How zip-top bags are made

On some lines, the zipper of a zip-top bag is extruded in the same pass as the bag itself.

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

The short version

Polyethylene is melted and blown into a tube of film. In bag lines with an integral zipper, the die lays down two thicker ridges that will become the interlocking zipper halves, and those ridges are cooled harder than the rest. The tube is then sealed across at intervals and cut into individual bags.

Melt the pellets

Polyethylene pellets are heated in an extruder until they flow like thick syrup.

Goes in
Polyethylene
Blow a tube with ridges

The melt exits a ring die and is inflated into a bubble of film. Slots in the die let two thickened ridges form on the inside, one for each half of the zipper.
Chill the ridges first

The thick ridges hold heat longer than the thin film. A jet of cool gas or radiant cooling aimed at them sets their shape before they can sag.
Seal the zipper ends

At the end of each bag the ridges are squashed flat with a heat seal, so the zipper cannot snag or leak at the corner.
Seal the sides and cut

Heated bars weld the flattened film across at each bag length, and a blade parts the bags from one another.
Count and box

Finished bags are counted into stacks and packed in cartons.

Goes in
Cardboard box

End of the lineZip-top bag

Worth repeating

Plastic bags were once too costly to sell to shoppers, until Dow engineers pushed line speed from 30 feet per minute to 300 by 1972.
The companies behind the plastic zipper, Dow among them, obtained over 300 patents for zippers, zipper bags and the machines that make them.