Cooled mold system for making a preform

A mold system comprising:

• a hollow outer mold part extending along a mold axis;
• a hollow and thin-walled inner mold part inside the outer mold part, substantially less rigid than the outer mold part, and forming therewith an elongated cup-shaped cavity adapted to hold a plastic mass for the production of a preform;
• a cooling tube extending coaxially inside the hollow inner part and forming a passage having an inner portion inside the tube and an annular-section outer portion outside the tube and between an outer surface of the tube and an inner surface of the inner mold part;
• means for circulating a coolant fluid through the passage;
• four angularly substantially equispaced and generally axially extending support ribs extending radially from one of the surfaces and bearing radially on the other surface in the outer passage portion;
• and
• a plurality of generally axially extending cooling ribs projecting inward from the inner-part inner surface, between each of the support ribs and an adjacent one of the support ribs, and each having an inner edge spaced from the tube outer surface.