Tablet press set-up can be more than just following prior practice; by understanding fit and functionality for setting lower punch ejection height, you can choose how far to chase perfection.
Lower punch position at tablet take-off is a critical setting as it affects how well tablets are ejected from a tablet press.
Position the lower punch too low and tablets may bind in the die during ejection and be damaged, and positioning the lower punch too high may cause the machine to crash fom lower punch tips contacting the feeder or take-off bar.
Two complicating factors are die table runout and variation in lower punch overall length.
The adjustment of lower punch ejection height varies by press make and model, with some using shims under the ejection cam and others using an adjusting screw or nut.
Most OEMs recommend setting the punch tip to be perfectly level with the die table at the point of ejection, though others recommend the lower punch protrude up to 0.3mm (0.012”) above the die table.
Die tables, when they rotate, do not stay perfectly level. Rather, one side will slightly rise and fall versus the other.
It's helpful to identify and mark the low spot and the high spot and to measure the runout. If the runout exceeds OEM specification, then the turret should be refurbished (new bearings or bushings).
The shortest and longest lower punch can easily be identified with new tooling as their lengths are recorded in their certification report. For used tooling, it is best to measure their lengths.
To ensure that lower punches are at least flush with the top of the die during ejection, it's ideal to use the shortest lower punch for setting ejection height with the die table at its highest point from runout during rotation.
To verify your lower punches will not contact the feeder or take-off bar and to help set feeder clearance, it is ideal to check again using the longest lower punch positioned with the die table at its lowest point from table runout. Always rotate your press by hand (or with jog if press requires) to verify free punch travel before start-up.
Most companies don’t go to this extreme in press setup to use both the shortest and longest lower punch to set ejection height. Many may just use any lower punch and not bother checking table runout. When making Minitablets in particular, it can be worth the extra effort. What do you choose?
Optimising your lower punch ejection height allows you to run at the highest possible press speed without tablet defects for greater productivity and minimise your feeder clearances which can increase your product yield.
Contact Natoli if you have any insights or questions at 636-926-8900 or https://natoli.com.
[Image credit: Natoli]
