An essential step to designing a cushioned package system is to determine the severity of the environment in which it will be shipped. The general idea is to evaluate the method of distribution to determine the hazards which are present and the levels at which they are present. These may include such things as accidental drops during handling, vehicle vibration, shock inputs, temperature extremes, humidity levels, and compression loads. This text will focus on the areas of shock and vibration, but it is important that other areas also receive proper consideration during the package design process.
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 The difficulty here is that the data is usually outdated, and was not originally recorded from the environment through which you will actually be shipping your package.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 the package design process.
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.
Shocks may result from many types of events, but it is generally agreed that the most severe shocks a package will receive occur during handling operations. These include the times when a package is dropped while being loaded or unloaded from a vehicle, sorted or staged for further distribution, or when bulk is being made or broken. It is important, therefore, to identify the drop height from which the package will be expected to fall.
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.
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.
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.
|Package Weight||Type of Handling||Suggested Drop Test Heights|
|0-20 lbs||one man throwing||42 inches|
|20-50 lbs||one man throwing||36 inches|
|50-250 lbs||two men carrying||30 inches|
|250-500 lbs||light equipment||25 inches|
|500-1000 lbs||light equipment||18 inches|
|over 1000 lbs||heavy equipment||12 inches|
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.
- The probability of a package being dropped from a high height is minimal.
- Most Packages receive many drops at low heights while relatively few receive more than one drop from higher heights.
- Unitized loads are subjected to fewer and lower drops than individual packages.
- Most packages are dropped on their bases. In most studies, base drops have averaged over 50% of the total number of drops.
- The heavier the package, the lower the drop height.
- The larger the package, the lower the drop height.
- Handholds reduce the drop height by lowering the container relative to the floor during handling.
- Labels such as fragile and handle with care have some effect, but can be considered minor.
It is virtually impossible to travel in a vehicle without experiencing some form of vibration. The rotation of engine and wheels induce vibration to the frame. Inconsistencies in the travel medium cause the suspension system to respond and the frame to flex. These inconsistencies may be semi periodic in nature such as expansion joints in a road or rail joints in train tracks, or they may be purely random occurrences such as potholes or railroad crossings. In any event, all these types of vibration become mixed together to form a composite input to the package.
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.