Introduction to the processing technology and blow

2022-10-02
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Introduction to the processing technology and blow molding equipment of PET plastic bottles

Pet blow molding 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. Tea beverage bottle is a modified PET bottle mixed 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 ℃; Water bottles are cold bottles, and there is no requirement for heat resistance. Hot bottles are similar to cold bottles in molding process. The author mainly discusses the molding technology of pressurized beverage bottles in cold bottles

1 equipment

with the continuous progress of science and technology and the large-scale production, the automation of PET bottle blowing machine is getting higher and higher, and the production efficiency is also getting higher and higher. The production capacity of the equipment has been continuously improved, from thousands of bottles per hour in the past to tens of thousands of bottles per hour now. The operation is also developed from manual button type in the past to full computer control now, which greatly reduces the difficulty of process operation and increases the stability of the process

at present, the manufacturers of injection, drawing and blowing equipment mainly include Sidel company in France, Krones company in Germany, etc. Although the manufacturers are different, their equipment principles are similar, generally including five parts: billet supply system, heating system, bottle blowing system, control system and auxiliary machinery

2 blow molding process

pet bottle blow molding process

the important factors affecting PET bottle blow molding process include bottle blank, heating, pre blowing, mold and environment

2.1 bottle blank

when preparing blow molding bottles, PET chips are first injected into bottle blanks. It requires that the proportion of secondary recycled materials should not be too high (less than 5%), the number of recoveries should not exceed twice, and the molecular weight and viscosity should not be too low (molecular weight, intrinsic viscosity 0..85cm3/g). Injection molded bottle blanks need to be stored for more than 48h before they can be used. The bottle blanks that have not been used up after heating must be stored for another 48h and can be reheated for use. The storage time of bottle blanks shall not exceed six months

the advantages and disadvantages of bottle blanks largely depend on the advantages and disadvantages of PET materials. Materials that are easy to blow and shape should be selected, and a reasonable bottle blank forming process should be formulated. The experiment shows that the bottle preforms made of pet with the same viscosity are easier to blow molded with imported raw materials than those made in China; However, the blow molding process of the same batch of preforms may be quite different due to different production dates. The quality of the bottle blank determines the difficulty of the blow molding process. The requirements for the bottle blank are purity, transparency, no impurities, no discoloration, and the length of the injection point and the surrounding halo spots are appropriate

2.2 heating

the heating of the bottle blank is completed by the heating oven, and its temperature is manually set and automatically adjusted. In the oven, the far-infrared light tube emits far-infrared radiation to heat the bottle blank, and the fan at the bottom of the oven carries out 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 lamp tube is generally arranged in an "area" shape from top to bottom in the oven, with more at both ends and less in the middle. The heat of the oven is jointly controlled by the number of light tubes turned on, the overall temperature setting, the oven power and the distance ratio of each heating section for easy reading and landing. The opening of the lamp should be adjusted in combination with the pre blowing bottle

in order to make the oven play a better role, the adjustment of its height, cooling plate, etc. is very important. If it is not adjusted properly, it is easy to have defects such as expanding the bottle mouth (the bottle mouth becomes larger), hard head and neck (the neck material cannot be pulled open) during blow molding

2.3 pre blowing

pre blowing is a very important step in the two-step bottle blowing method. It refers to the beginning of pre blowing while the stretching rod drops during the blow molding process, so that the bottle blank begins to take shape. In this process, pre blowing position, pre blowing pressure and blowing flow are three important process factors

the shape of the pre blown bottle determines the difficulty of the blow molding process and the performance of the bottle. The normal pre blowing bottle shape is spindle shape, while the abnormal ones are sub bell shape, handle shape, etc., as shown in Figure 2. The causes of abnormal shape include improper local heating, insufficient pre blowing pressure or blowing flow, etc., while the size of the pre blowing bottle depends on the pre blowing pressure and pre blowing position. During production, the size and shape of all pre blowing bottles of the whole equipment should be consistent. If there is any difference, find the specific reason, and adjust the heating or pre blowing process according to the pre blowing conditions

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 production capacity of the equipment should adopt the following methods to protect the jaw, 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 differences in PET material properties, the required pre blowing pressure is also different. Glass Fiber Reinforced PET material, small pre blowing pressure can make the macromolecules at the bottom of the bottle correctly oriented; For other preforms with improper materials or molding process, there are a lot of stress concentration near the injection point, which is not easy to dissipate. If blow molding, it will often blow out at the injection point or burst and leak from the injection point in the stress test. According to the orientation conditions, at this time, the lamp can be moved out of the branch to open above the injection point as shown in, and the injection point can be fully heated to provide enough heat to promote its rapid orientation

for the preforms that have been heated for secondary use or that have been stored for an excessive time, due to the time temperature differential effect, the forming process of the two preforms is similar. Compared with the normal preforms, the required heat is less, and the pre blowing pressure can also be appropriately reduced

2.4 auxiliary machine and mold

auxiliary machine mainly refers to the equipment that maintains the constant temperature of the mold. Mold constant temperature plays an important role in maintaining the stability of products. Generally, the temperature of the bottle body is high and the temperature of the bottle bottom is low. For the cold bottle, because the cooling effect at the bottom determines the degree of molecular orientation, it is better to control the temperature at ℃; The temperature at the bottom of the thermos is much higher

the mold is an important factor affecting the PET bottle blow molding process. The quality of the mold shape will reduce or increase the difficulty of process adjustment, such as the stiffener, the radian of the transition zone and the heat dissipation condition at the bottom

2.5 environment

the quality of the production environment also has a great impact on the process adjustment. Constant conditions can maintain the stability of the process and products. PET bottle blow molding is generally better at room temperature and low humidity

3 other requirements

the pressure bottle should meet the requirements of stress test and pressure test at the same time. The stress test is an internal quality control to prevent cracking and leakage caused by the degradation of molecular chains during the contact between the bottom of PET bottles and the lubricating agent (alkaline) of the industrial standard jgj107 (2) 010 "technical specification for mechanical connection of reinforcement" edited by China Academy of Building Sciences when filling drinks; The pressure test is a quality control to prevent the bottle from bursting after being filled with a certain pressure gas. In order to meet these two needs, 213 the thickness of the center point of the outer window, skylight, door and curtain wall should be controlled within a certain range. Generally, the center point is thin, the stress test is good, and the pressure resistance is poor; The center point is thick, the pressure test is good, and the stress test is poor. Of course, the stress test results also have a lot to do with the accumulation of materials in the transition area around the center point, which should be adjusted according to actual experience

4 conclusion

pet bottle blow molding process is adjusted for the corresponding materials. If the materials are poor, the requirements for the process are very harsh, and even it is difficult to blow molding qualified bottles

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