Introduction to the blowing process of PET plastic bottles

2023-11-24 12:00

I. Principle of bottle blowing

The bottle blowing process of the bottle blowing machine is divided into two parts.


1. Warm up

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


2. Bottle blow molding

This stage is to place the preheated bottle preform into the already prepared blow mold and inflate it under high pressure. The preform (embryo) is blown into the required bottle.


The fully automatic bottle blowing machine combines the two operations of bottle blowing through the operation of the robot, eliminating the manual process of placing the preheated bottle preform into the blow mold. Greatly speed up the production speed.


bottle blowing machine


II. Process flow of bottle blowing machine

The blow molding process of the bottle blowing machine is a bidirectional stretching process. During this process, the PET chains extend, orient and arrange in two directions, thereby increasing the mechanical properties of the bottle wall and improving the tensile, tensile and impact strength. And has good air tightness. Although stretching helps to increase strength, it should not be over-stretched. The stretch-to-blow ratio must be controlled: 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 preform should not exceed 4.3mm.


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


The blow molding process includes: stretching - first blow - second blow. The three actions take a very short time, but they must be coordinated well, 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 the parameters: stretching starting timing, stretching speed, pre-blowing start and end time, pre-blowing pressure, pre-blowing air flow, etc. If possible, it is best to control the overall temperature distribution of the preform. The temperature gradient between 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 inflatable beverage bottles, it can resist internal pressure, which is beneficial, but for hot-fill bottles, it is necessary to ensure that it is fully released above the glass transition temperature.


III. The following is the bottle blowing process flow chart:

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


Tea beverage bottles are modified PET bottles blended with polyethylene naphthalate (PEN) or composite bottles of PET and thermoplastic polyarylate. They are classified as thermal bottles and can withstand heat above 85°C; water bottles are Cold bottles, no requirements for heat resistance. The molding process of hot bottles is similar to that of cold bottles.


 blow molding machine manufacturers



IIIV. Equipment

At present, foreign PET fully automatic blow molding machine manufacturers have very complete equipment, and domestic manufacturers are not inferior, such as Link-Tech machinery. Although the manufacturers are different, the equipment principles are similar and generally include five parts: blank supply system, heating system, bottle blowing system, control system and auxiliary machinery.


V. Blow molding process

PET bottle blow molding process.

Important factors affecting the PET bottle blow molding process include preform, heating, pre-blowing, mold and production environment.


2.1 Bottle preform

When preparing a blow molded bottle, the PET slices are first injection molded into a bottle preform. It requires that the proportion of secondary recycled materials cannot be too high (less than 5%), the number of recycling cannot exceed two times, and the molecular weight and viscosity cannot be too low (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85cm3/g). According to the National Food Safety Law, secondary recycled materials are not allowed to be used for food and pharmaceutical packaging. Injection molded preforms need to be stored for more than 24 hours before use. Unused preforms after heating must be stored for more than 48 hours before they can be reheated for use. Preforms cannot be stored for more than six months.


The quality of the preform largely depends on the quality of the PET material. Materials that are easy to inflate and shape should be selected, and a reasonable preform molding process should be developed. Experiments have shown that imported raw materials for PET material with the same viscosity are easier to blow mold than domestic materials. However, the same batch of preforms may have different production dates and the blow molding process may be quite different. The quality of the preform determines the difficulty of the blow molding process. The requirements for the preform are purity, transparency, no impurities, no abnormal color, appropriate injection point length and surrounding halo.


bottle blowing process


2.2 Heating

The heating of the preform is completed by a heating oven, whose temperature is manually set and automatically adjusted. In the oven, the far-infrared lamp emits far-infrared rays to radiate and heat the preform, and the fan at the bottom of the oven performs heat circulation to make the temperature in the oven uniform. The preform rotates while moving forward in the oven, so that the walls of the preform are evenly heated.


The arrangement of lamp tubes in the oven is generally in the shape of a "district" from top to bottom, with more at the ends and less in the middle. The heat of the oven is controlled by the number of lamps turned on, the overall temperature setting, the oven power and the heating ratio of each section. The opening of the lamp should be adjusted in conjunction with the pre-blowing bottle.


To make the oven function better, it is important to adjust its height, cooling plate, etc. If not adjusted properly, bottle mouth expansion (the bottle mouth becomes larger), stiff neck (the neck material cannot be pulled open), etc. are prone to occur during blow molding. defect.