What Is Blow Molding? Main Types and Technologies Explained

PET Technologies offers insights into the fundamentals of blow molding and how this process continues to evolve.

Blow molding is the quiet workhorse behind much of the world’s plastic packaging—from beverage bottles to household containers. Among its variants, stretch blow molding has emerged as a preferred process for producing durable, lightweight PET bottles. Drawing on its extensive industry experience, PET Technologies offers insights into the fundamentals of blow molding and how this process continues to evolve.

You can also read: What’s Powering the Future of Blow Molding?

Definition of Blow Molding

According to Collins Dictionary: “Blow molding is a process for forming plastic objects in which plastic is melted, put in a mold, and then shaped by having compressed air blown into it.”

Blow molding is all about creating hollow plastic parts, such as bottles, containers, and tanks. The process uses heat to soften plastic, shaping it into a mold using air pressure. It’s an essential technology that made mass production of plastic bottles and packaging possible – from the water bottle you drink, to the detergent bottle in your laundry room. Moreover, there are several different types of blow molding processes, each with its unique characteristics and applications.

The three main types are:

• Extrusion Blow Molding (EBM)
• Injection Blow Molding (IBM)
• Stretch Blow Molding (SBM)

Let’s take a closer look at each of them.

Extrusion Blow Molding (EBM)

In this process, the machine first melts the plastic and then extrudes it through a die, forming a hollow tube known as a parison. Once the parison reaches the desired length, the system clamps it inside a cooled mold. Then, compressed air blows into the parison, forcing the plastic outward against the mold walls to create its final shape. After the plastic cools, the machine opens the mold and ejects the finished product.

Typical Products Made with EBM:

• Milk jugs
• Detergent bottles
• Automotive ducts and tanks

Key Advantages of EBM:

• Well-suited for making large, hollow parts
• Handles a variety of plastics (HDPE, PP, etc.)
• Can create complex shapes with handles or wide mouths

Limitations:

• Not as precise in wall thickness control compared to other methods
• Not the preferred method for PET bottles, which require higher clarity and strength

Injection Blow Molding (IBM)

Injection blow molding (IBM) combines two main steps: injection molding and blow molding. First, a preform – a small, thick-walled tube – is created by injecting melted plastic into a mold around a core rod. Next, the still-warm preform is transferred to a blow mold, where compressed air inflates it into the final shape. This method is widely used for making small, high-precision containers where uniformity is important. IBM typically works with plastics such as polypropylene (PP), polyethylene (PE), and sometimes polyethylene terephthalate (PET), depending on the application.

Typical Products Made with IBM:

• Pharmaceutical bottles
• Cosmetic containers
• Small personal care products (travel-size bottles, etc.)

Key Advantages of Injection Blow Molding (IBM):

• Produces bottles with very consistent dimensions
• Ideal for small and medium-sized containers

Limitations of Injection Blow Molding (IBM):

• Not suitable for large containers or high-volume beverage production
• Slower cycle times compared to extrusion blow molding for bigger parts

Stretch Blow Molding (SBM)

Now we come to stretch blow molding – the type of blow molding technology that PET Technologies specializes in. Stretch blow molding is a process designed specifically for producing lightweight and durable PET bottles, widely used in the beverage, food and other industries.

In stretch blow molding, the plastic preform is first heated to the right temperature. Then, inside the blow mold, it is mechanically stretched (usually by a rod) and simultaneously blown with compressed air to take the shape of the mold. This stretching step is what gives PET bottles their excellent mechanical strength, clarity, and reduced material weight compared to other blow-molded containers.

There are two main types of stretch blow molding processes:

• One-Stage Stretch Blow Molding – the preform is injection molded, conditioned (cooled and reheated if necessary), stretched, and blown into a bottle — all in a single machine. This method is often used for small to medium production volumes or specialty bottle shapes.
• Two-Stage Stretch Blow Molding – the preform is produced separately, stored or transported if needed, and later reheated and blown into bottles using a blow molding machine. This is the method used in PET Technologies blow molding machines. Those machines are ideal for high-speed, high-volume PET bottle production. This approach provides greater flexibility in preform sourcing, ensures faster production speeds, and delivers highly consistent bottle quality.

Typical Products Made with SBM:

• Mineral water bottles
• Soft drink bottles
• Juice and dairy packaging
• Edible oil bottles

Key Advantages of Stretch Blow Molding (SBM):

• Lightweight bottles without compromising durability
• High production speeds
• Wide flexibility in bottle design
• Bottle strength and clarity due to the stretching process

Limitations of Stretch Blow Molding (SBM):

• Primarily limited to PET and a few specialty plastics
• Requires preform sourcing or in-house production

This article is a general introduction, aimed at helping you become familiar with the concept of blow molding and its main types. In the future, we will dive deeper into each method in dedicated articles.

Article written by PET Technologies, edited for online publication by Juliana Montoya.