The invention relates to a mould for injecting tubes having slim walls made of a plastic material. The mould, destined to make tubes having an outlet mouth for a product and a lateral wall delimiting the space destined to contain the product to be packed, comprises an upper punch and a lower punch which are inserted in an external body or matrix of the mould to define the space to be filled with the injected plastic material; the upper punch comprises a projection which occupies the internal part of the outlet mouth and extends below the lower end of the mouth; the lower punch comprises a cavity internally of which the projection is inserted; injection channels are afforded between the projection and the cavity, which channels flow into a single injection manifold are reach inlet points to a space of the mould destined to receive the injected material.