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Lansmont Six-Step Method for cushioned package development | |||||||||||||||
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Developed by Dale Root |
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INTRODUCTION Once we have identified what inputs to expect from the shipping environment, and to what extent the unpackaged product can withstand these, we can go about making up the difference between these two levels with a cushioned package system.
Try to visualize the general concept of product/package design as the bar chart depicted in Figure 1. The shaded background can be thought of as the level of environmental intensity or severity for a given distribution channel. The product has some inherent ability withstand this abuse, however it usually is not rugged enough to make it through shipment on its own. The role of the package, therefore, is to make up the difference between what the environment has to offer and what the product can withstand. The ideal case, as depicted by the first product/package system bar, is where the package exactly makes up the difference between the product ruggedness and the environmental inputs. If the package falls short, as depicted by the second product/package system bar, "under packaging" has occurred and damage in shipment will most likely result. If the package provides too much protection, as depicted by the third product/package system bar, "over packaging" has occurred and money is being wasted on protection which is not required. In certain instances it will actually be cheaper to ruggedize the product rather than put an expensive package around each unit. This product improvement is depicted in the fourth product/package system bar. STEP 1 - DEFINE THE ENVIRONMENT It would be nice to follow every package through the distribution environment and observe what actually happens to it. Usually, however, we must accept another approach. The next best thing to being there is using some sort of recording device to monitor the package and/or the vehicle during shipment. Provided we do this enough times, we begin to gain some sort of statistically valid information which can be used to describe that particular channel of distribution. The events will obviously change from trip to trip, but in general we have an idea of what to expect. This is the best approach for gaining information about a specific distribution channel. Probably the most widely used approach, however, is to study available published data. The difficulty here is that the data is usually outdated, and was not originally recorded from the environment through which you will actually be shipping your package. In general, however, it may provide the guidelines and rules of thumb necessary for package design. The importance of this environmental information cannot be over stressed. This information will eventually become the part of the package design requirement and if not described correctly the package may appear to fail in distribution even though the design goals were met. In addition, over packaging may result if the actual inputs are lower than those chosen for the design goal.
SHOCK Of course not all packages are handled exactly the same way, even when shipped by the same carrier over the same route. Some packages may never be dropped, while others will fall from a height many times higher than anticipated. Some may fall on the bottom, and others on a side, the top, a corner, or an edge. What this means is that there is a certain inherent variability with the manner in which packages are handled.
Drop height information tied to probability of occurrence is the most accurate way to theoretically design a package and tailor it to meet a certain damage rate. For example, if we wanted to design a package that would arrive with less than 1% of the products damaged, based upon figure 2 we would select a design drop height of about 32 inches. If, however, we were willing to accept a 4% damage rate, then we could reduce the design drop height to 20 inches. On the other hand, if we insisted on having a damage rate less than 0.1% then our design drop height would jump up to 42 inches. This type of evaluation allows trade-offs between damage costs and packaging costs. In most cases a certain amount of damage is acceptable because of the expense associated with trying protect each and every unit from the highest level event.
Although designing a package with this type of information is obviously the most
informed approach, rarely is this type of detailed data available for your
particular package and channel of distribution. The next best approach is to
fall back upon some the general rules of thumb which have been developed in the
packaging industry over the years. These include data like the following table
which is presented in ASTM D-3332. This table describes drop height as a
function of package weight and indicates that light packages will fall farther
because they can easily be picked up and tossed about. Heavy packages, on the
other hand, usually require mechanical handling and therefore will not be
dropped as far.
