Blow molding is a process used in the plastics industry to create strong, hollow containers such as plastic bottles, drums, and fuel tanks. The process involves melting plastic into a parison, placing it in a mold, and pumping pressurized air to expand it. The resulting container is thin yet strong due to the webbing effect of the polymer chains. The process is expensive but efficient for large-scale production.
Blow molding is a manufacturing method used in the plastics industry to create strong, hollow containers; some examples of products commonly created through this process are plastic beverage bottles, plastic drums, and fuel tanks. A typical blow molding process involves melting the plastic into a hollow tube, known as a parison, placing the tube between halves of a mold, and pumping pressurized air into the preform until it expands to fit the shape of the mold; the two halves of the mold then separate and the finished container is released.
Strength of plastic
Plastics are actually chains of polymers held together by a strong but fluid bond. The reason a thin soda bottle is strong enough to withstand the pressure of carbonated liquids is a result of the polymer chains in a plastic bottle forming in two directions; this generally creates a very strong webbing effect. The plastic can then be stretched without sacrificing strength, and blow molding experts exploit this property to make thin yet strong containers.
How does it work
Blow molding any type of container requires a preformed piece of plastic called a parison; this preform is usually extruded from a plastic injection mold positioned very close to the blow molding machinery. Hot parison comes as an upside-down plastic test tube, with a preformed collar and threads for the cap at the bottom. The parison is mechanically loaded onto a holder and two sides of a bottle-shaped metal mold are joined around it. The difference between the three processes lies at this point in the process.
At this point in the process, a hollow rod is injected into the center of the parison and pushed towards the top of the mold, stretching the hot plastic preform as it goes. The compressed air is then expelled in controlled stages at low pressure through the hollow wand, which forces the plastic shape to form on the sides of the mould; Because the stretch is done evenly, the plastic remains uniformly thin and strong. This process is expensive to perform, but it is very efficient when used for production in large quantities, making it ideal for things like water bottles.
The plastic behaves much like a latex rubber balloon: As long as the compressed air pressure is controlled, the material will expand uniformly and form the shape of the mold. Blow molding engineers who design new pieces of molding process equipment often need hands-on experience with the machinery to understand how to improve the molding equipment and process.
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