PET plastic bottle blowing process
PET blow molded bottles can be divided into two categories, one is pressure bottles, such as bottles filled with carbonated aerated beverages; the other is non-pressure bottles, such as bottles filled with water, tea, edible oil, etc.
Tea beverage bottle is mixed with polyethylene naphthalate (PEN) modified PET bottle or PET and thermoplastic polyarylate composite bottle, in the classification of hot bottles, heat-resistant 85 ℃ or more; water bottle is a cold bottle, no requirements for heat resistance. In the molding process of hot bottles and cold bottles are similar.
At present, the manufacturers of PET automatic blow molding machine are mainly imported from France, SIDEL, KRONES, and Fujian Quanguan. Although the manufacturers are different, but the principle of the equipment is similar, generally include the blank supply system, heating system, blowing system, control system and auxiliary machine five major parts.
Blow molding process
PET bottle blow molding process.
The important factors affecting the PET bottle blow molding process are the bottle blank, heating, pre-blowing, mold and production environment.
2.1 Bottle blank
When preparing blow molding bottles, the first PET slices injection molding for bottle blanks, it requires the proportion of secondary recycled material can not be too high (less than 5%), the number of recycling can not be more than twice, and the molecular weight and viscosity can not be too low (molecular weight 31000-50000, characteristic viscosity 0.78-0.85cm3 / g). According to the national food safety law, no secondary recycled material shall be used for food and pharmaceutical packaging. The injection molded bottle blanks should be stored for more than 24h before use. After heating the unused bottle blanks, they must be stored for more than 48h before reheating and use. The storage time of bottle blanks should not exceed six months.
The merits of the preform depend largely on the merits of the PET material, which should be selected for easy blowing and shaping, and a reasonable preform forming process should be developed. Experiments have shown that the same viscosity of PET material molding blanks, imported raw materials than domestic materials easy to blow molding; and the same batch of blanks, different production dates, blow molding process may also have significant differences. The merits of the blanks determine the ease of the blow molding process, the requirements of the blanks are pure, transparent, free of impurities, no foreign colors, the length of the injection point and the surrounding halo spot appropriate.
The heating of the preform is done by the heating oven, whose temperature is set manually and adjusted automatically. The oven is heated by far-infrared lamps emitting far-infrared radiation to the bottle billets, and the fan at the bottom of the oven carries out heat circulation to make the temperature in the oven uniform. The billets move forward in the oven and rotate at the same time, so that the walls of the billets are heated evenly.
The arrangement of the lamp in the oven from top to bottom is generally in the shape of a “zone”, with more at both ends and less in the middle. The heat of the oven is controlled by the number of lamps on, the overall temperature setting, the power of the oven and the heating ratio of each section together. The opening of the lamps should be adjusted in combination with the pre-blowing bottles.
To make the oven work better, its height, cooling plate and other adjustments are very important, if not properly adjusted, blow molding is easy to swell the mouth of the bottle (bottle mouth becomes larger), hard head neck (neck material can not be pulled) and other defects.
Pre-blowing is a very important step in the two-step blowing method, it refers to the blow molding process in the stretching rod down at the same time to start pre-blowing air, so that the initial shape of the bottle blank. Pre-blowing position, pre-blowing pressure and blowing flow are three important process factors in this process.
The shape of the pre-blow bottle determines the ease of the blow molding process and the bottle performance. The normal pre-blowing bottle shape is spindle-shaped, abnormal sub-bell-shaped, handle-shaped, etc.. The reasons for the abnormal shape are improper local heating, pre-blowing pressure or blowing flow is insufficient, etc., while the size of the pre-blowing bottle depends on the pre-blowing pressure and pre-blowing position. In production to maintain the entire equipment all pre-blown bottle size and shape consistent, if there are differences to find specific reasons, according to the pre-blown bottle to adjust the heating or pre-blowing process.
The size of the pre-blowing pressure varies with the bottle specifications, equipment capacity, generally large capacity, pre-blowing pressure to small; equipment with high production capacity, pre-blowing pressure is also high.
2.4 Auxiliary machine and mold
The auxiliary machine mainly refers to the equipment to maintain the constant temperature of the mold. Mold constant temperature has an important role in maintaining the stability of the product. Generally the bottle body temperature is high and the bottle bottom temperature is low. For cold bottles, because the cooling effect of its bottom determines the degree of molecular orientation, it is better to control the temperature at 5-8℃; while the temperature of the bottom of hot bottles is much higher.
PET bottle blow molding is generally better at room temperature and low humidity
Pressurized bottles should meet the requirements of both stress test and pressure resistance test. Stress test is an inherent quality control to prevent the bottom of PET bottles from cracking and leaking due to the degradation of molecular chain during the contact with lubricant (alkaline) when filling drinks; pressure resistance test is a quality control to prevent the bottle from bursting after filling with certain pressure gas. In order to meet these two needs, the thickness of the center point should be controlled within a certain range, and the general situation is that the center point is thin, the stress test is better and the pressure resistance is worse; the center point is thick, the pressure resistance test is better and the stress test is worse. 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.
PET bottle blow molding process is adjusted for the corresponding material, if the material is not good, the requirements of the process is very demanding, even to the extent that it is difficult to blow mold a qualified bottle.