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Roy Christopherson, REXAM Flexibles Ltd., Bristol, England
Typical syringe and packaging
The use of thin flexible films for the packaging of disposable sterile
medical devices is a large and growing part of the medical packaging market.
In most cases, the packaging format used for medical devices is a formed
pack produced using a thin polymeric film sealed to a top web of paper,
which permits the ingress of the sterilization gas but is resistant to
bacterial penetration post sterilization. In addition, to keep the cost
of the packaging to a minimum and to reduce environmental impact, it is
desirable to use as thin a polymeric web as possible. In the case of a
syringe pack, the film thickness may typically be 65–150m, reducing
to as low as 15–35m in the corners after the thermoforming process.
This is adequate for providing a sterile environment, but may not be sufficiently
rugged for the life and demands of the packaging. For instance, during
transit from the manufacturing site to the end user, it is important that
the package remains intact with no holes or pits forming in the film.
A small hole of 10 microns will allow airborne bacterial spores to ingress
into the pack, leading to a sterilization failure.
At REXAM, one of the top consumer packaging companies in the world,
transit tests have been devised to simulate and quantify levels of packaging
failure for syringe packs. The rates of failure typically average less
than 0.2%, with the two primary causes being abrasion and puncture by
the syringe. Failure due to puncture was of primary interest to REXAM
engineers. They wanted to develop a technique to predict failure accurately
and use this knowledge to “reverse engineer” their packaging,
so that it would be less prone to puncture. The approach they chose involved
two computational software packages: POLYFLOW, to model the thickness
distribution of the thermoformed pack; and MSC.Marc™, a stress analysis
code, to model the strain rate of the thermoformed packaging and predict
probabilities for puncturing the pack. When combined, these two simulation
techniques could be powerful predictors of mechanical strengths for a
given type of syringe packaging.
POLYFLOW CFD simulation of the “coffin” thermoformed product
packaging, showing the thickness distribution. Predictions for thickness
at five locations on the coffin surface were found to be in very good
agreement with experimental measurements for both materials tested.
REXAM engineers validated their modeling approach for a typical 10ml
syringe package using two different film packaging materials. Both films
were thermoformed into a “coffin” style die for the 10ml syringe.
In the experimental tests, randomly chosen packs were punctured using
a Lloyd Tensile Tester. CFD models for the two cases were set up in POLYFLOW,
using the physical properties, including the special rheological behavior,
of each material used. A membrane approximating approach was used to
simulate the thermoforming process in order to reduce the computational
time required. The CFD predictions were in excellent agreement with measurements
for one of the films, and in good agreement for the other. The puncture
resistance simulations using MSC.Marc were also in very good agreement
with measurements, thus confirming the suitability of this dual simulation
approach for analyzing this type of film packaging. REXAM believes that
the ability to assess material changes in the packaging design will lead
to significant time and cost savings in their manufacturing processes
in the future.
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