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The Tablet Press
A lack of understanding of die support can lead to cracked dies
Dies reside within pockets set in the die table. The die pocket restricts the side or translation motion of the die, but not the axial or the vertical motion, nor twisting or rotatary motion. These unwanted motions are restricted by the dowels. There may be one or two dowels securing each die, depending upon the design of the press.
During compression of tablet blend into a tablet, the die will attempt to expand radially. Expansion is expected to be minor(and is invisible to the naked eye), because of the supporting pocket and will be largely uniform. Where the dies have been overtightened, the expansion can no longer be uniform. This expansion drives the die harder against the dowel(s). As a result the die may crack with the dowels defining the axis of the crack. High pressing forces will aggravate the effect. It is more prone to occur with dies for large oval tablets.
Dowels should be tightened to the press manufacturer's specification using a torque wrench. Pressing forces should be limited to that actually required to compress tablets to specification. Where excessive pressing forces are apparently required, the reason should be investigated. Remember that the shape of a hardness versus pressing force relationship is rarely linear. Hardness may cease to increase with increasing pressing force, or may even decrease.
Excessive pressing force coupled with overly tight dowels can be a fatal combination.