As appeared in Tablets & Capsules September 2015 Copyright CSC Publishing www.tabletscapsules.

com

tablet tooling
Understanding maximum Kevin Queensen
compression force Natoli Engineering

W
As the number and variety of products made on tablet presses grow, man- ing how to calculate maximum compression force for vari-
ufacturers of tablet presses and tooling continue to advance the science of ous tablet configurations.
tablet compression. This article discusses how computer modeling helps Manufacturing a high-quality tablet punch starts with
determine maximum compression force of tablet tooling. high-quality tool steel, and the tabletting industry uses
many different types, each with unique properties that suit
hile pharmaceuticals and dietary supplements account
for the great majority of tablets, they’re also found in the different applications. In the USA, the American Iron and
chemical, food, confectionery, and energy industries. Each Steel Institute (AISI) sets the standards for tool steels, and
of these sectors has high standards and each demands well for many years, AISI S7 was preferred for many applications
engineered, well made tooling. To meet those demands, because it offers excellent shock resistance and thus good
tooling manufacturers have turned to computer modeling resistance to splitting/shearing stress. It also withstands rela-
to refine or replace some of the textbook standards, includ- tively high compression forces.
 Copyright CSC Publishing

Figure 1
Normal forces acting on round and oval punch-cup faces

Other tool steels, such as AISI D2, DC-53, K340, and These forces result in stress, which is a function of both
powder-metallurgy (PM) class steels offer excellent wear the cup’s area and its geometric profile. Basic stress relates
resistance because they can attain a higher hardness than to the force exerted and the area over which it is applied,
other steel types and have a high carbide content. Tool but calculating maximum allowable compression force
steels with a high chromium content, such as AISI 440C requires comparing maximum stress to the yield strength
and M340, are particularly useful when compressing corro- of a given material. In addition to force and area, stress
sive or sticky products. The general rule: As the Rockwell concentration factors must be factored in. Stress concen-
hardness of a material increases, wear resistance increases tration refers to small areas of significantly increased stress
and impact toughness decreases. Some PM-class steels, caused by sharp transitions in the geometric profile of the
however, exhibit both excellent wear resistance and rela- cup and/or punch face. Examples include bisects and the
tively high impact toughness due to their unique chemical blend radius where the embossing meets the cup radius.
composition and the distinctive forging/manufacturing Punches with stress concentrations have lower maximum
process used to make them. Reputable vendors of tablet compression force compared to tooling that makes plain
tooling offer punches and dies made from a variety of tool tablets.
steels, enabling the vendor to select the best steel, one that A punch face’s land—the flat edge at the perimeter of
exceeds the requirements of the desired compression force the punch cup where the cup radius ends—also plays a
and withstands the abrasive, sticky, or corrosive properties critical role in determining maximum allowable compres-
of the granulation to be tabletted.

Steel strength
sion force. Despite the small size of this feature, it’s an
important factor in calculating how much force a punch
tip can withstand before it fails. In general, the larger the
Whatever the application and whichever tool steel you land of a tablet or tool, the greater the maximum allow-
use, when calculating maximum tip force, the most impor-
able compression force. In almost all cases, increasing the
tant properties to take into account are tensile strength,
size of the land of a punch is one of the easiest and quick-
compressive strength, yield strength, and impact toughness.
est ways to increase its maximum compression force. Keep
Compressive, tensile, and yield strength depend on the
in mind, however, that as the tooling wears, the land
chemical composition of the steel and its Rockwell hard-
erodes and becomes smaller. That’s why new punches
ness. Compressive strength indicates how well a material—
with unworn lands withstand more force than punches of
in this case steel—resists deformation in pure compressive
the same design that have been used for many production
loading. Tensile strength refers to the maximum stress a
runs. That fact also illustrates why proper tooling mainte-
material can undergo when pushed or stretched before it
nance is so important: not only does it ensure quality
fails. Yield strength—the most important indicator—is the
maximum amount of stress a material can withstand before tablets, but it also helps to maintain the mechanical
plastic deformation occurs. Impact toughness describes the integrity of the tooling itself.

