The process for making polyethylene film and bags is called extrusion. This process starts with melting down small plastic pellets, ( called resin ), until they become molten and pliable.
The molten plastic is pushed, ( extruded ), through a circular die to form a continuous tube of plastic called the bubble. The bubble is inflated with air to the desired diameter and drawn vertically up a tower giving it time to cool before it is flattened to its lay flat width. The thickness of the film is controlled by the speed at which it is pulled from the die. The width of the film is controlled by the amount of air inserted in the bubble.
Film color can be changed in the molten process by adding resin pellets that contain colored pigment.
Many things can be done during this “inline” process. A printing press may be printing images, instructions, warnings, company logos, ect.. on the film. A bag making machine can seal and perforate the film to form varying lengths of bags on rolls. The film can be cut and separated for individually cut bags. You can also add vent holes, which are punched through the film in a variety of patterns and sizes.
The inline process has some further processing limitations. If the film requires more technical alterations then rolled film will be taken off the extrusion line to be further modified in what is known as out of line converting. Here is where you see Process printing and laminating, in addition to process of making sideweld bags, reclosable bags, and wicketed bags, is done out of line.