Contoured stamping die

A contoured metal die suitable for stamping heat exchange channel element wall members from thin sheet metal, of block-like form having vertically extending side surfaces and vertically extending front and back surfaces, with each of said side surfaces being parallel to the other side surface and perpendicular to said front and back surfaces, whereby said die has a rectangular cross-section in plan view over its entire vertical extent, with a top surface comprising ridge projections extending elongately across said top surface parallel to said side surfaces of said die with flat top surface portions at the extremities of said ridge projections, said flat top surface portions having an aspect ratio (d.sub.2 /d.sub.1) of at least 10, wherein d.sub.2 is the longitudinal dimension of the ridge projection flat top surface portion as measured in a horizontal plane containing said flat top surface portions and d.sub.1 is the transverse dimension of the ridge projection flat top surface portion as measured in said horizontal plane containing said flat top surface portions along a direction perpendicular to said side surfaces of said die, with said flat top surface portions being surrounded by top surface portions having a concave elevational contour of constant radius of curvature R.sub.1 as viewed in a vertical plane perpendicular to said side surfaces of said die, such that the dimensional ratio of said radius of curvature R.sub.1 to the flat top surface portion transverse dimension d is between 0.375 and 30,000, and with said ridge projections having smoothly contoured edge surfaces between said top surface of said die and the front and back surfaces thereof, including edge surface portions extending between and joining said front and back surfaces with said flat top surface portions, said edge surface portions each having an inwardly directed elevational contour from a lower extremity adjacent said front or back surface to an upper extremity adjacent said flat top surface portion, as viewed in a vertical plane parallel to said side surfaces of said die and passing through the flat top surface portion.