Blow molding is a manufacturing method used in the plastics and polymers industries to create hollow but strong containers for their clients. Plastic beverage bottles and fuel tanks are commonly created through blow molding. A typical blow molding machine set-up uses an extruded plastic preform and compressed air to fill the chamber of a divided mold. The two halves of the mold separate and the finished container is released.

Although many products are manufactured using the blow molding process, it might be simpler to examine what happens to one particular product -- a standard two-liter soda bottle.

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 phenomenon called 'biaxial orientation.' The polymer chains in a plastic bottle form in two directions, creating a very strong webbing effect. The plastic itself can be stretched out without sacrificing strength. Blow molding experts takes advantage of this property to make thin but strong containers.

Blow molding a two-liter soda bottle requires a preformed piece called a parison. This parison is usually extruded from a plastic injection mold placed very close to the blow molding machinery. The warm parison looks like a upside-down plastic test tube, with a preformed collar and threads for the cap at the bottom. The parison is mechanically loaded onto a stand and two sides of a bottle-shaped metal mold come together around it.

Before the parison cools down, a hollow ramrod is injected into its center and pushed to the top of the mold, stretching out the warm plastic preform as it goes. Compressed air is then forced out in controlled low-pressure stages through the hollow ramrod. The plastic form is forced out to the sides of the mold. Because the stretching is performed evenly, the plastic remains uniformly thin and strong. The soda bottle assumes the shape of the mold and is dropped out of the blow molding machine as the two mold halves separate. A new parison is extruded and the entire blow molding process begins again. The actual manufacture of a soda bottle takes only a few seconds.

There are other forms of blow molding, but the general principle is the same. The plastic acts much like a latex rubber balloon -- as long as the pressure from the compressed air is controlled, the material will expand uniformly and form the shape of the mold. This requires a fair amount of skill and experience on the part of the blow molding engineers who design new pieces of equipment.