Predictive modeling
maximum amount of energy from an impulse or shock load-
ing that a material can withstand. Because each type of tool
steel has unique or distinctive mechanical properties, the Like other manufacturers, many tooling vendors have
maximum compression force of each also differs. adopted finite element analysis (FEA) to better understand

Stress concentration
how different tooling designs affect performance. FEA is a
powerful tool, enabling engineers to apply any combination
During the compression phase of tabletting, forces are of forces and/or pressures to a solid model and see the
applied normally to all surfaces of the cup (Figure 1). result, including the stresses, strains, and displacement, as
 Copyright CSC Publishing

Figure 2 The model is then constrained to simulate how it would

Mesh created by FEA modeling software

act in a real-world installation. Last, a failure criterion is
selected, which is a set of values at which the material and
part combination would likely fail. With these parameters
in place, the analysis begins, and the FEA software exam-
ines each element of the mesh to analyze the stress states of
the model. The result: a detailed map showing where maxi-
mum stresses and resulting failures are likely to occur,
enabling engineers to modify the design.
For ductile materials, such as metals, von Mises stress—
also called equivalent tensile stress—is generally accepted
as the best failure criterion. It states that when an elastically
deformable body is subjected to three-dimensional loading,
it will develop a complex network of three-dimensional
stresses.
Von Mises stress can also be formulated in terms of von
well as other factors related to the safe use of the tooling or Mises yield criterion, which states that, even when none of
any part or assembly associated with it. With FEA, design the three principal stresses exceeds the yield strength, the
engineers can analyze complex cup geometry and see yield may still be reached as a result of the combination of
where stress concentrations develop and use that informa- stresses. This criterion works particularly well when FEA
tion to improve tablet and tool design. uses a mesh model because it narrows the almost infinite
FEA works just as its name implies, by defining discrete number of degrees of freedom. That is, the calculated von
elements within a larger model and analyzing them piece- Mises stress combines all stresses into an equivalent tensor
by-piece. To begin, the model is broken down into a finite value that engineers can compare to a material’s known
number of small pieces, the size of which the user controls. yield strength. Figure 3 shows the von Mises stress distribu-
This collection of elements is known as a mesh (Figure 2). tions of a flat-face, bevel-edge tablet tool and a flat-face,
Next a material, with all the necessary properties defined, is radius-edge tool. Note the stress concentrations (red areas)
assigned to the model, and the desired force vectors are around the bevel edge of the punch face.
applied to all surfaces of interest.

Figure 3
Von Mises stress distribution plots: Flat-face bevel edge versus flat-face radius edge

Flat-face bevel edge Flat-face radius edge

Section view of bevel edge Section view of radius edge

Von Mises (psi)

5,140.7 60,105.5 115,070.3 170,035.2 206,678.4 225,000.0
23,426.3 78,427.1 151,713.6 188,356.8
yield strength 275,000.0
 Copyright CSC Publishing
Stress buildup, bent tips, and safety
Note that the maximum compression force that vendors
list on tablet and tool drawings account for the fatigue of
the tooling, which could be used to make thousands if not
millions of tablets. Fatigue is an important consideration
because as compression tooling undergoes repeated loading
cycles during tablet production, residual stresses build up.
This phenomenon—known as fatigue stress—can shorten
the life of the tooling. In fact, it’s normal for the maximum
force that a punch can withstand to decrease over time
because of wear and the buildup of residual stress. This
means that the tooling can withstand more force when it
presses the first tablet than it can when pressing the
500,000th. Ask your vendors if their tooling is designed and
engineered for high-cycle loading.

As compression tooling undergoes

repeated loading cycles during tablet
production, residual stresses build up
and must be taken into account.

A final factor to consider when discussing maximum

compression force: Bending of the lower punch-tip
straights. Generally, it’s a concern only when using tooling
with tips 4 millimeters in diameter or smaller. When the
tips are that small, cup geometry is unlikely to be the limit-
ing factor when determining maximum compression force.
Rather, it will be the propensity of the tips to bend under
compressive load. To calculate the maximum compression
force of these small-tipped tools, the tooling vendor should
determine the compressive load required to bend the tips.

After accounting for all these factors, the appropriate

maximum compression force is determined by comparing
the calculated stress and force results to the appropriate fac-
tor of safety (FOS) for the intended application. The FOS
is simply the ratio of the calculated stress to the yield
strength of the material. An FOS of two means that the
force applied generates a stress in the material that is one-
half the yield strength of the material. The FOS value spec-
ified depends on the industry and the application, and it
can even vary within an industry from one vendor to the
next. That’s true of the tablet compression industry, where
different vendors cite different maximum compression
forces. The difference usually stems what value a particular
vendor deems an acceptable FOS. T&C

Kevin Queensen is a technical support engineer at Natoli Engineer-

ing, 28 Research Park Circle, St. Charles, MO 63304. Tel. 636
926 8900. Website: www.natoli.com. He holds a BS in mechanical
engineering, and his work focuses on designing specialty tablets and
tooling.