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Post- warp


Inspired by Star Wars and Harry Potter 😊

It has been a very strange year for all in 2020. But also, an interesting start in 2021 to have some time to reflect on post-warp.

We all know how to handle warp, this within the limitations of the material combinations used for a certain corrugated composition and within the corrugator parameters we can control.

If we have a corrugator for running heavy BC for fruit and vegetable, it will be more challenging to make a flat low grammage E flute….

This was a choice made at a time the corrugator was specified to have massive preheaters and maybe prolonged hotplate sections to have enough heat to run at higher speed.  Perhaps also glue rolls with an engraving that spits a lot of starch in an uncontrolled way.

If we have a corrugator with parallelism issues also S -warp or irregular CD warp will make our corrugating lives difficult. (You can find in this blog an older article about S-warp)

We are aware of all of this but sometimes people (or dark forces) make our lives even more difficult by sending us green rolls, rolls with humidity profile issues or thickness issues (tension). These can convert into all different interesting shapes except perfect flat board.

And then one day we tackled all the above challenges, and we are running perfect board. At least that is what comes out of the corrugator, right after corrugating.

On good days, the board stays flat until delivery at the end customer, but on other days the flat board seems to be influenced by external forces 😊 and turns into a warped board nevertheless it was flat right after corrugating. This is also known as post-warp, or in Jedi language: “darth vader-shaped”. The force awakens!

To understand this effect, one needs to gather more data about the absolute moisture content of the board coming out of the corrugator and the influences of the climate around the corrugator in the transportation zone in stock or work areas, or even in the warehouse of the customer who may be miles away from the corrugator. It is always interesting to know the absolute moisture of your board coming out directly after corrugating.  Is it dry or is it wet or to wet?  Is it 4% or7%, or is it 9.5%?

Too dry and you get cracking issues in converting or/and the shape of the sheets can change if stocked in a very wet area.

More commonly it will be too wet at production and the shape of the stack will adapt to the humidity of the environment where it is transported or stored, hence the sheets may change in shape. Moisture will get out of the sheets and condensate at the edges of the stacks which are colder than the inside of the stack, sometimes creating honeycomb at the edges of the sheets making it impossible to print a nice solid.

Too wet board will lose part of its strength properties and will be difficult to feed into further converting and in addition the shape can and will change!

The board that comes out at 70%C with an absolute humidity of 9.5% will be shocked by the climate that most probably will try to abruptly dry out the board and bring it to an equilibrium of around 7%. One can clearly see this happening very quickly with the top sheets warping up. Depending on the circumstances this can change the complete shape of the stack to an extent that it cannot be converted anymore.

Out of experience it is good to keep the absolute moisture of the board as close as possible to the absolute moisture of the top liner. If in single wall the topliner’s absolute humidity is 7% and the corrugated board’s humidity is around 7%, then the dimensions will stay stable and the top liner will create a lesser amount of wash boarding and honeycomb.

When the separate layers of the single wall have a big moisture difference i.e. liner 6%, fluting 9% and inner liner 8%, a lot of work will be done on the corrugating settings, glue-amounts and preheating to get a flat sheet out of the corrugator. Internal tensions will be created in the sheet as the 9% material will dry out to maybe 7.5%, the liner will get more wet to 7.5 % and the inner liner will dry out to 7.5% .  This means that all three layers will change in shape.  Let us hope that all these fighting forces end up in a zero force and produce a flat outcome. Studies revealed that if the difference between the different sheets is 2 % or more it is very difficult to make flat board.

To add complication, we deal with the climatical conditions in transport areas, stocking areas and converting areas…  lord Voldemort of the corrugating process changing the shape of the sheets drastically.

Harry potters magic wand

The magic wand to avoid the above problem is to control your plant environment as much as possible and avoid big humidity shocks moving board from one area to another. Keep doors closed in winter times. Avoid air displacement remoistening of area’s that are too dry.

Wand 1: moisturizing unit

Follow and monitor the final absolute humidity of the board coming out of your corrugator.

Wand 2: device to measure absolute moisture by weighing and drying board in a small oven .

Wand 3: Temperature and moisture logger.

