Justin asks,
As you may know I am a bit of a geek when it comes to board combinations. By picking and choosing what machines and what mill run our paper, we are now pretty consistently getting 38 ECT and above from 31-23-31. I confess that the 23# comes in at 24 or above, but the point stands. I have been doing much ruminating about all this.
We have a very long-time customer that went from 200# test to 32ECT, started having too many box failures due to stacking problems, and went back to 200# Mullen. Folklore at our company for years has been that this is an example of a customer use that better suits itself to Mullen than to ECT. Maybe there’s some Mullen magic? However, now I am coming around to the opinion that since our 42-23-42 generally tests a bit north of 44ECT, what we really had was success moving from 32ECT to 40 ECT. The Mullen probably had nothing to do with it.
Would you agree that it is probably rare that Mullen makes or breaks the success of a box?
Agreed. I was just on a case with another member whose boxes made from 100% recycled fibre failed in trailer storage after about three weeks. They performed in the past. As best I could determine, the linerboard had a low level of size and was subject to more degradation through swings in relative humidity (RH) and the box seemed to overhang one corner of the pallet. It’s all about an engineered approach to each customer’s total packaging system. Yours maybe more than Mullen verses ECT, are the recycled contents the same of the two sheets? ECT 40 is always a good compromise, but I know you like to get to the bottom of opportunities. Thanks for sharing the challenge.
Anyone one like to share their knowledge and experiences?
— Ralph
askralph 
April 27, 2016 at 3:27 pm
We all understand that BCT is a function of both ECT and bending stiffness of the combined board. For the most part, ECT is a simple sum of the Ring Crush of the liners and medium. Where the fibre is located is of little consequence to ECT. Bending stiffness however, is a primary function of the liners, the medium’s role is limited to keeping the liners parallel.
As such, your switch to the ECT combination not only reduced the ECT value but also the resulting bending stiffness of the combined board. Your BCT loss would be significant. Mullen is actually a pretty good indicator for BCT as liners with a higher burst will typically have greater flexural stiffness too.
Having said all that, there’s something very odd about the ECT results you’re getting with those combinations; there could be something off with your conditioning or methods. Maintaining a (statistically viable) minimum 32 ECT with a C-flute construction in 31 (23) 31 would be quite the trick. Recycled or virgin, I think I’d have to keep my thumbs on the platen to get a 38.