How plastic bags are made

A plastic bag starts as a tube of molten polyethylene, inflated into a bubble from a ring-shaped die.

stations: 6
things upstream: 55
raw sources: 17
steps from the ground: 10

The short version

Plastic bags are made by melting polyethylene pellets and extruding the melt upward through a ring-shaped die while air is blown in, which inflates a thin tube of film. The tube is cooled, flattened and rolled up. Another machine prints it, then heat seals and cuts it into individual bags.

Melt the pellets

Pellets of high- or low-density polyethylene, made from natural gas or petroleum and increasingly mixed with recycled plastic, feed into an extruder. The choice of grade sets the bag's strength, clarity and thickness.

Goes in
Polyethylene
Blow a tube of film

The melt is forced through a ring-shaped die as a continuous tube. Air inside inflates it into a bubble, stretching the plastic to the wanted width and gauge.
Cool and flatten

An air ring chills the bubble as it rises. Once solid, the tube is collapsed flat and wound up for converting.
Print the film

Flexographic or gravure presses lay logos and text onto the film. This step is optional, and plain bags skip it.

Goes in
Ink
Seal and cut

Converting machines cut the flattened film to length and heat seal the bottom of each piece. Handles can be cut in at this point.
Check and pack

Inspectors look at dimensions, seal strength and print quality. Bags that pass are packed tightly to save shipping space.

Goes in
Cardboard box

End of the linePlastic bag Next it goes into syringe, vacuum cleaner, electric kettle and 2 more.

Worth repeating

About 1 to 5 trillion plastic bags are used and discarded around the world every year.
In the late 1950s, Curt Lindquist, the CEO of Celloplast in Sweden, began experimenting with plastic, and a patent was awarded in 1965.
Bags can have gussets for extra volume, and stand-up pouches can stand on a shelf.