How Does Compressed Air Form Hollow Products In Blow Molding

2026-09-30

Compressed air is the part of the blow molding process that expands the hot parison into the mold cavity. In extrusion blow molding, the extruder first melts and plasticizes the material, then the die head forms a tubular parison. After the mold closes, compressed air enters the parison and pushes the softened plastic against the mold wall. This is how a hollow plastic product takes its required shape. Kingplas Machinery designs and manufactures extrusion blow molding machines for bottles, jerry cans, chemical barrels, automotive parts, and other hollow products.

How Does Compressed Air Form Hollow Products In Blow Molding-Kingplas Machinery

The Parison Provides The Starting Shape:

The blowing process starts before compressed air enters the product. The extruder melts the plastic and sends the melt to the die head. The die head then forms a tubular parison around the core rod.

At this stage, technicians need to pay attention to the parison diameter and wall thickness. These conditions affect how the material spreads after blowing. Kingplas describes die head adjustment as a way to control the parison wall thickness and diameter for different product requirements.

This is also why the same blowing pressure cannot simply be applied to every product. A small bottle, a jerry can, and a large barrel have different material and mold requirements.

Compressed Air Expands The Parison Against The Mold:

Once the parison reaches the required length, the mold closes around it. The blow pipe then enters the parison, and compressed air flows into the hollow tube.

The air pressure pushes the softened plastic outward until it contacts the inner surface of the mold. The mold cavity determines the outside shape, while the air provides the force needed to expand the parison. Kingplas explains that blowing pressure, blowing time, and air volume need adjustment according to product size and plastic material.

The principle is simple, but the actual setting needs to match the product. If the pressure or timing does not suit the material, some areas may not form correctly.

Air Pressure Depends On Machine And Product Requirements:

Kingplas lists 0.8 MPa air pressure for its K2L, K5L, K15L, and K30L continuous extrusion blow molding machines. The listed air consumption is 6 m³/h for the K2L and K5L, and 12 m³/h for the K15L and K30L. These figures are machine specifications and should not be treated as universal process settings.

The double-station K2D, K5D, K15D, and K30D models also list 0.8 MPa air pressure. Their air consumption ranges from 9 m³/h to 15 m³/h according to the model.

For production, technicians still need to adjust the blowing conditions around the actual plastic material, product volume, mold structure, and cycle.

How Does Compressed Air Form Hollow Products In Blow Molding-Kingplas Machinery

Cooling Allows The Product To Keep Its Shape:

Blowing does not finish the forming process. After the plastic reaches the mold wall, the product remains hot and needs time to cool.

Cooling channels in the mold remove heat from the plastic. As the material cools, it becomes stable enough to keep the required shape. Kingplas notes that insufficient cooling can cause deformation, while excessive cooling time can reduce production efficiency.

Therefore, blowing and cooling need to work as one process. A suitable air setting cannot compensate for an unsuitable cooling condition.

Product Size Determines The Overall Blow Molding Conditions:

Different hollow products require different machine configurations. Kingplas continuous extrusion blow molding machines cover product volumes from 2L to 30L in the listed K2L, K5L, K15L, and K30L models. The corresponding plasticizing capacities range from 20–35 kg/h to 140–180 kg/h.

For larger products, Kingplas also manufactures accumulator type extrusion blow molding machines. Its product range covers applications such as chemical barrels, IBC tanks, automotive parts, and other large hollow products.

The continuous extrusion blow molding machine is therefore only one part of the available equipment range. Machine selection should follow the product volume, material, mold structure, and required output.

FAQ:

Q1: What does compressed air do in blow molding?

A: Compressed air expands the hot parison against the mold cavity and gives the hollow product its required outer shape.

Q2: Does compressed air create the hollow space?

A: No. The parison already has a hollow tubular structure. Compressed air expands this tube inside the closed mold.

Q3: What affects blowing pressure?

A: Product size, plastic material, parison condition, mold structure, blowing time, and machine configuration all affect the required blowing conditions.

Q4: What air pressure do Kingplas continuous extrusion blow molding machines use?

A: The listed K2L, K5L, K15L, and K30L models use a rated air pressure of 0.8 MPa.

Q5: Can different hollow products use the same blow molding machine?

A: A machine can handle different products when the product size, mold, material, and machine capacity fall within the applicable range.

Why Choose Kingplas Machinery?

Has 30 years blow molding experience. Our blow molding machine portfolio is designed to handle a wide range of thermoplastic materials, including HDPE, PP, PE, PVC, PC, and PETG, for containers ranging from 10ml to 1250L.

Automatic Extrusion Blow Molding Machines: High-speed continuous extrusion systems (Single/Double Station) for cosmetics, lubricants, and household chemicals.
Accumulator Head Blow Molding Machine: Robust solutions for large-scale industrial parts, such as 200L drums, IBC tanks, and automotive components.
Multi-Layer Co-Extrusion Technology: Advanced 2-to-6 layer systems that provide superior barrier properties for agrochemical and fuel tank applications.
Turnkey Solutions: Integrated production lines including precision mold design and auxiliary equipment.