this improvement in injection blow molding machines provide simple and effective apparatus for adjusting the location of the clamping apparatus that holds a mold closed with the holding force applied at or close to alignment with the center of pressure of the air that is introduced into the mold cavity . the invention is primarily for use with the blowing mold where the center of pressure varies widely from one mold to another , depending upon the size and shape of the cavities in which the parison on a core rod is expanded to fill the mold cavity .