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 preform of the parison. . In these two molding methods, because the injection stretch blowing process is easy to control, the production efficiency is high, and the waste and defective products are less, so they are more versatile. PET blow-molded bottles can be divided into two categories, one is pressurized bottles, such as bottles filled with carbonated drinks; the other is non-pressurized 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 classified as a hot bottle and can resist heat above 80°C; water bottles are Cold bottle, no requirement for heat resistance. The hot bottle is similar to the cold bottle in the forming process. The following mainly discusses the forming process of pressurized beverage bottles in cold bottles. With the continuous advancement of science and technology and the scale of production, the automation degree of PET bottle blowing machine
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 the process operation and increases the stability of the process. At present, the main manufacturers of injection stretch blowing equipment include France's SIDEL company and Germany's KRONES 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. Blow molding process PET bottle blow molding process. Important factors affecting the PET bottle blow molding process include preforms, heating, pre-blowing, molds, and environment. 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 ( The molecular weight is 31000-50000, and the intrinsic viscosity is 0.78-0.85cm3/g). The injection-molded preform must be stored for more than 48 hours before it 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 molded from PET materials of the same viscosity are easier to blow molded from imported raw materials than domestic materials; while the same batch of preforms have different production dates and blow molding processes 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. 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. 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 blowing process, so that the preform will 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, a handle shape, etc., as shown in Figure 2. 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 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, due to the difference in the properties of PET materials, the required pre-blowing pressures are not the same. With glass fiber reinforced PET material, a small pre-blowing pressure can make the macromolecules at the bottom of the bottle correct orientation; other preforms with improper materials or improper molding process have a large amount of stress concentration near the injection point, which is not easy to subside. Blow molding often breaks at the injection point or bursts and leaks from the injection point in the stress test. According to the orientation conditions, at this time, the lamp can be moved out 2-3 as shown above the injection point to turn on, and the injection point is fully heated, and sufficient heat is provided to promote its rapid orientation.
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