Meaning
Pneumatic forces applied during the second stage of stretch blow moulding determine the final shape and surface finish of the container. This operating parameter, known as main blow pressure, involves injecting high-pressure compressed air to force the preform against the cold mould walls. It ensures that the polymer replicates the intricate details of the mould surface, including logos and base contours.
Inflation Dynamic
The activation of this high-pressure stage must be precisely timed with the extension of the stretch rod. During the application of main blow pressure, the polymer undergoes extremely rapid biaxial stretching. The air must enter quickly to freeze the shape before the material cools below its stretching temperature.
This rapid inflation is critical for achieving a uniform wall thickness across the bottle body. If the air pressure is applied too late, the polymer cools in contact with the stretch rod, leading to heavy bases and thin shoulders.
Material Orientation
Applying sufficient pressure forces the polymer chains into a state of high biaxial orientation, which increases the density and crystallization of the material. This structural change enhances the mechanical properties and barrier performance of the bottle. A bottle with insufficient pressure results in a thick, poorly oriented base that is susceptible to stress cracking.
This orientation also improves the overall top load strength of the container.
Pressure Optimization
The magnitude of the required air pressure depends on the wall thickness of the preform and the complexity of the bottle design. Heavy-duty containers or those with sharp rib details demand higher pressure to ensure complete reproduction. Managing the consumption of this compressed air is a major factor in the operating costs of high-speed blow moulding lines.