Monitor the climate in the corrugator area, transport zones and converting zones by using simple temperature and moisture loggers that provide measurements over time.

These cheap devices are provided with software and allows you to make graphs of your environment.

This drawing is an analysis made in a corrugating plant by placing the datalogger in several areas.  The temperature/humidity graph (top) matches the weather graph (bottom) in zone 2. This is the moment when the stacks just came out of the corrugator.  You can see a big increase in relative humidity in a first stage but then the stacks cool down and moisturize the environment roughly from close to 40% to above 52 %. During that day, the stacks came out flat and stayed relatively flat because we made sure that the absolute humidity of the stacks was close to 7%.

Some days earlier the same job was run but the outside conditions were completely different, it was freezing cold (zone 1). (Source KMI= Royal Meteorological Institution, showing outside climate for that week). Unfortunately, the data logger was not in place at this time, but I guess it would have measured an extreme dry environment as the building would be heated up to fight the outside cold. As an example, my home office is 22°C when it is freezing outside, and the relative humidity drops to around 30%. Big temperature and humidity differences have a big influence.

Most probably the stacks being produced that day where closer to 8.5% absolute humidity. The board being too wet and the climate being too dry created a lot of post-warp that day.

Example 1:

Sheets produced during climate zone 1,   checked after 12 hours

Sheets produced during climate zone 2-3 checked after 12 hours

Example 2:

Day one                                                                                       

Day two showing same stacks having post-warp

The Force Awakens (2015)

May the force be with you……  so you can measure monitor and take adequate conclusions based on measured values and statistical data.

Even Harry potter could not do any magic without his magic wand, absolute and relative humidity measuring devices we call them in corrugating language.

At the same time some magic spells from the big book of magic were needed and in corrugating language this is called some knowledge about absolute, relative moisture, dimensional stability of paper .

May the force be with you.

But you know, happiness can be found even in the darkest of times, if one only remembers to turn on the light.” ― Albus Dumbledore.

The cure to all corrugator diseases

Élixir d'AnversWhen I was young, every time I felt a bit under the weather, my mom gave me “Elixir d’Anvers”. It was the cure to all illnesses. If it was a flu or an indigestion or whatever… “Elexir d’Anvers” took away the pain. I even heard that farmers where giving it to their horses when they had some stomach problems. (image courtesy Wikipedia)

It was nothing more than a herbal short alcoholic drink that was invented by a medical doctor as a simple and effective cure in 80 percent of the cases (and the alcohol in it was an additional advantage).


In exactly the simular way that “Elixir d’Anvers” was a cure to all illnesses, a cure to all Illnesses in the corrugating process is ‘less heat and less starch’

Let us some up some corrugating diseases and there cure:


  • Washboarding
  • Slinging (glue splashing)
  • Blisters
  • Honneycomb
  • Brittle bond
  • Steam bubbles opening layers (secret blowing)
  • Post warp
  • Bad slitting/ bad cut off
  • (and others)


  • Less starch, less heat

For my job I do a lot of troubleshooting and problem solving and it is amazing how simple my job sometimes is (please do not tell this to my boss).

The very first thing I suggest, when confronted with a problem, is to reduce heat and starch to the minimum required, and in 80 percent of the cases the problem disappears, just like using Elixir d’Anvers.

I make it sound very easy, but to convince someone that is already using 30 years too much heat and too much starch is quite a challenge.

Some time ago I was at a company making heavy combinations for the fruit and vegetable market: the typical BC semi-chemical coated kraftliner and heavy paraffined brown kraft. They were complaining about honeycomb, wash boarding and bad bonding of the coated kraftliner.

Observing the double backer, I noticed that every time a certain combination came up with coated kraftliner, the operator put a 100% wrap around a small preheater just before the hotplates. I asked him: “Why do you put so much heat in this coated kraftliner?”. The answer I got was:  “Because someone told me to do so, and if I do not do it then we have a bad bond.”

I enquired if he understood the physics behind what he was doing, but even when explaining it to him; he didn’t want to change this behavior. It was just an automatism based on the trigger word “CKL” coated kraftliner. This resulted in a not so good looking board, I would even dear to call it plain waste.

