Understanding the Process of Making Plastic Bottled Water

The production of plastic bottles is divided into several phases. Bottles are typically made from one of four different types of plastic. PET (polyethylene terephthalate) is commonly used to make plastic bottles for holding potable water and other beverages because it is both strong and light. High-density polyethylene (PE) is used to make rigid plastic bottles like detergent bottles, and low-density polyethylene (PE) is used to make squeeze bottles.

The Materials:

1. Polyethylene Terephthalate (PET)

Water bottle companies like Crystal Beverage make use of Polyethylene Terephthalate as the main component for making plastic water bottles. It is a thermoplastic polymer that, based on the material composition, can be opaque or transparent. PET is made from petroleum hydrocarbons, like most plastics, by a reaction between ethylene glycol and terephthalic acid. PET is polymerized to form long molecular chains, allowing PET bottles to be produced later.

Polymerization is a complicated process that accounts for many of the differences in manufactured PET batches. During polymerization, two types of impurities are commonly produced: diethylene glycol and acetaldehyde. Although diethylene glycol is rarely produced in sufficient quantities to harm PET, acetaldehyde can be produced not only during polymerization but also during the bottle-making process. A high level of acetaldehyde in the PET used to make bottles can give the beverage inside an off-flavor.

The PET bottle manufacturing process can begin after the plastic has been manufactured. Several tests are performed after production to make sure that the plastic is suitable for use and that the bottles are carbon dioxide impermeable (which is important for bottles that carry soda). Transparency, gloss, shatter resistance, thickness, and pressure resistance are all closely monitored as well.

2. Low and High-Density Polyethylene (LDPE & HDPE)

Polyethylene is a thermoplastic used to make blow-molded milk and water jugs, detergent bottles, ketchup bottles, spray bottles, and other items. Thermoforming, blow molding, injection molding, and other processes are all possible with LDPE and HDPE. Because it has higher ductility than HDPE but lower strength, LDPE was one of the first plastics to be blow-molded, and it is still used to make squeezable bottles today. Many types of pourable bottles are made of HDPE. When thinned to the dimensions of milk bottles and the like, the material is usually white or black in its natural state and becomes translucent. Manufacturers can tweak the formula to improve tear strength, transparency, formability, printability, and other characteristics.

Polyethylene is a “homopolymer” because it is made up of only one monomer, ethylene. Because LDPE is amorphous and HDPE is crystalline, LDPE has more ductility and HDPE has more rigidity. Though the two thermoplastics share many applications, polyethylene is more expensive than polypropylene, which is the least expensive.

The Process: 

1. Blow Molding and Reheat Process

Injection molding is the first step in a typical 2-step Reheat and Blow Machine (RBM) bottle manufacturing process. Plastic pellets are plasticized in the barrel of an injection molding machine, where heat and shearing action from a feed screw melt the plastic. The plastic is then injected into molds with multiple cavities, taking the shape of long, thin tubes. The formed necks and threads that will be used to cap the bottles to come are usually included in these and are called “parisons.” PET “parisons,” or pre-forms, are much more compact than fully formed bottles, making them easier to ship to bottling facilities.

2. Extrusion Process

A continuous extrusion machine is one such machine, in which an extruder continuously produces parison. The parison forms vertically in the extrusion blow molding process, and the wall thickness is varied by changing the size of the orifice through which the parison extrudes. Close the mold halves around the suspended parison and transfer it to the blow molding station, where the bottle is formed as described in the second step of the RBM process.

The weight of the formed portion would otherwise stretch the hot and still-forming section above it, resulting in non-uniformity of the hanging parison. Varying the wall thickness solves this problem. As the parison forms, the wall thickness is increased to achieve a uniform thickness throughout the formation.

3. Reciprocating Process

The reciprocating blow molding machine is another manufacturing method. To accumulate a shot, these machines move the screw linearly within the injector barrel. The screw then pushes the shot over the mandrel to form the parison, which is then formed as usual.