How foam cups are made

A foam cup is about 95 percent air, held in place by plastic bubbles that were puffed up with steam.

stations: 5
things upstream: 12
raw sources: 4
steps from the ground: 7

The short version

Tiny polystyrene beads contain a gas called pentane. Steam makes the gas expand, puffing each bead up like popcorn. The puffed beads rest for a while, then go into a cup-shaped mold where more steam melts their skins together. The mold is cooled so the cup hardens, and then the cup is ejected.

Puff the beads with steam

Polystyrene beads with pentane trapped inside meet steam in a pre-expander. The heat softens the plastic while the pentane turns to gas and swells each bead into a closed-cell foam sphere.

Goes in
Polystyrene Pentane
Let the beads rest

The puffed beads sit in breathable bags or silos while air seeps into them and replaces some of the pentane. This keeps them from shrinking in the mold.
Fill the cup mold

Beads are blown into an aluminum mold shaped like the cup. How densely they are packed sets how stiff the wall will be.
Fuse them with steam

More steam softens the beads so they swell slightly and weld into one piece that fills the cavity.
Cool and eject

The mold is cooled with water spray or vacuum so the plastic stiffens before it leaves the cavity. A cup that comes out too warm swells and deforms.

End of the lineFoam cup

Worth repeating

Foam used in cups and coolers is typically 95 to 98 percent air.
Styrofoam is a trademark for a blue extruded building insulation, not for the white beaded foam in disposable cups.