Tablet Press Instrumentation
Michael Levin
Metropolitan Computing Corporation, East Hanover, New Jersey, U.S.A.
INTRODUCTION
This article is designed to facilitate the understanding of
the general principles of tablet press instrumentation and
the benefits thereof by the formulators, process engineers,
validation specialists, and quality assurance personnel, as
well as production floor supervisors who would like to
understand the basic standards and techniques of getting
information about their tableting process.
HISTORY OF TABLET PRESSINSTRUMENTATION
In 1952–1954 Higuchi and his group[1] have instrumented
upper and lower compression, ejection, and punch
displacement on an eccentric tablet press, and pioneered
the modern study of compaction process. This work was
followed by Nelson[2,3] who was the member of the
original group.
In 1966, a U.S. patent was granted to Knoechel and co-
workers[4] for force measurement on a tablet press. This
patent was followed by two seminal articles in Journal
of Pharmaceutical Sciences on the practical applications
of instrumented rotary tablet machines.[5,6] A number of
other patents related to press instrumentation and control
followed from 1973 on ward.[7 – 15]
Despite the availability of published designs for
instrumenting rotary presses, much of the early work on
compaction properties of materials was done on instru-
mented single punch eccentric presses primarily, due to
the relative ease of sensor installation, as well as
availability of punch displacement measurement.[16 – 18]
In mid-1980s, custom-made press monitoring systems
were described,[19,20] and the first commercial instrumen-
tation packages became available, including both the
systems for product development and press control. The
first personal computer-based tablet press monitor was
sold in 1987, and the first Microsoft Windows-based press
instrumentation package in 1995.
A new era of compaction research has begun with the
introduction of an instrumented compaction simulator[21]
in 1976, while a compaction simulator patent was issued
as recently as 1996.[22]
A new generation of “compaction simulators” was born
when a mechanical press replicator was patented in the
year 2000.[23]
A good expose of the early stages of press
instrumentation and resulting research is presented in
review articles by Schwartz,[24] Marshall,[25] and Jones.[26]
A comprehensive review can be found in a relatively
recent paper by Celik and Ruegger.[27]
For other historical information on the press instru-
mentation, the reader is encouraged to peruse Ridgeway
Watt’s[28] volume. There have been a number of papers
published on various disparate instrumentation topics, and
in a recent volume by Munos-Ruiz and Vromans,[29] there
are two good articles on the subject—but, unfortunately,
they deal with marginal issues of single station press and
instrumented punch only.
DATA ACQUISITION PRINCIPLES:FROM TRANSDUCERS TO COMPUTERS
To monitor and control a tablet press, certain sensors must
be installed at specific locations on the machine. These
sensors are called transducers. In general, a transducer is
a device that converts energy from one form to another
(e.g., force to voltage). Tablet press transducers typically
measure applied force, turret speed, or punch position.
Because the signals coming from such transducers are
normally in millivolts, they need to be amplified and then
converted to digital form in order to be processed by a data
acquisition system.
Piezoelectric Gages
Historically, high impedance piezoelectric transducers
that employ quartz crystals were used in early stages of
press instrumentation. When subject to stress, the crystal
accumulates electrostatic charge that is directly propor-
tional to the applied force. Both low and (more modern)
high impedance piezoelectric gages have high-frequency
response, but may exhibit signal drifting due to charge
Encyclopedia of Pharmaceutical Technology
DOI: 10.1081/E-EPT 120012030
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1
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leakage (approximately 0.04% decay per second can be
seen in modern piezoelectric transducers). Nowadays,
the preferred way of instrumenting tablet presses is with
strain gages.
Strain Gages
Strain gages are foil, wire, or semiconductor devices that
convert pressure or force into electrical voltage. When a
stress is applied to a thin wire, it becomes longer and
thinner. Both factors contribute to increased electrical
resistance. If an electrical current is sent through this wire,
it will be affected by the changes in the resistance of the
conduit. This principle is used in strain gage-based
transducers. Foil gages, known for robust application
range, useful nominal resistance, and reliable sensitivity
control, are most commonly used for instrumentation of
compression, precompression, and ejection forces. Semi-
conductor-based strain gages are inherently more sensitive
but suffer from high electrical noise and temperature
sensitivity. Such gages are not commonly used in tablet
press instrumentation except for measurement of die-wall
pressure and take-off forces.
Wheatstone Bridge
Wheatstone bridge (Fig. 1) is a special arrangement of
strain gages that is used to ensure signal balancing. The
“full” bridge is composed of two pairs of resistors in a
circle, with two parallel branches used for input and two
for signal output. By applying the so-called excitation
voltage (typically, 10 V DC) to the bridge input and
changing the resistance of different “legs” of the bridge by
adding special resistors, we can make sure that there is a
zero output voltage when no load is applied to the
transducer. This is called zero balance. Once the bridge is
balanced, a small perturbation in the resistance of any
“leg” of the bridge results in a nonzero signal output.
The output of the Wheatstone bridge is normally
expressed in millivolts per volt of excitation per unit of
applied force. For example, sensitivity of 0.2 mV/V/kN
means that applying, say, 10 kN force and 10 V excitation
will produce 20 mV output. To utilize such output, it
usually needs to be amplified several hundred times to
reach units of volts.
Another important function of the bridge is balancing
of temperature effects. Although individual strain gages
are sensitive to temperature fluctuations, Wheatstone
bridge arrangement provides for temperature sensitivity
compensation, so that the resulting transducer is no
longer changing its output to any significant degree when
it heats up.
Because the output of these bridges is in the range of
millivolts, the cables utilized to carry the signal are
normally shielded with a braided or foil-lined sheath
around individual wires. The shield, as a rule, is connected
to the amplifier, but never touches the actual instrumented
equipment (i.e., tablet press). If this rule is violated, a
ground loop may generate electrical noise and present a
dangerous electrical shock hazard.
Gage Factor
The gage factor is a specific characteristic of a strain gage.
It is calculated as the change in resistance relative to unit
strain that has caused this change. Strain gages that are
commonly used for tablet press instrumentation are made
from copper–nickel alloy and have a gage factor of about
2. In a typical bending beam application (such as
compression roll transducer), one side of the beam
experiences tension while the other side undergoes
compression. By mounting two gages on each side, the
sensitivity of the transducer can be doubled.
Strain gages have to be bonded to areas of machine
parts that are most sensitive to applied force. Such areas
can be identified with the use of polarized light technique
that “points out” the stress distribution.
Manufacturing a Force Transducer
Usually, instrumentation designs are proprietary and
specific detail drawings are held in fiducial capacity.
Each force transducer is custom designed for a particular
machine part. Overall specifications are taken from the
actual party and/or manufacturer approved drawings.
Duplicate steel members, such as pins and cams, are
normally made from A2 tool steel in a fully annealedFig. 1 A typical Wheatstone bridge arrangement of strain
gages.
