The bottle blowing process technology is widely used. I hope this article can give some inspiration to friends who want to understand this knowledge.——-Vito.
Brief introduction of bottle blowing process of PET plastic bottle!
Polyethylene terephthalate (PET) is referred to as polyester. The production of PET plastic 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 blow molding) according to the preform of the parison. Squeeze and blow). Among these two molding methods, the injection stretch blowing process is easy to control, high production efficiency, and less waste and defective products, so it is more common.
PET blow molding bottles can 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, cooking oil, etc.The tea drink bottle is a modified PET bottle mixed with polyethylene naphthalate (PEN) or a composite bottle of PET and thermoplastic polarity .It is classified as a hot bottle and can withstand heat above 85°C;Cold bottle, no requirement for heat resistance. Hot bottles are similar to cold bottles in the molding process. The author mainly discusses the molding process of pressurized beverage bottles in cold bottles.
With the continuous advancement of science and technology and the scale of production, the degree of automation of PET blow molding machines 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 the previous production of thousands of 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.
2 Blow molding process
PET bottle blow molding process.
Important factors affecting the PET bottle blow molding process include preform, heating, pre-blowing, mold and production environment.
When preparing blowmolded bottles, first, PET slices are injection-molded into preforms, which requires that the proportion of secondary recycled materials should not be too high (below 5%), the amount of recycling should not exceed two times, 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 are not allowed to be used in food and pharmaceutical packaging. Injection molded preforms need to be stored for more than 24 hours before use. Unused preforms after heating must be stored for more than 48 hours before reheating. Preforms should not be stored for more than six months.
The quality of the preform largely depends on the quality of the PET material. We should choose materials that are easy to inflate and shape, and formulate a reasonable preform molding process. Experiments have shown that imported preforms made of PET materials with the same viscosity are easier to blow mold than domestic materials; and the same batch of preforms may have different production dates, and 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 pure, transparent, free of impurities, no different colors, and the length of the injection point and the surrounding halo are suitable.
The preform is heated by a heating oven whose temperature is manually set and automatically adjusted. In the oven, the far-infrared light emitted by the far-infrared lamp radiates and heats the preform, and the fan at the bottom of the oven performs heat 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 the shape of a “zone” from top to bottom in the oven, with more at the two 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 bottle.
To make the oven work better, it is very important to adjust its height, cooling plate, etc.If the adjustment is not proper, it is easy to cause swelling of the bottle mouth (bottle mouth becomes larger), hard neck (neck material cannot be pulled apart) and so on during blow molding. defect.
Pre-blowing is a very important step in the two-step blow molding method. It refers to the pre-blowing that 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, pre-blowing pressure and blowing air flow rate 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 shape is spindle-shaped, and the abnormal ones are sub-bell-shaped, handle-shaped, etc.The reasons for the abnormal shape are improper local heating, insufficient pre-blowing pressure or insufficient blowing air flow, etc., and the size of the pre-blowing bottle depends on the pre-blowing pressure and pre-blowing position. During the production, the size and shape of all the pre-blowing bottles in the whole equipment should be kept the same. If there are differences, the specific reasons should be found, and the heating or pre-blowing process can be adjusted according to the pre-blowing conditions.
The size of the pre-blowing pressure varies with the bottle specifications and equipment capabilities. Generally, the larger the capacity, the lower the pre-blowing pressure; the higher the production capacity of the equipment, the higher the pre-blowing pressure.
Even if the same equipment is used to produce bottles of the same specification, due to the differences in the properties of PET materials, the required pre-blowing pressures are also different. For glass fiber reinforced PET material, a small pre-blowing pressure can make the macromolecules at the bottom of the bottle correctly oriented; for other preforms with improper materials or improper molding process, there is a large amount of stress concentration near the injection point that is not easy to subside. Blow molding, often blowing at the injection point or bursting and leaking from the injection point during stress testing. According to the orientation conditions, at this time, as shown, the lamp tube can be moved out 2-3 branches to the top of the injection point and turned on, to fully heat the injection point and provide enough heat to promote its rapid orientation.
For preforms that have been heated for secondary use or preforms that have been stored for longer than the standard, due to the time-temperature differential effect, the molding process of the two is similar. Compared with normal preforms, the required heat is less, and the pre-blowing pressure can also be appropriate. reduce.
2.4 Auxiliary machines and molds
Auxiliary machine mainly refers to the 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 bottom of the bottle is low. For cold bottles, 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 hot bottles 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. For example, the ribs, the curvature of the transition zone, and the heat dissipation of the bottom all affect the process adjustment.
The quality of the production environment also has a great influence on the process adjustment. 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 other requirements
The pressure bottle should meet the requirements of stress test and pressure test at the same time. stress test is an internal quality control to prevent cracking, leakage, etc. from the degradation of molecular chains during the contact between the bottom of the bottle and the lubricant (alkaline) when the PET bottle is filled with beverages; the pressure test is to prevent the bottle from being filled. The quality control is carried out by bursting after entering a certain pressure gas. In order to meet these two needs, the thickness of the center point should be controlled within a certain range. In general, 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 also have a lot to do with the accumulation of material in the transition area around the center point, which should be adjusted according to actual experience.
The adjustment of the PET bottle blow molding process is carried out according to the corresponding material. If the material is not good, the requirements for the process are very strict, and it is even difficult to blow out qualified bottles.
If you have any specific questions about the process outlined or would like to discuss your needs in more detail,
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