The key step for the elastic ball to not leak air or liquid is this step

Our elastic ball is made of TPU material and heat sealed. Although the sealing edge has undergone preliminary heat sealing, the molecular bonding of the material is not yet fully stable. Laying them out neatly and letting them sit (this is the current step) is to allow the edge sealing area to fully mature at room temperature, allowing TPU molecules to fuse more tightly, thus completely avoiding problems of subsequent air and liquid leaks.

It's like sticking things together. After applying the glue, we have to wait for it to dry completely before the bonding force is strong enough - the same goes for the edge sealing and curing of the elastic ball. Only by doing this step well can the sealing effect withstand repeated pressure