Plastic bags are found nearly everywhere. We incorporate them for such diverse uses as freezing food, sealing a sandwich, carrying groceries and toting books. The process for making plastic bags is similar across manufacturers, although some may add a few proprietary steps or use specialized equipment. Two steps make a basic bag, after which some may receive additional processing.
An extruder heats polyethylene plastic resin pellets to around 500 degrees Fahrenheit, enough to melt the pellets. A screw inside the extruder forces the molten plastic through the machine and pushes the material through a die that controls the thickness of the product. Air forces the emerging plastic film into a bubble that travels upward about three stories in a cooling process. After pinching out the air and flattening the bubble, the film is cut to size and wrapped on a spindle.
Converting Film to Bags
A conversion department unwraps the roll of film and slices it with a heated knife that both seals the sides of the bag and cuts it to size. The conversion department also adds any special characteristics needed for a completed bag. Dies cut out handles, wheels produce gussets, and zippered seals get attached with heat or by ultrasonic means. Any printing may be done after the bags have been converted, or in a separate department between extrusion and conversion.