1) The probability of a package being dropped from a high height is
minimal. The vibration encountered in the distribution environment is very complex in nature, consisting of intermixed frequency excitations emanating from a variety of sources. This type of vibration is often considered random in terms of the time domain because it is almost impossible to predict what will happen at any one instant. Yet in the frequency domain, a vehicle may display a very distinct signature which allows for the determination of the frequencies and levels which are present. Figures 3 thru 5 are derived from ASTM D-4728 and display the power spectral density characteristics for several types of vehicles. These plots define the vibration in terms of the average power associated with each frequency. It should be noted that these particular plots do not represent any one trip, but rather encompass the general characteristics of the vehicle type. It is generally thought that steady state vibration occurs at relatively low levels during shipment. ASTM D-4169 suggests that a 0.5 G sinusoidal input over a frequency range of 3 to 100 Hz can used as a rule of thumb to predict how a package will perform during truck shipment. STEP 2 - PRODUCT FRAGILITY ANALYSIS SHOCK: DAMAGE
BOUNDARY
When the velocity change of the input exceeds the critical velocity change of the product, however, the only way to avoid damage is to limit the input acceleration to a level below that of the product's critical acceleration. This is usually one of the functions that a cushioned package performs. It translates the high acceleration events experienced on the outside of the container to lower acceleration events experienced inside at the unit. To determine a damage boundary requires running two sets of tests. A step velocity test is used to determine the product's critical velocity change and a step acceleration test is used to determine the critical acceleration. 1) STEP VELOCITY TEST The critical velocity change can be equated to an equivalent free fall drop range (EFFDR). These numbers describe how far the unpackaged unit can fall onto a rigid surface before damage will occur. The process for calculating the EFFDR is demonstrated in figure 7. In essence, it describes the ranges of heights that the bare product may fall before damage will occur, based upon the type of surface it impacts. Undoubtedly, if you drop the unit onto something soft, you can drop it farther without damage than you could if you dropped onto something hard. This is why a range of heights is defined, rather than one specific height above which damage will occur and below which damage will not occur. 1) STEP ACCELERATION TEST The designer now has all the information necessary to set the shock protection requirements of his product. The critical velocity change shows the designer the maximum drop height the bare product can be subjected to before damage will occur. If that drop height is less than the design drop height specified in STEP 1, a package or interface material is necessary. If a package system is necessary, it must transmit less than the critical acceleration to the unit, when dropped from the design drop height. In a rigorous testing program, damage boundary curves are generated for each orientation of the unit. To do this requires damaging 2 units per orientation, one for the step velocity test and one for the step acceleration test. Rarely are this many units available for destructive testing in the prototype stage of a product's life, the most beneficial time to do this type of work. Compromises are often made to limit the number of units which must be damaged. In certain situations it is possible to perform the testing only along the three orthogonal axes. In addition, the unit can often be repaired between tests, so that one unit and a few spare parts may be used to perform all of the testing.
In addition to the engineering reasons, there are also the economic and practical reasons for using the square wave to determine the critical acceleration. As can be noted from figure 8, the square wave produces a flat horizontal line to bound its damage region while other waveforms tend to make scalloped upward sloped shapes. Because of the square wave's flat line, we can define the critical acceleration with just one set of tests. This is something that can be done in an afternoon. To define the actual shape of the damage region for other waveforms, requires performing tests along the entire length of the velocity change axis. This approach can damage hundreds of units and require weeks or even months to complete. Although it may provide more precise results for a given waveform, it says nothing about how other waveforms may cause damage. Usually the expense and effort to define the critical acceleration in this manner is not warranted because no practical benefits are gained. VIBRATION: RESONANCE SEARCH & DWELL
Typically, the ratio of the component response to the table input acceleration is plotted as a function of frequency and is called a transmissibility plot (see Figure 9). The transmissibility ratio (response divided by input) reaches a peak at the natural or resonant frequency of the component. The plotting of this ratio comprises the resonance search portion of the testing. Once the product resonant frequencies have been determined, the vibration system is tuned to those frequencies and the product if forced to dwell there for a predetermined length. This will identify those frequencies which are prone to induce damage or fatigue. If the unit is likely to be shipped in more than one axis, the vibration sensitivities of the product in those axes should be investigated as well. STEP 3 - PRODUCT IMPROVEMENT FEEDBACK Based upon the results of the fragility tests, it may be desirable to strengthen or ruggedize the product rather than ship each one inside an expensive package. Trade-offs