ing a barrier of protection above and beyond what
they deem necessary to pass the examination.
Here, fear of failure supersedes exacting optimization – with costly overpackaging as an unfortunate side effect.
Luckily, this guessing game can be largely
marginalized by simulating the barrier properties of various packaging types, toward the goal
of realizing the ideal material selection for the
stability test. This is a particularly attractive
option when a fairly wide range of film specifications for thermoform blisters are available from
which to choose; for example PVC/PVDC laminates with various coating weights or ACLAR/
A simulation is especially useful for thermoform
laminates, because the barriers of their formed
blister cavities can only be estimated to a limited
extent using the barrier properties indicated in
the technical datasheets provided by suppliers.
This is because these datasheets refer to the barrier values of the unformed, flat laminate, which
can be misleading: during cavity formation, the
laminate is made thinner and the barrier reduces
The situation is further complicated by the
fact that the laminate is not made evenly thinner;
some areas of the cavity are thinner than others.
The degree of thinning and the resulting barrier
must be simulated to distinguish truly reliable
Aluminum laminates are often superior in this
regard, providing a “trump card” of sorts. This is
because the aluminum in the laminate provides a
100% barrier against water vapor (and oxygen).
In addition, the barrier properties of aluminum
are independent of its thickness; a coldform foil
offers the same barrier against water vapor and
oxygen after deep drawing as it does before deep
Even if a coldform foil is chosen, such an examination can still make sense. As stated, coldform
foil offers the highest barrier against water vapor.
However, there are fine differences in coldform
barrier properties pertaining to the cross perme-ation effect. Namely, a very small amount of water
vapor may enter the cavity from the punched
edge of the blister.
A suitable simulation can demonstrate whether
it may be advisable to use coldform foil with polyethylene (PE) instead of polyvinyl chloride (PVC)
to diminish this possibility. Another viable option
may be a coldform foil with a desiccant to absorb
the moisture coming from cross-migration.
The forming performance of base laminates is limited, and this must be considered in the cavity design of blister packs. If a laminate is overstretched
during deep drawing – regardless of whether it is
a thermoform or coldform laminate – tears or pinholes could occur that critically diminish or even
entirely eliminate protection. Even if these errors
are detected in-line, a poor cavity design can result in escalated costs due to increased waste and
And then there’s the worse-case scenario: that
these defects are not caught – a daunting prospect considering that not all pinholes are created
equal. If pinholes form due to a defective cavity
design, coldform foil offers the advantage that
these defective blister packs can be identified
with a so-called “pinholes detector.” This type of
inspection does not exist for transparent thermoform blisters.
It is, then, critical to have a comprehensive
understanding of material limitations during the
development phase, especially as they pertain to
cavity forming. In most cases, FE-based (finite element) simulations can assist experts in making
This kind of simulation helps compensate for
the inconsistent degree of material stretching
that takes place during cavity formation. Here,
areas with the highest degrees of elongation cannot be predicted based on experience, but must
instead be calculated (simulated) as a large number of parameters are at play.
THE INFLUENCE OF TEMPERATURE
Typically, products being packaged are subjected
to a certain temperature influence because the lid
foil and base laminate are heated up during blister
sealing. The degree of heating is dependent upon
many parameters, such as the heat conductivity
of the laminate being used, its melting and sealing
temperatures, and its sealing time.
FE-based simulation tools can be used to estimate temperature influence – an important step
especially when packaging potentially tempera-ture-sensitive products. The results can affect not
only the laminate choice, but the specific foil/lam-inate combination.
For example, it could be beneficial to use coldform with PE over coldform with PVC, as the sealing temperature for PE laminates is lower.