What is blow molding & how it works?

We see plastic bottles, containers, and jars everywhere. We use them now and then. But have you come across how these shapes are formed? It is because of blow molding. Blow molding is the industrial manufacturing process for making hollow plastic parts. You can use the same process or joining the hollow plastic parts.

Inspection Line
Inspection Line

The idea behind plastic blow molding comes from glass blow molding. The worker heats the glass to boiling point and then blows air through a hollow tube to inflate the molten glass. Back in those days, the process was very cumbersome and not at all suitable for mass production.

Furthermore, the process evolved, and in the 1930s, commercial machines for mass production were available. As time went by, the invention of high and low-density polyethylene changed the molding industry altogether.

The process

The industry uses the blow molding technique to manufacture different kinds of plastic products. The process involves heating a plastic tube known as a parison (a tube). This process heats the tube to its melting point. This molten tube is then inserted into the cavity of the mold.

Furthermore, the air compressor compresses the air, sends it into the molten plastic tube, and inflates it. As a result, the molten plastic takes the shape of the mold. The molten plastic tube remains hollow from inside. As the plastic cools, it takes the shape of the mold and becomes rigid. The mold then opens up, and the final product is ready.

Besides, the channels carved inside the mold help to cool it faster by circulating water around it. The air pressure and amount of plastic decide the thickness of the finished product.

Types of blow molding

The molding process has three main types: extrusion, injection, and injection stretch blow molding. As we already explained above, extrusion blow molding is a basic process. Here, the plastic pallets are fade through the hopper. Then, manufacturers heat these pallets to make molten plastic. Then the formation of a plastic tube takes place. This tube is then transferred to the mold. The compressed air then inflates this plastic tube. As a result, it takes the shape of the mold. Once cooled, the machine ejects the product.

Furthermore, the injection and injection stretch blow molding replaces the parison (plastic tube). Instead, this process involves preform. Preform means a plastic tube with a hole is manufactured ahead of the final product manufacturing. These preforms are manufactured by injection molding. The manufacturers then further process these preforms to take the necessary shape.

The injection and injection stretch blow molding simplifies and streamlines the process for mass production. It also increases the overall speed of production.


First of all, this process is one of the cheapest processes in the industry. It is a cost-effective way to mold plastic into different products. The machinery required for manufacturing blow molded products is also on the cheaper side. It is best suited for manufacturing products that don’t have joints, which means single-piece products. This process also avoids plastic bleeding. So, it avoids the headache of cutting such bleeds before shipping the product.

In conclusion, blow molding is a fast process for molding plastic into different container shapes. This simple process revolutionizes our current packing industry.

Image Courtesy: Hammonton Mold Co, Inc

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