OVERPACKAGING. Throwing Away Money and Clogging Landfills in the Name of Safe Product Delivery PREPARED BY HERBERT H. SCHUENEMAN, CP-P/MH

Transcription

1 OVERPACKAGING Throwing Away Money and Clogging Landfills in the Name of Safe Product Delivery PREPARED BY HERBERT H. SCHUENEMAN, CP-P/MH PRESENTED BY Westpak, Inc. 83 Great Oaks Blvd. San Jose, CA (408) FAX (408)

2 It's generally accepted that electronics products are often overpackaged in the United States, resulting in an enormous amount of waste both in terms of dollars and bulging landfills. In this article we'll examine some of the reasons and some of the things that can be done about it. Much of the overpackaging comes from the conservative bias built into the 5-Step or 6-Step method (see below) for protective package engineering and testing. Let's examine each step in detail to see how this occurs. 5 STEP PACKAGING DEVELOPMENT 1. Define the Environment 2. Define Product Fragility 3. Choose the Proper Cushioning 4. Design and Fabricate the Prototype Pack 5. Test the Prototype Package This pioneering document was published in the early 70's by MTS Systems Corp. and became an instant success. It simplified the process of protective package design with the use of product fragility data. 6 STEP METHOD FOR CUSHIONED PACKAGE DEVELOPMENT 1. Define the Environment 2. Product Fragility Analysis 3. Product Improvement Feedback 4. Cushion Material Performance Evaluation 5. Package Design 6. Test the Product/Package System This six step was published by Lansmont Corporation originally in the mid 70's in order to avoid conflict with the five step. It is today the most widely used procedure for protective package design and testing. DEFINING THE ENVIRONMENT The first step involves defining the distribution environment through which a packaged product must travel. For shock inputs this amounts to defining the drop height that a package is likely to experience. Studies clearly show that package drop height decreases as weight increases, yet this relationship has never been defined very well. Many engineers use the drop heights for a lighter product when setting their specifications in order to be "more assured" of successful delivery. The result is conservatism in the environmental definition. In addition the drop height selected for an individual weight category is normally the 3-Sigma level which means that 99.4% of the impacts are below that level. Certainly there are some products for which a 3-Sigma level is appropriate, but for most products, this results in another conservative bias to the packaging equation.

3 PRODUCT FRAGILITY ASSESSMENT Another source of overpackaging comes from the method by which we determine product fragility. Shock fragility assessment uses the Damage Boundary test procedure according to ASTM D For cushioned (acceleration sensitive) products, the most important number from this test procedure is the critical acceleration determined by means of a trapezoidal waveform using ASTM D , Method B 2. The trapezoidal pulse is used because it excites both even and odd harmonics within the product resulting in the maximum response of suspended components and therefore failure at a lower input acceleration level. This is the first lesson we learn from Shock Response Spectrum (SRS) analysis. Figure 1: Shock Spectra, Various Pulse Shapes (Source 1) The trapezoidal pulse is also used because it results in a horizontal abscissa on the Damage Boundary plot. Therefore only one test is necessary to characterize the critical acceleration level over a wide range of velocity changes.

4 Figure 2: (Source 2) Perhaps the primary reason for using a trapezoidal pulse is that it's easy to program on most shock test machines by means of a pneumatic pressurized gas cylinder. Acceleration and velocity change levels are easily varied by adjusting the table drop height or pressure in the gas cylinder. On the other hand, programming precise half sine levels is often a tedious task of interchanging elastomeric pads and changing drop heights in order to get the correct acceleration, velocity change, and waveforms. The price we pay for this convenience may be enormous. Most waveforms transmitted through package cushion materials are half sine in shape, and I have personally never seen anything that approaches a trapezoidal pulse during package drop testing. Thus the use of a trapezoid to determine product fragility may be considerably more conservative than what we had originally believed. It's interesting to note that a much less conservative procedure formerly existed for product shock fragility assessment. ASTM D utilized product impacts onto cushion materials in order to generate the required excitation of the product. The thickness of the cushions was decreased in order to raise the acceleration level. This method was elegantly simple and resulted in a potentially more accurate fragility assessment due primarily to the fact that the waveform was the one that would actually be generated during an impact. However, ASTM withdrew this standard in Copies are still available, but it is not a sanctioned or approved method.

