An investigation of some factors influencing plug formation and fill weight in a dosing disk-type automatic capsule-filling machine.

The progressive formation of plugs resulting from successive tamps in a dosing disk-type automatic capsule-filling machine was investigated for three fillers: anhydrous lactose, dicalcium phosphate, and microcrystalline cellulose. An evaluation of seven compaction parameters revealed that all tamping stations and all piston positions within a station, with the exception of station #1, contribute equally to plug formation. Model calculations suggesting that fill weight could be achieved in only three tamps were supported by direct experimental observation. The effective available fill volume was shown to be greater than the nominal volume of the dosing disk cavity. This was verified by using a modified scrape-off at station #5, which eliminated the powder head over the disk at that station. Here, increases in tamping force resulted in harder plugs, as measured with an improved plug mechanical strength tester, with fill weights remaining unchanged. However, plug segments did not increase in mechanical strength when additional segments were compressed over them using the same force. For one excipient (microcrystalline cellulose), the fill weights and the mechanical strengths of systems containing 0, 0.10, 0.25, and 0.50% magnesium stearate were measured. The 0.10% level was found to be optimum in that it resulted in maximum plug mechanical strengths and fill weights, regardless of the number of tamps used and/or their intensity.