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Tablet Tools – How To Measure Your Tooling

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1 August 2016 No comments

Tablet manufacturers often look at the punches more than other parts of the tooling equipment especially when inspecting new or in-process tools. They would generally measure the overall punch length, cup depth by a digital indicator that is mounted on a granite stand, and subtract the numbers to obtain the punch working length.

Although this is common practice, it can lead to errors as the tolerance for the overall punch length and cup depth is at +/- 0.003 inch, which is loose compared to the tolerance of the punch working length that is at +/- 0.002 inch. That said, when one is calculating using the units mentioned, some tools might be out of spec when in truth, it is in perfect calibration.

The correct way to know if there is a deviation is to measure the working lengths against the set of punches. To start measuring, choose the reference punch or datum from where you are measuring. Then compare with the dimension of the other punches to see if there is a deviation within the 0.002 from the shortest to the longest. If there is a deviation, replace the punches that fall out of the specified range.

Punch Tip Wear

After checking, it is important to also check other parts of the tooling equipment such as the punch tips for premature wear on it. Wearing of the tip can cause defects on the tablet.

Common causes of punch wear:

  • Using new punches with worn dies
  • Worn die pockets that can cause dies to misalign
  • Worn punch guides and keyways

When new punches are used together with old dies, this can cause problems, as the gap between the two is huge allowing powders to slip through the void and causing flashing during the compression process. The loose powder will cause heat and friction. The friction can then wear the upper punch prematurely and in certain instances lead to the upper punches to bind resulting to the wearing of the upper raising cam.

On the other hand, if the die pocket, keyway and punch guides are worn out, it can also cause the die and punch to be misaligned causing a j-hook. J-hook is a small burr or curl that can form in the punch’s edge and can cause tablet capping. Capping happens when the top portion of the tablet separates completely or partially from the body. It is easy to identify aJ-hook problem; one can simply use their fingernails to pick at the inner edge of the punch cup to feel the curled edges. If J-hook is detected, the punch must be repaired first before using the tooling equipment.

The best way to repair the punch is to polish the surface of the punch lightly and use a cotton buffing wheel and white rouge to finish it. One should not try repairing the problem with a drag finisher, as it would over-polish the tool resulting to dull corners, where a sharp corner is needed by the punch to create tablets.

Assessing the Dies

It is normal for die bore to wear especially when compressing powders and ejecting tablets especially if the powder is highly abrasive which can increase the wear. Even though one cannot eliminate the wearing of the die bore, one can reduce this by using complementary steels or lining the die with carbide or ceramics. One can also consult with their tooling vendor on ways to reduce premature wearing. Lining or choosing the right steel type can also reduce the amount of force that is needed to eject tablets resulting to reduced friction force and

premature wearing. Aside from these, one can also improve the tablet quality; prevent sticking and chipping of the tablet and lamination issues.

Inspection

Inspecting for premature wearing starts with a visual check of the tooling equipment. Using a split ball gore connected to an indicator as well as a digital handheld gauge can help you spot small or miniature wear.

The allowed deviation from the standard depends on the powder’s formulation as it varies from one product to another. Example would be a wear ring of 0.004 inch. Although it deviates from the norm, but if it does not affect the quality of the tablet, one can deem the wear okay.

However, if the gauge indicates that the wear exceeds the tolerance level then it is much better to replace the die. If the wear happens on the higher level or area of the die, one can flip it over and use the other half to compress the tablet. That said, if the die has a single taper, flipping will not work and would only lead to poor tablet quality.

It is important to regularly maintain and check the punch and die for any signs of premature wearing. By checking frequently, one would be able to save money in less frequent replacement of the tooling equipment. Maintain a SOP so that other inspectors would be able to use the same guidelines in checking the tool.