A while ago a customer was constantly facing s-warp on his recent corrugator (2800 mm).

In the picture you see E-flute with s-warp. The s-warp was only noticeable when running E-flute.The s-warp was so extensive that certain jobs were considered as broke. This had for sure a cost.

At the beginning we thought the paper was the cause. After turning around reels end changing positions the s-warp remained in the drive side so it became clear that the problem was caused by the corrugator.

During several weeks we verified a lot on the corrugator.

A first thing we noticed was an accumulation of dirt on the first pressure rollers on the hotplates. This accumulation was especially at the dirve side. These rollers where cleaned but still the s-warp remained.

In the picture below you see the pressure rollers. At the left hand side you see the dirt accumulation, and on the right side you clearly can see the damage on the hotplates by the grinding action of the belt.


I was convinced that it was a parallelism issue but the question was where did it happen in the corrugator?

So a big job was done by checking everything starting from the single facers up to the belt.

Step by step the glue-machines in the SF where checked and double checked. All the preheaters, and rap arms where double checked. All preheating cylinders wherechecked, as well on parallelism as on heat distribution.

Iodine images and also absolute humidity measurements, where helpful in this process.

Many times we found a difference between operator side and drive side. After readjustment, we still had the same s-warped board. It improved a bit but the problem was still not gone.

Same action was taken on the unwinders by checking the parallelism.

After a visit of the corrugator technicians (supplier), they suspected a different porosity of the upper belt between operator side and drive side.The different porosity could also explain the faster contamination of the first pressure rollers on the belt.

A brand new belt was installed but still the problem remained.

At this time we got a bit desperate because we thought we checked everything.

The hotplate positioning in height however and the belt pressure system was not yet checked….

We asked a belt supplier who uses an acupressure system to check this up. It took a while before we could organize this but at the end we were able to get the technicians to come and perform this mapping.

A pressure sensitive material is put between the cooled down hotplates and the belt, and then step by step the whole hotplate section was checked up.


We first tried the pressure for EB and that gave a rather equal image. However, when we gave in the E-flute settings and about 50% of pressure, we recognized a big difference between operator side and drive side.


We decided to do the measurement based on the E flute settings. We noticed a higher pressure at the operator side compared to the drive side. The pressure was higher on the operator side on both the pressure roler and the shoes throughout the whole hotplate section.


As the images where rather similar on each row of shoes we did two tests to make sure that the measuring tool was not faulty.

I came up with a very professional test which you can see in the following picture. It is the “ISO Samsung S3 test”.


On the pressure sensitive material I had put my mobile phone and then a can of 5kg lubricating oil was put on my phone. On the picture at the right you can see the readout on the computer. One can even recognize the rounded corners of my mobile.

As a second test we turned the complete measuring device around to make sure the pressure sensitive sensors weren’t invoking a difference. But the image we got was exactly the same. So it was clear that what we could see on the screen was the reality.

So it was very clear that or the pressure system or the hotplates needed to be re-adjusted.

After that re-adjustment the problem improved drastically. To be a 100% sure one should redo the acupressure measurement.


The before and after picture…….

Another solution would have been to install a zonal moisturizing device, but it would be a bit strange to try to correct a problem with an investment

Published by CORRADDICT

Addicted to corrugating Consulting possible

2 thoughts on “S-warp

  1. Hi interesting blog. I am experiencing a very similar issues regarding s/warp. all of the above you mentioned except for the heat survey on the hot plate section. although we do have a new hot plate section that is not more than 2years old. I am at a total guess at this stage and will gladly take any advice.

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