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Frequently asked questions about PET blow molding and solutions
By CHUMPOWER  2017/09/12
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Blow molding is a bi-directional stretching process in which the PET chain undergoes bi-directional extension, orientation and arrangement, thereby increasing the mechanical properties of the bottle wall, improving the stretching, tensile, and impact strength with good air tightness. Although stretching helps improve the strength, excessive stretching will cause the bottom or part of bottle turn white.

For better control over the stretching and inflating ratio, it should be noted that the radial direction must not exceed 3.5-4.2, the axial direction must not exceed 2.8-3.1, and the thickness of the bottle preform must not exceed 4.5mm.
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The blow molding is carried out between the glass transition temperature and the crystallization temperature, and is generally controlled between 90 and 120℃. In this range, the PET has extremely high stretch elasticity for rapid blow molding and results in the formation of a transparent bottle after cooling. For operation in one stage PET blow molding machine, this temperature is determined by cooling time of the injection process.

During rapid blow molding and cooling, induced stress occurs at inner bottle wall, which is good forcarbonated drink bottlesbecause it can resist internal pressure. However, for hot-filling bottles, it is necessary to ensure that the above glass transition temperature is reached to allow pressure to be fully released. 
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Frequently asked questions and solutions are explained as follows:

Vaporific point at the lateral position:
The pre-heating temperature of bottle blowing is too low. Increase the heating box temperature.

Generation of lacteous crystal:
The pre-heating temperature of bottle blowing is too high. Reduce the heating box temperature.

The entire bottle is cloudy (opaque):
Insufficient cooling.

Upper bottle body is thick and lower is thin:
Delay the pre-blowing time, or reduce the pre-blowing pressure to reduce air flow.

Upper bottle body is thin and lower is thick:
The opposite of the above.

Buckling appears under the bottleneck:
Pre-blowing is too late or pre-blowing pressure is too low, or cooling of the bottle preform is abnormal.

Partially turns white:
The temperature is too low or pre-blowing too early, or it has touched the stretch bar.

Bottom turns white:
The preform is too cold. The pre-blow is too early or pre-blow pressure is too high.

Magnifying phenomenon on bottle bottom:
Excessive material used on bottle bottom. Pre-blow is late or pre-blow pressure is too low.

Buckling inside bottle bottom:
Bottom temperature is too high (sprue cooling is abnormal). Pre-blow is late and pre-blow pressure is too low with insufficient flow.

Abnormality on bottle bottom:
It may be related to the bottle preform temperature, stretching, pre-blowing, and high-pressure blowing. Try to reduce the temperature of bottle preform and check the gap between the draw head and bed die. Accelerate stretching speed. Delay pre-blowing and reduce pre-blowing pressure. Postpone high-pressure blowing.


CHUMPOWER PET Blow Molding Machine has excellentstability and is soldworldwide. No matter in cold or tropical climates, all CHUMPOWER PET Blow Molding Machine are still in stable operation in our customers' factories. Even if there is a problem, CHUMPOWER's after-sales service team has more than two decades of rich manufacturing experience, and can effectively and rapidlyeliminating problemsto become the most powerful backing for our customers.


CHUMPOWER machinery Corp. possesses the copyright of this article. It is not allowed for any form of reprinting or distribution without the authorization and indication of source.Wereserve the right to claim for its legal right. Contact us if you have any questions or intents to reprint this article. Marketing Department: hanna@chumpower.com

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