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(softened) state. Original machine parts are first annealed,
if required. The member is then machined, hardened to
Rockwell 60–64, tested, and finally, ground to specified
dimensions.
It is highly advantageous to determine stress dis-
tribution using polarized light beams identifying the areas
of maximum strain yet avoiding areas of uneven strain.
A typical procedure of transducer manufacturing in a
professional gaging lab might be as follows. Foil type
strain gages are bonded to the steel member utilizing a
high-performance 100% solid epoxy system adhesive
under controlled heating rate conditions. After intrabridge
wiring is completed using solid conductor wiring (to
prevent electrical noise), multistrand wire cabling with a
combination of foil and braided insulation is connected to
the bridge. Wire anchoring is achieved utilizing epoxy
adhesive and protected with a combination of latex-based
adhesives and/or epoxy resins. Lead wire cabling is
protected by Teflon outer shield, as well as inner braided
wire shield. The Teflon to steel joint is sealed with epoxy
but should not be subject to stresses that would cause the
cable to kink or yield in such a way as to expose the inner
braided wire shield. Protective coatings are then applied
and final postcure heating is performed for at least 2 hr at
a temperature approximately 508C above the transducer
normal operating temperature (approximately 175 or
105 8C). A silicon-based adhesive, such as RTV, is used to
fill large cavities while maintaining a low modulus of
elasticity, preventing undue influence upon the actual
strain measurement. The coating is resilient to moderate
mechanical abrasion, as well as most solvents, oils,
cleaners, etc. It is not intended for protection against
penetrating sharp objects.
A high-quality connector is then attached to the cable.
The next step is to perform offset zeroing with fixed, 1%
precision, low-temperature coefficient resistors, followed
by NIST (National Institute for Standards and Technol-
ogy) traceable calibration.
It is worth noting that, in general, the duplicate members
are not made of corrosion-resistant steel, because high
tensile strength and ability to be easily machined are
required. Prior to storage, the surface should be treated
like any high-grade tablet press punch steel will be treated;
a thin coating of oil should be used after wiping with
alcohol.
Load Cells
In some cases, where appropriate, a load cell can be used
in place of bonded strain gages.
A load cell is a strain gage bridge in an enclosure
forming a complete transducer device.
Like any properly made transducer, it produces an
output signal proportional to the applied load. Unlike
custom-made transducers that require application of strain
gages directly to press parts, load cells are self-contained
and can be placed on a press in specially machined cavities
to be easily replaced or serviced. As a drawback, load cells
generally are less sensitive or less suited to measure the
absolute force than custom-made strain gage transducers.
Load cells can be used on punch holders in a single station
press. Another example of load cell use is a die assembly
for calibration of existing traducers on a press.
Linear Variable Differential Transformer
Linear variable differential transformer (LVDT, Fig. 2) is
a device that produces voltage proportional to the position
of a core rod inside a cylinder body. It measures
displacement or a position of an object relative to some
predefined zero location. On tablet presses, LVDTs are
used to measure punch displacement and in-die thickness.
They generally have very high precision and accuracy, but
there are numerous practical concerns regarding improper
mounting or maintenance of such transducers on tablet
presses.
Proximity Switch
Proximity switch (Fig. 3) is a noncontact electromagnetic
pickup device that senses the presence of metal. On tablet
presses, it is widely used to detect the beginning of a turret
Fig. 2 Linear variable differential transformer (LVDT).
Tablet Press Instrumentation 3
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revolution, to identify stations and to facilitate peak
detection in tablet force control applications.
Instrumentation Amplifier and Signal
Conditioning
Signal conditioning involves primarily an amplifier that
provides the excitation voltage, as well as gain (a factor
used in converting millivolt output of the strain gage
bridge to the volt-based range of the input of the data
acquisition device).
Instrumentation amplifier is different from other
types of amplifiers in that the signal from each side of
the transducer bridge is amplified separately and then
the difference between the two amplified signals is, in
turn, amplified. As a result, noise from both sides of the
sensors is reduced.
Some instrumentation amplifiers also offer filter
functionality. A filter circuit combines resistors and
capacitors that act to block the undesirable frequencies.
It is a fact, however, that a good transducer should
provide a clean signal. Use of analog or digital filters
may cause the loss of some important portions of the
signal that one is attempting to measure. Frequency
filters may cause the measured compression force peak,
for example, to be skewed toward the trailing edge of
the peak and can yield a lower than actual peak force.
Because filters distort the signal, they must be
avoided unless absolutely necessary. In many cases,
better electronics may make filter use obsolete. For
example, the so-called antialiasing filter that is used to
condition a high-frequency signal for a slow sampling
rate is generally not required for tablet press
applications if modern fast speed data acquisition
devices are used for signal processing.
In addition to amplification, excitation, and filtering,
signal-conditioning devices may provide isolation, voltage
division, surge protection, and current-to-voltage
conversion.
Instrumentation Terms: Definitions
Several important terms may be now defined with respect
to transducers and signal conditioning:
. Full Scale (FS): The total interval over which a
transducer is intended to operate. Also, it can define
the output from transducer when the maximum load is
applied.
. Excitation: The voltage applied to the input terminals
of a strain gage bridge.
. Accuracy: The closeness with which a measurement
approaches the true value of a variable being
measured. It defines the error of reading. Good tablet
press transducers have at least 1% accuracy (with this
level of accuracy, for example, a compression force
transducer with 50 kN FS will produce at most an error
of 500 N).
. Precision: Reproducibility of a measurement, i.e., how
much successive readings of the same fixed value of a
variable differ from one another. If a person is shooting
darts, for example, the accuracy is determined by how
close to the bull’s eye the darts have landed, while the
precision will be indicated by how close the darts are
to each other.
. Resolution: The smallest change in measured value
that the instrument can detect.
. Dynamic range of a transducer: The difference
between the maximal FS level and the lowest
detectable signal. Measured in decibels (dB), it
indicates the ratio of signal maximum to minimum
levels:
dB ¼ 10 log10ðSmax=SminÞ
Some press sensors may have a rather narrow dynamic
range not necessarily correlated with accuracy. For
example, a very accurate compression roll pin designed
to measure 50 kN force may not detect 5 kN signal.
. Calibration: Comparison of transducer outputs at
standard test loads to output of a known standard at the
same load levels. A line representing the best fit to data
is called a calibration graph.
. Calibration factor: A load value in engineering units
that a transducer will indicate for each volt of output,
after amplifier gain and balance. Calibration factor is
usually expressed in relation to FS.
. Shunt calibration: A procedure of transducer testing
when a resistor with a known value is connected to one
leg of the bridge. The output should correspond to the
voltage specified in the calibration certificate. If it does
not, something is wrong and the transducer needs to be
inspected for possible damage or recalibrated.
Fig. 3 Proximity switch.
Tablet Press Instrumentation4
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. Sensitivity: The ratio of a change in measurement
value to a change in measured variable. For example, if
a person ate a 1 lb steak and the bathroom scale shows
2 lb increase in the body weight, then the scale can be
called as insensitive (ratio is far from unity).
