressure, pre-blowing flow rate, etc. If possible, it is best to control the overall temperature distribution of the preform and the temperature gradient of the inner and outer walls of the preform.

During the rapid blow molding and cooling process, induced stress is generated in the bottle wall. For carbonated beverage bottles, it can resist internal pressure, which is good, but for hot-filled bottles, it is necessary to ensure that it is fully released above the glass transition temperature.

Classification of PET blow-molded bottles PET blow-molded bottles can be divided into two categories, one is a pressure bottle, such as a bottle filled with carbonated carbonated beverages; the other is a pressure-free bottle, such as a bottle filled with water, tea, edible oil, etc.

Tea beverage bottles are modified PET bottles mixed with polyethylene naphthalate (PEN) or composite bottles of PET and thermoplastic polyarylate. They are classified as hot bottles and can withstand heat above 85°C; water bottles are cold bottles and have no requirements for heat resistance. Hot bottles are similar to cold bottles in molding process. ‍

In addition, pressure bottles should meet the requirements of stress test and pressure test at the same time.

Stress test is an internal quality control to prevent the degradation of molecular chains and cracking and leakage during the contact between the bottom of the PET bottle and the lubricant (alkaline) when filling beverages; pressure test is a quality control to prevent the bottle from bursting after a certain pressure gas is filled in.

In order to meet these two needs, the thickness of the center point should be controlled within a certain range. Generally, if the center point is thin, the stress test is better and the pressure resistance is poor; if the center point is thick, the pressure resistance test is better and the stress test is poor.

Of course, the result of the stress test is also closely related to the accumulation of materials in the transition area around the center point, which should be adjusted according to actual experience.

Why the Right Equipment Is Key to a Successful PET Blow Molding Process

Understanding the principle and process of PET blow molding—from preform heating and stretching to high-pressure blowing and final cooling—helps manufacturers optimize bottle quality, reduce waste, and improve production efficiency. However, no matter how well the process is designed, it can only be executed effectively with the support of precise and reliable machinery.

To ensure consistent results and high-speed production, choosing the right equipment is essential. For example, the PET mineral water bottle blow molding machine is specifically designed for efficient and stable production of mineral water bottles, offering seamless integration with filling and packaging lines.

For operations that require flexible bottle size options—from 250ml to 2000ml—the Plastic PET Bottle Blowing Machine is a reliable choice, combining fast changeover times with accurate mold control.

Additionally, if your production focuses on high-volume mineral water bottling, the Mineral Water Blow Molding Machine offers a complete solution for 500ml to 2L bottles, ensuring uniformity and durability at every stage.

In short, mastering the PET blow molding process is only half the equation — the other half is selecting the right machine. With the right equipment, your production line will run smoother, faster, and with greater precision.