Measuring feeder accuracy
PharmTech: How is the accuracy of a gravimetric feeder determined?
Nowak: Weigh-feeder accuracy, regardless of the feeder type or design, is measured by weighing a series of timed catch-samples of
material discharged from the feeder. The term 'weigh-feeder accuracy' refers to the combined effect of two distinct, but related,
performance factors: linearity and repeatability.
Linearity, as the word implies, is a measure of the feeder's ability to deliver, on the average, the desired flow rate throughout
the feeder's full range of operation (see Figure 2). A linearity measurement, therefore, reveals the difference between the actual and desired average sample weight at various
flow settings. Repeatability, on the other hand, is a measure of the degree to which the feeder discharges a constant flow
of material over a specified time period at a given flow-rate setting. Usually made at the intended nominal operating flow
rate, a repeatability measurement indicates the level of scatter or dispersion (around the average sample weight) of the group
of weighed catch samples.
Figure 2: Linearity measures the feeder’s ability to deliver a desired flow rate throughout the feeder’s range of operation.
A feeder's linearity measurement quantifies how well or poorly it delivers the desired average rate at each of various points
throughout the feeder's complete operating range. Perfect linearity is represented by a straight-line relationship between
the setpoint and the actual average feed rate throughout the feeder's specified turndown range from its design, full-scale
In addition, feeders should be designed to maintain accuracy during the refill phase, in which material is replenished in
the feed hopper and the feeder is momentarily not being controlled by loss in weight. In the past, feeders were operated by
a constant metering speed during refill, but because the bulk density of the material can change during refill, this often
led to overfeeding. A more accurate method (Smart Refill Technology, K-Tron) stores trending data of the weight-to-speed ratio
obtained while the hopper is emptying and uses this data to gradually change metering speed during refill. The speed correction
allows the mass flow to remain constant during refill. Material characteristics (e.g., bulk density, particle size and shape,
angle of repose, and gas permeability) and the refill hopper size also affect the refill process and its accuracy.
Using gravimetric feeders in continuous manufacturing
PharmTech: Continuous manufacturing is predicted to grow in use for solid-dosage manufacturing. What technology is needed to feed continuously?
Nowak: Gravimetric feeders are typically the technology of choice for continuous pharmaceutical processing, such as hot-melt extrusion
or continuous direct compression, because the loss-in-weight controller is a real-time device that provides the accuracy needed
for continuous process control. In a continuous process, the feeder sets the precise throughput for the downstream equipment,
and feeding performance largely affects the performance of subsequent unit operations.
Due to the shorter residence times in continuous pharmaceutical processes, automatic sampling of feeder performance is often
performed at smaller time intervals, from 15 seconds down to 5-second and even 1-second sampling. For this reason, it is imperative
that the control system of the feeder chosen for continuous operations has fast response times. Although use of gravimetric
feeders for continuous processing in the pharmaceutical industry is fairly new, these feeders have been an integral part of
continuous processing in the food and plastics industries for decades.
Gravimetric feeders in tablet-press lubrication
PharmTech: How are gravimetric feeders used in tablet-press lubrication?
Nowak: Recently, gravimetric feeders have been used for direct, external lubrication of tablet presses, in which magnesium-stearate
lubricant is blown into the press, which can reduce stearate use by as much as 97%. This significant reduction in the amount
of lubricant added in the blending stage can drastically improve the blend properties, making it more free flowing as well
as reducing the possibility of the formulation sticking to the tablet-press tooling. This concept was illustrated in a poster
presented at an AAPS (American Association of Pharmaceutical Scientists) meeting (1). The use of a loss-in-weight feeder to
control the rate of lubricant to the press allows processors to precisely measure how much is going in; by measuring how much
stearate remains after processing, the amount of stearate in the formulation can be quantified (1). Typical feed rates of
magnesium stearate for this application are 0.2–2 kg/h, and microfeeders allow rates as low as 50 g/h.
1. J. Nelson, S. Bell, M. Roy, J. Chu, and K. Waterman "Consistency of Magnesium Stearate Content Using External Lubrication
in Tablet Compression," poster presentation at AAPS Annual Meeting & Exposition (Atlanta, Georgia, 2008).