缓冲包装设计5步法.doc

32 FIVE STEPS FOR PACKAGE CUSHION DESIGN 缓冲包装设计缓冲包装设计 5 5 步法步法 Introduction Better 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 damage 2 Over design or poor design more protection than 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 utilization 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 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 ruggedness 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 Packaging 1 Define The Environment Shock choose the most severe drop height you wish to protect against Vibration Determine a representative acceleration vs frequency profile 2 Define Product Fragility Shock Determine the product s shock damage boundaries Vibration Determine the product s critical resonant frequencies 3 Choose The Proper Cushioning Select the most economical cushioning to provide adequate protection for both shock and vibration 4 Design and Fabricate The Prototype Package 5 Test The Prototype Package Shock Use the Step Velocity test method Vibration Verify adequate protection 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 product s needs for protection from these hazards should also be incorporated In some cases only minor modifications may be 33 required to account for these other factors after a sound basic design for shock and vibration has been completed and tested Step 1 Define the Environment Shock It is generally 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 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 similar 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 percentage of loss which can be tolerated must be considered Vibration The 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 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 Shocks You may select acceleration levels and frequency ranges from environmental data and 34 acceleration 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 experience 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 Railcar The actual shape of the acceleration frequency profile is not as important as being able to sufficiently excite the critical components over the range of frequencies occurring in the transportation environment Generally 1 200 Hz or greater 35 Figure 4 Railcar Frequency Spectra Summary of PSD data In summary the first step in the package design is to select a design drop height and an acceleration frequency profile Step 2 Define Product Fragility Shock Shock 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 g s the item can withstand When a dropped package strikes the floor local accelerations at the container surface can reach several hundred g s 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 times longer It is the package designer s 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 They permit construction of a damage boundary curve like that shown in Figure 5 Figure 5 Typical Damage Boundary Curve The 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 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 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 plot under the curve damage does not occur even for large velocity changes drop heights That s 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 acceleration value to the right of the vertical line of the 36 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 normally cannot be predicted a trapezoidal pulse shape is typically used Figure 6 Damage Boundary for Pulses of Same Peak Acceleration and Same Velocity Change The damage boundary generated with use of a trapezoidal 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 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 standards such as ASTM1 D3322 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 controlled 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 table 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 compressed gas pressure in the cylinder The velocity change is controlled by adjusting drop height Conducting a fragility test To 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 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 increased to provide a higher velocity change The second drop is made and again the specimen is 37 examined 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 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 Figure 7 Velocity Damage Boundary Development In some cases it is sufficient to determine only this vertical line of the 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 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 at 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 programmer 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 damage 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 is 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 7B 38 shows a typical damage boundary plotted by this method Figure 7B Damage Boundary Line Development Two 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 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 each direction in each of the 3 axes and a total of 6 damage boundaries established Vibration It 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 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 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 This is the frequency range where damage is most likely to occur To actually determine a product s 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 frequencies A test method often used to accomplish this is ASTM Standard Method D3580 Vibration Vertical Sinusoidal Motion Test of Products 39 Figure 8 Typical Resonant Frequency Transmissibility Curve The resonance search 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 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 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 frequencies are identified Step 3 Choose the Proper Cushioning Until 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 to selecting the most economical cushion protection is the use of cushion curves Two types of data are needed and must be used simultaneously Shock Cushioning Curves and Vibration Transmissibility Data Shock A Shock Cushioning Curves maximum transmitted shock acceleration vs Static stress A typical 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 square 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 or lower than the damage g level determined during fragility testing in Step 2 You must also 40 consider the most economical cushion configuration i e full item area coverage only partial area coverage using corner pads etc Figure 9 Polyethylene 2 pcf 36 Drop Height Shock Cushion Curves A 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 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 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 is read directly from the oscilloscope trace or from the wave form analyzer Static stress is calculated by dividing the weight of the platen in pounds by the cushion area in square inches The peak transmitted acceleration level data points for the static stress loadings are recorded for each cushion thickness and drop height and provide the basis of establishing the cushioning curves Vibration B Vibration Transmissibility Data Vibration natural frequency vs static stress A typical example of this type curve is shown in Figure 10 It shows the natural frequency of the product cushion combination for a specific cushion static loading combina