When a worn die is used with a new station of punches or a new die used with a worn set of punches, it’s nearly impossible to obtain the proper clearance between the punch tooling tip and die wall because of the large gap, in where the powder will flow into the void, and thus creates “flashing”. The wayward powder will also create heat and friction that causes premature worn of upper punch and in certain circumstances upper punches bind, which, in turn, will also make upper raising cam wore.
If the die pockets, punch guides, and key-ways are worn out and give rise to misalignment of punch and die, a J-hook can gradually develop on the punch tip, especially when the cup is deep and the land is little. A J-hook is a small curl or burr that forms at the edge of punch. Capping is often happened due to the J-hook which is easy to identify. Use your fingernails to pick at the inner edge of the punch cup to feel whether it has a curled edge. If so, the punch must be repaired immediately before you return it in service.
The effective way to correct is to lightly polish the surface of the punch cup and use a cotton buffing wheel to finish the job. Do not attempt to remove J-hook with a drag finisher, as it may over-polish the tool and cause dull corners, where, however, a sharp corner is needed by the punch to create tablets.
It is normal to see the die bore wear as a result of frequent friction during compression and ejection process. The situation becomes worse when the abrasive powders, such as those used in some dietary supplements, are tabletted. It is nearly impossible to eliminate the occurrence of die bore worn, but you can reduce the damage by using complementary steels or lining the die with carbide or ceramics. Such measurement will somehow reduce the force required to eject out a pill, and, therefore, reduce the friction and minimize premature cam wear. Most importantly, tablet quality is apparently improved because of little occurrence of tablet sticking, chipping, capping, and tablet lamination.
You could inspect the wearing conditions with a visual check at first, but you do need a tool to help determine the extent of wear. A split-ball gauge and a digital handheld gauge are regarded as useful tools here.
The allowable deviation from standard is dependant on the powders formulation as it varies from product to product. For an instance, a wear ring of 0.004 inch is deviated from the norm, but it may be permitted if it does not affect the quality of tablet press tooling. However, if a split-ball gauge indicates a large deviation－exceeding the tolerance level, you’d better replace a new one. If the wear develop on high area of the die, it’s possible to flip it over and compress tablets with the fine half of die. If the die has a single taper, flipping will not work and would only lead to poor tablet quality.
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