The blow molding process is a two-way stretching proces […]
The blow molding process is a two-way stretching process. In this process, the PET chains are stretched, 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 3.5-4.2, and the axial direction should not exceed 2.8-3.1. The wall thickness of the preform should not exceed 4.5mm.
The blow molding machine is carried out between the glass transition temperature and the crystallization temperature, and is generally controlled between 90-120 degrees. In this interval, PET behaves as a high-elasticity, rapid blow molding, and a transparent bottle after cooling and setting. In the one-step method, this temperature is determined by the length of the cooling time in the injection molding process for the PET blow molding machine, so the relationship between the injection and blowing stations must be connected.
In the blow molding process, there are: 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 blow molding machine. 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. Temperature gradient. There are common problems and solutions for induced stress in the bottle wall during rapid blow molding and cooling.
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, too late pre-blowing, and too low pre-blowing pressure.
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 and 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 preform temperature, speed up the drawing speed, check the gap between the tie rod head and the bottom mold, postpone pre-blowing, reduce pre-blowing pressure, postpone high-pressure blowing, and check whether the preform is eccentric.