The flash is the "waste product" of the blow molding process, created by excess plastic being pressed to the edges of the mold. When the two halves of the mold press onto the suspended plastic, the excess plastic is squeezed to the top and bottom of the mold, where it stays during blowing. After the bottle is blown, the flash stays attached to the bottle during the cooling process, and must be removed.
This isn’t really “waste” though! After the flash is removed, it can be reground and introduced back into the extrusion process. This can help provide a cost-effective and environmentally sustainable alternative to the “virgin,” or new material, which comprises parts of the bottle.
Plastic Extruded with an Open Mold
The plastic, pictured in green, is first extruded from the head with the mold open.
Mold Closed and Plastic Blown
Then, when the mold closes around the suspended plastic, the plastic is blown to the edges of the mold.
Bottle with Neck and Bottom Flash
Here you can see the excess plastic at the bottom and neck of the bottle.