5 CUSHION MATERIAL EVALUATION The procedures used to develop a cushion curve are likewise very conservative. Most procedures call for 5 impacts with the cushion curve drawn from averaged data. Procedures such as ASTM D actually require that the first impact data be thrown out and that the curve be constructed of the average of the last 4 drops. For almost all cushion materials, especially in their optimum loading range, the first impact normally results in the lowest amount of transmitted deceleration with the levels increasing for each impact thereafter. The result of these procedures can be an enormously conservative estimate of the transmitted deceleration. (See Figure 3) Figure 3 In addition, environmental data tells us that a product is likely to experience only one impact from the designed drop height. Therefore, designing packages using multiple impact data is extremely conservative. (This word shows up a lot!) In addition, cushion curves are normally available only in multiples of 1-inch thickness. Thus if an optimum cushion response wound up being between two given thicknesses, an engineer would normally opt for the greater thickness material resulting in, again, a conservative package design.

6 PACKAGE PERFORMANCE TESTING Procedures used to verify the performance level of the package for shock protection is perhaps most conservative of all. The "design drop height" or specified level from which the package is dropped is normally the 3-Sigma level from the environmental study. Sometimes it's even higher. However, the number of impacts specified is normally a minimum of 8 to 10, and in some cases, as high as 26. The environmental data clearly tells us that the chances of one impact at this level are very slim (1% or less), and the chances of 10 impacts from the design drop height are about equal to winning the California lottery twice in a row. The placement of the response accelerometer used to determine transmitted deceleration is also very critical. If the accelerometer is on any flexible component, the indicated deceleration will tend to be higher and therefore exaggerated. However, it's very difficult to find a rigid component in most of today's electronic products. The requirement of a high number of flat impacts is also conservative (and you thought the Republicans were all gone!). When transmitted deceleration is absorbed through one axis only, higher readings result. With corner or edge impacts, the deceleration level is spread over several axes resulting in lower levels overall. For new designs the package performance test is normally done on a prototype product. In many instances these prototypes are more fragile than those produced after the manufacturing process has matured. The net result is more protection built into the package for initial product production which is not needed later on. Again, more conservatism. (Is the trend apparent yet?) The following suggestions may help reduce overpackaging. 1. Use first drop cushion impact data for package design purposes. For most cushion materials in their optimum loading range, this will be just fine. Reusable packages are the exception here. 2. Use realistic package test specifications. This includes drop heights that aren't artificially inflated to gain that "extra bit of protection." It also means that the number of impacts should be reasonable. One specification that we've seen uses a lower drop height for the flat impacts and a slightly higher impact for corner and edge impacts. A strong case can be made for this. 3. During the package drop test, monitor transmitted deceleration at the same location that was monitored during the product fragility analysis. In this case, it is not the input shock level that becomes product fragility but rather the response at the monitored location. This is normally a higher acceleration number and will result in a less conservative package design requirement.

7 4. Use realistic levels of filtering for the instrumentation on a package drop test. Many products have suspended components that will impact one another during a drop test resulting in high spikes or peaks on the response deceleration waveform. These are product response numbers rather than package input numbers. Intelligent filtering can help eliminate the ambiguity between these two. CONCLUSION Overpackaging is a hidden cost believed to be very pervasive and very expensive. Intelligent use of the tools available to the packaging engineer can help reduce this. References: 1) Newton, Robert E., "Fragility Assessment Theory and Test Procedure", U.S. Naval Postgraduate School, Monterey, California, ) ASTM D "Standard Test Methods for Mechanical-Shock Fragility of Products, Using Shock Machines" 3) ASTM D "Assessment of Mechanical-Shock Fragility Using Package Cushioning Materials" 4) ASTM D "Standard Test Method for Dynamic Shock Cushioning Characteristics of Packaging Material"