Most tablet manufacturers focus their inspection procedures for both new and in-process tools on the punch.
Written procedure usually dictates the age-old method of using a digital indicator on an extension pole affixed to a granite stand. With this equipment, they measure the overall length minus the cup depth to determine the working length for that punch. However, this method is flawed because the tolerance of the overall length and cup depth are greater than that of the working length, so they cannot be used to calculate the working length with its much tighter tolerance.
A better method of nmeasurement prescribes using a datum (reference punch) from the set. One can then measure
the rest of the tools against that one punch to ensure all tools are within .002in from the shortest to the longest, across either the lowers or the uppers, using a 'pass/fail method'.
While this method is best for measuring working length, there are additional critical wear patterns that can affect tablet production. For example, punch tip wear is a common issue that can be identified through inspection of a couple of areas.
Causes of punch tip wear are: die pockets leading to misaligned dies; worn punch guides and keyways; and running new punches with worn dies.
When an old set of dies is used with a new set of punches, it’s nearly impossible to realise a proper clearance between the punch tips and the die wall as there will be a gap. It’s within this void that excess powder begins ‘flashing’ or slipping around the edges during compression. This travelling powder creates a tight space for the upper punch that generates bfriction and heat, and results in premature wear to the punch tip, upper punch binding and can lead to premature wear to the upper raising cam as well.
A common deformity
Tip wear, which can occur from worn die pockets (especially on tools with deep cups and little land), includes ‘J-hook’, which appears when the edge of the punch tip curls in to create a burr. This common deformitycan create tablet capping and lamination (where the upper cup of the tablet is removedpartially or completely from theremainder of the tablet.) J-hook.
Smart inspection procedures help reduce wear on both punch tips and the die wear to the upper punches can be easily identified through the ‘fingernail method’- this is done by picking at the inner edge of the cup and feeling for the curled edge that is indicative of J-hook.
A technician can repair the J-hook and return the tool to service by lightly polishing the surface of the punch cup and performing a quick blended polish on an un-sewn buff wheel. This repair cannot be achieved by using a drag finisher.
Die bore wear occurs during compression of product and ejection of the tablet from the die. This wear pattern is exacerbated when compressing abrasive products such as nutraceutical tablets. Die borewear cannot be completely eliminated but may be drasticallyreduced through implementation of a complementary steel type, die bore taper, or the use of a carbide or ceramic lining in the die. (Contact your tooling vendor for their recommendations on combating troublesome products). These options can also help reduce ejection forces of the tablet from the die, which in turn will reduce friction, premature cam wear, and most importantly tablet quality issues such as sticking to the lower cup, chipping at takeoff and laminating.
Inspection procedures
Inspection of die wear can be done visually for the obvious appearance of wear rings, a pattern within the die that allows prominent wear to be visible to the naked eye. Although this method is common, it does not indicate how much wear is present. Other inspection procedures include using a split ball bore gauge on an indicator or digital handheld gauge to physically measure deviations within a die bore.
The amount of allowable deviation within a die bore is determined by the characteristics of the powder, this can vary from product to product. For example: if it’s known that with a wear ring of .004 you are still manufacturing a quality tablet with light flashing and no negative reaction to the tablet press, this measurement could be acceptable. However, if it’s determined through the use of a split ball bore gauge that the tolerance is exceeded, then the best option is to replace the dies.
Often, if the wear occurs high enough in the die, it can be flipped over to compress tablets in the other half of the die. This would not work if the die has a one-way taper as it may not achieve the same finished product results and can cause production issues.
Monitoring punch and die wear can bring decreased cost and fewer tableting production issues.
For best results, implement inspection procedures into SOPs wherever possible.
For more information visit www.scientistlive.com/eurolab
Stephen Natoli is with Natoli Engineering
