It's a Gas

OMB of Cambiago (Milan, Italy) has become specialized over the years in the design of moulds for major-name designer chairs. One of the earliest products, dating to the late 1990s, is the one-piece Air Chair, designed by Jasper Morrison for Magis and produced via gas-assisted injection moulding.
More recently, OMB developed the full production process – from mould construction to casting the final product – of the Vanity Chair, designed by Stefano Giovannoni, again for Magis, and made of transparent polycarbonate, again using gas-assisted moulding.
The gas is used not only to compact the material, but also to ensure optimal moulding filling, driving the polymer from the zones of greatest thickness (the legs and their junctions with the seat) to thinner areas (such as the back). The injection channels are relatively short to accommodate a material with low flow index which has to be injected directly without stresses.