Cast extrusion and blown extrusion are two distinct processes used in the manufacturing of plastic films and sheets. Here are the key differences between cast extrusion and blown extrusion:
Process Overview:
Cast Extrusion: In cast extrusion, also known as flat die extrusion, the molten plastic is extruded through a flat die to form a thin, flat sheet. The extrudate (molten plastic) is then cooled rapidly on large polished rolls (chill rolls) to set the shape and dimensions of the sheet. The cooling process typically happens quickly, allowing for precise control over the thickness and properties of the sheet.
Blown Extrusion: In blown extrusion, the process involves extruding a tube of molten plastic vertically upward through a circular die. As the tube emerges from the die, it is inflated (or blown) with air to expand and stretch the tube to the desired dimensions. The inflated tube is then cooled with air and collapsed into a flattened shape before winding onto rolls. Blown extrusion allows for the production of tubular films, which can be slit open to form flat sheets.
Product Characteristics:
Cast Extrusion: Produces flat, uniform sheets with excellent flatness and surface quality. The process is suitable for producing thin films (as thin as 10 microns) with consistent thickness and minimal thickness variation across the width of the sheet.
Blown Extrusion: Typically used to produce tubular films, which can be slit open to form sheets. Blown films have characteristics such as good toughness and puncture resistance due to the orientation of molecules during the stretching process. They are often used for packaging applications where strength and durability are important.
Applications:
Cast Extrusion: Commonly used for applications requiring high clarity, such as food packaging films, shrink films, and industrial liners. The precise control over thickness and surface quality makes it suitable for applications where aesthetics and barrier properties are critical.
Blown Extrusion: Widely used for applications requiring strength and impact resistance, such as trash bags, agricultural films, and heavy-duty packaging. The ability to produce films with varying thickness and properties across the width of the film (thanks to the stretching process) makes it versatile for different packaging needs.
Equipment Differences:
Cast Extrusion: Utilizes flat dies and large, polished chill rolls for rapid cooling of the extruded sheet. The process requires precise control over temperature and cooling to maintain sheet quality.
Blown Extrusion: Involves a circular die and an air ring to cool and shape the inflated tube. The process also includes collapsing frames and winding equipment to produce flattened films.
