The invention relates to a mold base (1) for a mold that is used for the blow molding or stretch-blow molding of thermoplastic containers (2) having a petaloid-shaped base. The mold base (1) includes several cavities (11) separated by radiating ridges (13) with concave crests (14), the bases (15) of the cavities (11) being distributed along a circular contour. As seen radially on either side of the deepest point thereof, each cavity (11) is defined towards the exterior by a curved surface portion (18) having a circular arc radial cross-section and towards the interior by a broken-plane surface portion (19) with at least two slopes that intersect the curved surface portion (18) at a straight edge (20). The plane (T) which contains the aforementioned edge (2) and which is tangent to the curved surface portion (18) is perpendicular to the axis (12) of the mold base. Moreover, the portion of plane (21) adjacent to the curved surface portion (18) has a slope (a) of between 12° and 8° in relation to the tangent plane (T).