Views: 1 Author: Huan Machinery Publish Time: 2021-08-26 Origin: Huan Machinery
The blow molding process is a two-way stretching process. In this process, the PET chains are extended, oriented, and arranged in two directions, thereby increasing the mechanical properties of the bottle wall, improving the tensile, tensile, and impact strength. Good air tightness. Although stretching helps to increase the strength, it should not be overstretched. The stretch-inflation ratio must be controlled: the radial direction should not exceed 3.5 to 4.2, and the axial direction should not exceed 2.8 to 3.1. The wall thickness of the preform should not exceed 4.5mm.
Blowing is carried out between the glass transition temperature and the crystallization temperature, generally controlled between 90 and 120 degrees. In this interval, the PET exhibits a highly elastic state, and becomes a transparent bottle after rapid blow molding and cooling and setting. In the one-step method, this temperature is determined by the length of the cooling time during the injection molding process (such as the Aoki bottle blowing machine), so it is necessary to connect the injection-blowing two stations.
The blow molding process includes: stretching-blowing once-blowing twice. The three actions are very short, but they must be well coordinated, especially the first two steps determine the overall distribution of the material and the quality of the blown bottle. Therefore, it is necessary to adjust: stretching start timing, stretching speed, pre-blowing start and end timing, pre-blowing pressure, pre-blowing flow, etc., if possible, it is best to control the overall temperature distribution of the preform, bottle The temperature gradient of the inner and outer walls of the blank.
In the process of rapid blow molding and cooling, induced stress is generated in the bottle wall. For gas-filled beverage bottles, it can resist internal pressure, which is beneficial, but for hot-filled bottles, it is necessary to ensure that it is fully released above the glass transition temperature.
Common problems and solutions
1. Thick top and thin bottom: Delay the pre-blowing time, or reduce the pre-blowing pressure to reduce the air flow.
2. The bottom is thick and the top is thin: the opposite of the above.
3. There are wrinkles under the bottleneck: the pre-blowing is too late or the pre-blowing pressure is too low, or the blank cooling here is not good.
4. White bottom: the preform is too cold; too stretched; pre-blowing too early or pre-blowing pressure is too high.
5. There is a magnifying glass at the bottom of the bottle: too much material at the bottom of the bottle; pre-blowing is too late, and the pre-blowing pressure is too low.
6. There are wrinkles in the bottom of the bottle: the bottom temperature is too high (the cooling at the gate is not good); the pre-blowing is too late, the pre-blowing pressure is too low, and the flow is too small.
7. The whole bottle is turbid (opaque): insufficient cooling.
8. Partial whitishness: Excessive stretching, the temperature here is too low, or the pre-blowing is too early, or the stretch rod is hit.
9. Eccentricity of the bottom of the bottle: It may be related to the temperature of the preform, stretching, pre-blowing, high-pressure blowing, etc. Reduce the temperature of the preform; speed up the drawing speed; check the gap between the tie rod head and the bottom mold; postpone the pre-blowing to reduce the pre-blowing pressure; postpone the high-pressure blowing; check whether the preform is eccentric.