The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected. The cost of blow moulded parts is higher than that of injection – moulded parts but lower than rotational moulded parts.
The process principle comes from the idea of glassblowing. Enoch Ferngren and William Kopitke produced a blow molding machine and sold it to Hartford Empire Company in 1938. This was the beginning of the commercial blow molding process. During the 1940s the variety and number of products was still very limited and therefore blow molding did not take off until later. Once the variety and production rates went up the number of products created followed soon afterwards.
The technical mechanisms needed to produce hollow bodied workpieces using the blowing technique were established very early on. Because glass is very breakable, after the introduction of plastic, plastic was being used to replace glass in some cases. The first mass production of plastic bottles was done in America in 1939. Germany started using this technology a little bit later, but is currently one of the leading manufacturers of blow molding machines.
In the United States soft drink industry, the number of plastic containers went from zero in 1977 to ten billion pieces in 1999. Today, even a greater number of products are blown and it is expected to keep increasing.
For amorphous metals, also known as bulk metallic glasses (BMGs), blow molding has been recently demonstrated under pressures and temperatures comparable to plastic blow molding.