. Traceability: The step-by-step transfer process by
which the transducer calibration can be related to
primary standards. During any calibration process,
transducer is compared to a known standard. National
or international institutions usually prescribe the
standards. In the United States, such governing body
is the NIST.
. Measurement errors: Any discrepancies between the
measured values and the reported results over the
entire FS. Such errors include, but are not limited to:
. Nonlinearity: The maximum deviation of the
calibration points from a regression line (best fit
to the data), expressed as a percentage of the rated
FS output and measured on increasing load only.
. Repeatability: The maximum difference between
transducer output readings for repeated applied
loads under identical loading and environmental
conditions. It indicates the ability of an instru-
ment to give identical results in successive
readings.
. Hysteresis: The maximum difference between
transducer output readings for the same applied
load. One reading is obtained by increasing the
load from zero and the other reading is obtained
by decreasing the load from the rated FS load.
Measurements should be taken as rapidly as
possible to minimize creep.
. Return to zero: The difference in two readings:
one, at no load, and the second one, after the FS
load was applied and removed.
A good transducer is one with the combined (or
maximum) error of less than 1% of the FS.
Analog-to-Digital Converters
In order to convey analog output (in volts) from a
transducer to a computer, it has to be converted into a
sequence of binary digital numbers. Modern analog-to-
digital converter (ADC) boards are sophisticated high-
speed electronic devices that are classified by the input
resolution, as well as the range of input voltages and
sampling rates.
Resolution of an ADC board is measured in bits.
Bit (abbreviation for binary unit) is a unit of information
equal to one binary decision (such as “yes or no,” “on or
off”). A 12-bit system provides a resolution of one part in
212 ¼ 4096; or approximately 0.025% of FS. Likewise, 16
bits correspond to one part in 216 ¼ 65; 536; or
approximately 0.0015% of FS (for tablet press appli-
cations, such resolution is usually excessive).
Thus, resolution of ADC board limits not only the
dynamic range but also the overall system accuracy.
Alternatively, a higher resolution may be required to retain
a certain level of accuracy within a given dynamic range.
For example, a 0.5% accurate transducer with 80 dB
dynamic range requires at least 12-bit ADC resolution.
Amplifying a low-level signal by 10 or 100 times
increases the effective resolution by more than 3 and 6 bits,
respectively. On the other hand, increasing an ADC
resolution cannot benefit the overall system accuracy if
other components, such as amplifier or transducer, have a
lower resolution.
When an input signal change is smaller than the
system’s minimum resolution, then that event will go
undetected. For example, for an FS of 10 V (corresponding
to, say, a compression transducer output of 50 kN), using a
12-bit ADC, any signal that does not exceed 2.44 mV
(12.2 N) will not be seen by the system.
The ADC boards also differ by the effective sampling
rate range. Sampling rate speed is measured in Hertz
(times per second). The signals coming from a tablet press
have a frequency of not more that 100 Hz (compression
events per second). To avoid aliasing (losing resolution of
the incoming signal due to slow sampling rate), the
sampling rate should be at least twice the highest
frequency of the signal. Most ADC boards used for data
collection in tableting applications have a sampling rate of
5–20 times larger than signal frequency. That is why
antialiasing filters are not required.
Computers and Data Acquisition Software
Overall accuracy of a data acquisition system is
determined by the worst-case error of all its components.
One should be aware of the fact that most system errors
come neither from transducers (0.5%–1% accuracy) nor
from A/D converters (0.025% accuracy) but from the
software analyzing the data (round-off errors, improper
sampling rate, or algorithms).
The speed and capacity of a data acquisition system
depend on the computer’s processor and hard drive
specifications. The real-time data from transducers is
streamlined to both the screen (for monitoring) and the
disk (for replay and analysis). Generally speaking, “real-
time” processing means reporting any change in the
phenomena under study as it happens. Interestingly, but
Tablet Press Instrumentation 5
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a high-speed data collection from a tablet press and a
bookkeeping home finance program used on a monthly
basis to balance the checkbook can both be related to as
“real-time” software. The difference is in the time frames.
For a tablet press, we are collecting data that need to be
sampled and processed in milliseconds (a typical
compression event may take 25 msec), while for home
bookkeeping once a month will do.
Most vendors supplying transducers, signal condition-
ing, and computer hardware adhere to practical standards,
e.g., there are some accepted norms for strain gage factors,
combined errors, sensitivity, ADC resolution, and
sampling rate. The difference between vendors is apparent
when we compare software because there are no
universally established standards of user interface. Yet
the hardware is “transparent” (i.e., invisible) to the end
user—day in and day out the user is facing the screen,
keyboard, and mouse. The ease of software use, bug-free
analysis of signals coming from transducers, reliability of
statistical computations, quality of graphs and reports, and
validatability of the system—all of the above contribute to
the quality of the data acquisition software.
Proper validation tests of a data acquisition system
should include calculation of an overall system error when
the input is known and controlled (e.g., an NIST traceable
signal generator providing a sinusoidal signal with a
known amplitude and frequency, to simulate compression
events on a press). Comparing the output (for example,
peak heights as reported by the software) to the known
input, the overall system error can be reliably established.
TABLET PRESS INSTRUMENTATION FORPRODUCT AND PROCESS DEVELOPMENT
R&D Grade Instrumentation
In a production environment, typical tablet weight control
mechanism is driven by signals that are coming from a
compression force transducer. Strain gages may be
installed on a column connecting the upper and lower
compression wheel assemblies. This transducer measures
what is generally known as “main compression,” i.e., a
diluted average of the forces acting on the upper and lower
punches. Alternatively, a load cell may be positioned in
some convenient location to register a transmitted
compression force. It is measured away from the force
application axis, and some force is lost in the transmission.
The resulting measurement may be adequate for tablet
weight control, but, even if properly calibrated, it does not
represent the absolute value of the compression force.
The R&D grade instrumentation, in contrast, requires
placing the strain gages as close to the punch tips as
possible in a vertical alignment with the direction of force
application.[30] In practice, it means placing the gages in a
compression roll pin, so that the resulting measurement
would reflect the absolute force. Thus, we can differentiate
between the force on the upper and lower punch, and
moreover, we can compare readings of the compression
force from different tablet presses.
Compression Force Measurement
On a typical R&D grade compression transducer, a
compression roll pin is machined with incisions made for
placing strain gages (Fig. 4). The actual form of these
cavities constitutes the very art of the transducer design
that is usually proprietary and is based on the know-how of
instrumentation vendor.
It hast to be noted that there could be an upper or lower
instrumented compression roll pin.
The resulting measurement of a compression force is
highly correlated with a variety of tablet properties. As
compression increases, so does tablet hardness and weight
(at constant thickness and true density), along with a force
required to eject a tablet. Many variables affect the force
of compression: press settings, press speed, punch length
variation, punch wear, and damage, formulation and
excipient properties.
