Method for manufacturing a cushion of a motor vehicle seat, said method comprising the following steps: • providing a flexible cladding cover, • providing a mould comprising a first and a second subassembly actuatable for closing so as to define a moulding cavity, said mould having the following features: ∘ said first subassembly comprises a first and a second element partly defining the wall of said cavity, ∘ said elements can be separated from each other from a close-together configuration in which they are separated from each other by a slot, allowing a gripping of said cover, to a separated configuration, in which they are separated from each other by said broadened slot, enabling said cover to pass, ∘ said second element is provided with a strip extending along said slot and projecting into said cavity so as to partially compartmentalise it, • said elements being in separated configuration, passing said cover through said broadened slot, so that said cover has a middle zone, disposed in said cavity facing said first element, and a lateral zone projecting from said cavity, • actuating said elements to their close-together configuration so as to grip said cover between said zones, • injecting a first elastically compressible polyurethane-foam precursor mixture onto said middle zone and a second elastically compressible polyurethane-foam precursor mixture onto the cavity-wall part defined by said second element, said first mixture leading to a foam of less compressive strength than said second mixture, so as to form a stuffing block by expansion of said foams in said cavity, • moving said parts into a separated configuration and removing said cushion obtained from the mould, said cushion having said middle zone moulded onto by said block and said lateral zone not moulded onto by said block, said method being characterised in that, prior to the injection of said mixtures, said strip is surrounded by a flexible reinforcing band intended to have said block moulded onto it, in order to reinforce said block faced with the risk of tearing in a trench defined by said strip in said block once moulded. • providing a flexible cladding cover, • providing a mould comprising a first and a second subassembly actuatable for closing so as to define a moulding cavity, said mould having the following features: ∘ said first subassembly comprises a first and a second element partly defining the wall of said cavity, ∘ said elements can be separated from each other from a close-together configuration in which they are separated from each other by a slot, allowing a gripping of said cover, to a separated configuration, in which they are separated from each other by said broadened slot, enabling said cover to pass, ∘ said second element is provided with a strip extending along said slot and projecting into said cavity so as to partially compartmentalise it, ∘ said first subassembly comprises a first and a second element partly defining the wall of said cavity, ∘ said elements can be separated from each other from a close-together configuration in which they are separated from each other by a slot, allowing a gripping of said cover, to a separated configuration, in which they are separated from each other by said broadened slot, enabling said cover to pass, ∘ said second element is provided with a strip extending along said slot and projecting into said cavity so as to partially compartmentalise it, • said elements being in separated configuration, passing said cover through said broadened slot, so that said cover has a middle zone, disposed in said cavity facing said first element, and a lateral zone projecting from said cavity, • actuating said elements to their close-together configuration so as to grip said cover between said zones, • injecting a first elastically compressible polyurethane-foam precursor mixture onto said middle zone and a second elastically compressible polyurethane-foam precursor mixture onto the cavity-wall part defined by said second element, said first mixture leading to a foam of less compressive strength than said second mixture, so as to form a stuffing block by expansion of said foams in said cavity, • moving said parts into a separated configuration and removing said cushion obtained from the mould, said cushion having said middle zone moulded onto by said block and said lateral zone not moulded onto by said block,