1、32FIVE STEPS FOR PACKAGE CUSHION DESIGN缓冲包装设计 5 步法IntroductionBetter Package and Product Design Saves Money and Improves Customer Satisfaction.Packaging can be unnecessarily expensive in a couple of ways:1. Inadequate design results in shipment damage2. Over-design or poor design (more protection th
2、an is required or materials being incorrectly used ) results in excessive material cost.High cost of damage in shipment should be unacceptable to those who are aware of the claims costs and the lost customers. Conversely, the cost of waste resulting from over-packaging (poor and unneeded material ut
3、ilization) is less visible and more difficult to aggressively pursue. This total waste, estimated at billions of dollars, could be significantly reduced if packages were properly designed for shock and vibration protection.This text describes a basic procedure for logically designing and testing cus
4、hioned packages. The techniques outlined here are not new. Nevertheless, the logical, step-by-step procedures are not yet universally used by all package designers . Increasingly the theories and techniques presented here are also being used by product designers to evaluate and improve the ruggednes
5、s of products. Indeed, often it is more economical to permanently improve products than to provide temporary cushioning which will later be discarded.The procedure can be broken down into five basic steps. This 5 Step Method was developed in conjunction with the Michigan State University School of P
6、ackaging.1. Define The EnvironmentShock: choose the most severe drop height you wish to protect against. Vibration: Determine a representative acceleration vs. frequency profile.2. Define Product FragilityShock: Determine the products shock damage boundaries. Vibration: Determine the products critic
7、al resonant frequencies.3. Choose The Proper CushioningSelect the most economical cushioning to provide adequate protection for both shock and vibration.4. Design and Fabricate The Prototype Package5. Test The Prototype PackageShock: Use the “Step Velocity” test method.Vibration: Verify adequate pro
8、tection at the critical frequencies.This chapter discusses only shock and vibration. Other environmental factors such as compression, humidity, temperature, and other potentially destructive forces should also be considered in designing and testing a package. A similar, logical treatment of the prod
9、ucts needs for protection from these hazards should also be incorporated. In some cases, only minor modifications may be 33required to account for these other factors after a sound, basic design for shock and vibration has been completed and tested. Step 1 Define the EnvironmentShockIt is generally
10、agreed that, regardless of the transportation mode, the most severe shocks likely to be encountered in shipping result from handling operations. These result from dropping the package onto a floor, dock or platform. Of course, many kinds of drops are possible (flat, corner, edge, etc.), but we know
11、that the most severe transmitted shock occurs when a cushioned package lands flat on a nonresilient horizontal surface. It is reasonable, then, to design cushioned packages for this flat drop.In designing for shock protection, the first consideration is selecting the design drop height. Charts simil
12、ar to the one shown in Figure 1 will be helpful. The chart takes into consideration both the package weight and the probability of drops occurring from specified heights. When selecting the probability level, factors such as the relative costs of products and package, shipping costs, and the percent
13、age of loss which can be tolerated must be considered.VibrationThe transportation vibration environment is complex and random in nature. The basic method of testing for package design is not to simulate the vibration environment, but rather to simulate its damage-producing capabilities. Thus, a proc
14、edure which identifies the product and component resonant frequencies, and which leads to protection at those frequencies, can be expected to produce effective result .Figure 1 Probability Curves for Handling ShocksYou may select acceleration levels and frequency ranges from environmental data and 3
15、4acceleration-frequency profiles such as shown in Figure 2, from a vibration acceleration envelope like that in Figure 3, or, from a power spectral density summary plot as shown in Figure 4. Acceleration levels and frequency ranges you select must be consistent with the available additional data, ex
16、perience, judgement, and knowledge about the product.Figure 2 Frequency Spectra for Various Probabilities-Railroad(vertical direction, composite of various conditions)Figure 3 Vibration Acceleration Envelope-RailcarThe actual shape of the acceleration-frequency profile is not as important as being a
17、ble to sufficiently excite the critical components over the range of frequencies occurring in the transportation environment (Generally 1-200 Hz or greater).35Figure 4 Railcar Frequency Spectra-Summary of PSD dataIn summary, the first step in the package design is to select a design drop height and
