PET bottle blowing process
Beverage bottles are modified PET bottles mixed with polyethylene naphthalate (PEN) or composite bottles of PET and thermoplastic polyarylate. They are classified as hot bottles and can withstand heat above 85 ° C; water bottles are cold Bottles, no requirements for heat resistance. The hot bottle is similar to the cold bottle in the forming process.
1. Equipment
At present, the manufacturers of PET fully active blow molding machines import mainly from SIDEL of France, KRONES of Germany, and Fujian Quanguan of China. Although the manufacturers are different, their equipment principles are similar, and generally include five major parts: billet supply system, heating system, bottle blowing system, control system and auxiliary machinery.
2. Blow molding process
PET bottle blow molding process.
Important factors affecting the PET bottle blow molding process are preform, heating, pre-blowing, mold and production environment.
2.1 Preform
When preparing blow-molded bottles, the PET chips are first injection molded into preforms. It requires that the proportion of secondary materials recovered cannot be too high (less than 5%), the number of times of recovery cannot exceed twice, and the molecular weight and viscosity cannot be too low (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85cm3 / g). According to the National Food Safety Law, secondary recovery materials shall not be used for food and pharmaceutical packaging. Injection molded preforms can be used up to 24h. Preforms that have not been used up after heating must be stored for more than 48 hours to be reheated. The storage time of preforms cannot exceed six months.
The quality of the preform depends to a large extent on the quality of the PET material. Materials that are easy to swell and easy to shape should be selected, and a reasonable preform molding process should be worked out. Experiments have shown that imported preforms made of PET materials with the same viscosity are easier to blow mold than domestic materials; while the same batch of preforms have different production dates, the blow molding process may also be significantly different. The quality of the preform determines the difficulty of the blow molding process. The requirements for the preform are purity, transparency, no impurities, no color, and the length of the injection point and the surrounding halo.
2.2 Heating
The heating of the preform is completed by the heating oven, the temperature of which is set manually and adjusted actively. In the oven, the far-infrared lamp tube announces that the far-infrared radiantly heats the preform, and the fan at the bottom of the oven circulates the heat to make the temperature inside the oven even. The preforms rotate together in the forward movement in the oven, so that the walls of the preforms are heated uniformly.
The placement of the lamps in the oven is generally in the shape of a “zone” from top to bottom, with more ends and less middle. The heat of the oven is controlled by the number of lamp openings, the overall temperature setting, the oven power and the heating ratio of each section. The opening of the lamp tube should be adjusted in conjunction with the pre-blown bottle.
To make the oven function better, the adjustment of its height, cooling plate, etc. is very important. If the adjustment is not correct, it is easy to swell the bottle mouth (the bottle mouth becomes larger) and hard head and neck (the neck material cannot be pulled open) during blow molding And other defects.
2.3 Pre-blowing
Pre-blowing is a very important step in the two-step bottle blowing method. It refers to the pre-blowing that starts when the draw bar descends during the blow molding process, so that the preform takes shape. In this process, the pre-blowing orientation, pre-blowing pressure and blowing flow are three important process elements.
The shape of the pre-blow bottle shape determines the difficulty of the blow molding process and the quality of the bottle function. The normal pre-blow bottle shape is spindle-shaped, and the abnormal ones include sub-bell shape and handle shape. The reason for the abnormal shape is 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 orientation. In production, the size and shape of all pre-blow bottles in the whole equipment must be kept in common. If there is a difference, detailed reasons should be found. The heating or pre-blow process can be adjusted according to the pre-blow bottle conditions.
The size of the pre-blowing pressure varies with the bottle size 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.
2.4 Auxiliary machine and mould
Auxiliary machine mainly refers to equipment that keeps mold temperature constant. The constant temperature of the mold plays 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 at the bottom determines the degree of molecular orientation, it is better to control the temperature at 5-8 ° C; and the temperature at the bottom of the hot bottle is much higher.
2.5 Environmenta
The quality of the production environment also has a greater impact on the process adjustment. The stable temperature conditions can maintain the stability of the process and the stability of the product. PET bottle blow molding is generally better at room temperature and low humidity.
3. Other requirements
The pressure bottle should satisfy the requirements of stress test and pressure test together. The stress test is to prevent the cracking and leakage of the molecular chain during the contact between the bottom of the bottle and the lubricant (alkaline) during the filling of the PET bottle. The pressure test is to avoid the filling of the bottle. Quality control after bursting into certain pressure gas. To satisfy these two needs, the center point thickness should be controlled within a certain range. The general condition is that 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 results of the stress test are also closely related to the accumulation of material in the transition area around the center point, which should be adjusted according to practical experience.
4. Conclusion
The adjustment of the PET bottle blow molding process is based on the corresponding data. If the data is poor, the process requirements are very stringent, and it is even difficult to blow mold the qualified bottles.