Rick asks,
I am seeking written accepted warp standards for corrugated. I’m looking primarily for standards on C, B and E flutes.
I seem to recall 1/4″ per running foot as acceptable for B and C.
1. Is the same true for E Flute and other “micro”
2. Is there anything for “S” warp, as that seems to be issue with lower profile flutes.
I have not been able to locate “written” warp standards for E flute and finer corrugated board. And S warp is just a matter of compound warp in both the MD and CD directions.
Yes, the Fibre Box Handbook recommends that warp of corrugated board should not exceed 1/4” (6mm) over 12 lineal inches (305mm) of the material’s surface. However, there does not seem to be anything specifically relating to micro or finer flute board. I would think due to the nature of material and its uses that the warp standards would be different than that of larger flute board.
One has to look to setting standards on paper moisture variations, glue application rates, and corrugator process controls. It’s these input variables that a paper buyer, corrugator supervisor, and control system need to establish. Without getting the front end communicated there is not much chance of flat board.
Hey followers… We’re always open to your input. Please share your knowledge and experience on this subject with us.
— Ralph
askralph 
August 26, 2016 at 2:11 am
I worked for a digital printer supplier (c) in the last years dealing with warp for many years.
I think it is time to create different standards for contact and none contact print.
For contact print it is mainly for Flexo with minimal exception with micro flute on offset.
For none-contact print we talk about the new developed digital market that will expand more and more in the coming years.
There are two directions to work on
1. Reduce warp in production of corrugators
2. Improve media handling at the printing and finishing equipment
1. Reduce warp at production:
a. Create standards of board acclimatization (very problematic in some parts of the word like NA)
b. Create acceptable easy to measure standards during production and along the manufacturing steps of the final product and the printer / converter
c. Create standards also at the manufacturing site (humidity, temp etc)
2. Machine manufacturer can gain allot from the jungle above and gain market advantage if they will know how to deal with extreme warp. IT is a good reason to by this or another machine.
I think the industry understand very much that tight standards will influence the price of the corrugated product immediately. There are no free meals here as well!
Yes corrugators should set higher standards not sure if the market will be able to cover the cost involved in it….
November 3, 2016 at 8:43 pm
It seems to me that this 1/4″ warp spec is one of the more useless guidelines in our industry. Myself, I would have no patience for any MD or S-warp at all simply because this is primarily a tension issue. We can see that happen as it happens and the remedy should be quick. If it’s a paper issue then perhaps you need a polar angle spec more than a warp spec.
CD warp is (for the most part) a moisture imbalance and post-warp (usually upwards) can be tricky especially with small flutes, coated liners, out-of-balance liners (weight and/or moisture), mixed recycled and virgin liners, etc. Yes, there are certain customer applications that require flat, flat, flat board. For the most part, however, if your sheet is flat enough to feed through the converting press(es) without a negative OEE impact (or jacking-up the vacuum on the in-feed table) then it’s probably flat enough for the external customer too. Given that, figure out what the press needs and there’s your spec.