The present invention relates to a bag made of sheets of heat weldable plastics assembled by welding lines along their edges, and comprising a base also made of a sheet of heat weldable plastics allowing the bag to be placed in an upright position. The bag further comprises an armature extending substantially from its base to its opposite upper edge, which armature comprises a vertical rod of length approximately equal to the height of the bag and two crosspieces, an upper crosspiece along the upper edge of the bag, and a lower crosspiece along the base of the bag.