Instrumentation of an Automatic Capsule-Filling Machine

Abstract

Techniques similar to those used in instrumenting tablet presses were applied to an automatic capsule-filling machine. The dosing unit was modified to allow the bonding of strain gauges to the compression piston. The gauges formed the arms of a Wheatstone bridge circuit. Thus, compression and ejection events were monitored by measuring the bridge unbalance voltage using a suitable amplifier-recording system. The instrumented piston was calibrated in a physical testing machine. Dosing unit rotation required the interposing of a mercury contact swivel between the amplifier and instrumented piston. The instrumentation system required only one minor, permanent modification to the machine, a cut in the dosator tube. Since the same dosator piston sensed both compression and ejection, only one of the two dosing units was instrumented and the other was removed from the machine. A solenoid switching system was devised which only permitted the feeding of empty gelatin shells into the filling cycle for the instrumented piston. Representative fillers were run at constant powder bed and piston heights. Oscilloscope tracings showed two stages in slug formation: (a) “precompression,” representing the force sensed during dipping of the dosator into the powder before actual compression, and (b) actual piston compression. Generally, the maximum slug compression force fell to zero rapidly on retraction of the piston, but in some cases a retention force was noted, possibly due to elastic rebound of the slug against the retracted piston. A negative deflection due to binding of the piston also was observed in tracings of one material. Lubricated batches exhibited ejection forces of less than 1 kg. ·