Sticking and picking are two of the most common problems tablet manufacturers face during produc- tion. Sticking occurs when powder
formulation sticks to the punch cup surface
and picking occurs when formulation is pulled
from the tablet surface because a portion of
the formulation has become trapped in the
engraving cut or the embossing characters.
There are many possible causes of these
issues, including formulations that have excessive moisture, insufficient main compression
force, improperly maintained tools that have
defects in the cups, and engraving cuts that
are too deep or narrow.
When all other possible causes have been
investigated, sometimes it comes down to the
formulation and whether it can be adjusted.
Sometimes the formulation cannot be adjusted and the problem is better addressed by
changing the tablet design, the engraving cut
depth/radius, or perhaps the materials of
construction for the tool.
Recent research indicates that there are
other options for reducing sticking and picking issues that may help when traditional fixes
don’t provide adequate correction.
When a tablet and its requisite tooling are
first designed, the likelihood of problems pertaining to sticking and picking are often not
addressed. The fortunate development scientist runs into sticking and picking problems
during initial trials and scaling up clinical supply. Unfortunately this is not always the case,
and when these problems arise during scale
up, corrections are required. If the problems
arise after the product has been registered and
commercial scale production is required, then
the problem is difficult to retract and compensating actions are needed.
Traditionally, change of steel type and
specialty punch cup coatings have been proposed to reduce issues such as sticking and
picking. Steel chemistry can play a part in
reducing sticking issues.
Elemental compositions in steel can provide
benefits such as improved wear and/or durability. As it relates to sticking and picking, steel
with a high chrome content usually improves
product release. Steel type is a decision that
should be carefully considered as there are
usually trade-offs. For example, a higher
chrome content may produce steel that is less
Coatings can also be utilized to address certain tablet quality issues. Coating the tips of the
tools with chrome is an often utilized approach
to address sticking problems. With proper polishing technique, a chrome surface can have
an average surface roughness of less than 0.5
Other coating techniques deposit chromium
nitride, titanium nitride or diamond like carbon
coatings on tools to address sticking and abrasion problems experienced by tablet manufacturers. These coatings also impart a very
smooth abrasion-resistant surface to the punch
cups. Caution should be taken when using coatings as all coatings do wear (reducing tool life)
and should be used as a last resort to solve a
It has been a universally accepted hypothesis
that sticking is minimized when the surface of
the metal punch cup is as smooth as possible.
This helps minimize the surface adhesion forces
by reducing the opportunity for the molecules
making up the powder to interact with asperities
on the surface.
Influence of Humidity
Humidity also plays a role in the adhesive
forces involved in the sticking of formulation
to the punch cup as it can play a role in the
hydrogen bonding component of adhesion. In
a recent screening of a customer’s complex formulation that was prone to cause tablet defects
and sticking, Natoli Engineering Company
manufactured tools with a punch cup that had
a surface that was lightly abraded.
This surface could be described as a matte
finish as opposed to the usual mirror-like
surface that is generally preferred on a punch
cup. The matte finish was applied using a
process that is similar to other processes that
can be utilized to remove coatings from metal
surfaces or to remove rust from steel, which
left the cup with a less smooth finish.
To test the effectiveness of the matte finish
versus other possible corrections, Natoli includ-
A Sticky Situation
New tableting technology can help with sticking issues
n By Dale Natoli, President of Natoli Engineering Company, Inc.