I had to be in the same plant the next day. I was a bit devious and closed the valve at the drive side that fed the steam in to this small preheater. After a while it was around 40 degrees Celsius and was only brought to this temperature because of the preheating of the liner in the triple stack big preheaters. Nobody noticed me doing this.

The next day when the same “challenging” combination came up the same operator again put a 100 percent wrap around the small preheater. The board was looking better today then yesterday and the operator called me and explained to me that having that 100 percent wrap around is essential for having nice board. Proudly he showed me the 100 percent wrap around and the board that looked a lot better compared to yesterday.

I listened to his story and nodded from time to time. Then I took out my IR camera and showed him the fact that the small preheater was completely cold ( as I had closed the valve the day before). He panicked first a bit and then he suddenly made the link to the better board and the less heat combination. In the coming runs he even put the wrap around on 10 % since he realized that someone had given him the wrong explanation.

It was probably a lucky shot, thirty years ago, but the quality standards for making board have also changed drastically during that time.

I would like to challenge all of you that are running with a lot of starch and heat to try the following.

Let us say that you are running with the first hotplate on 6 bar (single wall, just to keep it simple).

Reduce it to 3 bar and see what happens. Most probably nothing… Reduce it to 1 bar. Remember that with thick hotplates it takes about 12 to 15 minutes before temperatures go down.

If it changes nothing to the final board, then the question is: “Why are you running with those high temperatures in the first section?”

Give your corrugator some “Elixir d Anvers”, less starch and heat, and you will be amazed how many problems it solves.



corraddict funding

Dear Readers,

I hope you enjoy and value my posts filled with corrugating advice. Corraddict.com gives me a way to share knowledge which hopefully you can use in your daily practice.

This blog is funded entirely with my private money. If you have found my articles interesting and usefull, I would appreciate a small token of appreciation by means of a donation (AXABBE22 – BE44 7506 0219 3845). This will enable me to keep on paying for the costs involved with this blog.

This is of course entirely voluntary and does not restrict at all the acces to corraddict.com.

Yours sincerely,

Koen Verplancke



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.

Picture11As 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”.

Picture6On 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



Cross company cooperation at customer

I recently had the honour to work together at a customer to do trial runs and operator process training. We teamed up with two specialists Mr H. Medemblik from BHS corrugated and with Mr D. Murfitt from Crespel & Deiters (starch specialists).

This resulted in a very successful cooperation and very good light weight board production in different combinations. This also gave us the possibility to give a joint training pointing out several things to the operators and management. With as main topic, less heat, less starch.

The corrugator we used was not the youngest one and could be described as a middle aged lady with a couple of face lifts and all the other cosmetic stuff. A steam system that is not always very reliable and E flute corrugator rollers that are also due for a full cosmetic rebuild 🙂

We focused in getting flat E flute board, with no post-warp and no wash boarding neither honeycomb.

The papers used where Modo Northern light  Flexo 120 as coated fully bleached (100 percent fresh fibre), combined with 80 gsm Modo Northern Light / 100 MNL and as inner liner we used 100 MNL

2014-09-03 11.03.03 This pictures shows 120/80/100 combination. Nice board and no postwarp.

As you can see these are very low grammages and these fully bleached virgin fibre liners have the tendency to pick up heat very fast and have an build in natural humidity barrier. ( so they do not like water/ starch to much).  So the risk one has poor bonding due to excess starch is there always.

When we started the corrugating we first monitored the settings and state of the equipment. We noticed relatively high amounts of starch and a bit to much preheating an about all positions (SF and DB) whilset running Flexo preprint coated liner 120gsm combined with MNL 100 as fluting and MNL 100 as inner liner.

After one day we ended up using a gap of 0,12 on the  ( in stead of 0,20 ) DF with 0% preheating so liner went in to hotplates at around 27 to 40 degrees Celcius..

Same actions were taken at the single facer where we went to minimum possible gluegap and reduced drastically the preheating of liners and fluting.