Precompression Force Measurement
Similar to instrumented compression pin, there is a way
to instrument an upper or lower instrumented precom-
pression roll pin. Precompression, if it exists on a press,
is used for de-aerating and initial tamping of the powder
mass in the die and usually helps to achieve the desired
hardness without capping or lamination.[30,31] The force
Fig. 4 Compression force transducer.
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of precompression is normally a fraction of the
compression force.
Ejection Force Measurement
There are many ways to instrument an ejection cam.
A preferred arrangement of strain gages (the so-called
“shear force” design) does not require any discontinuities
in the cam surface, and, most importantly, it provides for a
very good linearity of the resulting signal (Fig. 5).
Larger ejection forces may lead to an increased wear on
tooling and eject cam surface. Ejection force may also be
used to evaluate the effectiveness of lubrication (of both
the press and the product) and punch sticking. Sensitivity
and linearity of an ejection transducer are design
dependent: shear force designs are always preferable
over split cam or cantilevered beam designs.
Take-Off Force Measurement
Take-off force is monitored by mounting a strain gage to a
cantilever beam on a press feed frame (in front of the take-
off blade, Fig. 6). It is done to measure adhesion of tablets
to lower punch face. Such adhesion is indicative of
underlubricated granulation, poor tooling face design, die-
wall binding, and tablet capping.[32,33]
Speed Measurement and Station IDDetermination
Station identification and press speed are usually obtained
by means of a revolution counter (proximity switch). It is
installed on the press in order to mark the beginning of
a turret revolution and thus enable station identification
and speed calculations by system software.
In addition, a linear arrangement of proximity switches
can mark the beginning and end of a compression event
(information that is vital for tablet weight controllers) and
also indicate missing punches.
Other points of instrumentation on a tablet press are as
follows:
. A rotary press pull-up cam can be instrumented to
measure the upper punch pull-up force (the force
required to pull up the upper punch from the die).
Likewise, the lower punch pull-down force is
measured on a bolt holding the pull-down cam.[34] It
is useful in determining the smoothness of press
operation (extent of lubrication, cleanliness of the
machine, and long batch fatigue buildup).
. Punch displacement measurements are easily done on
a single station press by attaching LVDT to the punch.
On a rotary press, such measurements can be done by
means of slip ring, telemetry, or instrumented punch.
Punch displacement profiles may be used in conj-
unction with compression force to estimate work of
compression and work of expansion (measure of
elasticity). Because capping tendency increases with
the punch penetration depth, it may be desirable to
monitor actual punch movement into the die. The
shape of a force–displacement curve is an indication
of the relative elasticity or plasticity of the material;
whereas plastic deformation is desirable for stronger
tablets, excess plasticity usually results in tablets that
tend to cap and laminate.[35 – 37]
. Radial and axial die-wall force measurements also
provide an insight into the compaction mechanism of
the material and may indicate a die-wall binding
(sticking) that is, in effect, a negative pull on lower
punch. The radial die-wall pressure due to friction is
material-specific and is more evenly distributed inside
the die with an addition of a lubricant.[38 – 44]
Instrumentation of the die presents a technological
challenge because pressure is distributed nonlinearlyFig. 5 Instrumented ejection cam.
Fig. 6 Instrumented take-off bar.
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with respect to tablet position inside the die and
depends on tablet thickness.[45,46]
. In-die temperature can be monitored for heat-sensitive
formulation, such as ibuprofen.[47,48]
Instrumented Punch
Several vendors offer instrumented punch, i.e., a punch
that has strain gages and other instrumentation built-in.
The data are then accumulated or transmitted via telemetry
to a stationary computer. Such devices are versatile
enough to report compression forces and either punch
displacement or acceleration, and, at least in theory, they
can be easily moved from press to press.[49,50] However,
one should keep in mind that each instrumented punch is
limited to one size and shape of the tooling, and is limited
to one station, compared to roll pin instrumentation that
reports data for all stations and any tooling. In addition,
instrumented punches are either rather cumbersome to
install, or else they report a useless measurement of punch
acceleration instead of punch displacement. Attempts to
calculate displacement from acceleration so far could not
be validated.
Single Punch Press for R&D
Single punch eccentric presses are often used in early
stages of formulation development because they do not
require a large amount of powder. Another benefit is that
they allow relatively inexpensive measurements of die
forces and punch displacement (there is no rotation of die
table and therefore no need to use expensive telemetry
methods). This is the primary reason why so much basic
research and product development were done on eccentric
machines.[51] Negative considerations are that a special
tooling is required (usually, F tooling), and also that speed
of compaction is too slow compared to rotary presses. As it
will be seen later, the speed is a crucial factor in tableting
process and therefore the results obtained on single punch
presses do not directly correlate with tablets made on
rotary machines.
Benefits of Press Instrumentation
Among many benefits of press instrumentation, formu-
lation fingerprinting is perhaps the most obvious. Com-
pressibility and ejection profiles, as well as dissolution and
disintegration curves related to compression force, are
unique for each formulation and can be used as a batch
record. For process optimization, one can include com-
pression or precompression force and speed factors in
experimental design. Compactibility and ejection profiles
can be used for excipient and lubricant evaluation. Other
useful product development and optimization tools include
response surface, Heckel, and force–thickness plots.
Scientifically reliable process scale-up cannot be
achieved without instrumentation data providing scale-up
parameters such as dwell time, density, and energy of a
tablet.
In a pilot plant and on the production floor, proper
instrumentation can be used for press troubleshooting, to
warn about tooling irregularities, worn-out cams, sticking
punches, underlubricated dies, and so on. Finally,
instrumentation is widely used for tablet weight control.
Instrumentation for Formulation Development
Much of the current body of knowledge about
compaction properties of pharmaceutical materials
came from instrumented tablet presses.[52 – 56] Many
tablet properties, such as tensile strength (hardness) and
porosity can be predicted from force profiles.[57 – 59]
Work of compaction (a scale-up parameter) can be
obtained with proper instrumentation.[60,61] Information
about the plasticity of materials can be derived from
force–time curves.[62 – 64]
The phenomenon studied with the help of instrumenta-
tion is the so-called “lag time” (the time difference
between peak of compression and maximum punch
penetration). The extent of this lag is indicative of
compaction mechanisms of the powder being com-
pressed.[65,66]
Typical waveform
Let us have a look at the actual waveforms that one may
obtain from an instrumented tablet press (Fig. 7). The
black proximity switch trace that marks the beginning of a
revolution should be noticed. On this press, precompres-
sion and compression events coincide in time (they relate
to different punches, of course). They are followed by the
ejection and take-off.
Compactibility profile
When the average tablet hardness is plotted against the
average compression peak force, we get the so-called
compactibility profile that allows us to compare different
formulations or different processing speeds. Referring to
Fig. 8, which formulation is better? Well, formulation
No. 2 makes harder tablets for the same compression force,
and this would mean less wear and tear on the production
press is required to achieve desired hardness. On the other
Tablet Press Instrumentation8
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hand, if the hardness tolerance limits are exceptionally
narrow, the steeper slope of formulation No. 2 may be a
detriment.
