What is Blow Molding?

You use them all the time. They come in a wide range of plastic food and beverage containers and many other types of containers. But have you ever wondered how hollow plastic bottles are made? These parts are made by standard or custom blow molding processes and there is more than one method used. Here is information on blow molding and why it is so important to both industry and consumers today.

What Does Blow Mold Mean?

Blow molding comes from processes like old fashion glass blowing. A hot material is formed into a cylinder or other shape and then air is injected or blown inside to create the desired shape. A machine for this process was first introduced in the 1930s. It worked well for producing hollow body shapes but took a few years for the method to catch on.

Part of the reason for the slow process was the fragility of glass, but the time the 1940s began, the US was starting to manufacture plastic bottles. Yet it still took time for the soft drink industry to adopt this technology. For example, during most of the 1970s there were virtually no plastic soft drink bottles, but by the dawn of the 21st Century there were 10 billion plastic bottles manufactured per year.


Standard and custom blow molding is very popular today. There are three ways materials are manufactured. Those methods are extrusion, injection and stretch injection.


To create hollow containers, materials are melted down and turned into tubes by extrusion. To extrude shapes, material is forced through a die and comes out in the form of tubes or parisons. The parison is then placed into a mold and air is blown into it while it is still hot. It is then allowed to cool to form the desired shape.


When you need vast quantities of hollow containers, injection molding may be the best option. Instead of extruding, the parison it is created via injection molding. It is then transferred to an area where air can be injected to literally inflate the parison.

Stretch Injection

Imagine blowing up a heated parison into a hollow bottle. Now, stretch the parison as you blow it up. This is the basic principle behind stretch injection. This method can produce various bottle shapes (with necks and many other features) in high quantities. In addition, some companies produce parisons which can then be blown into bottles by manufacturers or used for custom blow molding. The process can be done in several stages and can produce cost effective bottles and containers.

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