The manufacture of plastic bottles takes place in stages. Typically, the plastic bottles used to hold potable water and other drinks are made from polyethylene terephthalate (PET), because the material is both strong and light. To understand the manufacturing process it’s helpful to first examine the composition of PET and how this affects plastic bottles.
Polyethylene Terephthalate (PET)
PET is a thermoplastic polymer that can be either opaque or transparent, depending on the exact material composition. As with most plastics, PET is produced from petroleum hydrocarbons, through a reaction between ethylene glycol and terephthalic acid. To produce plastic bottles, the PET is first polymerized to create long molecular chains.
Polymerization itself can be a complicated process, and accounts for many of the inconsistencies between one batch of manufactured PET and another. Typically, two kinds of impurities are produced during polymerization: diethylene glycol and acetaldehyde. Although diethylene glycol is generally not produced in high-enough amounts to affect PET, acetaldehyde can not only be produced during polymerization, but also during the bottle manufacturing process. A large amount of acetaldehyde in PET used for bottle manufacturing can give the beverage inside an odd taste.
Once the plastic itself has been manufactured, the bottle manufacturing process can begin. To ensure that the PET is appropriate for use, numerous tests are done post-manufacturing to check that the bottles are impermeable by carbon dioxide (which is important for bottles that carry soda). Other factors, such as transparency, gloss, shatter resistance, thickness and pressure resistance, are also carefully monitored.
The first stage in bottle manufacturing is stretch blow molding. The PET is heated and placed in a mold, where it assumes the shape of a long, thin tube. (The process by which the plastic is forced into the mold is called injection molding.)The tube of PET, now called a parison, is then transferred into a second, bottle-shaped mold. A thin steel rod, called a mandrel, is slid inside the parison where it fills the parison with highly pressurized air, and stretch blow molding begins: as a result of the pressurized air, heat and pressure, the parison is blown and stretched into the mold, assuming a bottle shape. To ensure that the bottom of the bottle retains a consistently flat shape, a separate component of plastic is simultaneously joined to the bottle during blow molding.
The mold must be cooled relatively quickly, so that that the newly formed component is set properly. There are several cooling methods, both direct and indirect, that can effectively cool the mold and the plastic. Water can be coursed through pipes surrounding the mold, which indirectly cools the mold and plastic. Direct methods include using pressurized air or carbon dioxide directly on the mold and plastic.
Once the bottle (or, in continuous manufacturing, bottles) has cooled and set, it is ready to be removed from the mold. If a continuous molding process has been used, the bottles will need to be separated by trimming the plastic in between them. If a non-continuous process has been used, sometimes excess plastic can seep through the mold during manufacturing and will require trimming. After removing the bottle from the mold and removing excess plastic, the bottles are ready for transportation.