18、an acceleration-frequency profile.Step 2 Define Product FragilityShockShock damage to products results from excessive internal stress induced by inertia forces. Since inertia forces are directly proportional to acceleration (F=ma), shock fragility is characterized by the maximum tolerable accelerati
19、on level, i. e, how many gs the item can withstand.When a dropped package strikes the floor, local accelerations at the container surface can reach several hundred gs. The packaging material changes the shock pulse delivered to the product so that the maximum acceleration is greatly reduced (and the
20、 pulse duration is many times longer). It is the package designers goal to be sure that the g-level transmitted to the item by the cushion is less that the g-level which will cause the item to fail.Shock Spectrum and Damage Boundary Theory are techniques for characterizing the resistance of products
21、 to handling shocks. They permit construction of a “damage boundary” curve like that shown in Figure 5.Figure 5 Typical Damage Boundary CurveThe horizontal line of the boundary is at the peak acceleration value of the minimum damaging shock pulse. The vertical line of the boundary is at the minimum
22、velocity change (drop height), necessary to cause damage. A plot like this can be determined for any product. A shock pulse which falls within the shaded area (sufficient acceleration and velocity change), will produce damage. No damage will occur for pulse with less velocity change or lower peak ac
23、celeration.The low-velocity portion of the plot (at the left) is that area where damage does not occur even with very high accelerations. Here the velocity change (drop height) is so low that the item acts as its own shock isolator. Below the acceleration boundary portion of the plot (under the curv
24、e), damage does not occur, even for large velocity changes (drop heights). Thats because the forces generated (F =ma) are within the strength limits of the products.Figure 6 shows that the velocity change boundary (vertical boundary line), is independent of the pulse wave shape. However, the acceler
25、ation value (to the right of the vertical line) of the 36damage boundary curve for half sine and sawtooth pulses depends upon velocity change. Use of this damage boundary would require accurate prediction of drop heights and container/ cushion coefficients of restitution. Since they normally cannot
26、be predicted, a trapezoidal pulse shape is typically used. Figure 6 Damage Boundary for Pulses of Same Peak Acceleration andSame Velocity ChangeThe damage boundary generated with use of a trapezoidal pulse encloses the damage boundaries of all the other waveforms. This is a great advantage, since th
27、e wave shape which will be transmitted by the cushion is usually unknown. By using the trapezoidal pulse to establish the acceleration damage boundary rating, the package designer can be sure that actual shocks transmitted by the cushion will be equal to or less damaging than the test pulse. Fragili
28、ty testing is the process used to establish damage boundaries of products. It is usually conducted on a shock testing machine. The procedure has been standardized and incorporated into several standards such as ASTM1 D3322-85. Use of a shock machine provides a convenient means of generating variable
29、 velocity changes and consistent, controllable acceleration levels and waveforms. Typically, the item to be tested is fastened to the top of a shock machine table and the table is subjected to controlled velocity changes and shock pulses. The shock table is raised to a preset drop height. It is then
30、 released, free falls and impacts against the base of the machine; it rebounds from the base and is arrested by a braking system so that only one impact occurs. A shock programmer between the table and the base controls the type of shock pulse created on the table (and the test item mounted on it) d
31、uring impact. For trapezoidal pulses used in fragility testing, the programmer is a constant force pneumatic cylinder. The g-level of the trapezoidal pulse is controlled simply by adjusting the compressed gas pressure in the cylinder. The velocity change is controlled by adjusting drop height .Condu
32、cting a fragility testTo conduct a fragility test, shock machine drop height is set at a very low level to produce a low velocity change, and the product is secured to the table surface. Either a half sine or a rectangular pulse may be used to perform this test, since the critical velocity portion i
33、s the same. A half-sine shock pulse waveform programmer is normally used for convenience. The first drop is made and the item examined to be sure damage has not occurred. Drop height is then increased to provide a higher velocity change. The second drop is made and again the specimen is 37examined.
