What is blow moulding?

Iranpolymer/Baspar  PET Technologies manufactures two-stage stretch blow moulding machines for PET bottle production. But what do these words actually mean? What is “two-stage”? What is “blow molding”? The company dives into the world of blow moulding to understand what it is, what types exist and how plastic containers are made using different methods.

Definition of blow moulding

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

Blow moulding is all about creating hollow plastic parts, such as bottles, containers, and tanks. The process uses heat to soften plastic, shaping it into a mould using air pressure. It’s an essential technology that made mass production of plastic bottles and packaging possible – from the water bottle you drink from, to the detergent bottle in your laundry room.

There are different types of blow moulding processes, each with its own unique characteristics and applications. The three main types are:

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

1. Extrusion Blow Moulding (EBM)

In this process, plastic is first melted and then extruded – meaning pushed – through a die to form a hollow tube called a parison. Once the parison reaches the right length, it is clamped inside a cooled mould. Compressed air is then blown into the parison, pressing the plastic outward against the mould walls and giving it its final shape. After cooling, the mould opens and the finished product is ejected.

Typical products made with EBM include:

• Milk jugs
• Detergent bottles
• Automotive ducts and tanks

Key advantages of EBM include:

• 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

2. Injection Blow Moulding (IBM)

Injection blow moulding (IBM) combines two main steps: injection moulding and blow moulding. First, a preform – a small, thick-walled tube – is created by injecting melted plastic into a mould around a core rod. Next, the still-warm preform is transferred to a blow mould, 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 include:

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

Key advantages of IBM include:

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

Limitations:

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

3. Stretch Blow Moulding (SBM)

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

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

There are two main types of stretch blow moulding processes:

• One-Stage Stretch Blow Moulding – the preform is injection moulded, 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 Moulding – the preform is produced separately, stored or transported if needed, and later reheated and blown into bottles using a blow moulding machine. This is the method used in PET Technologies blow moulding 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 SBM:

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

Limitations:

• 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 moulding and its main types.

interplasinsights