Victor asks,
I read your answer to a question from Josh regarding the formula to calculate ECT.
There you answered: A “normal” 200 C linerboard and medium combination must include 42# linerboards with any grade of fluting. Mullen is based on the burst strength of the combined board. Burst strength is a contribution by the liners alone without regards to the medium. So 200# test board can be engineered with 18# to 47# medium. NOW, THE ECT VALUES ARE VERY DIFFERENT!
In a separate answer I found a contribution from one of your blog readers Rohit Chawla.
There he uses the following formula (Liner + (33% Media) + Liner) to predict Bursting Strength, but if I apply that formula, your example of two 42# would yield 195 C.
Could you please share the formula/method to predict bursting strength from liner combinations and how you reached the 200 C from two 42# liners?
I have read the paper and would disagree with Rohit’s formula. If they are using liner for Medium in India then the assumption might be correct. In North America and Mexico, we do not even test medium for Mullen or Burst at the paper mills or box plants because it adds nothing to the combined board Mullen. Our shipping regulations in the US for Burst boxes is based on the liners only and do not include the medium. Of the various properties that the American Forest and Paper Association reports from member mills, Mullen is not associated with medium.
Combined board 200# Mullen is reached by 2 42# liners that each have 100# of Mullen.
You can find this information in chart form in the Fibre Box Association Handbook. The charts show US shipping regulations for truck and rail and the combined weight of facings for each combined board grade.
– Ralph
askralph 