between product cost, product reliability, and packaging costs should be identified and ranked for effectiveness. Often times it is possible to raise the fragility level of a product with minor modifications or design changes. This may add a slight cost to each product, but if the packaging requirements drop significantly the total system price goes down. The ability to get product modifications implemented can vary widely depending upon the atmosphere within each company, and the position of the individual trying to get it accomplished. For some companies, this type of feedback to product designers is a formal step used in developing all new products. This allows the product to become more reliable, of better quality, and also keeps packaging costs to a minimum. In other situations, particularly when packaging is being developed by an outside supplier, it can be almost impossible to convince a company that making changes to the product is in their own best interest. It is, however, still important to present these ideas and take the role of educator where needed. If possible, try to identify the trade-offs between minor product changes, reliability and repair costs, and packaging expenses. STEP 4 - CUSHION MATERIAL PERFORMANCE EVALUATION 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
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. VIBRATION 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. STEP 5 - PACKAGE DESIGN VIBRATION Once the static loadings which appear to provide adequate shock and vibration protection have been identified, material and thickness selections can be made. The actual static loading which is chosen for the package is dependent upon several factors, however designing at the highest possible static loading means using less material. When other considerations such as compressive creep are important, designing at the lowest possible static loading may be warranted. Figure 14 displays the method for calculating the amount of material which must be used around a product to reach a desired static loading. DESIGN CONSIDERATIONS STEP 6 - TEST THE PRODUCT/PACKAGE SYSTEM SHOCK Flat drops are usually thought to be the most severe drops possible in terms of the acceleration level transmitted to the product. Flat drops focus all of the input along one axis of the unit and little energy is lost to the crushing of corners or edges of the package, or to package rotation. Flat drops, therefore, are used to measure the performance of the package system. Corner and edge drops, however, often cause damage to the structure of the package which similar flat drops do not. These types of drops are often used as part of a test sequence to verify the package's ability to hold together during shipment. VIBRATION To run the package response vibration tests, the product
is again fitted with a response accelerometer attached at a rigid location. The
product is then placed in the prototype package system and secured to the table
of a vibration test machine. The package is subjected to a low level sinusoidal
input over Once the package resonant frequencies have been determined, dwell tests are performed at those frequencies. In addition, dwell tests may be performed at each of the product resonant frequencies. Random vibration tests may also be conducted in which the vibration system is programmed to mimic the real world motions of transportation vehicles. SUMMARY |
Ideally, the package system will provide enough protection
to exactly match the requirements of the product and distribution environment.
There are, however, two pitfalls which may occur if a systems approach to
package design is not adopted. In the first situation, the package falls short
of the protection requirements and a significant amount of damage occurs during
shipment. This "under-packaging" is fairly obvious to detect, but is avoidable
and easily corrected with changes to the method of shipment, package design,
product design, or combinations of the above. In the second situation little or
no damage occurs, but the product is "over packaged". In effect, the package is
providing more protection than is required. Just as "under-packaging" wastes
money through damaged product and loss of customer good will, "over-packaging"
siphons money directly from a company's bottom line.
Figures 2 and 3 are examples of a few
the ways in which drop data may be presented. Figure 2 describes drop height for
a particular package in terms of its probability of occurrence over a given
distribution route. This chart indicates that low level drops occur frequently,
while very high level drops are rare. Although this is just an example plot, the
general thrust of the data is valid. Many small drops can occur during normal
handling when the package is picked up, set down and just plain jostled around.
Large drops, however, usually only result from accidents such as a package
falling off the top of a stack or loading platform.
The following are some of the general conclusions which were presented by Ostrem and Godshall
in "An Assessment of the Common Carrier Shipping Environment" published by the
Forest Products Laboratory, U.S. Department of Agriculture in 1979.
A typical
damage boundary curve can be found in Figure 6. It is a plot of the damage
causing parameters of a shock pulse and defines the region where certain
combinations of acceleration and velocity change will cause damage. If the
combination of acceleration and velocity change fall in the clear band outside
the damage region, no damage occurs. For example, if the velocity change of the
input is below that of the product's critical velocity change, then the
acceleration level of the input can be in the 100s G's, 1000s G's, 10,000s G's,
or even infinite without causing damage. In practical terms, to achieve these
very high accelerations means that the duration of the shock must be very short.