Lubricant profile
A certain quantity of lubricant must be present in the
granulation to reduce the friction that occurs at the die wall
as the tablet is being ejected, as well as to prevent sticking
of the tablet to the face of the punches. Without
instrumented ejection cam and take-off bar, no objective
estimate of an optimal lubricant level is possible, and the
“best” formulation is usually the last one prepared.
The plot shown in Fig. 9 will help a formulator to
determine the optimal amount of lubricant. Obviously, one
would try to minimize the ejection force (again, to reduce
wear and tear on the ejection cam of the production press),
and yet to avoid the pitfalls of having too much lubricant in
the formulation.
Because of the natural association of lubricant proper-
ties with lipophilic materials, formulations containing high
levels of lubricant can show retarded dissolution of the
active ingredient and the slow dissolution rate could
adversely affect the in vivo bioavailability of the drug.
Lubricant study
Another important use of ejection force transducer is the
evaluation of lubricants (either different chemically, or
similar lubricants coming from different vendors).
As shown in Fig. 10, the preferred lubricant is No. 1 as
it results in a smaller ejection force. Early lubricant studies
are a must. When lubricant problems occur later on in the
scale-up process, corrective measures not only require
additional materials and development time, but may also
require a supplement of an approved new drug application
(NDA).
Response surface plot
Formulation and process optimization can be done
statistically with the use of experimental design for
estimates of the best processing parameters and excipient
Fig. 7 Typical compression, precompression, and ejection waveforms.
Tablet Press Instrumentation 9
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and lubricant levels. Controllable variables in tableting are
mainly the precompression and compression forces and
tablet press speed, as well as the formulation component
levels. Response variables include the ejection force,
tablet hardness and friability, dissolution rate, and drug
stability. The purpose of an experimental design is to
perform a series of experiments in order to determine some
levels of factors that will allow us to achieve an optimal
level of dependent variables. The experiments should be
designed so as to minimize effort and maximize statistical
reliability of the results. Published work in this area deals
mostly with response surface designs that produce a
predictor polynomial equation for each response variable
under consideration. A multidimensional surface is then
searched for the best ranges of factor variables that, when
plugged in the equations, result in the optimal value of the
Fig. 8 Compactibility profile.
Fig. 9 Lubricant profile.
Tablet Press Instrumentation10
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responses. In this plot (Fig. 11), the optimal (highest)
hardness is obtained when the compression force is in the
range 3000–4000 lb with as much MCC in the formulation
as possible.[67]
Compactibility study—elastic recovery
Many powders, especially with viscoelastic compaction
mechanism, such as starch or avicel, exhibit large degree
of stress relaxation (with time-dependent deformation).
If you monitor punch displacement and compression
force, you can make pretty accurate assessment of the
compactibility of your material. If LVDTs are attached to
both upper and lower punches, it is possible to actually
measure in-die thickness of the tablet at various
compression levels. In Fig. 12, one can see how the tablet
thickness rapidly decreases in the compression stage and
then gains some thickness during the decompression stage
of the tableting cycle. The degree of this increase in
Fig. 10 Lubricant study.
Fig. 11 Response surface plot.
Tablet Press Instrumentation 11
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thickness is indicative of the elastic recovery as the
pressure is removed from the tablet.
Elastic recovery and work of compaction were studied
extensively using instrumented tablet presses.[68]
Compactibility study—Heckel plot
In 1961 Heckel[69,70] postulated a linear relationship be-
tween the relative porosity (inverse density) of a powder
and the applied pressure. The slope of the linear regression
is the Heckel constant, a material-dependent parameter
inversely proportional to the mean yield pressure (the
minimum pressure required to cause deformation of
the material undergoing compression). Large values of the
Heckel constant indicate susceptibility to plastic defor-
mation at low pressures, when the tablet strength depends
on the particle size of the original powder. The intercept of
the line indicates the degree of densification by particle
rearrangement (Fig. 13).
TABLET PRESS INSTRUMENTATION FORPROCESS TROUBLESHOOTING
Typical problems in tableting include weight variation,
capping, lamination, tooling irregularities, die binding,
picking, and sticking. Most of such problems can be
detected and/or resolved using proper instrumentation.
Upper and Lower Compression
Fig. 14 shows a typical set of upper and lower compression
profiles. One can see that the lower trace is smaller than the
upper. On a single station press, only the upper punch is
usually moving, and the difference is caused, mainly, by
the friction of the compressed powder inside the die.
On a rotary press, both punches are moving and
therefore the friction of the punches inside the turret and
the die causes the difference. The lower punch fits the
turret with a larger standard clearance compared to the
upper punch. In addition, the lower punch is never leaving
the die, while the upper punch leaves and enters the die
with each stroke. This results in a comparatively better
alignment and lower friction. The close fit of the upper
punch does not allow it to penetrate the die smoothly and
this can cause the increase in friction. Thus, an excessive
difference between the two peaks may indicate an
underlubricated die or some punch misalignment problem.
Histogram of Punch Performance
Similar irregularities in punch tolerances become even
more visible on a bar histogram where each bar
corresponds to peak force produced by each punch
(Fig. 15). As the new revolution arrives, the bar lengths
Fig. 12 Compactibility study—elastic recovery.
Fig. 13 Heckel plot. Fig. 14 Upper and lower compression force vs. time.
Tablet Press Instrumentation12
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are adjusted. If a particular bar persists in being taller or
smaller than the rest, it means the presence of a long or
short punch.
Press Monitor—Real-Time Screen
On this real-time screen (Fig. 16), one can clearly see
that there is an abnormal ejection event in the first station
after the proximity switch. This is a clear indication of a
sticking punch or a similar problem. In addition, the first
compression peak is somewhat taller than others. This
may be caused by a long punch.
TABLET PRESS INSTRUMENTATIONFOR PROCESS SCALE-UP
Scale-Up Factors
One of the main practical questions facing formulators
during development and scale-up is: Will a particular
formulation sustain the required high rate of compression
force application in a production press without lamination
or capping? Usually, such questions are never answered
with sufficient credibility, especially when only a small
amount of material is available and any trial and error
approach may result in costly mistakes along the scale-up
path.[71]
Fig. 15 Histogram of compression force per punch.
Fig. 16 Real-time screen.
Tablet Press Instrumentation 13
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As formulations are moved from small-scale research
presses to high-speed machines, potential scale-up
problems can be eliminated by simulation of production
conditions in the formulation development lab. One way to
eliminate potential scale-up problems is to develop
formulations that are very robust with respect to
processing conditions. A comprehensive database of
excipients detailing their material properties may be
indispensable for this purpose. However, in practical
terms, this cannot be achieved without some means of
testing in production environment and, because the initial
drug substance is usually available in small quantities,
some form of simulation is required on a small scale.
