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INSTRUMENTATION OF TABLET MACHINES
Contents
 Introduction
 Isolated punch and die assembly
 Compaction simulators
 Single station press
 Strain gauged punches
 Poisson arrangement
 Calibration of instrumentation
 Multi station press
 Signal processing
 Role of instrumentation in production
Introduction
 Provide
information on the inherent compaction
characteristics of components, effect of additives upon them.
 Tabletting process involves the utilization of unsatisfied bonds
at the solid surface.
 The process can be enhanced by the generation of large areas
of clean surface, if pressed together as might occur if
appreciable brittle fracture and plastic deformation were
introduced into the system.
 Behavior on decompression can affect the characteristics of
finished tablets because the structure must be strong enough to
accommodate the recovery and ejection induced stresses.
 Ability to monitor ejection forces leads to valuable information
on lubricant efficiency.
 Measurement of punch and die forces , relative displacement of
the punches can provide raw data which when suitably processed
and interpreted, facilitate evaluation of many of these tabletting
parameters.
 The sensitivity of the formulation being tested to loading rate
should be determined by compression at different speeds and by
monitoring of any changes in tablet properties.
 Because of this rate factor several workers have elected to
instrument isolated punch and die sets and to carry out
compression experiments using these sets in conjunction with a
compression/tension testing machine.
Isolated punch and die assembly
Compaction simulators
 Recently a sophisticated system has been described, capable
of mimicking the precise compression cycle means it has
single station tooling and follows rotary press action.
 The system contains micro processer representing the precise
compression cycle of the press, which in turn controls the
movements of isolated punches, reproduces the exact loading
profile of the press.
 This assembly facilitate
compressional studies at various
compressional rates,results in maximum amount of information
while using the minimum amount of material and time.
 They are now often referred to as
“compaction simulators”.
Single station presses
 Strain gauge networks as
transducers were used for
measuring the magnitude of the forces operating during the
compression cycle.
 Resistive gauges are usually preferred.
 They should be bonded as near to the active site like punch
surfaces so as to eliminate the lack of correlation between
signals obtained from the remote regions of the machine and
actual forces present in tooling.
 The bonding must be over the entire area of the correctly
aligned gauge, at a site where the elastic change in linear
dimension of the stress bearing member can be measured.
 Die wall instrumentation requires machining of die wall to
accommodate the gauges and reduce the thickness to a point
at which the adequate sensitivity is achieved, original
geometry is restored with silicone rubber or similar material.
 The
foregoing procedure necessitate annealing and
subsequent rehardening and tempering of the die, and this
treatment should not change the precise geometry of bore.
Strain gauged punches
 These assemblies may be connected so as
to conform a
wheat stone bridge network which is normally energized by
an AC amplifier system the attenuated out put giving a DC
voltage proportional to the force being applied.
 The changes in the resistance are small so a full bridge with
strain gauges in all four arms is preferable.
 A stabilized supply is essential since the system is a input
voltage dependent, and this arrangement is mounted as
Poisson configuration compensation is provided for both
temperature and bending of piece.
Poisson arrangement
 Transducers based on piezo electric effect in certain crystals
like quartz may be used.
 When subjected to external forces these materials develop an
elctrical charge proportional to the electric force.
 Transducer is connected to a charge amplifier by a high
impedance cable, converts the electrical charge into a
directly proportional DC voltage.
 Disadvantage of such transducers is that the charge is
inevitably dissipates with time, and since the difference is
being measured they are unsuitable for measuring static
forces.
 Small piezo electric load washers can be easily mounted on
upper and lower punch holders of single station presses.
 Radial die wall measurement has been achieved by means of a
special holder for the transducer.
Piezo electric die wall
instrumentation
 They may give rise to misleading data because strain gauge
equivalents only monitor radial forces over a localized region
of the die wall.
 They have to be sited at same level on the die wall as the
region at which the tablet is being compressed.
 The advantage of piezo electric devices include high
sensitivity, robust construction, no bonding to machine fabric
which allows easy changing of tooling.
Calibration of
instrumentation
 Calibration of instrumentation with punch assemblies was
achieved by use of a calibrated load cell which was placed on
die table, its signal can be compared with signal from
instrumented punches as they are simultaneously loaded.
 Alternative method was, entire punch assemblies can be
removed from the press and mounted in an accurate
compression/ tension test press.
Calibration using load cell
 The normal procedure with die wall instrumentation is less
straightforward.
 But the necessary measurements are made by sealing the
open ends of the die cavity then pumping in oil at known
pressures and noting the response.
 A close fitting rubber plug or rubber powder in the cavity
how ever may be regarded as a perfectly elastic material and
therefore can be substituted for oil.
 The preferred form of transducers for measuring punch
displacements is based on the differential inductor principle and is
commonly a linear variable differential transformer(LVDT).
 The movable ferrous core of the transducer is rigidly connected
to the punch by mechanical link so that movement unbalances the
secondary circuit .
 The out put is attenuated to produce a DC voltage directly
proportional to punch displacement.
Typical LVDT circuit
Multi station presses
 Major differences in the instrumentation of multi station as
opposed to single station presses is the inherent difficulty in
retrieving electrical signals from a revolving turret.
 This was over came by employing radio telemetry to transmit the
force signal from upper and lower punches to external recorders.
 This was unsuitable for normal production conditions, since only
few stations are operative, and only low machine speeds are
possible.
 Ho and co workers how ever overcome this and a significant
proportion of the stations remain active as in telemetry
system.
 Other workers have been less reluctant to utilize remote
stationary sites on the machine frame in particular the upper
and lower compression carrier systems.
 The response of a strain gauge is entirely a function of the
change in linear dimension of the machine part to which it is
bonded.
