The bottle stress cracking test showed good results, but there were many bottle explosions during large-scale filling. What is the reason?
Question
PET bottles showed excellent results in stress cracking tests, but a large number of them exploded on the customer’s filling production line. What could be the reason? Is it a quality issue with the bottles or a problem with the bottle design? How can this issue be resolved?
Answer
There are many factors that can cause PET bottles to crack during production and storage. The reasons for PET stress cracking can be summarized as follows:
The viscosity of the PET chips is too low.
Poor base mold design. Bottles with base cups do not experience stress cracking issues because the base cup isolates the bottle bottom from the lubricants on the filling line. Additionally, bottles with base cups have a hemispherical bottom rather than a complex petal-shaped design. A hemispherical bottom is formed with almost no internal mold stress, allowing for sufficient stretching and orientation, thus providing higher tensile strength. Complex bottle bottom designs tend to accumulate stress, preventing adequate stretching and limiting molecular orientation, which in turn restricts the improvement of tensile strength.
Sudden changes in the thickness of the bottle bottom during molding. Stress concentration areas mainly occur in regions where the bottle wall thickness changes abruptly.
The use of inappropriate lubricants. Chemical erosion caused by lubricants on the bottle bottom reduces tensile strength. The more aggressive the lubricants on the filling line, the lower the initial point of bottle cracking.
Impurities in the bottle wall.
Bottles being stored for too long.
Rough outer walls.
Stress caused by the pressure of carbonated gas inside the bottle.
To reduce stress cracking in PET bottles, the following measures can be taken:
Increase the intrinsic viscosity of the raw resin to improve the material’s tensile strength.
Seek less aggressive lubricants for the filling line.
Redesign the bottle bottom to reduce stress concentration.
Improve storage methods and production processes, such as avoiding sunlight exposure, reducing foam formation from production line lubricants, and minimizing the dwell time in heaters.
Thoroughly dry the raw materials to prevent a decrease in intrinsic viscosity during the conversion of resin into preforms. The presence of moisture can lead to molecular chain breakage, causing an excessive drop in intrinsic viscosity during processing and making PET bottles prone to cracking.
Regularly clean or maintain molds to avoid foreign particles in the bottle wall or the formation of orange peel defects.
