How are plastic bags manufactured in the factory?

Question

Serious and specific answer, thanks!!

Hi, thanks so much for your answers! Both are good so i'm very sorry i've no idea who to give the best answer points to...

Answer 1

The polymer that you're making the bag from (like polyethylene) is usually provided in the form of solid pellets and fed into an extruder. This melts the polymer and extrudes it as a film in the desired form. The film can then be shaped like a bag and heat-sealed.

Interestingly enough, many plastic bags are actually layers of different polymers. They'll have a strong polymer on the outer layer, and one that doesn't heat seal so easily, and a weaker polymer with a lower temperature for heat sealing on the inner surface. that's why when you buy some sealed bags of food, for example, you can pull the bag open without tearing the entire thing apart. You'll really just separating the inner layer which was heat sealed, while the stronger outer layer remains intact.

Depending on the qualities the manufacturer wants the bags to have (i.e. strength, temperature resistance, sealability, ability to have graphics printed on them, etc.) different polymers or layers of polymer are chosen.

Answer 2

First, polyethylene resin (plastic pellets delivered in cardboard drums) is melted in an extruder. An extruder has a barrel with a long screw (worm gear) in it. Heat applied electrically from the outside of the barrel helps melt the plastic. Shear energy (the energy of rubbing plastic molecules against one another in the screw and barrel) adds energy to melt the plastic. The screw keeps turning, moving plastic melt ever upwards. (For convenience in plastic bag manufacture, the extruder barrel is on the "bottom," oriented vertically, with the plastic melt always delivered "up." In other processes, like making white pipe for water in houses, the extruder barrel is horizontal.) Anyway! The plastic melt is pushed through a die. The die looks like an O with an o in the middle. The space in the middle between the inside of the O and the outside of the o is the thickness of a plastic bag. The plastic flows between the outer O and the inner o. The result is a cylinder of plastic film pulled upwards from the die. The cylinder begins at the bottom (the die) and extends up many feet. The sight is really impressive, and it would be good if you could get a photo from someplace showing it done. The endless cylinder of plastic film is extruded from below and pulled from above, with plastic bags made by sealing off from the bottom and cutting off from the top. The bags are pressed flat, which is why you have to wave them around when they're new to get air into them and open them up to put things into them.