varies for example between 4 mm and 6 mm depending on the size on the machine the internal wall 18 is fixed on the upper face of the external wall 2 through two rectilinear shims 24 a positioned along the longitudinal edges of both walls and of both shims in a circular arc 24 b fig3 positioned along the curved edges of both walls shims 24 a and 24 b may notably be welded onto the upper face of external wall 22 after which the edges of internal wall 18 are themselves welded onto shims 24 a 24 b a closed chamber 26 sealed relatively to the outside world is thereby delimited between walls 18 and 22 this chamber 26 is at least partly filled with a heat carrying fluid this heat carrying fluid may comprise any fluid usually used in dryers ironers the cup of which is heated by a heat carrying fluid this fluid is for example and in a non limiting way a liquid such as oil in order to take into account thermal expansions of the heat carrying fluid contained in chamber 26 the latter may be connected with an expansion vessel not shown also integrated into the machine as illustrated schematically in fig2 heating means 16 are placed outside cup 10 below the latter and directly close to wall 22 therefore heating means 16 directly heat the thick external wall 22 of the cup heat is then transferred to the internal thin wall 18 by the heat carrying fluid contained in chamber 26 for the heat to uniformly diffuse over the internal face 20 of wall 18 during ironing the dryer.