Bottle blowing process and analysis of common problems

The blow-molding process is a biaxial stretching process in which the PET chain is bi-directionally extended, oriented and aligned, thereby increasing the mechanical properties of the bottle wall, increasing the tensile, tensile, and impact strength, and Good airtightness. Although stretching helps to increase the strength, it must not be excessively stretched, and the stretch blow ratio must be controlled so that it does not exceed 3.5 to 4.2 in the radial direction and 2.8 to 3.1 in the axial direction. The wall thickness of the preform should not exceed 4.5 mm.

Blowing is carried out between the glass transition temperature and the crystallization temperature and is generally controlled between 90 and 120 degrees. In this range, PET exhibits a high elasticity, and is quickly blow molded and cooled to become a transparent bottle. In the one-step process, this temperature is determined by the length of the cooling time in the injection molding process (such as Aoki Blower), so it is necessary to connect the injection-blow two-station relationship.

In the blow molding process, there are: stretching - blowing once - blowing twice. The time for the three actions is very short, but it must be well coordinated. Especially the first two steps determine the overall distribution of the material and the quality of the blowing bottle. Therefore, it is necessary to adjust well: starting time of stretching, stretching speed, starting and ending timing of pre-blowing, pre-blowing pressure, pre-blowing flow, etc. If possible, it is better to control the overall temperature distribution of the preform, bottle The temperature gradient inside and outside the billet.

In the rapid blow molding and cooling process, induced stress occurs in the bottle wall. For carbonated beverage bottles, it is good for internal pressure resistance, but for hot-fill bottles, it must be ensured that it is fully released above the glass transition temperature.

Common problems and solutions

1. Thin and thick: Delay the pre-blowing time or reduce the pre-blowing pressure to reduce the air flow.

2. Thickness and thinness: contrary to the above.

3. Wrinkles under the neck of the bottle: Pre-blowing is too late or the pre-blowing pressure is too low, or the billet does not cool well here.

4. Bottom hair: The preform is too cold; it is excessively stretched; the pre-blow is too early or the pre-blow pressure is too high.

5. There is a magnifying glass phenomenon at the bottom of the bottle: too much material at the bottom of the bottle; the pre-blowing is too late, and the pre-blowing pressure is too low.

6. The bottom of the bottle has wrinkles: 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 rate is too small.

7. The entire bottle is turbid (opaque): cooling is not enough.

8. Partial whitening: excessive stretching where the temperature is too low, or premature blowing, or hit the stretching rod.

9. Eccentricity of the bottom of the bottle: It may be related to the temperature of the preform, stretching, pre-blowing, high-pressure blowing and so on. Reduce the preform temperature; speed up the drawing speed; check the gap between the head and the bottom mold; delay the pre-blowing to reduce the pre-blowing pressure; delay high-pressure blowing; check the preform for eccentricity.