緩沖包裝設(shè)計(jì)5步法

1、FIVE STEPS FOR PACKAGE CUSHION DESIGN緩沖包裝設(shè)計(jì)5步法IntroductionBetter Package and Product Design Saves Money and Improves Customer Satisfaction.Packaging can be unnecessarily expensive in a couple of ways:Inadequate design results in shipment damageOver-design or poor design (more protection than is requ

2、ired 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 utilization)

3、 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 cushioned pac

4、kages. 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 ruggedness of produc

5、ts. 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 Packaging.De

6、fine The EnvironmentShock: choose the most severe drop height you wish to protect against.Vibration: Determine a representative acceleration vs. frequency profile.Define Product FragilityShock: Determine the producfs shock damage boundaries.Vibration: Determine the producfs critical resonant frequen

7、cies.Choose The Proper CushioningSelect the most economical cushioning to provide adequate protection for both shock and vibration.Design and Fabricate The Prototype PackageTest The Prototype PackageShock: Use the “Step Velocity” test method.Vibration: Verify adequate protection at the critical freq

8、uencies.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 producfs needs for protection fr

9、om these hazards should also be incorporated. In some cases, only minor modifications may be required to account for these other factors after a sound, basic design for shock and vibration has been completed and tested.Step 1Define the EnvironmentShockIt is generally agreed that, regardless of the t

10、ransportation 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 that the most severe transmitted

11、 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 similar to the one shown in Figure 1

12、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 percentage of loss which can be tolerat

13、ed 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 procedure which identifies the produ

14、ct 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 andgsu-rM 工 ao5acceleration-frequency

15、 profiles such as shown in Figure 2, from a vibration acceleration envelopelike 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, experience, judgement, and

16、 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 able to sufficiently exc

17、ite the critical components over the range of frequencies occurring in the transportation environment (Generally 1-200 Hz or greater).Figure 4 Railcar Frequency Spectra-Summary of PSD dataIn summary, the first step in the package design is to select a design drop height and an acceleration-frequency

18、 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 acceleration level, i. e, how many

19、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 pulse duration is many t

20、imes longer). It is the package desi gners 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 to handling shocks. Th

21、ey permit construction of a “damage boundary” curve like that shown in Figure 5.5 xraoldVelocity Change. Inches/SecondsFigure 5 Typical Damage Boundary CurveozThe horizontal line of the boundary is at the peak acceleration value of the minimum damaging shock pulse. The vertical line of the boundary

22、is at the minimum 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 chang

23、e or lower peak acceleration.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 pl

24、ot (under the curve), damage does not occur, even for large velocity changes (drop heights). Thafs 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. Ho

25、wever, the acceleration value (to the right of the vertical line) of the damage 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

26、normally cannot be predicted, a trapezoidal pulse shape is typically used.Terminal Peak Sawtooth PulseHalf Sine Pulse -Trapezoidal PulseVelocity Change. Inches/SecondFigure 6 Damage Boundary for Pulses of Same Peak Acceleration and Same Velocity ChangeThe damage boundary generated with use of a trap

27、ezoidal pulse encloses the damage boundaries of all the other waveforms. This is a great advantage, since the 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

28、that actual shocks transmitted by the cushion will be equal to or less damaging than the test pulse.Fragility 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

29、standards such as ASTM1D3322-85. Use of a shock machine provides a convenient means of generating variable 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 co

30、ntrolled velocity changes and shock pulses. The shock table is raised to a preset drop height. It is then 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 tab

31、le and the base controls the type of shock pulse created on the table (and the test item mounted on it) during 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 co

32、mpressed gas pressure in the cylinder. The velocity change is controlled by adjusting drop height .Conducting 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

33、half sine or a rectangular pulse may be used to perform this test, since the critical velocity portion is 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 incre

34、ased to provide a higher velocity change. The second drop is made and again the specimen isexamined. 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

35、 testing is stopped, since the minimum velocity necessary to create damage has been established as well 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.400- 1st Drop Caus

36、ing Damage350-300-250-200-150-3rd Drop2ndDrop11st Drop .DamageLast Drop Without Damage10050-I I I I I Ii050100150200250300350400A V In/SecFigure 7 Velocity Damage Boundary DevelopmentIn some cases, it is sufficient to determine only this vertical line of the damage boundary. If the velocity change r

37、equired 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 for shock protection will be required. This

38、 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 at least 1.6 times the critical velocity. The

39、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 programmer pressure is increased to provide a

40、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 damage boundary curve. The damage boundary line f

41、alls 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 is actually rounded, not square. In most cases,

42、 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 7Bshows a typical damage boundary plotted by th

43、is method.Damage1st Drop Causing DamageLast Drop Without Damage:1st DropFigure 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 protectio

44、n is needed.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 p

45、erformed in 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

46、to occur when 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

47、may be represented 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.

48、 This is the frequency range where damage is most likely to occur.To actually determine a producfs 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 reson

49、ant frequencies. A test method often used to accomplish this is ASTM Standard Method D3580, Vibration (Vertical Sinusoidal Motion) Test of Products.Frequency. HzResponse AccelerationInput Acceleration 1ProductNatural FrequencyFigure 8 Typical Resonant Frequency Transmissibility CurveThe resonance se

50、arch is run on a vibration test machine (shaker). The item to be tested is fastened to 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 o

51、bserved 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 b

52、e recorded.In general, tests should be performed in each of the three axes, and three sets of critical frequencies recorded. If the product is mounted on a definite skid base, only the vertical axes need to be analyzed.To summarize Step 2, damage boundaries are determined and plotted, and critical f

53、requencies are identified.Step 3Choose the Proper CushioningUntil now, shock and vibration procedures have been separated. In Step 3, however, their effects must be considered simultaneously: the designer must specify cushioning which provides adequate protection for both shock and vibration.The key

54、 to selecting the most economical cushion protection is the use of “cushioncurves”.Two types of data are needed and must be used simultaneously: Shock Cushioning Curves and Vibration Transmissibility Data.ShockA. Shock Cushioning Curves; maximum transmitted shock acceleration vs. Static stress.A typ

55、ical example of this type of curve is shown in Figure 9. The cushion curve shows the peak acceleration that will be transmitted by various thicknesses of the cushion for different values of static stress (static stress is the weight of the packaged item in pounds divided by the cushion area in squar

56、e inches).To select the most economical cushion to use, you should review cushion curves for the same drop height as you selected in Step 1 as the design drop height. From these curves, select the cushion type and thickness to limit the peak transmitted acceleration to a level which is the same as,

57、or lower than, the damage g-level determined during fragility testing in Step 2. You must alsoconsider the most economical cushion configuration, i.e., full item area coverage, only partial area coverage, using corner pads, etc.Static Stress W/A (psi)Figure 9 Polyethylene, 2 pcf, 36 Drop Height Shoc

58、k Cushion CurvesUopedA large number of cushion curves have been generated and reported in the literatures concerned. In many cases, you can use existing curves. At times, particularly where newer materials are involved, it may be necessary to generate new data by conducting dynamic cushion tests to

59、develop cushion curves.Cushion tests are typically run in accordance with ASTM Test Method D1596-78 , ASTM Test Method D4168-82 or MIL-C-26861. Standard 8 inch x 8 inch cushion samples are normally used either as flat sheets or encapsulating designs. Both vertical drop and shock tests machines have

60、been employed for these tests. A dummy load or platen with an adjustable weight is used. The drop height is adjusted and the drop weight is instrumented with an accelerometer so the acceleration pulse during impact on the cushion can be recorded.Each test results in one data point. Peak acceleration