Analysis on the blowing process of the hottest PET

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Analysis of PET bottle blow molding process

the production of polyethylene terephthalate (PET) blow molding bottle can be divided into injection stretch blow molding (injection stretch blow molding for short) and extrusion stretch blow molding (extrusion stretch blow molding for short) according to different preforms. In the two molding methods, the injection, drawing and blowing process is easy to control, the production efficiency is high, and the waste and defective products are few

pet blow molding bottles can be divided into two categories. One category is pressure bottles, such as bottles filled with carbonated beverages; The other is non pressure bottle, such as bottle 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 poly (aryl ester). It is classified as a hot bottle and can resist heat above 80 ℃; The water bottle is a cold bottle and has no requirements for heat resistance. Hot bottles are similar to cold bottles in molding process. The author mainly discusses the forming technology of the pressed beverage bottle in the cold bottle

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 the manual button type in the past to the 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 of France, Krones company of 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 equipment

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 preform

when preparing blow molded bottles, PET chips are first injected into preforms. It requires that the proportion of secondary recycled materials should not be too high (less than 5%), the number of recycling times should not exceed twice, and the molecular weight and viscosity should not be too low (molecular weight, intrinsic viscosity 0..85cm3/g). The injection molded bottle blank needs to be stored for 48h before it can be used. The bottle blanks that have not been used up after heating must be stored for another 48h so that they can be reheated for use. The storage time of bottle blank 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 developed. The experiment shows that the imported raw materials of PET Preforms with the same viscosity are 3 Multifunctional formaldehyde tester: it can detect the content and release of formaldehyde. Domestic materials are easy to blow molding; For the same batch of preforms, the blow molding process may be quite different due to different production dates. The advantages and disadvantages of the bottle blank determine the difficulty of the blow molding process. The requirements for the bottle blank are purity, transparency, no impurities, no discoloration, appropriate injection point length and surrounding halos

2.2 heating

the heating of bottle blank is completed by heating oven, and its temperature is set manually and adjusted automatically. 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 tubes are 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 opened, the overall temperature setting, the oven power and the heating ratio of each section. The opening of the lamp tube shall be adjusted in combination with the pre blowing bottle

in order to make the oven work better, the adjustment of its height and cooling plate is very important. If the adjustment is not correct, defects such as bottle mouth expansion (bottle mouth becomes larger) and hard head and neck (neck material cannot be pulled open) are easy to occur during blow molding

2.3 pre blowing

pre blowing is a very important step in the two-step bottle blowing method. It means that during the blow molding process, the pre blowing starts at the same time that the stretching rod drops, 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 pre blown bottle determines the difficulty of blow molding process and the performance of 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 construction of abnormal green plastic additives should be based on the principle of life cycle analysis (LCA) and the idea of source control and zero emission. The reasons 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 shall be consistent. If there is any difference, the specific reason shall be found. The heating or pre blowing process can be adjusted 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 equipment has high production capacity and high pre blowing pressure

even if the same equipment is used to produce bottles of the same size. 3. There must be less dust in the room. 1. Due to the different properties of PET materials, 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 oriented correctly; For other preforms with improper materials or forming 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 tube can be removed from the branch above the injection point as shown in the figure, 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 over time, due to the time temperature difference effect, the forming process of the preforms is similar. Compared with the normal preforms, the preform requires less heat and the pre blowing pressure can be appropriately reduced

2.4 auxiliary machine and die

auxiliary machine mainly refers to the equipment that maintains the constant temperature of the die. 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 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 shall meet the requirements of stress test and pressure test at the same time. Stress test is an internal quality control to prevent cracking and leakage caused by degradation of molecular chains in the contact process between the bottom of PET bottles and lubricants (alkaline) when filling beverages; 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, the thickness of the center point 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 voltage withstand test is good, and the stress test is poor. Of course, the stress test results are also closely related to 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 corresponding materials. If the materials are poor, the requirements for process are very strict, and even it is difficult to blow qualified bottle

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