PET bottles will 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.
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 classified as a hot bottle and will be resist heat above 85°C; water bottles are Cold bottle, no requirement for heat resistance. The hot bottle is similar to the cold bottle in the forming process.
1 Equipment
At present, the manufacturers of PET fully automatic blow molding machines mainly import from France SIDEL, Germany's KRONES, and the domestic ones include Fujian Quanguan Company. Although the manufacturers are different, their equipment principles are similar, generally including five parts: blank supply system, heating system, blowing system, control system and auxiliary equipment.
2 Blow molding process
PET bottle blow molding process.
Important factors affecting the PET bottle blow molding process include preforms, heating, pre-blowing, molds, and production environment.
2.1 Preform
When preparing a blow-molded bottle, firstly, the PET chip is injection molded into a preform. It requires that the proportion of secondary recycled materials should not be too high (less than 5%), the number of recycling should not exceed two, and the molecular weight and viscosity should not be too low (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85cm3/g). According to the National Food Safety Law, secondary recycled materials shall not be used for food and medical packaging. The injection-molded preform must be stored for more than 24 hours and can be used. The preforms that are not used up after heating must be stored for more than 48 hours to be reheated for use. The storage time of preforms cannot exceed six months.
The pros and cons of preforms largely depend on the pros and cons of PET materials. Materials that are easy to inflate and shape should be selected, and a reasonable preform molding process should be formulated. Experiments have shown that the preforms made of PET material with the same viscosity are easier to blow molded from imported raw materials than domestic ones; while the same batch of preforms have different production dates, and the blow molding process may also be quite different. The pros and cons of the preform determine the difficulty of the blow molding process. The requirements for the preform are pure, transparent, free of impurities, no different colors, and the length of the injection point and the surrounding halo are suitable.
2.2 Heating
The heating of the preform is completed by a heating oven, and its temperature is manually set and automatically adjusted. In the oven, the far-infrared lamp tube emits far-infrared rays to radiate and heat the preform, and the fan at the bottom of the oven circulates heat to make the temperature in the oven uniform. The preform rotates while moving forward in the oven, so that the preform wall is heated evenly.
The arrangement of the lamp tube in the oven is generally in the shape of "zone" from top to bottom, with more at both 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 tube should be adjusted in conjunction with the pre-blowing.
To make the oven work better, the adjustment of its height, cooling plate, etc. is very important. If the adjustment is improper, it is easy to expand the bottle mouth (bottle mouth becomes larger), hard head and neck (neck material cannot be pulled open), etc. defect.
2.3 Pre-blowing
Pre-blowing is a very important step in the two-step blowing method. It means that the pre-blowing is started while the stretch rod is lowered during the blow molding process to make the preform take shape. In this process, the pre-blowing position, pre-blowing pressure and blowing flow are three important process factors.
The shape of the pre-blow bottle determines the difficulty of the blow molding process and the performance of the bottle. The normal shape of the pre-blowing bottle is a spindle shape, and the abnormal shape is a bell shape and a handle shape. The reasons for the abnormal shape include improper partial heating, insufficient pre-blowing pressure or blowing flow, etc. The size of the pre-blowing bottle depends on the pre-blowing pressure and pre-blowing position. In the production, the size and shape of all pre-blowing bottles in the whole equipment should be maintained. If there are differences, specific reasons should be found. The heating or pre-blowing process can be adjusted according to the pre-blowing situation.
The size of the pre-blowing pressure varies with bottle specifications and equipment capabilities. Generally, the capacity is large and the pre-blowing pressure is small; the production capacity of the equipment is high, and the pre-blowing pressure is also high.
2.4 Auxiliary machine and mould
Auxiliary equipment mainly refers to 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. Generally, the temperature of the bottle body is high and the temperature of the bottle bottom is low. For the cold bottle, since the cooling effect at the bottom determines the degree of molecular orientation, it is better to control the temperature at 5-8°C; while the temperature at the bottom of the hot bottle is much higher.
2.5 Environment
The quality of the production environment also has a greater impact on the process adjustment, and constant temperature conditions can maintain the stability of the process and the stability of the product. Blow molding of PET bottles is generally best at room temperature and low humidity.
3 requirements
The pressurized bottle shall meet the requirements of stress test and pressure test at the same time. The stress test is the internal quality control to prevent the degradation of the molecular chain during the contact between the bottom of the bottle and the lubricant (alkaline) when the PET bottle is filling the beverage, and the cracking, leakage, etc.; the pressure test is to prevent the bottle from being filled Quality control for bursting after entering a certain pressure of gas. To meet these two needs, the thickness of the center point must be controlled within a certain range. Generally, the center point is thin, the stress test is better, and the pressure resistance is poor; the center point is thick, the pressure test is better, and the stress test is poor. Of course, the results of the stress test have a lot to do with the accumulation of materials 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 carried out for the corresponding material. If the material is not good, the process requirements are very strict, and it is even difficult to blow out qualified bottles.
