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What is Blow Molding and How Does it Work?

What is Blow Molding?

You’ve probably seen glass molding before. The videos can be captivating, watching someone take a glob of metal and then expertly create any number of artistic designs with it. A similar technique, known as blow molding, can be leveraged to mold plastic quickly and easily for manufacturing purposes. While creating a two-liter bottle or a milk carton might not be as artistically satisfying as beautiful and unique vases and art pieces, it utilizes similar processes to create a sort of simple elegance of its own.

Blow molding is the process of creating one-piece hollow objects, such as in manufacturing bottles for use. Most gallons of milk and two-liter bottles you see have been blow-molded into existence. Other items, such as some children’s outdoor toys and stadium seats may also be made by blow molding. It is a highly versatile manufacturing process that is essential for many products we use on a daily basis.

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This process creates objects by taking a heated liquid material, which is then pushed into a mold cavity under pressure. To do this, raw plastic is melted and shaped to create a parison (a rounded mass of the molten material). The parison can then be secured to a mold, at which point, pressurized air is forced into the parison, stretching it until it conforms to the interior of the mold.

Blow molding has several advantages that make it particularly lucrative in the manufacturing sector. It is cost-effective, especially when compared to injection molding. Both production and machinery are usually cheaper for blow molding.

Blow molding also usually creates one-piece items, which means objects do not require assembly later down the line. This means that containers that need exterior threading can be made even more effectively.

This process also reduces flash, the little bits of plastic bleed that can be found around seams of products present in other forms of molding. The excess plastic on the seams often requires additional finishing work on the product to remove before shipping the products, adding the cost of labor as well as the additional manufacturing time and cutting into profits. Blow molding reduces and in some cases, eliminates flash, allowing for faster turnaround and quicker profits.

Types of Blow Molding

There are three common types of blow molding: Extrusion blow molding, injection blow systems, and injection stretch blow molding. All three offer their own benefits and unique fits for products.

Extrusion Blow Molding (EBM)

Extrusion blow molding is the process of melting plastic and then extruding it into a parison. The parison is then caught onto a cooled metal mold, at which point it is inflated with air to create a hollow plastic shape. This works best for creating hollow bottles, containers, and parts that must be made in one piece, such as milk containers, watering cans, or industrial drums.

This particular type of blow molding is advantageous thanks to reduced tool and die cost, the quick rate of production, the ability to create more complex pieces, and the fact that handles can be designed.

There are a few disadvantages as well: It can only create hollow parts. Parisons often are created with mixed materials to help with stability, which often causes them to no longer be recycled. The products also tend to have lower tensile strength when compared to other techniques like traditional injection molding.

Injection Blow Systems (IBS)

The injection blow system is useful for producing hollow plastic or glass objects rapidly in larger quantities. There are three steps that go into this form of molding: Injection, blowing, and then ejection. The polymer is injection molded onto a core pin. The core pin can then be rotated to a blow molding station, where it is then inflated and cooled. This is less popular than extrusion or injection stretch blow molding but can be especially useful for small medical or single-serve bottles.

This is particularly useful when an injection molded neck is needed for accuracy purposes. However, because it is only suitable for smaller capacity bottles, it is not as common as the others. Handles also can’t be incorporated into the designs of the bottles.

Injection Stretch Blow Molding (ISBM)

Injection stretch blow molding can be completed in either a single-stage or double-stage process. Each of these works slightly differently.

Single-stage processes require both preform manufacture and bottle blowing to be completed in just one machine. Instead of using a four-station method for injection, reheating, stretch blowing, and ejection, the single-stage system uses three stations instead, eliminating the reheat stage by utilizing the latent heat from the preform stage. As a result, energy costs and tooling costs are reduced. This is beneficial when a low volume needs to be produced quickly. Because the preform is not released, the thickness can be shaped as well.

The double-stage process involves first molding plastic into a preform with the injection molding process. The preforms can then be produced with necks of bottles, along with threads, on one end. Then, the preforms are packed away and after cooling, are put into a reheat stretch blow molding machine. They are then blown into bottles with highly pressurized air and blow molds. Though more expensive and requiring more floor space than other methods, this does allow for very high volumes to be produced with very little restriction on designs. They can create cylindrical, rectangular, or ovular bottles.

Popular Materials in Blow Molding

Though we talk about plastic as all being plastic, there are many different types suitable for very different purposes. Blow molding can make use of many different materials, depending upon what’s being manufactured and the methods being used.

Acrylonitrile Butadiene Styrene (ABS)

ABS is primarily used for household goods, containers, and housings. But, it is limited due to lacking the same degree of chemical resistance that other blow molding plastics often offer.

High-Density Polyethylene (HDPE)

HDPE is particularly useful thanks to its chemical and temperature resistance as well as a higher strength-to-density ratio. It is one of the most popular molding materials for this reason. HDPE is often easily molded and can be used for consumer and industrial goods. It is commonly used for several bottles and containers.

Low-Density Polyethylene (LDPE)

LDPE is more flexible and softer than HDPE, which makes it useful for plastic bags or squeeze bottles that require some give to them. Like HDPE, LDPE is chemically inert. It’s also very easy to mold.

Polypropylene (PP)

Polypropylene is used similarly to HDPE but is stiffer and less dense. PP is more resistant to higher temperatures as well.

Polystyrene (PS)

Polystyrene is most commonly used in Styrofoam, but when kept in its solid form, it is a very rigid plastic that can be used for food containers, cases, and lab equipment. It is cost-efficient, but also brittle.

Polyurethane (PU)

Polyurethane is easily molded, making it useful for many products that are used in industrial facilities or marine environments. It is more rigid than many other options and can be made temperature resistant, making it useful for rugged environments.


Santoprene acts similarly to rubber but is a thermoplastic. It is both flexible and durable, making it useful in household appliances, medical equipment, or electronics.


Kostrate is temperature resistant and durable, making it useful for food and beverage storage and packaging. It’s also regularly used for toys.

K-Resin Styrene-Butadiene Copolymer (K-Resin SBC)

K-Resin SBC is a resin that works similarly to Kostrate, granting both durability and rigidity. It’s used for toys, medical equipment, and packaging.

Blow Molding vs. Injection Molding

A lot of people might interchangeably use both blow molding and injection molding, but these do refer to different processes. Both blow molding and injection molding are useful processes to manufacture plastic products, but they do serve different purposes.

While both use some sort of liquid material, usually plastic, which is forced into a mold cavity, the process of doing so differs. Blow molding creates hollow objects, such as bottles, but injection molding does not have this limitation. With injection molding, solid objects are created instead. Blow molding is a fast and effective solution for hollowed objects that are relatively simple. Injection molding is best for more complex shapes and products.


Most of the time, when you choose the wrong product, material, or manufacturing process, you will not be working as efficiently as you could be. Bottles that are not made of the right material or with the right processes may leak or react to whatever is inside of them. A container that is supposed to be one solid piece may fall apart if it has seams holding it together. Objects may not be as uniform as they need to be. Blow molding solves all of these issues. With blow molding, one-piece, hollowed, uniform containers can be created quickly and easily.

For manufacturers, blow molding is a versatile asset. Without it and the hollowed bottles and containers it provides us, manufacturing costs would likely be higher. Thanks to the streamlined processes of blow molding, bottles, containers, and other plastic hollow pieces can be created quickly and easily, cutting costs for everyone.

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