the density of the foam is reduced and the thickness of the sheet obtained by extrusion / expansion is increased by grasping the sheet between confining walls which are permeable to gas and movable together with the sheet , and which diverge from each other in the direction of advance of the sheet , and by applying a subatmospheric pressure to the faces of the sheet and through the said walls while the sheet is in its thermoplastic state , after which the foam is gelled by cooling to stabilize the thickness achieved .