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To manufacture bags, industry uses a process known as Blown Film Extrusion. This process is used to manufacture not only garbage bags but also items such as; vapor barrier, bread bags, grocery bags, or any one of thousands of different items that you would see packaged for resale.
The plastic is fed in pellet form into the machine hopper (this machine is known as an Extruder), the plastic is conveyed forward by a rotating screw inside a heated barrel and softened by both friction and heat. The softened plastic is then forced upwards through a circular die in a shape of a hollow tube.
This is a continuous process where the tube is expanded with air above the die, and collapsed by the take-off or nip rollers, the volume of air inside the bubble, the speed of the nip rollers and the extruders output rate all play a role in determining the thickness and size of the film.
The tube or “web” of film is then continuously rolled up by take-off rollers, or the web of film may be fed directly into a bag-machine in an in-line process. The tube is heat-sealed across its width to form the bottom of the bag and cut across further up the tube to form the opening.
Cast Film
The extrusion of Cast Films by the flat die technique is becoming more and more important in view of its superior performance: throughput of more than 1 ton per hour against a few hundred kilogram’s per hour for the circular die technique, and better control of the distribution of layer thickness in the transverse direction.
As in the case of bi-oriented films, the technique of multi-layer co-extrusion allows for 3, 5 or 7 layers within complex structures.
The material temperature recommended for manufacturing is situated between 230 and 280°C when using a flat die and between 180 and 210°C for a circular die, with water-cooling of the tubular film ; higher temperatures must be avoided as they may cause degradation of the additives.
Cast polypropylene films are used for the packaging of foodstuffs, textiles, the production of stationery, simulated leather goods, as well as for the manufacture of complex films by lamination or co-extrusion coating.
