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What is Blow Molding?

Michael Pollick
Updated May 17, 2024
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Blow molding is a manufacturing method used in the plastic industry to create strong, hollow containers; some examples of products commonly created through this process are plastic beverage bottles, plastic drums, and fuel tanks. A typical blow molding process involves melting plastic into a hollow tube, known as a parison, placing the tube between to halves of a mold, and pumping pressurized air into the parison until it expands to fit the form of the mold; the two halves of the mold then separate and the finished container is released.

Strength of Plastic

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 result of the polymer chains in a plastic bottle forming in two directions; this generally creates a very strong webbing effect. The plastic can then be stretched thin without sacrificing strength, and blow molding experts takes advantage of this property to make thin, but strong, containers.

The Process

Blow molding any type of container requires a preformed piece of plastic 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 an 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. The difference between the three processes lies at this point of the process.

At this point in the process, a hollow ramrod is injected into the parison's 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, which forces the plastic form to form to the sides of the mold; because the stretching is performed evenly, the plastic remains uniformly thin and strong. This process is expensive to do, but is very efficient when used for producing mass quantities, making it ideal for such things as water bottles.

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. Blow molding engineers who design new pieces of equipment for the molding process often need first-hand experience with the machinery in order to understand how to improve the equipment and molding process.

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Michael Pollick
By Michael Pollick
As a frequent contributor to About Mechanics, Michael Pollick uses his passion for research and writing to cover a wide range of topics. His curiosity drives him to study subjects in-depth, resulting in informative and engaging articles. Prior to becoming a professional writer, Michael honed his skills as an English tutor, poet, voice-over artist, and DJ.
Discussion Comments
By pinkandred — On Apr 27, 2011

Is large plastic object like buckets and trashcans blow molded, or are they made by some other process?

By honeysuckle — On Apr 26, 2011

I worked in a plastic factory. We made food, medical, and chemical plastic bottles and containers. All of our blow molded products were made from high quality plastic. Some of the containers were colored, others were clear, especially the medicine bottles.

The blow mold machines had to be cleaned and maintained on a very strict schedule. If they were dirty, they made dirty bottles and wasted a lot of money.

The machines also ran at over 300 degrees to melt the plastic. The thermostats on the machines had to be in top working order to make sure the plastic would melt. If not, the plastic would partially melt and clog the injectors, making the whole line shut down.

Michael Pollick
Michael Pollick
As a frequent contributor to About Mechanics, Michael Pollick uses his passion for research and writing to cover a wide...
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