If we think of it in a graphical sense, we have a certain amount of area or
velocity change which we can re-arrange to produce a shock pulse. If we decide
to make the pulse very tall, i.e. very high in acceleration, then because of the
limited amount of area that the pulse can contain it must also be very short in
duration. In fact, the duration is so short that the product cannot respond the
acceleration level of the event, only the energy input. Because the input
velocity change did not exceeded the critical velocity change of the product, no
damage has occurred.
To run the step velocity test, the unit is fixtured to the
table of the shock machine and subjected to a short duration pulse with a
relatively low velocity change content. It is important that the duration of the
event be so short that the unit cannot respond to the acceleration level of the
shock, only the velocity change. Most commercial machines intended for use in
this type of testing produce a half sine pulse 2-3 msec in duration. Following
the input, the test unit is examined for functional, physical and aesthetic
damage. If none has occurred, it is given an input with a slightly larger
velocity change component, but with roughly the same duration. This process is
repeated until damage to the unit has occurred. The last non-failure input
defines the critical velocity change for the unit in the orientation in which it
was tested.
It should be noted that the square wave used to determine
the critical acceleration provides conservative results. In general, a square
wave of a given acceleration and duration is the most severe waveform possible.
It contains not only the fundamental frequency associated with the pulse
duration, but also all the higher harmonics associated with the quick rise and
decay of the waveform. What this means is that square wave can cause damage, at
a given acceleration level and velocity change, while other waveforms do not.
Figure 8 displays examples of the damage regions which various waveforms may
produce. As can be observed form this plot, the square wave encompasses all
damage regions produced by other waveforms. This is useful to the package
designer, because in the early stages of development it is not known what shape
waveform the package will transmit to the product. By using a square wave for
this test, we gain confidence in our final package design. We know that if the
unit can pass a square wave of a given velocity change and acceleration level,
we can pass any waveform that our package may produce of the same velocity
change and acceleration level.
To run the vibration test, the unit
is secured to the table of a vibration test machine and subjected to a low level
sinusoidal input over a broad frequency range. The product can be observed for
resonances either visually, audibly, or fitted with response accelerometers
attached to its critical components. If the unit has been instrumented, the
table input and component responses are monitored through out the test.
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.
The package designer now has all the information necessary to adequately protect
the product during distribution. Step 1 defined the types of inputs that the
package will receive during shipment. Step 2 determined the ruggedness of the
product and thus its ability to withstand the environmental inputs.
Step 3
evaluated the ruggedness to allow for product redesign. Step 4 defined the
performance characteristic of packaging materials. Its now time to combine this
information into a package design.
For vibration consideration, we need
to collect amplification/attenuation curves for the selected materials. Locate
the lowest product natural frequency on each of the curves and draw a horizontal
line across the plot. Any portion of the line which extends into the attenuation
zone indicates the static loading range where the material should attenuate
vibration at the frequencies where the product is most sensitive, see figure 13.
The package design must be able to achieve both
the shock protection requirements and the vibration protection requirements of
the unit. This can sometimes present a challenge since the best design from a
shock standpoint is rarely the best design from a vibration standpoint, and vice
versa. Often times, due to material limitations, compromises need to be made.
When this is the case, intelligent decisions can be based upon the facts and
techniques used for Steps 1 and 2. We know, for instance, that vibration is a
certainty. We will encounter vibration no matter which method of shipment is
used because it is inherent to vehicles as they travel. Drops, on the other
hand, have a certain probability associated with them. Not all packages will be
dropped and certainly not all will be dropped from the design drop height. In
addition, the critical acceleration of the unit was determined in a conservative
manner. The package system will most likely not transmit a rectangular waveform
to the product. Thus the product should be able to withstand somewhat higher
accelerations than were predicted by the step acceleration test because the
actual transmitted waveform is less damaging. What this means is that it is
usually best to lean toward vibration protection when compromises need to be
made. Of course this will depend upon the individual situation, however, in
general this is the optimum approach.