Tyler asks:
I am a seasoned QA manager, but new to the corrugated industry. I am trying to understand warp and how it is measured. I understand from the Fiber Box Handbook ¼” per 12” is allowed. Since our converting equipment doesn’t see the sheet in 12” sections I would like to measure the whole board and extrapolate the warp measure from that. .
Is the allowed warp a linear measure, i.e. ½” per 24”, ¾” per 36”, etc ?We are developing a Go/No Go gage for use line side and I want to make sure I set it up correctly.
Yes, warp is linear and can also be considered diagonal when measuring a corrugated sheet. You need to consider the worst case of dimensional deformation. There are very simple gauges made that are twelve inches wide with a small stylus in the middle to measure the deflection from the plane of the substrate. This one size gauge should be sufficient to give you a go/no go measurement. Let me know if you need a link.
-Ralph
askralph 
November 9, 2011 at 3:51 pm
Just to clarify, I can interpret 24” should be no more than ½” warp
I am debating this with my corrugated supplier right now, based on the attached chart. The chart suggests the curve resulting from ¼” per foot gives a fixed curve (radius of 72.125) and warp allowance is based on this..
This fixed radius changes the warp values such that allowable warp is;
12” = ¼” warp
18” = ½” warp
24” = ¾ “ warp
Etc.
Two problems are arising;
1) The warp calculation of Warp divided by say a 36” sheet converted to a 12” warp number does not match the straight 12” number.
2) Based on the allowed warp using the chart, we are dealing with some serious warp that will cause us to slow down our high speed equipment.
Current process is;
• Measure Length of sheet
• Measure warp deflection
• Divide deflection by length to get warp per 1”
• Multiply by 12 to get warp per 12”
I am working on a full sheet go/no go gage that mimics what the machine sees, i.e. the machine sees the full width sheet, not 12” sections.
I hope you can help clarify what the intended standard is.
April 23, 2013 at 10:48 am
I take a random sheet, cut a 12″ section out of it, and use that as my measurement. I also cut a section out to weigh the liners as well.
March 9, 2015 at 7:35 pm
I am manufacturing aluminum warp gauges that are very easy to use. They eliminate any question of passing or failing. Has 1/4″ / ft. curve on top and bottom so that it can be used on the concave or convex side of the board. I will be listing them on Ebay shortly.
October 20, 2015 at 2:35 pm
I use a Mitutoyo digital warp gauge, which does eliminate any guess work. However, the weight of the gauge will reduce the measured warp to some degree. Could I put 2 warped sheets together which should reduce the deflection caused by the gauge or is there another technique to use?