3 Common Problems and Solutions Faced by Stretch Blow Molding Manufacturing Companies

If you use stretch blow molding in your manufacturing process to make PET bottles, you likely face several challenges in your facility. For example, what do you do when your PET bottles turn out hazy or have insufficient top-load strength? Luckily, there are solutions to common stretch blow molding issues.

1. Bottle Whitening

Pearlescence and haze both cause a whitening of the bottle, but they are issues of the opposite nature. Pearlescence, or stress-whitening, happens when the PET material is stretched too much. The PET microstructure breaks, creating tiny white circles. The temperature and the wall thickness are factors to the overstretching. 

When using a PET stretch blow molding machine, remember to let the very thin area cool down while heating the part underneath to move more material to the whitish area of the bottle. If the material remains thick, increase the temperature level to stretch it some more.

Haze happens when you heat the PET higher than 115 degrees Celsius during the injection or blow phases, and then the material cools down for a short time. With the high temperature, the PET molecules form crystals. In a two-stage (reheat) blow molding process, the solution to haze is to turn down the responsible lamp or increase ventilation to bring the preform temperature to lower than 115 degrees Celsius.

2. Not Enough Top-load Strength

Top-load strength is becoming a major issue because of increasing efforts to save money and materials by reducing the weight of PET bottles. The bottle design and wall thickness determine the top-load performance of bottles. Knowing the bottle’s weakest area, which may be the body, shoulder, or bottom, that will cause its collapse is the first step to creating a solution. After determining where the bottle could fail, processors can move more material into this part.

3. Rocker Bottom

The center of every bottle’s bottom curves inward to some degree, which is called a push-up. This is important because it keeps the bottle sitting on the outer rim of the base. The center push-up always comes to shrink outward. But when the material is too hot after going through the stretch blow molding machine, the center push-up can go out beyond the outer rim’s limits. Then, the bottle will rock instead of staying upright.

Some machines use cooling jets to blow air to the bottoms of the bottles to avoid the rocker bottom problem. Processors can also increase the cooling time or reduce the heat on the bottom of the preform.

Choose the Best PET Stretch Blow Molding Machine

The right PET stretch blow molding machine will go a long way toward preventing issues. That’s why Pet All Manufacturing, Inc. offers superior quality machines. Contact us today to learn more.

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