by: Jake Szarcosi
Blow molding, also known as blow forming is the process used in the manufacture of plastic hollow objects. The principle of this process has been used since the first century BC by Syrian glass workers. The method used at that time involved placing a long tube, into a blast furnace containing liquid glass, and removing a blob of white hot liquid glass and spinning it and then blowing into a mouthpiece on the other end of the tube. The process was refined in Europe and can still be used nowadays in traditional glass workshops. With the introduction of polyethylene plastics in the early 1990s the process was commercialized for use in the production of plastic bottles and containers and due to mass production of such products the process has been made more and more automated and geared towards mass production over the years.
Nowadays three main types of blow molding exist: extrusion blow molding, injection blow molding, and stretch blow molding.
The basic process has two fundamental phases. The blow molding process begins with melting down the plastic and forming it into a preform of tubular shape with a hole in one end in which compressed air can pass through. This preform is then transferred to a metal mold and a pressurized gas, usually air, is then used to expand the hot preform and press it against a mold cavity. When the plastic has cooled and hardened the mold opens up and the part is ejected.
Although the perform manufacture and blowing process are the same for all three main types of blow molding, slight differences exist in the overall process. Extrusion blow molding is the aforementioned process and is the most simple process and widely used process for blow molding. Examples of parts made by the extrusion blow molding include shampoo bottles and hoses/pipes. The machinery used in such blow molding include rotary wheel blow molding systems, shuttle machinery, reciprocating screw machinery and accumulator head machinery.
With injection blow molding the molten plastic is injected into a hollow hot preform mold. The preform mold forms the external shape of the plastic container and is clamped around a core rod which forms the internal shape. The preform itself will form the body. The preform mold is opened and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod is then opened and compressed air is blown into the preform, inflating it required container shape. After cooling the blow mold is opened and the core rod is rotated in order to allow the finished container to be is stripped off the core rod. Injection blow molding is used in the large scale manufacture of plastic products for example soft drinks and mineral water bottles.
In stretch blow molding the preform is formed in a similar way to the injection blow molding method. The plastic container is then formed by stretching the preform in both the hoop direction and the axial direction as the preform is blown into its desired container shape. New methods of blow molding are still being experimented on with the newest machines using 3D molding where the preform is stretched in 3 dimensions.
Jake is a freelance writer with over 10 years of editorial experience. Jake is currently engaged as a gazetteer for our b2b site and has become the resident expert on plastic processing equipment. Visit our great website for more information on Blow Molding Machines and Plastic Blow Molding Machine.