34、Additional drops are made with drop height gradually increasing until failure occurs. The velocity change and peak acceleration are recorded for each impact. Once damage occurs, the velocity boundary testing is stopped, since the minimum velocity necessary to create damage has been established as we
35、ll as the velocity change portion of the damage boundary curve (See Figure 7). The damage boundary line falls between the last drop without damage and the first drop causing damage. Figure 7 Velocity Damage Boundary DevelopmentIn some cases, it is sufficient to determine only this vertical line of t
36、he damage boundary. If the velocity change required to damage the product will not be encountered from normal drops expected in the environment, no cushioning will be needed. However, if the product is damaged at levels which will be encountered in the environment, product improvements or cushioning
37、 for shock protection will be required. This indicates a need to establish the horizontal line of the damage boundary. Determining the acceleration boundary line requires that a new test specimen be attached to the shock table. The drop height is set at a level which will produce a velocity change a
38、t least 1.6 times the critical velocity. The programmer compressed gas pressure is adjusted to produce a low g-level shock which is lower than the level which you anticipate will cause damage to the product. Again, a first drop is made and the item is examined for damage. If none has occurred, the p
39、rogrammer pressure is increased to provide a higher g-level impact from the same drop height. Another drop is made and again the specimen is examined. The procedure is repeated with gradually increasing g-levels until damage occurs. This level establishes the level of the horizontal line of the dama
40、ge boundary curve. The damage boundary line falls between the last drop without damage and the first drop causing damage. You can plot the damage boundary curve by connecting the vertical velocity boundary line and the horizontal acceleration boundary line. The corner where the two lines intersect i
41、s actually rounded, not square. In most cases, this rounded corner will not be in the range of interest and a square corner can be used. If, however the corner is in the range of interest, the shape of the corner can be determined by calculation or by running an additional test in the area. Figure 7
42、B 38shows a typical damage boundary plotted by this method.Figure 7B Damage Boundary Line DevelopmentTwo things may be learned from the damage boundary plot.1.If the velocity change which the packaged item will experience is below the critical velocity, no cushioning for shock protection is needed.
43、2.If the velocity change which the packaged item will experience is above the critical velocity, a cushion should be designed so that it transmits less acceleration than the critical acceleration level.In most cases, where a product might be dropped on any of its sides, tests should be performed in
44、each direction in each of the 3 axes, and a total of 6 damage boundaries established. VibrationIt is generally accepted that the steady-state vibration environment is of such low acceleration amplitude that failure does not occur due to nonresonant inertial loading. Damage is most likely to occur wh
45、en some element or component of a product has a natural frequency which is excited by the environment. If this tuned excitation is of sufficient duration, component accelerations and displacements can be amplified to the failure level. Response of a product or component to input vibration may be rep
46、resented by a curve similar to that shown in Figure 8.You can see that for very low frequencies, response acceleration is the same as the input; for very high frequencies, the response is much less than the input. But in between, the response acceleration can be many times the input level. This is t
47、he frequency range where damage is most likely to occur.To actually determine a products vibration fragility would involve complexities which are probably not justified in terms of greatly improved results. The product test method, then, involves identifying the product and component resonant freque
48、ncies. A test method often used to accomplish this is ASTM Standard Method D3580, Vibration (Vertical Sinusoidal Motion) Test of Products.39Figure 8 Typical Resonant Frequency Transmissibility CurveThe resonance search is run on a vibration test machine (shaker). The item to be tested is fastened to
49、 the shaker table and subjected to vertical sinusoidal motion according to the acceleration-frequency profile selected in Step 1. As the frequency is slowly varied between lower and upper limits, the test item is observed for resonances. Sometimes, if non-critical product panels, etc. , or other shielding external components are removed, resonant effects can be seen or heard directly. At other times, use of a stroboscope and/or various sensors may be necessary. The critical frequencies and components should be recorded.In general, tests should be performed in each of the three axes,
