Applying the process to tableting problems
Common problems can be identified and resolved by using the seven-step process.
Damage to the punch-tip edge.
Punch-tip edges are very delicate, and damage in the form of nicks and bruises can cause burrs and occasionally chipping
(see Figure 1). These defects can lead to more serious failures, such as punch-tip breakage. Experience tells us that approximately 80%
of damage to punches and dies is usually caused accidentally when handling the punches during the production and tool-care
processes. Damage can occur, for example, when loading or unloading the tools in the tablet press, during transportation of
the tooling, during storage, or while cleaning or polishing the tools. By adopting Steps 1 (Clean) and 2 (Assess) of the seven-step
process, this type of damage can be identified and, by repeating the assessment throughout the process, we may recognize the
main causes. If the damage is only slight, it may be possible to use Step 3 (Repair) to repair the damage, by removing the
burrs and repolishing the tip edges to a good usable condition, thus ensuring consistent tablet appearance.
Figure 1: Typical handling damage to a delicate punch-tip edge.
Those areas of a punch that are in contact with other parts of the tablet press and are subjected to high-speed, frictional
and compaction forces are typically prone to wear. An example is the head "dwell" flat, which is in contact with the precompression
and main-compression rollers (see Figure 2). Premature wear should be investigated. Wear may be attributable to various causes, but commonly it is due to punch tightness
and/or poor lubrication. Again this can be identified during Step 2 (Assess). Any light wear can be removed by remedial polishing
through Step 3 (Repair). Polishing should be followed by measurement of the critical working length during Step 4 (Measure)
thereby ensuring that the punches are within an acceptable tolerance range for good tablet weight and thickness control. Wear
to the head angle from the cam track can also be a result of tight punches or lack of lubrication, and this wear can also
be monitored and repaired as above.
Figure 2: Typical head wear.
Die-bore wear or "ringing" (see Figure 3) is another common defect that can lead to problems such as tablet "capping" and tablet-ejection issues, which result in
wasted product and reduced tablet output. Ringing is caused by abrasive wear and deformation from continuous forces acting
on the face of the die bore. This damage is accelerated when compressing hard and abrasive granules found in some formulations,
particularly vitamins and minerals. This wear can be identified using visual inspection during Step 2 (Assess), but precise
equipment is also available to measure and monitor this wear Step 4 (Measure). Physical die-condition monitoring can result
in early diagnosis before the defect has a negative effect on tablet output.
Figure 3: Typical die-bore ringing.
Sticking and picking.
When in a new condition, the surfaces of tablet-punch faces are usually polished to a high mirror finish. These surfaces,
however, can deteriorate over a period of time due to the continuous compaction of granules, which can lead to tableting defects
such as "picking" and "sticking" (see Figure 4) brought about by adhesion of the product to the now rough finish. This deterioration can be identified during assessment
(Step 2, Assess). The product can be cleaned off with an efficient cleaning system (Step 1, Clean), and the finish can be
improved/repaired by using a remedial polishing process as in Step 3 (Repair). With frequent light polishing using an automated
polishing system (Step 5, Polish), a smooth and efficient surface can be maintained.
Figure 4: Sticking (i.e., product adhering to the punch tip face).
The seven-step process is a logical sequence to follow when tooling has been used and removed from the tablet press. Each
of the steps can be conducted in isolation, if necessary, and go some way towards extending tooling life, while at the same
time allowing for the planned and scheduled replacement of tooling.
Trevor Higgins is a technical expert at I Holland, tel. 44 0 115 9726153, email@example.com