Introduction to Compression Estimation
Estimation of case failure loads
The formula which are dealt with here are for one piece type, flanged cases, i.e. 0201/2/3 etc.
Concerning the estimation of compression strengths in die-cut designs or fitments, great care must be taken in how you rearrange the compression figures, and laboratory tests always made to confirm your findings.
Effects of stacking
Most cases are palletised both for storage and transportation. The way in which the cases are stacked and the type of pallet used will have a significant effect on the final case failure load (C.F.L.).
The final case compression strength of a box is related to the case perimeter. If only 50% of the case perimeter is supported the pallet, you can only expect 50% of the estimated case compression strength. This may seem obvious, but it is a very easy trap to fall into. When cases overhang the pallet it is usually easier to find the overall perimeter and then divide by the number of cases to find a suitable perimeter figure for use in estimation. The way in which the cases are stacked on the pallet will also have an effect. A reduction of up to 45% may take place if an interlocking pallet pattern is used.
If cases are stacked more than one pallet high, then again care must be exercised, as frequently pallets have only two or three runners on their bottom, but fully covered tops. This means that, in some instances, the highest weight per case perimeter is not the bottom layer of the bottom pallet, but the top layer of the bottom pallet.
A pack may be loaded onto and off of a lorry several times before it reaches its final destination. At each of these times, when it is being handled, damage is likely to occur. This means that the more handling a pack receives the stronger it must be. The exact amount of required strength is, of course, impossible to predict without detailed observation. However, it is possible to generalise by saying that sea transport is worse than rail transport and rail transport is worse then road. Also that mechanical handling (forklifts etc.) is better than hand.
After a case failure load has been calculated a safety factor must always be added. The higher the number of unknown factors, the higher the safety factor must be. They normally range from 1.5 - 6 to 1. The standard safety factor, which should always be used, except in special circumstances, is 2 to 1.
Laboratory testing is only a guide, NOT a definitive result.
When compression testing hand-made samples, at least five samples should be used and then a mean average taken. Any figures which, compared to the others, are abnormally high or low, should be discard before an average is taken.
Hand-made samples are usually between 5 and 10% stronger than samples taken from a production run. This must be taken into account in your final calculations.
When estimating the compression strength of a case, a compression test of a minimum, of five samples should be carried out as a means of cross-checking results, but what these results represent should always be borne in mind