Plastic bottles are used for a variety of different liquid products across the world, but are particularly popular containers for water and fizzy drinks. Plastic is a great material because it is strong but also light. Before plastic was invented, bottles were made out of glass which is very heavy to carry around and can break easily. A plastic bottle is made from PET (Polyethylene Terephthalate) which is made from petroleum hydrocarbons. The plastic forms when a re-action occurs between the ethylene glycol and terephtalic acid. The process of making plastic bottles has three stages: Blow molding, stretch molding and cooling. This is called, "Polymerisation."
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Firstly, the PET material is molded in order for the bottle to be shaped correctly. The process of putting the PET material into the mold is called "injection." This turns the PET material into a tube. The mold is long and thin which determines the shape and size of the tube created. These tubes are like test tubes and are called "preforms." It is not a bottle shape at this stage. Most manufacturing companies order preforms and mold it again into the required shape. The base of the bottle is secured to the preform at this stage.
Next comes stretch molding, which gives the bottle it's shape. At this stage, the molded PET material is now called "Parison." To start the stretch molding, a rod made of steel is inserted into the parison. The rod fills the parison with highly pressurised air, heat and causes the mold to change and become what looks like a bottle shape. An example of a bottle made this way is the 2-litre bottle used for carbonated drinks. Stretch blow molding has pros and cons. It is virtually unbreakable, but also it might be damaging to the environment as it takes hundreds years to decay.
The final stage is cooling. The mold needs to cool to allow the shape to set. There are two ways to do this: One is by using cold water in the pipes surrounding the mold. The other is to use pressurised air or carbon dioxide which is applied directly onto the mold. Once the mold is cool, the bottle can be separated from the mold. There may be excess plastic on the bottle which, if found, is usually removed. This is known as "trimming." This can happen if bottles are produced on a line. Even if produced separately, PET can leak through the mold and cause excess plastic. The bottles are then ready for transportation.
The process of bottle making
Tips and warnings
- PET can be harmful to the environment. If you use them, make sure you recycle them. Bio-plastic bottles are a great alternative to PET, as they are better for the environment.
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