Why is the bottom of a plastic bottle never flat?

Most of the current plastic bottles, especially the soft drink bottles, have the bottom of the bottle designed bumpy, with 5 protrusions in the form of folds, never as flat as glass bottles.

These folds are designed not to create a highlight for plastic bottles but to increase the strength of the entire water based bottle. For example, with a sheet of paper, you can easily fold it many times but when the paper is rolled it will be more difficult to fold. In short, bending any material contributes to its strength and stiffness.

Currently, most plastic bottles of fresh water or spring water are added to the gas and served chilled. This changes the volume of the liquid in the bottle, resulting in the bottle being under great pressure. To regulate the pressure of the liquid inside the bottle, the bottom of the bottle is designed so that the corner or top of the bumpy part can expand or contract.

The protrusions of the bottom of the plastic bottle also contribute to absorbing shocks when you accidentally drop the bottle on the ground. This is evident when you step on a plastic bottle, the top of the bottle is easy to deform while the bottom is very difficult.

The non-flat bottom of the bottle combined with the fact that the bottle is designed to reduce the size to the top of the bottle also improves the stability of the entire bottle. The bottom of the soda can, which is bent inward, has the same effect.