How polystyrene foam is made

Foam cups and blue insulation board are the same hard, brittle plastic, puffed full of gas.

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

The short version

Polystyrene foam starts as solid polystyrene, a plastic that turns runny above about 100 °C. Extruded board is melted, mixed with a gas, and squeezed through a die so it swells as it cools. Molded cups and packing are fused from small pre-expanded beads.

Feed in polystyrene pellets

Solid pellets are melted in a heated barrel. The plastic is glassy at room temperature and flows above its glass transition at about 100 °C.

Goes in
Polystyrene
Mix in a blowing agent

A gas or volatile liquid is blended into the melt under pressure. It will form the bubbles.
Squeeze it through a die

As the mix exits the die, the pressure drops and the gas expands. The plastic puffs into a closed-cell foam and hardens as it cools.
Cut the board to size

The continuous slab is trimmed and sawn into boards of standard dimensions.
Or expand beads in a mold

For cups and packing, beads that were pre-expanded with a hydrocarbon such as pentane are fed into a mold. Steam fuses them together into one piece.

Goes in
Pentane

End of the lineStyrofoam

Worth repeating

A patent for pre-expanded polystyrene beads was issued to BASF and Fritz Stastny in 1949.
Foam polystyrene is not biodegradable and builds up as litter along shores and waterways.
Styrofoam is a brand name for extruded polystyrene insulation, though many people use it for foam cups.