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What
is a Creep Test?
Pressure vs. Time Relationship to Seal Strength
Recommend Pressure Level and Time
A Go/No Go Test
The
Creep test is a timed test that holds a constant
pressure in the package at some predetermined
critical level. The pressures and times chosen
for the test are very much dependent on the package
design, including its materials, seal design and
processes of manufacturing.
The
Creep test is more analogous to the tensile pull/
peel test in that the forces are applied in a
manner to attempt to peel the seal interface apart
in a given cycle. In this manner, the test provides
a measure of the ability of the seal to resist
sustained pressure differentials such as occur
in sterilization cycles or air transport. The
creep test is best used in conjunction with a
visual analysis of the part. The visual observation
can note the degree of peel and the location of
the weakened seal.
This
graph shows the relationship of the pressure and
time. As you can see, the pressure rises and is
maintained at a constant level during the test
time. If the seal does not rupture during the
test time, then the package is considered to PASS.
Flow rate is not an important parameter in this
test other than to control the rate of rise preceding
the start of the test cycle time. The flow rate
does not want to be too fast to create a "hammer"
effect on the seal, nor too slow to cause an effective
increase in test time that the seal is under pressure.
The
industry has generally used a Pressure value of
80% of the Burst test pressure to perform a Creep
test. This value is empirically derived and used,
I suspect, because it generally works! The value
provides a sufficient load on the seals to be
significant while meeting the time restrictions
for a rapid test.
Regarding
the time used in Creep testing, a time of 15 to
20 seconds has generally been used in the industry
for the Creep test. This time in conjunction with
the Pressure value provides a fast and convenient
and Consistent test of the seal resistance to
induced loads.
The
Creep test is a Go/ No Go attribute test. The
package is said to Pass if it does not rupture
or open during the test time. This test does not
provide a quantitative measure of seal strength
as found in the Burst test. In fact, an interpretation
of the results of the test may be necessary by
visual observation of the degree of peeling in
the seal area from package to package.
This paper is presented courtesy
of TM Electronics, Inc.
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