This is a very good example why we should all be calculating the "End Point of Sieving". Let me first say that the more elongated a material is or the flakier a material is the more important it is to determine the end point of sieving because these shapes present problems in the mechanical sieving process. In general, when a specification gives a time to run the sieve analysis (i.e. 5 min, 10 min. etc.) this is based on the assumption that no more material will be transfered from one sieve in the stack to the next, however, this can be a bad assumption. In order to find out how long we should be sieving for a given sieve stack we should perform the following procedure:
- Start with the minimum shaking time provided in the specification being used and perform the analysis
- At the end of the minimum shaking time, remove each sieve from the stack and weigh it with the content of the sieve. Record this amount.
- Place the stack of sieves back in the shaker and shake for one (1) aditional minute.
- Remove the sieve stack and weigh each of the sieves with its content. Record this amount.
- If you see a change of .1 gram between the first recorded weight and this one on any given sieve in the stack we have not reached the end point of sieving and should repeat the above steps until we see a change of less than .1 gram between sieves.
Once we have performed this procedure we will know precisely how long we should be shaking for each applicable sieve stack and media type. One should keep in mind that the time set in the specification may be to short of a time or to long of a time.
In conclusion one predetermined shaking time does not fit all appliations. In order to get the most accurate and repeatable sieve analysis you shoud find out what "your" end point of sieving is.
