Read the PET bottle blow molding process

PET blow molding bottles can be divided into two categories, one is a pressure bottle, such as a bottle filled with carbonated beverages; the other is a non-pressure bottle, such as a bottle filled with water, tea, oil, and the like. 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 into a hot bottle and can be heat resistant to 80 ° C or higher; Cold bottles do not require heat resistance. The hot bottle is similar to the cold bottle in the forming process. The following mainly discusses the molding process of pressurized beverage bottles in cold bottles.

equipment

With the continuous advancement of technology and the scale of production, the PET bottle blowing machine is becoming more and more automated and the production efficiency is getting higher and higher. Equipment production capacity continues to increase, 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 evolved from the past manual button type to the current full computer control, which greatly reduces the difficulty of the process operation and increases the stability of the process.

Blow molding process

PET bottle blow molding process. Important factors affecting the PET bottle blow molding process include preforms, heating, pre-blowing, molds and the environment.

Preform: When preparing a blow molded bottle, the PET slice is first injection-molded into a preform, which requires that the proportion of the secondary regrind should not be too high (less than 5%), the number of recovery should not exceed two, and the molecular weight and viscosity should not be too low ( The molecular weight is 31000-50000, and the intrinsic viscosity is 0.78-0.85 cm3/g). Injection molded preforms must be stored for 48 hours to be used above. The preforms that have not been used up after heating must be stored for another 48 hours to be reheated. Preforms should not be stored for more than six months. The quality of the preform depends largely on the quality of the PET material. The material that is easy to inflate and easy to set should be selected and a reasonable preform molding process should be established.

Experiments show that the preforms of PET materials with the same viscosity are more easily blow molded than the domestic materials; while the same batch of preforms has different production dates, the blow molding process may also have a big difference. The advantages and disadvantages of the preform determine the difficulty of the blow molding process. The requirements for the preform are pure, transparent, free of impurities, no discoloration, suitable length of the spot and surrounding vignet.

Heating: The heating of the preform is done by a heating oven, the temperature of which is manually set and automatically adjusted. In the oven, far infrared rays are emitted from the far-infrared tube to heat the preform, and the fan at the bottom of the oven performs thermal circulation to make the temperature in the oven uniform. The preform is rotated while moving forward in the oven to heat the preform wall uniformly. The arrangement of the lamps in the oven is generally "zone" shape from top to bottom, with more ends and less in the middle. The heat of the oven is controlled by the number of lamps opened, the overall temperature setting, the oven power, and the heating ratios of the sections. The opening of the lamp should be adjusted in conjunction 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 it is not properly adjusted, it may easily appear in the bottle opening (the bottle mouth becomes larger), the hard head neck (the neck material cannot be opened), etc. during blow molding. defect.

Pre-blowing: Pre-blowing is a very important step in the two-step blowing method. It refers to the pre-blowing of the preform while the stretching rod is lowered during the blowing process, so that the preform has a shape. The pre-blowing position, pre-blowing pressure and blowing flow rate in this process are three important process factors. The pros and cons of the shape of the pre-blowing bottle determines the difficulty of the blow molding process and the performance of the bottle.

The normal pre-blowing bottle has a spindle shape, and the abnormal one has a bell shape, a handle shape, and the like, as shown in FIG. 2 . The cause of the abnormal shape is local heating improperly, pre-blowing pressure or insufficient blowing flow, and the size of the pre-blowing bottle depends on the pre-blowing pressure and the pre-blowing position. In the production, it is necessary to maintain the same size and shape of all pre-blowing bottles of the whole equipment. If there are differences, find the specific reason, and adjust the heating or pre-blowing process according to the pre-blowing conditions. The pre-blowing pressure varies with bottle specifications and equipment capabilities. Generally, the capacity is large and the pre-blowing pressure is small; the equipment has high production capacity and the pre-blowing pressure is also high.

Even if bottles of the same specification are produced from the same equipment, the required pre-blowing pressure is different due to the difference in performance of the PET material. Glass fiber reinforced PET material, the small pre-blowing pressure can make the macromolecule at the bottom of the bottle correctly oriented; other improperly used or improper preforming process, a large amount of stress concentration near the injection point is not easy to subside, if Blow molding, often blows at the point of injection or bursts and leaks from the point of injection during the stress test.

According to the orientation conditions, the lamp can be removed from the 2-3 rods as shown above to be opened above the injection point, and the spot is sufficiently heated to provide sufficient heat to prompt its orientation. For preforms that have been heated for secondary use or preforms that have exceeded the storage time, the molding process is similar due to the time-temperature difference effect. Compared with the normal preform, the required heat is less, and the pre-blowing pressure is also appropriate. reduce.

Auxiliary machine and mold: Auxiliary machine mainly refers to the equipment to maintain the mold constant temperature. The mold constant temperature plays an important role in maintaining the stability of the product. Generally, the temperature of the bottle is high and the temperature at 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; the temperature at the bottom of the hot bottle is much higher. Mold is an important factor affecting the blow molding process of PET bottles. 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 all have an impact on the process adjustment.

Environment: The quality of the production environment also has a greater impact on process adjustment. Constant conditions can maintain process stability and product stability. PET bottle blow molding is generally preferred at room temperature and low humidity.

other requirements

Pressure bottles should meet the requirements of stress test and pressure test. The stress test is an internal quality control to prevent cracking and leakage of the molecular chain during the contact between the bottom of the PET bottle and the lubricant (alkaline) during the filling of the PET bottle; the pressure test is to prevent the bottle from being filled. Quality control after bursting into 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 withstand voltage is poor; the center point is thick, the withstand voltage test is good, and the stress test is poor. Of course, the results of the stress test are also highly dependent on the accumulation of material in the transition zone around the center point, which is adjusted according to actual experience.

Conclusion

The adjustment of the PET bottle blow molding process is carried out for the corresponding materials. If the materials are not good, the requirements on the process are more severe, and it is even difficult to blow out the qualified bottles.