Plastic science: one article to understand the PET bottle blow molding process

The production of polyethylene terephthalate (PET) blow-molded bottles can be divided into injection stretch blow molding (referred to as injection stretch blow molding) and extrusion stretch blow molding (referred to as extrusion stretch blow molding) according to the preforming of the parison. . Among these two molding methods, the injection-stretch-blowing process is easy to control, the production efficiency is high, and the waste and defective products are few, which are more common. PET blow-molded bottles can be divided into two categories, one is a pressure bottle, such as a bottle filled with carbonated beverages; the other is a non-pressure bottle, such as a bottle filled with water, tea, oil, etc. The tea beverage bottle is a modified PET bottle mixed with polyethylene naphthalate (PEN) or a composite bottle of PET and thermoplastic polyarylate. Cold bottle, no requirement for heat resistance. The hot bottle is similar to the cold bottle in the molding process. The following mainly discusses the pressurized beverage bottle forming process in the cold bottle. With the continuous advancement of technology and the scale of production, the automation of PET blow molding machines is getting higher and higher, and the production efficiency is getting 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 been developed from the manual button type in the past to the current full computer control, which greatly reduces the difficulty of process operation and increases the stability of the process. At present, the main manufacturers of injection stretch blowing equipment are SIDEL in France and KRONES in Germany. Although the manufacturers are different, their equipment principles are similar, generally including five parts: blank supply system, heating system, bottle blowing system, control system and auxiliary machines. Blow molding process PET bottle blow molding process. Important factors affecting the blow molding process of PET bottles include preform, heating, pre-blowing, mold and environment. Preform: When preparing blow-molded bottles, firstly inject PET chips into preforms, which requires that the proportion of secondary recycled materials should not be too high (below 5%), the number of recycling should not exceed twice, and the molecular weight and viscosity should not be too low ( Molecular weight 31000-50000, intrinsic viscosity 0.78-0.85cm3/g). Injection-molded preforms need to be stored for more than 48 hours before they can be used. The preforms that are not used up after heating must be stored for more than 48 hours before they can be reheated for use. Preforms should not be stored for more than six months. The quality of the preform depends largely on the quality of the PET material. Materials that are easy to inflate and shape should be selected, and a reasonable preform forming process should be formulated. Experiments show that preforms formed from PET materials with the same viscosity are easier to blow molding with imported raw materials than domestic ones; while preforms in the same batch have different production dates, the blow molding process may also 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 discoloration, and the length of the injection point and the surrounding halo are suitable. 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 radiately heat the preform, and the fan at the bottom of the oven conducts thermal circulation 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 lamps in the oven is generally from top to bottom."Area"Glyph, more at both ends, 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 bottle. In order to make the oven function better, the adjustment of its height and cooling plate is very important. If the adjustment is improper, the bottle mouth will be swollen (the bottle mouth will become larger), and the hard neck (the neck material cannot be opened), etc. defect. Pre-blowing: Pre-blowing is a very important step in the two-step bottle blowing method. It means that the pre-blowing starts when the stretch rod is lowered during the blow molding process to make the preform take shape. In this process, the pre-blowing position, the pre-blowing pressure and the air flow rate are three important process factors. The pros and cons of the pre-blown bottle shape determine the difficulty of the blow molding process and the pros and cons of the bottle performance. The normal shape of the pre-blown bottle is a spindle shape, and the abnormal one is a bell shape, a handle shape, etc., as shown in Figure 2. The reasons for the abnormal shape include improper local heating, insufficient pre-blowing pressure or air flow, etc. The size of the pre-blowing bottle depends on the pre-blowing pressure and the pre-blowing position. In the production process, the size and shape of all pre-blowing bottles in the entire equipment should be kept the same. 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 the bottle specification and equipment capacity. Generally, the capacity is large and the pre-blowing pressure is small; the equipment production capacity is high, and the pre-blowing pressure is also high. Even if the same equipment is used to produce bottles of the same specification, the required pre-blowing pressures are not the same due to the differences in the properties of PET materials.
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