So finally we arrived to following process parameters:

      Process temperatures and glue gaps

  • 200 m/min
  • SF liner 80- 90°C
  • Fluting 75- 85°C
  • web 85-95°C
  • Glue gap on mechanical minimum.
  • Temperature liner before hotplates 27 (uncoated) 40 degrees coated
  • DF 117- 122°C Sensor  (130-130-135-135°C hotplate temps)
  • DB gluegap: 0,12 mm

before web
before: web



before: liner







before: fluting





after lining

after fluting


after: liner






after: fluting








zero wrap


Too much wrap around







Zero wrap






The very stable one bag mix from Crespel & Deiters performed very well – the parameters for this high-performance wheat starch based glue were 55-60 seconds Stein Hall viscosity (18-20 sec. Lory cup), 57 DegC gel. point and (oven) dry substance of 22.70%. Even at what we would consider a bit high temperature of the starch in the glue pan (38 degrees) it sill performed very well.

This cooperation has again taught us that using iodine pictures is so very helpful.

While running at 180 m/min making perfect board we increased the speed to 200 meters and the board started to delaminate:

Iodine pictures showed us again what was happening:



At 170 meter







At 200 meter





So the iodine pictures showed it why we had a bad bond at 200 meter. Too much glue and not enough heating. First reaction of operators before making iodine picture was putting even more glue…..

This irregular glue-lines on the liner where caused by an excess vibrating of the arm holding the interfic shoes.








After fixing this the problem disappeared and the road is open to higher speeds than 200 m/min.

So making iodines is very important, it show the soul and conscience of the corrugator and help us to take the correct decisions.
Another tool that is also very handy and I dare to say a must is an absolute humidity measuring device.
This will give us peace of mind and we will not be afraid of postwarp.

This corrugator still had some issues like a doubt on the parallelism and some irregular belt porosities but these where just some things to add to the list to be checked along the road.

I would like to thank Mr Medemblik and Mr Murfitt for their perfect help for the commen goal: GOOD BOARD!



Litholaminators with in line single facer.

These machines are awesome! Especially for a corraddict since these machines are so open – unlike the single facer with negative or positive pressure – that they allow you to see and measure everything.

One would expect that just because these machines are so accessible that they would be in a very good mechanical state, but I see more of the opposite in the field, not too good maintained equipment with serious maintenance issues.


Incoming steam pressure in the single facer at the litholaminator.

Let’s start with the incoming steam. Most cases one runs with 14 bar incoming steam. That’s in most cases just too much and the more one goes to smaller flutes the more difficult this becomes with this amount of heat. At one customer we were able to go to 9 bar without seeing a drastic change. So even going lower in pressure when running N flute is possible.

Quote:  “We cannot run with steam pressures lower than 14 bar”.

Question (me) : “Can you explain why ?”

Answer:  “Because it is so…. “

Question: “Did you try to run with lower heat/ lower starch amount?”

Answer: “ No”, or “Board will not glue” or “We will have too much warp” etc.

A most interesting answer : “We can’t  apply less heat on the single facer because then we need to use less glue and this results in too dry board that will cracks in converting.” ?!

We all know that higher temperatures /steam pressures results  automatically in a higher starch demand, ending up with at the end of the day a wetter single facer web.

And how do we get then a flat sheet after laminating with Poly Vinyl Acrylic glue (PVA glue)?

Mostly we will apply too much glue to enable us to create flat board. (The 50% water in the PVA glue will balance the sheet with the high humidity of single faced web). So then we get almost perfect flat sheets but very wet and with a huge amount of washboarding.

extreme washboarding

So one creates very wet board that one also needs to flip flop to get flatness.

An additional problem with this way of running is that through the hydrodynamic forces of the glue one tends to apply more glue in the middle then on the edges. Many mechanical solutions have been tried to overcome this, but one this phenomenon cannot completely be suppressed.

To be able to apply low amounts of glue on has to have a single facer in perfect mechanical state.

As an example I will show you some pictures of what can cause serious problems in a litholam line:

The problem: 2014-04-02 11.01.51


The result:2014-04-02 10.59.40


effect of scorelines

The iodine coloring of the fluting in the picture above does not look good. In the light colored area’s one could obtain a lack of glue. If that happens the operator has no other choice then to increase the glue, but then this will result in an overload of glue in the darker areas.

This is resulting in applying more PVA glue on the PVA glue station to get the board back in balance. (flat).

