A lower mould for a heat-sealing machine for food containers, configured to receive one or more food containers moved by a conveying unit of a heat-sealing machine along a feeding direction towards a sealing station, said lower mould comprising: - a frame having connection portions configured to reversibly engage reciprocal support portions of said machine during a positioning of said lower mould in a respective housing seat, and a flat upper surface arranged horizontally and in alignment with said feeding direction when the lower mould is housed in the respective housing seat, said upper surface having one or more openings configured to house one or more of said containers; - one or more plates parallel to the upper surface; - one or more vertical pistons supporting said plates, slidingly connected to the frame and vertically movable with respect to said frame between a raised position, wherein said plates engage with said openings to arrange themselves flush with said upper surface and a lowered position, wherein said plates are arranged below said upper surface so that said openings define processing seats for said containers, each piston having a lower end configured to abut against a respective abutment element of said machine when said lower mould is housed in the housing seat, a reciprocal movement between said support portions and said abutment elements determining a vertical movement of said pistons between the raised position and the lowered position; characterised in that each piston has a respective engagement portion, and said lower mould comprises a locking element, positioned below said plates, slidingly connected to said frame between an engagement position, wherein said locking element interferes with the engagement portions of the pistons to prevent a downward movement of said pistons, and a disengagement position, wherein said locking element allows a vertical movement of said pistons between the raised position and the lowered position. said lower mould comprising: - a frame having connection portions configured to reversibly engage reciprocal support portions of said machine during a positioning of said lower mould in a respective housing seat, and a flat upper surface arranged horizontally and in alignment with said feeding direction when the lower mould is housed in the respective housing seat, said upper surface having one or more openings configured to house one or more of said containers; - one or more plates parallel to the upper surface; - one or more vertical pistons supporting said plates, slidingly connected to the frame and vertically movable with respect to said frame between a raised position, wherein said plates engage with said openings to arrange themselves flush with said upper surface and a lowered position, wherein said plates are arranged below said upper surface so that said openings define processing seats for said containers, - a frame having connection portions configured to reversibly engage reciprocal support portions of said machine during a positioning of said lower mould in a respective housing seat, and a flat upper surface arranged horizontally and in alignment with said feeding direction when the lower mould is housed in the respective housing seat, said upper surface having one or more openings configured to house one or more of said containers; - one or more plates parallel to the upper surface; - one or more vertical pistons supporting said plates, slidingly connected to the frame and vertically movable with respect to said frame between a raised position, wherein said plates engage with said openings to arrange themselves flush with said upper surface and a lowered position, wherein said plates are arranged below said upper surface so that said openings define processing seats for said containers, each piston having a lower end configured to abut against a respective abutment element of said machine when said lower mould is housed in the housing seat, a reciprocal movement between said support portions and said abutment elements determining a vertical movement of said pistons between the raised position and the lowered position; characterised in that each piston has a respective engagement portion, and said lower mould comprises a locking element, positioned below said plates, slidingly connected to said frame between an engagement position, wherein said locking element interferes with the engagement portions of the pistons to prevent a downward movement of said pistons, and a disengagement position, wherein said locking element allows a vertical movement of said pistons between the raised position and the lowered position.