Studies of tableting process on a class of equipment,
generally known as compaction simulators, are designed
to facilitate the development of robust formulations.
However, simulators are rarely used to simulate tablet
presses for reasons that will be explained later.
In any process transfer from one tablet press to another,
one may aim to preserve mechanical properties of a tablet
(density, and, by extension, energy used to obtain it) as
well as its bioavailability (e.g., dissolution that may be
affected by porosity). However, a formulation that was
successfully developed on a single station or small rotary
press may not stand up to the challenges of scale-up
because tablets that were meeting all specifications in the
lab or clinical studies may exhibit capping or lamination at
higher speeds.[72,73]
Compression force magnitude and the rate of force
application are the most important variables in tableting
scale-up.
Force factor
The compression force is the dominant factor of the
tableting process. It is directly related to tablet hardness
and friability, and is correlated with the phenomena of
lamination and capping.
It was also shown to have effect on disintegration times
and dissolution profile.
Speed factor
As the punch speed increases, so does the porosity of
tablets and their propensity to capping and lamination. The
tensile strength of compacts tends to decrease with faster
speeds, especially for plastic and viscoelastic materials,
such as starch, lactose, avicel, ibuprofen, or paracetamol.
Such materials have the tendency to cap or laminate at
higher speeds.[74 – 88]
The notions of dwell time and contact time, to be
discussed in detail later, are the common indicators of
press speed and the rate of force application.
Force profile factor
In addition to the level of force and the rate of force
application, the shape of the compression force vs. time
curve is of a paramount importance because it directly
affects tablet properties such as hardness and friability.
It is a known fact that the compression part of the
compression cycle (during the “rise time” of the force–
time profile) is 6–15 times more important than the
decompression part as a factor contributing to capping and
lamination. On the other hand, reducing the decompres-
sion part of the cycle results in the increase of tablet
hardness by reducing the extent of elastic recovery.[89]
Alternatively, reducing the compression part of the cycle
results in no change of tablet strength for viscoelastic
materials and increased hardness for brittle materials.
Other Considerations
Numerous other factors may affect the scale-up process.
The quality of the measurements, variation in tooling,
powder properties, and tablet weight are some of those
factors.
. Instrumentation grade.
. Measurement of speed.
. Measurements of mechanical strength.
. Tooling variation.
. Powder flow variation.
. Excipient/raw material variation.
. Tablet weight variation.
Dwell Time and Contact Time
Matching tablet press speed (rpm) of the research and
production presses has, of course, no meaning, because of
different number of stations and pitch circle diameter. It is
vital, therefore, to translate the rpm into dwell time or
contact time.
Many investigators have reported the effect of dwell
times and strain rate sensitivity on the compaction of
various excipients, especially viscoelastic materials.[90 – 94]
There are at least two definitions of dwell time in practice
today.
Functional definitions
Functionally, the effective dwell time (EDT) at 90% level
can be defined as the time it taken by the force–time curve
Tablet Press Instrumentation14
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to traverse the 90% of the peak height. Likewise, the
effective contact time (ECT) is the time between points at
10% of the peak height. The shape of the force curve
depends, as we know, on the deformation mechanisms of
the powder and therefore, all other variables being equal,
EDT and ECT will be different for brittle and plastic
materials. Although somewhat useful for material
characterization, such variables should not be confused
with the universally accepted definitions of contact time
(time of contact) and dwell time (time of immobility).
Mechanical definitions
Mechanical Definitions of dwell and contact times
disregard material properties and concentrate on press
and punch geometry (Fig. 17). Contact time can be defined
as the time the punch is in contact with the compression
wheel. Dwell time is defined as the time the flat portion of
punch head is in contact with the compression wheel (time
at maximum punch displacement, or time when the punch
does not move in vertical direction). In dwell time
calculations, the length of the punch head flat and
horizontal component of punch speed (as determined by
RPM and pitch circle diameter) are used. In case of a round
head tooling, the dwell time, as defined here, is zero. But it
should be kept in mind that mechanical definition is given
here as a convention, a yardstick, or a common measure, to
compare press speeds for different presses, and its absolute
value is meaningless. A proposed convention to quantify
linear speed of a press is to use an Industrial Pharmaceu-
tical Technology (IPT) Type B tooling with a known punch
head flat as a standard for press speed comparisons.
In what follows, we will use the mechanical, rather than
functional, definitions because they serve as an objective
material-independent measure of compaction speed.
Dwell time comparison for different presses
Comparing the dwell time ranges for a number of presses
can be a gratifying experience. Here one can see that even
for the same brand name, there is a wide distribution of
ranges. It has to be noted that proper scale-up will be
possible only when the ranges overlap. Dwell time range
may also be used to classify or identify tableting equipment.
The dwell time ranges vary considerably in various tablet
presses. As shown in Fig. 18, Manesty Betapress is
positioned well within the range of production speeds of the
high-speed presses. That is, probably, why this press is
often used for R&D work. On the other hand, small presses
such as Korsch PH106 or Piccola do not even come close to
benchmark production speeds of 6 msec–15 msec in terms
of dwell time. The MCC Presstere can reach the
production dwell times while making one tablet at a time.
Dwell time vs. production rate
For a benchmark production speed of 100,000 tablets per
hour (Fig. 19), dwell time distribution follows an inverse
power relationship, which is expected because dwell time
is a reciprocal function of the press speed. In general, all
production presses should be qualified with respect to
dwell and contact times per benchmark output. Ideally,
product development must be done on a laboratory press
that can match (in terms of the dwell time) the target
production output.
Dimensional Analysis of Tableting Process
Dimensional analysis is a method for producing dimen-
sionless numbers that completely characterize the process.
It is widely used in many areas, including chemical
engineering and pharmaceutical industry.[71] Because all
Fig. 17 Dwell time and contact time—mechanical definitions.
Tablet Press Instrumentation 15
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the dimensionless numbers necessary to describe the
process in similar systems must have the same numerical
value,[95] matching such values on different scales is a sure
way to success in any scale-up operation. This dimension-
less space in which the measurements are presented or
measured will make the process scale invariant.
In tableting applications, the process scale-up involves
different speeds of production in what is essentially the
same unit volume (die cavity in which the compaction
takes place). Thus, one of the conditions of the theory of
models (similar geometric space) is met. However, there
are still kinematic and dynamic parameters that need to be
investigated and matched for any process transfer.
Scientifically sound approach would be to use the
results of the dimensional analysis to model a particular
production environment to facilitate the scale-up of
tableting process, by matching several major factors,
such as compression force and rate of its application
(punch velocity and displacement) in their dimensionless
equivalent form.[96]
TABLET PRESS AND COMPACTIONSIMULATORS
It can be seen that, as a rule, tablets are formulated at
speeds that are very slow compared to production. If
Fig. 18 Dwell time comparison for different presses.
Fig. 19 Dwell time vs. production rate.