Another risk of  applying too much heat and starch, caused by damaged equipment, is splashing or spraying of glue in the flute valley, where there is even no contact with the liner.

exttreme splashing

So the message here is to make sure that the equipment is in good shape.


Running Lower grammages as fluting and inner liner

There is a tendency to use lower grammages. The prerequisites to be able to run low grammages like 70 g fluting and 60 g liner are:

  • Positive mentality of the crew to do its best, and accept change.
  • Good starches and good PVA glue (mostly not the cheapest)
  • Equipment in a perfect state:  glue rollers, blades, corrugating rollers

The same prerequisites are valid for running aereated glues. These allow you to save big time in glue consumption but also in the total drying time of your laminated sheets (less PVA glue used, less water, hence less drying time). But mostly one does not succeed because one of the four conditions above have not been met.

So one ends up using 230 g as a top sheet in order to cover up the imperfections in the laminator or the crew’s mentality. Have you ever made the calculation if you could run 160 g as top sheet?


pool car

My corrugating philosophy

I have been doing customer trials in a 1000 places around the world.

After many years I can already tell after 5 seconds of contact with people if they want it to make it happen or not. Are they open for new ways of doing things or not…

Sometimes bosses enforce trials upon their people and they should know this will never work. New stuff brings operators – people – out of their comfort zone, so they are not always enthusiastically waiting for me.

As an example, I was in Russia and I needed to do a trial on the corrugator. They had massive hotplates that take about 20 minutes to cool down. I received a window on the corrugator of 5 minutes, so obviously the test failed.

But when the senior VP of Walmart wakes up one morning with a vision and says ‘I want to reduce packaging consumption with 20%’, then corrugating plants have to follow

In order to meet the new Walmart standards, corrugators will not only need to run their equipment in optimal states, they will need to adopt the state of mind of their operators as well.

One cannot make an omelet without breaking eggs. Change means disruption.

We may understand this, but the challenge is in transmitting this message to the operators who often consider us as the ‘bad guys’. We ask them to run faster, we ask them to run more difficult stuff, we ask them to have less cigarette breaks…

I strongly believe that we need to involve them a 100% in the change process, even the lady cleaning the floors should be involved as she is as important as the guy adapting the software of the million euro equipment. Training is the key; people have to feel confident in what they are doing.

In a well-organized project, there is special attention for change management. I can only plead to carefully plan the change management for your teams running the corrugators.

And when it comes to teams… Is your night shift running the same final quality as your day shift? Do you sometimes hear statements like ‘I hope that this combination is for the other shift’. Do your shifts communicate with each other? Do you create the opportunity to allow them to communicate? Do your shifts compete on waste and starch consumption?

Competition isn’t a bad thing, but should be positive. Your teams shouldn’t benchmark against each other but against achievable new standards. Sometimes it is just about changing your communication style and which graphs you post on the whiteboard. Don’t make them run against each other but provide them with a common enemy. Make sure people focus on achieving new quality levels instead of beating the other team.

Think positively.


Washboarding in all its aspects.

Time to post something new…

I have been in many countries since the last post and I am looking forward to my X-mass holiday. Spending some time with my family I have been neglecting so heavily the last years:-)

washboardIf i would have the cure to wash boarding i would be a rich guy……But unfortunately putting 250 grams liner on N  flute is not daily business…:-) This is apparently too expensive for some people.

Finding a flexo plate system that has a perfect solid print, and can go up to 70 lines per cm in the screened process colours, with no dot gain on the flute-tip compared to the valley, and still be able to print at 10000 sheets per hour, is an illusion.

So OK, I think we all agree. I am personally convinced that the cure to stripyness is understanding what is causing it in a print.

strypiness versus no stripiness

We could say that a certain amount of stripyness is acceptable, but please be aware that on flexo preprint one does not have any stripyness in the print.

Neither when one prints in offset on solid board etc….. Also inkjet has this problem less. So there is a need to control and minimize the stripyness to prevent losing business.

The stripes one observes on flexo post printed material is often named wash boarding. I prefer to call it stripyness in print. I think one needs to make a distinction on the different mechanisms of wash boarding.

