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STEP 4 - CUSHION MATERIAL PERFORMANCE EVALUATION
Material performance data should generally be available from the manufacturer of
the material. In certain instances, however, it may be necessary to generate
this type of data. This involves examining both the shock absorbing and
vibration transmission characteristics of the materials.
It should be noted that the data generated by these
methods is applicable to the cushion material only, and may not necessarily be
the same as the response obtained in a complete pack. In addition, specimen
area, thickness, loading rate and other factors will affect the actual
performance of the material in any given situation. What this means is that the
data can be used to provide a scientific best guess for the initial package
design, but some fine tuning may still need to occur.
SHOCK
A shock cushion curve describes the material in terms of the deceleration
transmitted to an object falling on that material at different static loadings.
One cushion curve is generated for each material type, material thickness and
drop height combination.
 The test
procedure is basically one of dropping a platen of specified weight from a known
drop height onto a cushion of predetermined bearing area and thickness. The
deceleration experienced by the platen at impact is monitored and recorded by an
accelerometer. Five drops from a particular drop height are performed on a
sample at a given static stress loading. The average of the deceleration
readings from the last four of these drops is the value used in plotting each
cushion curve point. By adding weights to the platen, the static stress on the
cushion material can be changed. Through a series of tests at various static
loadings, data is generated and presented in the form of cushion curves(see
Figure 10). A minimum of five static loadings are tested to plot each curve,
with a new sample being used at each loading.
It should be noted that these curves are "best fit"
curves. This is because they are generated from averaged drop data at each
static stress point on specific cushion samples. There is a certain inherent
variability in the manufacture of the material as well as the judgment involved
in drawing smooth curves from "non-classical" data. The effects of sample
variability, averaged data and curve fitting cannot be ignored, and therefore,
the curves must be properly interpreted.
Most cushion curves tend to have what is generally referred to as a "smile"
shape, see figure 10. At low static loadings, the materials transmit relatively
high accelerations. In this area, the impacting object does not have sufficient
force to deflect the material, thus the material does not act much like a
cushion. As the static loading increases, the transmitted accelerations tend to
drop. In this region, the object now is able to deflect the material and cause
it to work like shock absorber. At higher static loadings, the object deflects
the material so far that it bottoms out. This is what causes the transmitted
acceleration to rise along the right end of the curve.
VIBRATION
The amplification/attenuation curve defines the frequencies at which a cushion
material will amplify vibrational input and the frequencies at which it will
filter out or attenuate the vibration. One amplification/attenuation curve is
generated for each material type and material thickness combination.
To run the test, a block is monitored with a response
accelerometer and loaded with weight until it reaches the desired static stress
level when resting on a cushion sample. One cushion sample is placed below the
test block and another is placed above it. This whole configuration is then
placed in a corrugated container and secured to the table of the vibration test
machine. A resonance search test is performed and a transmissibility plot of the
cushion response is generated. The weight in the test block is then changed to
obtain the next desired static stress loading and the test is repeated with
fresh cushion samples. This process is repeated until the desired range of
static loading has been explored. A minimum of five static loading test points
are use to generate an amplification/attenuation curve.
Once all the transmissibility plots have been generated,
the data is plotted on the amplification/attenuation curve as shown in figure
11. The amplification/attenuation curve describes the vibration performance of
the material as a function of static loading and can be though of as a "top
view" of a series of related transmissibility plots.
In general, the shape of the amplification/attenuation curve slopes downward as static loading increases. This results from the basic characteristics of spring-mass systems. As static loading increases, the amount of weight supported by a given area of cushion increases. Since the cushion/spring characteristics have not changed, the natural frequency of the system tends to decrease.
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