TABLETING AND LAB CONSUMABLES
predict accurately how much an increase in dwell time will decrease compression force. The effect of dwell time on compression force can best be quantified during the research and development stage, and understanding the relationship of dwell time to compression force can mitigate issues that commonly arise during scaleup to large production presses. Geometrically, the design of the
extended head flat differs only slightly from a TSM domed head or an EU head. Other than the increased head flat, one notable difference is a reduction in the head’s thickness to allow the larger flat to fit through the same cam profile as that of a standard TSM domed or EU head. Head wear and the propensity of
surface origin fatigue (i.e. head pitting) are affected by the head profile design. The smaller the outside head radius, the higher the contact stress and likelihood of accelerated wear. The difference in stress can be calculated analytically using Hertzian contact stress equations.
USING THE HERTZIAN STRESS EQUATION For example, using the cylinder-on- cylinder Hertzian stress equation to approximate a comparison between TSM Std, TSM Domed, and TSM Extended head profiles assuming a 10” (254mm) diameter pressure roll: Note that contact stresses are reduced with larger pressure rollers and are higher with smaller pressure rollers. For this reason, the old TSM standard head design with a small 5/16” radius between the outside head angle and the head flat was replaced with the domed head design. Punches with extended heads will be less resistant to head wear than domed punches, so care in selection is advised when high compression forces are needed. The limiting factor in the head flat’s
size is the neck diameter of the punch. The neck transfers the force from the head to the barrel and tip and then to the granulation. As you can see in Figure 4, if the head flat gets larger than the neck’s diameter, it won’t have the support required to transfer the force generated when it contacts the pressure roll, which can cause the head or neck to fail. For mini-tablet production, the size of the head flat affects the side loads
Fette 2090 with Pitch-Circle Diameter of 410mm
being applied to the punch tips during compression and the likelihood of punch tips bending or buckling. Larger head flats increase side loads.
Therefore, a reduced head flat (i.e. 6.35mm) may be advised.
OVAL AND ELLIPTICALLY SHAPED HEAD FLATS Extended head flats can also be oval or elliptically shaped. Although this design extends the dwell time exactly as a round extended head flat does, it has some drawbacks. The primary drawback is that the oval head flat can pass under the pressure rolls only in one direction along the major axis of the oval or ellipse to extend dwell
time. For that reason, punches with an oval head flat must be keyed on the upper and lower punches, even when round, to prevent them from rotating as they pass under or over the pressure rolls. With upper and lower punches requiring keys for oval or elliptical head flats, punch heads can experience accelerated wear because of repeated contact between the punch head and the compression roller at the same spot and no punch rotation.
Additionally, if a turret doesn’t have
lower key slots, then round tooling with an oval head cannot be used because the benefit of the extended head flat will be lost if the lower punch rotates. Another limitation is that if two presses have turrets with key slots of different angles, the oval head flat’s punches cannot be interchanged because the different angle will alter the orientation of the head flat with respect to the pressure rolls. Punch head profiles and the resulting
dwell time play an important role in the compaction characteristics of many drug products. Using extended head flat punches can provide a quick and reliable way to increase dwell times and, in some cases, reduce compression force without the need to modify the tablet press or cam tracks. Round, extended head flat punches don’t require keyed tooling for round shapes, as oval ones do, and that allows them on many makes and models of tablet presses interchangeably. n
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Comparison between the TSM Domed and Extended head profiles
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