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Rabu, 31 Desember 2014

Blow Molding Shapes Raw Plastics Into Usable Forms

By Genevive B. Mata


Artisans create beautiful objects by manually forcing air into melted glass while it is still hot, forming hollow shapes. That process has existed for centuries, and has been adapted to the industrial production of many commercial plastic items. These include containers of all sizes for liquids, automobile parts, toys, and many other applications. The development of blow molding made this transition possible.

The process begins with a heated raw plastic tube called a parison, a word that originally referred to a mass of unformed, melted glass. Once the parison is sealed carefully inside a mold, air is forced through under pressures that range from 25 to 150 psi, evenly forcing the material onto the inner shape. The plastic is spread throughout the mold at a precise thickness, and cools quickly.

The materials used to create a parison consist mainly of polypropylene, polyethylene, or polyvinyl chloride pellets. All are considered thermoplastics, which become malleable at high temperatures, but do not turn to liquid like other varieties. The tubes are made to fit a particular order, and different sizes can be added to the production line for rapid turnover.

Once inside, the parison can be shaped in several ways. Extrusion methods employ a screw-shaped device that evenly forces the soft mass into a mold. Inside that space, pressurized air fills the cavity from the center of the plastic outward, forming the soft material into an exact duplication of the interior walls, and exactly reproducing the originally engineered shape.

Depending on what is being made, extrusion is continuous or intermittent. Variations are commonly used to make large bottles for milk or juice, but some containers are more efficiently produced using an injection process. The plastic is injected into a core pin, air is forced in, cooled, and the entire item ejected in one continuous operation.

Stretch injection is a similar process that is used mainly to make individual serving containers and other small objects. Injectors create a preform, which is cooled, reheated and extended using a core rod. During this process, air under high pressure is carefully blown in to extend the shape within a mold made of metal. All of these processes can use recyclable plastics.

Plastics are derived from hydrocarbons, and there is no way to ignore their environmental impact. Less than five percent of total oil production goes to plastic production, however, and intensified recycling efforts help to balance the equation. Plastics are an integral part of modern life, and he cost benefits of using these production methods are proven.




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