Artists create objects of astonishing beauty by carefully blowing air into a mass of melted glass to form various shapes. Glass blowing originated hundreds of years ago, but the basic principles of this art are used today in plastics manufacturing to create containers, toys, medical devices, and many other items. This transition was made possible through the emergence of blow molding.
This production method begins with a tube of raw plastic material called a parison, a word taken from artisans describing a mass of melted glass. The parison is carefully sealed between the sections of a mold, and pressurized air is forced in at 25 to 150 psi. This makes the soft plastic assume the inner shape. It coats the interior with a uniform thickness of material, and rapidly cools.
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.
A parison can be shaped in a variety of ways. Screw-like machinery forces these masses into their molds during a process called extrusion. Almost immediately, pressurized air is sent through a tube into the center of the material and expands outward, creating precise shapes and surface details that can be rapidly and accurately reproduced.
Extrusion can be intermittent or continuous, depending on the style and quantity of the product. Large milk or juice containers are commonly made using variations of this method, but others are produced most effectively using injection molding. Injection forces the polymer into a core pin, which is inflated with air, allowed to cool, and then ejected.
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.
There is no escaping the fact that plastics come from hydrocarbons. Even though the material accounts for less than five percent of all petroleum production, the numbers are significant. Today, improved methods of recycling and re-use greatly are reducing the chances of this material ending up in a land fill or floating for years at sea.
