Issue link: https://www.e-digitaleditions.com/i/316458
32 May 2014 Tablets & Capsules per using these strength values and bulk density measure- ments. We also divided the arching dimension calculated in this manner by the span of the hopper (Figure 9). This gave a dimensionless number that describes the arching tendency based on how it relates to the hopper width. If this ratio becomes greater than 1.0, then the material will arch over the hopper span that corresponds to the point where the ratio grew past 1.0. It is clear from Figure 9 that the data from the higher- pressure strength measurements (traditional testing) pre- dicts arching in the feed hopper. In fact, that method pre- dicted a 139-mm arching span. However, the dimensionless arching ratio for the strength measure- ments—calculated using the new low-pressure strength measurement technique—never exceeded 1.0, suggesting that the material will flow from the hopper without hang- ing up. The highest arching ratio was at the 50.8-mm hopper outlet, which registered a dimensionless arching ratio of 0.564. That figure suggests that arching in the conical hopper may be near 28.6 mm when data from the low-pressure technique are used. This is more in line with reality (direct observation) of an arching dimension of 38 mm, ±9 mm. Conclusion The combination of acquiring strength measurements at low stress levels and calculating the stress values in actual hoppers helps give more accurate predictions of the arching dimension in process equipment. With better predictions, formulators and engineers can better adapt their materials and equipment. A similar analysis can be done for die filling, enabling engineers to determine if and when a force feed frame is required. Finally, this analysis suggests that Jenike's old flow factor technique of estimating hangups in small process equipment may be in error. Further study should be done, however, before we dismiss the method. T&C Kerry Johanson, PhD, is chief operating officer of Material Flow Solutions, 7010 NW 23rd Way, Suite A, Gainesville, FL 32653. Tel. 352 303 9123, fax 352 379 8878. Website: www.matflowsol.com. Figure 8 Expected strength in tablet press feed hopper Figure 9 Arching ratio as a function of position in the tablet press feed hopper 0 100 200 300 400 500 From high pressure tests From low pressure tests Unconfined yeild strength (Pa) 0 100 200 300 400 Height (mm) 0 100 200 300 400 500 From high pressure tests From low pressure tests Arching ratio (A1/D) 0 0.5 1 1.5 2 2.5 3 Height (mm) What's new? Our articles are written by technical professionals for technical professionals. Ask about the types of articles we're looking for, as well as tips on preparing an article for consideration. Contact: Matthew Knopp, Editor Tablets & Capsules Tel. +1 651 287 5618 Fax +1 651 287 5650 E-mail: mknopp@cscpub.com h-Johanart_28-32_Masters 5/14/14 10:17 AM Page 32