One can have a sheet of corrugated board with no visual wash boarding or severe height differences between flute tip and valley, but still have stripyness in print .

This could be caused by a wrong plate/mounting system (too hard) in combination with a rough paper surface and a relatively low anilox volume printing on bad quality white test.

One also needs to make a difference in approach between solid areas and screened areas.

Without trying to be posh, i would like to introduce some terminology, so we all talk about the same thing and can immediately visualize it.

Mechanical wash boarding: height difference between flute tip and flute valley expressed in microns. (I also would recommend to read my earlier post on ‘a new device have been born’).


Stripyness in print: uneven print on flute valleys compared to flute tips, this can be caused by mechanical wash boarding but also by other peripheral circumstances.

Typically one sees higher density (ink coverage) on flute tip then flute valley. In certain circumstances one can have negative stripyness, in these cases the ink coverage on the flute tip is lower than in the valley.


negative stripyness

This negative stripyness can have different causes.





2013-12-15 07.28.11

Cockling or honeycomb: too high moisture will cause the fibres to expand. This will happen in proportion a lot more in the width then in the length. The fibre direction of the liner is always perpendicular to the flutes.

As the fibre groups are already fixed on the flute tips, the fibre expansion will cause an unevenness, also known as honeycomb or cockling. A simple test to create honeycomb is to take a sponge and wet the surface of a piece of corrugated board. Wait for a while, you will see the honycomb appearing. Later, after penetration of the water in the fibres and the evaporation of the surface water, the surface will not return to its initial state, it is irreversibly damaged.

Crushed board/damaged board surface: in-feed-systems in flexo postprint presses can damage the sheets even before print and that damage will mostly be more severe on the flute tip then on the valley. Also this can cause a stripy structure in the print. In some cases I have seen this caused by a transport roller in the corrugator (i.e. MHI dry end) which has been roughened to have grip on the board. The surface of the roller looks a bit like sanding paper and will damage the coating on the flute tips.


The in feed systems from flexo postprint presses mostly have a rough metal roller and a rubber roller. This rubber roller will lose its grip after many prints. To still have the sheets transported, the operators increase the pressure or reduce the gap and start to damage the flute tips. In these cases one should replace the rubber roller or interchange the rubber roller with the metal roller. (i.e. metal rough roller only touching the inner liner and not the coated top liner).

Banding stripyness: In double wall one can notice some repetitive bands on the board which are caused by the frequency differences between different flute types. i.e. BC flute.

BC banding

Please notice that the flutes are sometimes supporting each other (green in the image) and sometimes not (red). This happens with every flute combination that has not been mathematically sequenced. One solution is for instance to run EE. If the profile of the two E is identical no banding will occur. Another solution is to adapt the flute profiles so they are a mathematical multitude of each other.

banding  unadapted  flute profiles

adapted ratiobanded

Stripyness caused by post glazing: the corrugating process is a physically very rough process. Take a coated liner, moisten it, heat it and then drag it for about 15 meters under high pressure on metal heated plates. This can and will change the coating structure on the flute tips and will result in a different way how inks are settling or penetrating. You can compare post glazing with what happens if you iron the pants of your best suit too warm…your pants start to shine.

Stripyness in print can also be caused by too hard fluting. You can perform the following test. Make a composition with a high grammage semichemical fluting and one with a softer recycled fluting. Optimize printing pressure in the flexo press and compare the results.

Even the so called econoflutes (with lower take up factor) are not helping us as the flute tip starts to become a knife.

Mechanical wash boarding finds its root cause in the mechanism of the drying of the starch. When the starch dries it has the tendency to retract and it pulls down the liner. It is able to do so as the top liner is humidified by the condensation of the vapour on the backside of the top liner. Also the sheets in a pallet communicate with each other and the inner liner transfers moisture to the top liner of the next sheet.

So far the terminology lesson of the day.


Pictures of BC flute with conventional and adapted ratio: courtesy BHS.

A new device to measure washboarding is born!

About a year ago I was invited at Proflex to act as a speaker at the DFTA in Stuttgart.

I presented the mainly German Flexo audience with some philosophy about corrugating and advice on making good corrugated board for flexo postprint.