Telemetry system
 There fore such changes must be sufficient to induce an
adequate change in gauge resistance while not exceeding
elastic limit.
 Cast iron components are unsuitable because of variability in
their modulus of elasticity and Poisson ratio, so parts only
constructed with steel should be selected.
 More popular arrangements on modern high speed presses is
to attach strain gauges or to incorporate piezo electric load
cell into one of the tie rods.
 Instrumenting the normal ejection cam on a rotary tablet machine
by attaching strain gauges to its bolts is of limited value, because
the resulting signal is a summation of the effects of the several
lower punches on it at any instant.
 The solution adapted by Wray employed a two part cam so that
the region responsible for tablet ejection is separate and in the
form of a beam fixed at one end.
 But this method necessitated minor modifications to other parts
of the machine and there is no affect on the normal operation.
 Flexure of beam caused by lower punches during ejection was
monitored by strain gauges and was found to mimic the ejection
response of an instrumented lower punch.
 Alternatively, the normal cam can be cut into three sections each
clamped to the frame by a bolt, two of which are fitted with a
piezo electric load washer for instance.
 The division should be such that there is only one punch on each
section at a time.
 The first transducer then monitors the force to initiate
ejection and the second monitors the force necessary to push
the tablet clear of the die.
 This arrangement minimizes the fulcrum effect as the
punches move over the cam surface toward and then away
from the actual transducer location.
 Certain aspects like sticking of the lower punches due to
frictional effects can also be detected by sensitive
instrumentation of this type.
 Mitrevej and Augsburger have recently described a system to
measure the adhesion of tablets to the lower punch face by
attaching strain gauges to a small cantilever blade mounted
on the feed frame in front of the sweep off attachment.
 They found that the force of adhesion did not necessarily
reflect the ejection force or the lubricant activity of the
formulation but system did appear to be sensitive to batch
variations in the anti adherent quality of magnesium stearate.
Instrumentation to measure sweep
off force
 Regardless of selection of remote site for instrumentation the
response should always be checked periodically against
signals obtained from directly instrumented tooling over the
whole working range of the machine to ensure constancy in
response
relation ships.
Signal processing
 Instrumentation described here are usually DC voltages and can
therefore be retrieved, stored, processed.
 Displaying the signals on a cathode ray oscillograph(CRO) enables
the instant visualization of instrumentation out put.
 Displays were often photographed to provide permanent record
but UV oscillographs provide better definition of traces and can
facilitate large number of simultaneous recording channels.
Signal processing
 Inexpensive microcomputers
that are currently available can
remove much of the tedium in reducing raw data from the
recorders previously described there fore they are the method of
choice.
 The analog signals(DC voltages) can be fed by
an A to D (analog to digital) convertor to memory locations in
the computer.
 This digital data can be recalled, manipulated and outputted in a
wide range of graphic or tabular formats. So that active
compounds and excipients can be fingerprinted for their
compactional characteristics.
I.T.M micro processor linkage
Role of instrumentation in
production
 Tablet production department seeks innovation because of
the trend toward direct compression methods, availability of
machines with increased out put and to reduce tablet to
tablet variations.
 Design of instrumentation that exercises some degree of
control over the tabletting process is an attractive goal
because it reduces labor involvement.
 Attempt to limit weight and hardness deviations based upon
the premise that compressional force is directly proportional
to tablet weight provides
1. The formulation is homogeneous(i.e., has uniform
density).
2. The compressional force/ tablet weight function is
constant.
3. The volume of die cavities at the point of maximum
compression is constant.
 The third supposition is valid only when over all length of punches
and tip geometry are constant , the die bores are uniform and the
pressure rolls are perfectly cylindrical and mounted centrally.
 The out put from the electronic unit of most force measuring
systems is a DC voltage which produces series pulses of short
duration each proportional to the weight of an individual tablet.
 Signals can be conditioned to provide a wide range of monitoring
and control facilities of increased complexity.
 The individual compression pulses can also be used to drive
counting mechanisms and to provide a reliable figure for the
number of tablets made.
 These devices are uncomplicated, inexpensive, and there is a
desire to extend the signal conditioning system to control the
process to some extent.
ex: one can set upper and lower limits for acceptable tablet
weight and then distinguish pulses from tablets lying out side
these thresholds.
 When the frequency of these out of specification tablet exceeds
some present value a relay can be tripped to activate an alarm and
the machine can automatically stopped.
 Machines can be fitted with mechanical accept/reject gates at
machine out let so that individual out of specification tablets can
be diverted to separate container.
 This function requires a high level of sophistication because the
defective tablets must be memorized until they reach the outlet.
 A second approach is to take the amplified out put signals and feed
them into an averaging network.
 This average DC voltage is compared with reference voltage and
any difference is converted into an AC signal which was amplified
and used to drive a two phase servomotor.
 The motor can be connected to the weight or pressure adjustment
control of the press so that any change in the average
compressional force is reflected in an adjustment of either the
weight or force control.
 Regardless of which transducer system, sites, forces selected
it is essential to ascertain that the response of
instrumentation is a direct function of the property needing
to be monitored.
 The work of Wray and his colleagues is important in that it
establishes that stresses generated in certain parts of machine
frame are directly proportional to the punch forces which in
turn are related to compressional weights.
 Machine out puts are now exceeding 12,000 tablets per
minute which means the frequency of force pulses is
approximately o.1kHz.
 Since the detection of small differences in individual pulses
may be necessary all units should have flat responses well
beyond this level up to approximately 1.0kHz.
 Instrumented tablet machine technology is advancing rapidly.
 Useful for better under standing of the tabletting process, in
turn will assist in formulation development and batch quality
control .
References
 The theory and practice of industrial pharmacy by Leon
lachman, Hebert A.leiberman
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