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simulation of a production press is required to minimize
the scale-up effort, some way of speeding up the tableting
process in development is required. Once the linear speed
of the punch is attained, the rate of force application (i.e.,
the instantaneous change in compression) should also be
matched. This is, of course, an infinitely more difficult task.
Hydraulic Compaction Simulator
A small number of devices known as compaction
simulators exist in the world. Invented more than 20 years
ago, they become popular in basic compaction research.
A wealth of studies have been generated in the last
20 years.[97 – 117]
Compaction simulators (Fig. 20) were designed to
mimic the compression cycle of any prescribed shape by
using hydraulic control mechanisms that are driving a set
of two punches (upper and lower) in and out of the die. All
hydraulic compaction simulators are similar in design and
construction. A compaction simulator consists of several
main units: the load frame (column supports and cros-
sheads with punches), the hydraulic unit (pumps and
actuators that move the crossheads), and the control unit
(electronic console and computer). Usually, a simulator
accepts F tooling only, but can be retrofitted to use
standard IPT B tooling. Under computer control, the
hydraulic actuators maintain load, position, and strain
associated with each punch.
The simulation can be achieved by one of the two
procedures: matching either the force (load control) or the
movement of punches (position control) at any given
moment of time. Thus, when running a simulator, one has
a choice: to mimic the force/time path (compression
profile) or the motion of the punches (punch displacement
curves). It is impossible to mimic both at the same time on
any hydraulic compaction simulator.
Load control profile
Matching the force–time profile of a production tablet
press is the primary goal of any tablet press simulation.
However, the rate of force application and the shape of
the resulting signal are not usually known in advance.
The compression profile (force vs. time) is impossible to
calculate theoretically because it has a different shape for
different materials and tooling.
Using an instrumented punch to collect force data is
cumbersome because it is limited to a particular punch size
and shape. Recalculation to pressure values is not always
adequate. One can, however, monitor and record force
waveform from a properly calibrated R&D grade
compression transducer. Once the production press
brand, model, speed, and tooling are specified, a waveform
can be recorded and then fed into hydraulic simulator.
This approach is impractical for formulation develop-
ment work because one would need to record force
waveform on a rotary press using formulation similar to
the one being developed. Usually, there is no such powder
available and the actual formulation being developed is
available in limited quantities.
Position control profile
Because load control profiles are not practical, users of
hydraulic compaction simulators overwhelmingly prefer to
utilize punch displacement profiles in hope that, once the
punches are forced to move in the same pattern as in the
production press, the force–time curve will follow. For
example, a recently built laboratory compaction simulator
does not have a force control functionality at all.[118]
There are three possible sources of the punch position
trace:
. Prerecorded data.
. Artificial profiles.
. Theoretical profiles.Fig. 20 Schematic depiction of a compaction simulator.
Tablet Press Instrumentation 17
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To obtain a position control profile from any tablet
press, one has to record the movement of the punches
using LVDT. Besides the technological challenge that this
objective may present, the punch movements on a press
depend on many factors, including brand name and model
of the press, speed of the turret, shape of punches and die,
size and shape of the tablet, and most importantly,
compaction properties of the powder. The problem with
this is that data from production presses are inevitably
obtained using material other than the one being
developed. It is a vicious circle: the profiles before
developing a formulation and the formulation before
obtaining punch displacement profiles are needed.
Many compaction studies were done on compaction
simulators using artificial punch displacement profiles, for
example, the so-called “single-ended” profile, i.e. when
the lower punch is stationary (like in a single station
press). Other studies were using a “saw tooth” profile, i.e.,
a constant speed profile where punch displacement speed
is constant at any given time interval under load. It is
obvious that such profiles have nothing to do with
simulation, although they provide a degree of uniformity
for basic compaction studies. The very name “compaction
simulator” is a misnomer as is acknowledged by a number
of researchers in the field. The machine is best described as
a compaction research system because it is well suited for
the basic compaction studies (densification and bonding
properties of materials).
To simulate tablet presses, compaction simulator users
most frequently employ the theoretical position control
profiles. Theoretical path is calculated from the geometry
of the press and punches, using the radius of the com-
pression roll, the radius of the curvature of the punch head
rim, the radius of the “pitch circle” (distance between
turret and punch axes), and the turret angular velocity.[119]
The resulting sinusoidal equation is used in order to
“simulate” punch movement in a tablet press.
In practice, theoretical and actual punch displacement
profiles on a rotary press have very little in common because
the theoretical profile does not account for several mecha-
nical factors, such as punch head flat. Moreover, the punch
movement equation was derived for a punch moving in and
out of an empty die. The effect of material resistance to
pressure and elastic recovery is not accounted for in the
equation. The discrepancies between the calculated and
real punch movements are rather striking.[120 – 122]
In addition, it was shown that the lower and upper
punches may not move synchronously. Moreover,
maximum force does not coincide in time with the
minimum punch gap.[123] These and other considerations
(press deformation, contact time, etc.) make the effort of
simulating a production press on a hydraulic compaction
simulator rather impractical. That is why, to quote from a
paper by Muller and Augsburger,[124] “Although compac-
tion simulator have been designed to mimic the
displacement time behavior of any tablet press, they
rarely have been used in that fashion.”
Literature sources reporting the use of compaction
simulators in simulating actual tablet presses are rather
scarce. Some studies suggest that, for whatever reason,
tablets made on a Manesty Betapress were significantly
softer than those made on a compaction simulator using the
theoretical Betapress punch displacement profiles.[120,125]
To summarize, one can say that hydraulic compaction
simulators are ideally suited for basic compaction research
but are not very practical for simulation of production
presses.
Tablet Press Replicator: The Presstere
Recently, a new type of machine was introduced to mimic
production presses on a small scale. Known under a brand
name of “Presster”—kind of an agglomeration of the
words “press,” “presto,” and “tester”—this machine can be
classified as a mechanical compaction simulator.
The Presstere is a high-speed single station press that
is also a tablet press simulator (Fig. 21). It was designed to
simulate production presses without any use of hydraulic
controls, and, consequently, there is no need to feed in any
artificial, theoretical, or prerecorded punch displacement
profiles. Built around a linear carriage that moves a set of
punches and a die between two compression rolls, it can
mimic press geometry by matching the compression
wheels match press speed using a variable speed motor
drive match tablet weight and thickness by adjusting depth
of fill and the distance between the rolls match tooling by
installing standard IPT or any special tooling.
Thus, using mechanical similarity, all of the scale-up
fact are matched, namely, the compression force, the
speed, and the shape of the force profile. To use Presster,
first a production press to be simulate should be selected,
and the compression wheels with a matching diameter
should be installed. Then, production speed should be
selected in terms of tablets per hour, RPM, or dwell time.
The Presstere will mimic the selected production press
speed and compression force profile and will allow us to
make one tablet at a time.
