PET blow-molded 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, edible 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 85°C; water bottle is Cold bottle, no requirement for heat resistance. The hot bottle is similar to the cold bottle in the forming process. The author 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 degree of 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 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 preforms, heating, pre-blowing, molds and production environment.
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 (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85cm3/g). According to the National Food Safety Law, secondary recycled materials shall not be used for food and pharmaceutical packaging. The injection-molded preform must be stored for more than 24 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 imported raw materials are easier to blow-mold the preforms made of PET material with the same viscosity than the domestic ones; and the same batch of preforms have different production dates and the blow molding process 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, suitable for the length of the injection point and the surrounding halo.
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 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 evenly heated.
The arrangement of the lamps in the oven is generally in the shape of "zone" from top to bottom, with more ends 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, the bottle mouth (bottle mouth becomes larger), hard head and neck (neck material cannot be pulled), etc. will easily occur during blow molding. defect.
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 blow molding process to make the preform 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 pre-blowing bottle shape is a spindle shape, and the abnormal shape is sub-bell shape and handle shape. The reasons for the abnormal shape include improper local 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. During production, the size and shape of all pre-blowing bottles in the entire 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 production capacity of the equipment 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 pressure is 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.
For the preforms that have been heated for secondary use or the preforms whose storage time exceeds the standard, due to the effect of time-temperature equal difference, the forming process of the two is similar. Compared with the normal preforms, it requires less heat and the pre-blowing pressure can also be appropriate. reduce.
d. Auxiliary machine and mould
Auxiliary equipment mainly refers to equipment that maintains the constant temperature of the mold. Mold constant temperature plays an important role in maintaining product stability. Generally, the temperature of the bottle body is high and the temperature of the bottle bottom 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 the hot bottle is much higher.
The mold is an important factor that affects the PET bottle blow molding process. The shape of the mold will reduce or increase the difficulty of process adjustment. For example, the ribs, the curvature of the transition zone and the heat dissipation at the bottom have an impact on the process adjustment.
The quality of the production environment also has a greater impact on process adjustment. Constant temperature conditions can maintain the stability of the process and the stability of the product. PET bottle blow molding is generally best 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. The stress test is the internal quality control to prevent the degradation of the molecular chain during the contact between the bottom of the bottle and the lubricant (alkaline) when the PET bottle is filling the beverage, and the cracking, leakage, etc.; the pressure test is to prevent the bottle from being filled Quality control for bursts after entering a certain pressure of gas. 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 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 have a lot to do with the accumulation of materials in the transition zone around the center point, which should be adjusted based on actual experience.
The adjustment of the PET bottle blowing process is carried out for the corresponding material. If the material is not good, the process requirements are very strict, and it is even difficult to blow out qualified bottles.