Proflex also hosts small booths to give people who are active in flexo the opportunity to show their novelties.

I passed by a combined booth of FAG and Peret, and my left eye fell on a USB device that could correctly measure braille and also creases in FBB (carton blanks produced on folding boxboard). So we started to talk, since I have been searching already for a long while for a device that can accurately measure washboarding. When I want to do corrugator settings optimization I have to depend on subjective assessments about the amount of washboarding. In the past I tested equipment that is used in the quality control of the production of metal parts, but it did not give me a perfect repeatability and the setup just didn’t fit my suitcase.

Objectivity versus subjectivity… What is perfect board? For one person the board is bad, for the other… I wanted to find a device that could give an objective, correct and repeatable measurement of mechanical washboarding.

The existing braille device has an accuracy of 3 microns and makes a topographic image of a surface. I asked Lukas and Philippe, “can you measure washboarding with it?”. As both are no corrugating experts, I went to look for a sheet of corrugated board to explain them what I had in mind.

Lukas was very interested and immediately started to develop ideas in his mind… I talked to my friend Daniel at Bobst and also he was immediately interested I such a device. One year later the CORRCHECK prototype was ready and I could show it to people in the industry.

The CORRCHECK measures washboarding by giving a value which is based on the average distance between the highest point and the lowest point on the surface. This value is based on the average of 5 measurements. A lower value relates to a lower washboarding amount.


I was impressed by the repeatability of the measurements. The final version will even have a better accuracy than the prototype.


As the device accuracy is 3 microns, it also measures the roughness of the paper surface. Hence, measuring a sheet of corrugated board with a cheap and rough recycled liner results in a value that would indicate heavy washboarding. My French colleague Hervé pointed out to me that it would usefull to calibrate the device on the medium itself.

Doing more tests with the CORRCHECK prototype, we figured out that the functionality available in one of the other devices was perfect for checking the flute profiles of single faced material.

This made us think further… The device would be ideal to monitor the mechanical wear of corrugator rollers over duration of time. By measuring the single faced material (coming out of the single facer) at the same positions on the web, on regular basis, and storing the dated measurements for later comparison, one can easily monitor the state of the mechanical wear of the corrugator rollers.

Many potential users are enthusiastic about this function.

O, G, N, F and E flute can be measured accurately with this small portable CORRCHECK device.

Last week I talked to a corrugating manager and one of his problems is that scores performed on the corrugator are good until a customer complains because the folding fails on the erecting/filling line. It is very painful and extremely costly to figure this out at the moment the customer has refused and returned your full lorry.

Measure the crease after you’ve replaced the scores, store the values somewhere and after feedback of the customer that the boxes worked perfectly, you can keep these measurements as a guideline. On regular intervals you retake an image when you are running the same box. Based on the saved values you can make for yourself a tolerance of when you need to replace the score line so you never get costly lorries returning back to your company anymore.

As an example you can find below a graph of the measurements of all the combinations of boards of our recently printed swatch book. The measurements reflect nicely the amount of wash boarding that is visible. The results are very logical and are confirming our thoughts: E flute is better than B flute, the higher the grammage the lower the wash boarding, the surface roughness (Brite uncoated versus Lite + coated ) has an influence, etc.


So about the CORRCHECK…
We expect to finalize the final version by the end of this calendar year. The device is small, USB connected, comes with a practical user interface for Windows, measures accurately and most importantly it fits my suitcase J. More information can be found here http://www.fag.ch/products/pdf/FAG_CORRCHECK_METSA.pdf

Just for the record… Lukas is the guy developing the device, Philippe from FAG is selling it and I’m there for testing in the field.

Did we now solve wash boarding? Not at all! The mechanical wash boarding value is only one small part of the jigsaw puzzle.

Keep up the good board!

I once was at a plant and I observed the older corrugating manager put his hand in a stack of fresh board. He thinks for a second and then decides that the board is too wet and takes actions based on these tactile sensations. I asked him if he is Spiderman. He smiles says ‘Why?’. I say, if you can feel the absolute humidity of board by just putting your hand in a stack then you need to have special sensors in your hand which feed via your nerve system in to your brain.