As a high-speed single station press, mechanical
compaction simulator will be able to plot compressibility
profiles, Heckel graphs, calculate work of compaction, and
virtually any other imaginable variable that is of interest to
formulators. Tensile strength of tablets made on a
Betapress and The Presstere was similar.[126]
Tablet Press Instrumentation18
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Precompression and ejection steps of the tableting cycle
can be included in simulation. Some current limitations of
Presster should be pointed out: It will neither follow any
artificial punch movement profile, nor will it address, at
least in its present implementation, the issues of feeding
and die fill at high speeds, or speed-related temperature
fluctuations.
PRESS INSTRUMENTATION AND CONTROLON THE PRODUCTION FLOOR
Tablet Weight Control and Tablet ForceControl
To keep tablet weight within the prescribed tolerance
limits, the required instrumentation includes compression
transducer and several proximity switches for station
identification and pinpointing the compression event.
Tablet weight controller can be just one, albeit a major,
unit of a larger press automation system. Press automation
system may include:
. Tablet weight control.
. Material handling interface.
. Feeding system.
. Collection system.
. Packaging system.
. Supervisory control (SCADA) station.
The latter can be a computer positioned on the
supervisor’s desk that monitors the performance of each
press on the production floor, with timely status report for
each batch.
There are many reasons why it is imperative to control
tablet weight variation:
. Production costs are lowered because there is less
waste.
. Productivity is increased because there is a better
equipment utilization.
. Batch-to-batch variability is minimized for obvious
reasons.
When the cost of each out-of-spec wasted tablet is
significant, the savings produced by weight control
quickly add up. Product quality is improved when there
is less of a chance to get an out-of-spec tablet into the
acceptable batch. Last, but not least, is the improved safety
because the automated process requires less human
intervention.
Fig. 21 Tablet press replicator: The Presstere.
Tablet Press Instrumentation 19
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Control mechanisms
For constant tablet thickness, in a small target area of
tablet weight, compression force is directly proportional to
tablet weight. Force control systems maintain compression
force within prescribed limits by adjusting the depth of fill
cam. The limits are established empirically for each
formulation recipe.
Alternatively, a weight control system would require
an expressed correlation between force and tablet weight.
A few tablets at different force levels can be made to
correlate the resulting tablet weights with the force values.
Next, one can express tablet weight tolerance limits in
terms of the compression force. The weight control system
can adjust the dosing to keep the tablet weight within the
desired limits. For this control theory to work consistently,
one needs to recalibrate force–weight relationship
periodically, as the powder properties and tablet
mechanical condition can vary in time.
Control functions include alarm and shutdown (when
any individual force peak or revolution average exceeds
preset limits), force or weight control (usually done on
revolution average only), and rejection of out-of-spec
tablets. With a mechanical gate, usually several tablets are
rejected at a time, with the bad tablet being rejected along
with the adjacent good tablets. With a fast pneumatic–air
stream rejection mechanism, the control system can
pinpoint and reject one bad tablet only.
Control criteria
Control decisions are usually done as follows:
1. Adjust depth of fill cam if revolution average is
outside some redefined acceptable limits.
2. Reject each tablet that exceeds individual limits or
if the press speed slows down beyond some level.
3. Sound an alarm if the same station repeatedly
produces bad tablets or if revolution average
exceeds alarm limits.
4. Shut down the press if any tablet is made with a
force exceeding the shutdown limits, or if
revolution average is outside the shutdown limits.
Control algorithms
One-point control is a description of a control when any
deviation of the compression force from a preset target
level results in a corrective action of the dosing cam. This
action can be proportional (P) to the deviation from the
target, or it could be correlated with the integral (I) of
the deviation over some time, or it could be tied in with the
rate of change (D, for derivative) of the compression force.
This would correspond to proportional, integral, or
derivative control types, respectively. There also can be
a combination of the control types. Thus, one can have P, I,
P þ I, P þ D, and P þ I þ D control algorithms.
Alternatively, a two-point control is enacted when the
compression force is outside an acceptable band outlined
by the upper and lower tolerance limits. Thus, there are
separate control limits, rejection limits, alarm limits, and
shutdown limits, and no respective action is taken when
the signal is inside these limits.
Strictly speaking, one-point control can be viewed as a
special case of the two-point control when the bandwidth
of the control limits is tightened to approach zero.
Control Systems
Original equipment manufacturer (OEM) controlsystems
Almost each tablet press manufacturer offers a system that
is designed to control the press. Some of these systems are
very sophisticated devices that monitor and control a vast
array of tableting functions. If, however, there are several
brand names on the production floor, any standards in the
control system implementation for different manufacturers
should not be expected. Likewise, software interfaces
exhibit quite a range of user-friendliness.
In one brand name press, tablet weight control is
achieved by regulating the dosing cam based on powder bed
thickness in the precompression cycle. This clever approach
is possible because precompression force is kept relatively
constant by means of pneumatic compensating mechanism.
Under these conditions, tablet weight is directly pro-
portional to thickness. The subsequent compression cycle
can be done to constant thickness, like on any other press.
Generic control systems
Analternative toOEM control systems isgeneric controllers
that may offer plug-in compatibility with the brand name
controllers and may provide a degree of standardization.
However, such generic controllers may lack the
degree of sophistication and versatility of the controllers
that are made by the press manufacturers. One thing to
keep in mind is that not all control systems are created
equal, but all control systems use the same principles.
A decent tablet weight control system should be based
on product recipes, provide instant display of compression
force distribution, control charts and batch reports on
Tablet Press Instrumentation20
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demand, archiving data for subsequent analysis or
documentation, give some measure of standardization,
be fully compliant with current validation requirements,
and provide a multilevel security, e.g., password
protection for operator and supervisor when they need to
change recipes, etc. Some examples of displays, available
to operators of instrumented production presses equipped
with controllers, follow.
Recipe example
In Fig. 22, an example of a typical control recipe with
dosing cam adjustment, rejection, alarm, and shutdown
limits expressed as a percent deviation from the target
tablet weight is shown. The software then converts all
values into the corresponding compression force levels for
control purposes.
Bar histogram
This chart is similar to one used in tablet press monitoring
for press troubleshooting (Fig. 15). It is vital for any
production press operator.
Control chart
Control chart is a simple graph of peak compression force
vs. time. Each point on the chart corresponds to the
Fig. 22 Recipe example.
Fig. 23 Statistical process control X-bar chart.
Tablet Press Instrumentation 21
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average of N tablets made, with N ranging from 1 to
several revolutions. The horizontal lines would indicate
the control limits.
Statistical process control chart
Statistical process control (SPC) chart (Fig. 23) of the
averages is another “must have” real-time display. Each
point on the chart represents a revolution average of the
compression forces or corresponding tablet weights.
The limit lines are calculated at one standard deviation
of the mean, and there are certain rules that are used to
determine when and if the process gets out of control.
These rules are available in any textbook on the SPC.
ACKNOWLEDGEMENT
Selected excerpts and figures from M. Levin, “Tablet Press
Instrumentation for Product Development, Process Scale-
Up and Production Quality Control, Education Anytime,
CD-ROM Short Course, AAPS 1999” are reprinted with
permission.
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©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016