Abstract: A process for producing a seat element from a portion of an outfit (1) including a relatively compact plastic covering layer, a network formed by a continuous sheet of metal springs (5), the intermingled turns of which, on one side, come substantially flush against the lower face of the covering (1), and an intermediate connecting layer (3) made of a plastic foam which partially encases the turns of the sheet of springs (5).According to the invention, the peripheral edge (7) of the said outfit portion is gripped in an injection mold (11), this mold having a cavity (9) into which the said edge (7) penetrates, a thermoplastic to which an expanding agent has been added is injected into the said cavity at low pressure, and this thermoplastic is allowed to harden there.