2.3 pre-blowing Pre-blowing is a very important step in […]
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 the pre-blowing bottles of the whole equipment. If there are differences, it is necessary to find the specific reason, and the heating or pre-blowing process can be adjusted 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 improperly formed preforms, 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.
2.4 auxiliary machine and mold
The auxiliary machine mainly refers to the equipment that maintains the temperature of the mold. 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 advantages and disadvantages 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 at the bottom all have an impact on the process adjustment.
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.
3 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 better, 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 strongly related to the accumulation of material in the transition zone around the center point, which is adjusted according to actual experience.
The adjustment of the PET bottle blowing process is carried out for the corresponding materials. If the material is not good, the requirements on the process are very demanding, and it is even difficult to blow out a qualified bottle.