PET bottle blowing process analysis

The production of polyethylene terephthalate (PET) blow-molded bottles can be divided into injection stretch blow molding (abbreviated injection stretch blow) and extrusion stretch blow molding (abbreviated as pultrusion blowing) according to preform preforms. . Among these two molding methods, the injection-pull blowing process is easy to control, and the production efficiency is high. The waste defective is less and more general.

PET blow-molded bottles can be divided into two categories. One is pressurized bottles, such as bottles filled with carbonated beverages; the other is non-pressure bottles, such as bottles filled with water, tea, oil, etc. The tea beverage bottle is a modified PET bottle blended with polyethylene naphthalate (PEN) or a composite bottle of PET and thermoplastic polyarylate. It is a thermos bottle on the classification and can withstand heat above 80°C; the water bottle is Cold bottles, no requirement for heat resistance. Hot bottles and cold bottles are similar in the molding process. The author mainly discusses the pressure bottle molding process in cold bottles.

1 Equipment

With the continuous advancement of science and technology and the scale of production, PET bottle blowing machines have become more and more automated, and their production efficiency has also become higher and higher. The production capacity of the equipment has been continuously improved from the previous production of several thousand bottles per hour to the current production of tens of thousands of bottles per hour. The operation has also evolved from the manual button style used in the past to the current full computer control, greatly reducing the difficulty of process operations and increasing the stability of the process.

At present, the manufacturers of injection blow blowing equipment mainly include SIDEL in France and KRONES in Germany. Although different manufacturers, but the principle of their equipment is similar, generally include the billet system, heating system, blowing system, control system and auxiliary machine five major parts.

2 Blow molding process

PET bottle blow molding process.
Important factors affecting the blowing process of PET bottles include preforms, heating, pre-blowing, molds, and the environment.

2.1 preform

When preparing blow-molded bottles, the PET chips are injection molded into preforms, which require that the proportion of secondary recycled materials is not too high (less than 5%), the number of recycling times cannot exceed two, and the molecular weight and viscosity cannot be too low (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85 cm3/g). Injection molded preforms need to be stored for more than 48 hours before use. The unused preforms after heating must be stored for more than 48 hours before reheating. The preforms must not be stored for more than six months.
The merits and demerits of preforms depend to a large extent on the merits of the PET material. The materials that are easy to inflate and easy to set should be selected, and a reasonable preform molding process should be established. Experiments show that the same viscosity of the PET material preform, the imported raw materials than the domestic material is easy to blow molding; and the same batch of preforms, the production date is different, the blow molding process may also have greater differences. The quality of the preform determines the ease of the blow molding process. The requirements for the preform are pure, transparent, free of impurities, no color, and the length of the injection spot and the surrounding halo are appropriate.

2.2 Heating

The heating of the preform is accomplished by heating the oven. The temperature is set manually and automatically adjusted. In the oven, far-infrared rays are emitted from far-infrared lamps to radiantly heat the preforms. The heat is circulated by the fan at the bottom of the oven to make the temperature in the oven uniform. The preform is rotated while moving forward in the oven, so that the preform wall is heated evenly.
The arrangement of the lamps in the oven is generally "zone" shaped from top to bottom, with more than two 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 power of the oven, and the heating ratio of each section. The opening of the lamp should be adjusted in conjunction with the pre-blowing.
To make the oven function better, the adjustment of its height, cooling plate, etc. is very important. If it is improperly adjusted, the expansion bottle mouth (bottle opening becomes large), hard head and neck (the neck material cannot be pulled open), etc. defect.

2.3 Pre-blowing

Pre-blowing is a very important step in the two-step blow-molding process. It refers to the fact that the blow-molding process begins with pre-blowing at the same time as the stretching rod is lowered, so that the preform takes shape. The pre-blowing position, pre-blowing pressure and blowing flow in this process are three important process factors.
The quality of the pre-blow shape determines the ease of the blow molding process and the performance of the bottle. The normal pre-blowing shape is spindle-shaped, and the exceptions are sub-bells, handles, etc., as shown in FIG. 2 . Caused by the abnormal shape of the local heating is not properly, the pre-blowing pressure or insufficient air flow, etc., and the size of the pre-blowing depends on the pre-blowing pressure and pre-blowing position. In the production, it is necessary to maintain the same size and shape of all pre-blowed bottles in the entire equipment. If there are differences, it is necessary to look for specific reasons. The heating or pre-blowing process can be adjusted according to the pre-blowing conditions.
The size of the pre-blowing pressure varies with the bottle size and equipment capacity. Generally, the capacity is large and the pre-blowing pressure is small; the equipment has high production capacity and the pre-blowing pressure is also high.
Even if the same device is used to produce the same size bottle, the required pre-blowing pressure is not the same due to the difference in the performance of the PET material. Glass fiber reinforced PET material, the small pre-blowing pressure can make the macromolecules at the bottom of the bottle to be correctly oriented; other materials used in improper or improperly shaped preforms, there is a lot of stress concentration near the injection point is not easy to fade, if Blow molding often breaks at the injection point or bursts or leaks from the injection point during the stress test. According to the orientation conditions, the lamp can be removed as shown by 2-3 branches above the injection point to open, giving sufficient heating at the injection point to provide enough heat to promote its rapid orientation.
For preforms that have been heated for secondary use or preforms that have been stored for an excessive amount of time, the molding process is similar due to the equal temperature difference effect. Compared with normal preforms, less heat is required and the pre-blowing pressure may be appropriate. reduce.

2.4 Auxiliary machine and mould

Auxiliary equipment mainly refers to the equipment that maintains the constant temperature of the mold. The constant temperature of the mold plays an important role in maintaining the stability of the product. The general bottle temperature is high and the bottom temperature is low. For cold bottles, because the cooling effect at the bottom determines the degree of molecular orientation, it is better to control the temperature at 5-8°C; the temperature at the bottom of the thermos bottle is much higher.
Mold is an important factor affecting the blowing process of PET bottles. The shape of the mold will reduce or increase the difficulty of process adjustment. For example, stiffeners, the curvature of the transition area, and the cooling conditions at the bottom all affect the process adjustment.

2.5 Environment

The quality of the production environment also has a great influence on the process adjustment. The constant conditions can maintain the stability of the process and the stability of the product. Blow molding of PET bottles is generally preferred at room temperature and low humidity.

3 Other requirements

A pressure bottle should meet the requirements of stress test and pressure test at the same time. The stress test is to prevent the internal quality control of cracking and leakage caused by the degradation of the molecular chain during the contact of the PET bottle with the lubricant (alkaline) during the filling of the beverage; the pressure test is to prevent the bottle from filling. Quality control by bursting after entering a certain pressure gas. To meet these two requirements, the center point thickness should be controlled within a certain range. Generally, the center point is thin, the stress test is better, and the pressure resistance is poorer; the center point is thicker, the pressure resistance test is better, and the stress test is poorer. Of course, the results of the stress test also have a lot to do with the accumulation of material in the transition zone around the center point, which should be adjusted based on actual experience.

4 Conclusion

The adjustment of the PET bottle blow molding process is performed on the corresponding materials. If the materials are not good, the requirements for the process are very severe, and it is even difficult to blow out qualified bottles.