The first stage of bottle manufacturing is stretch blow molding. The PET is heated and placed in a mold in which the PET is in the shape of an elongated tube. (The process of pressing a plastic into a mold is called injection molding.) The PET tube, now called a Pet Preform, is then transferred to a second bottle mold. A thin steel bar called a mandrel slides inside the parison, where it fills the parison with high-pressure air, and the stretch blow begins: the parison is blown and stretched due to pressurized air, heat and pressure In the mold, the shape of the bottle is presented. To ensure that the bottom of the bottle maintains a uniform flat shape, a separate plastic part is simultaneously attached to the bottle during the blow molding process.
The mold must be cooled relatively quickly so that the newly formed parts are properly set. There are several direct and indirect cooling methods that effectively cool the mold and plastic. Water can indirectly cool the mold and plastic through the tubing that surrounds the mold. Direct methods include the use of pressurized air or carbon dioxide directly on the mold and plastic.
Once the bottle (or, in continuous manufacturing, the bottle) is cooled and solidified, it can be removed from the Pet Bottle Preform. If a continuous molding process is used, the bottles need to be separated by trimming the plastic between the bottles. If a discontinuous process is used, sometimes excess plastic will penetrate the mold during the manufacturing process and needs to be trimmed. After removing the bottle from the mold and removing the excess plastic, the bottle can be transported.