Saturday, August 23, 2014

How Blow Molding Helps Produce Plastic Products

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.

This industrial process starts with a basic, raw plastic form called a parison, a name taken from artisans for an unformed glob of melted, malleable glass. The parison is placed inside a mold via machinery and air is pumped in at pressures ranging from 25 to 150 psi, creating an even coating over the interior. That layer has a uniform thickness, and cools down rapidly.

The raw tubes of plastic are made primarily from pellets of polyethylene, polypropylene, or polyvinyl chloride. These are thermoplastics, which melt at a rate and consistency making them ideal for mass production. The tubes can be made to order in virtually any size and quantity needed for a particular job, and are inserted into the molds in rapid assembly-line fashion.

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.

Injection stretch molding is a similar method used mainly to create small items or individual serving bottles. Preforms are created through injection, cooled, and then reheated and stretched using the core rod. High pressure air is blown in to help form the shape within a metal mold. All three processes can be completed using plastics that can be recycled.

Although it is derived primarily from hydrocarbons, less than five percent of oil production becomes plastic. While that is a large number, recycling and other green practices can help prevent containers from becoming landfill or ocean debris. The inherent advantages of using these methods to make reusable containers helps balance environmental concerns.




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