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).

http://www.elixir-danvers.be/en/150-years-of-tradition

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:

Symptons:

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

Cure:

  • 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.

Koen

 

Advertisements

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

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.

stripyness

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.

Koen

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

Even heat transfer


Today it was a terrible day to travel.  Due to an autumn storm I got rebooked and delayed. This prevented me from getting some work done on the corrugator this afternoon, so I’m using the time wisely to talk about another parallelism challenge or better put ‘controlled heat transfer’.

It is very important that an equal amount of heat transfer happens as well in machine direction as in cross direction.

A corrugator (“is not an evaporator”) has plenty places where heat is transferred to the liner fluting and corrugated board.  I already talked about the parallelism challenge from reel-stand towards the next stage in the unwinding in my first blog ‘parallelism’. That type of parallelism mostly prevents the forming of creases. Irregular tensions, however, can also cause irregular heat transfer. One only has heat transfer if there is a contact between paper and preheater, or hotplates, etc.

Once it comes to preheating, irregular web tension will result in irregular heating, causing warp at OS or DS depending where the irregular heating occurs.

IR_0126With a simple pyrometer (= an infra-red thermometer) one can check this.  Even a better tool is an IR camera, this can easily show differences between OS and DS in machine direction caused by irregular tensions. The rule here is to aim for only minor differences.

Online systems can also help us but their usefulness is as good as the correctness of the measurements. When using thermometers it is necessary to understand that one needs to adapt the sensitivity of the device depending on the materials that are measured (shiny steel, mat steel, coated liners, brown liners, white liners, etc.).

So lets now go in detail in to the double-backer.  We all are aware of the fact that the more shoes we apply – or rollers we engage – the more contact we are making, the more heat we are transferring. A key question is: do you know if the heat is equally transferred at OS and DS or even in MD?

I will gladly entertain you with some terribly disappointing (shocking?) images of irregular heat transfers on the double-backer.

Psi accupressure1

For these examples I am using real images I have ‘stolen’ from Alabany which is using a system called “Accu-pressure” to check the double-backer rollers, the shoes, the Shortt-press or combinations pressure towards the corrugated board.

The identification used is the following: dark colours show high pressure areas, light and yellow shades identify low pressure areas.

A special film is put under the belt and the shoes/rollers. Once the shoes/rollers are engaged, the amount of pressure à contact à heat transfer can be visualized in an image looking like the one underneath.

Accupressure-RT Norcro(4

 
Every red square represents a shoe. You don’t have to look close to see that on drive side there is no contact and in the middle there is an area with heavy contact that can cause scratches.Accupressure-RT Norcross (3)

In this way the whole double-backer section can be mapped and in this example above, one can clearly see that there is a difference between operating side and drive side.

On top of the previously mentioned pitfalls you also should take into account the simple mechanical system of siphons, valves that transport the steam in the cylinders and hotplates. All of this has to function correctly to make sure that we have no irregularities in heat amounts. Problems can easily be caused by corrosion, damage or failure.

I know, corrugating is complicated, so many things that can go wrong…  and yet also so many things that can work perfect.

Even glue application


 One of the hidden problems in corrugators are uneven glue application along the width and even sometimes in machine direction. Let’s concentrate here on the doublebacker glue machines.

Luckily there is such thing as a wet film glue metering gauge that helps you checking the evenness of the glue application. The instrument is simple and actually brilliant.

glue metering

The glue metering gauge is a high precision tool. Protect it well so it doesn’t get damaged. The two side metal circles are perfect circles. In the middle there is an oval where the height difference corresponds with the markings on the wheel. There where we have 100 microns, this means at that particular place there is a 100 microns difference between the two side circles and the oval.  If you put the gauge on the glue roll one will be able to read the exact glue amount you have on the glue roll. This does not tell you anything about the amount of glue that is transferred to the flute tip. Other factors that influence the amount of glue are the glue roll engraving and the amount of pressure applied with the small shoes or rider roll on the singlefaced material pushing it to the glue roll. The rheology of the glue is also a factor and for sure the capability of the flute tip to absorb the glue or at least the water.

So if you run a non-absorbing liner with a very absorbing flute, playing around with the borax can help…

At least the metering gauge learns you how much glue you have on the glue roller and you can then calibrate your digital settings. It also allows you to measure the parallelism of your glue machine.  If you have more glue OS then DS then you need to do something about it. It is normal that there will be more glue in the middle of the web then at the two sides.

When running C flute – and even B flute – with for instance 10 microns difference between OS and DS will not be catastrophic but you will end pushing more starch then needed. To have enough glue on one side you will need to open the glue machine with an extra 10 microns…..so that is a waste of money and also a risk to have warped board or excessive washboarding.

The measuring with the glue metering gauge should happen at a speed above 150 meters per minute. If you need to open the glue pan leave it a bit running before you measure so that dried starch is dissolved again.

Unfortunately more and more safety instructions and also the construction of glue machines make it more difficult to access the glue machine for measuring. So hopefully someone invents quickly a device that can measure accurately without any impact. Maybe it exists already in other industries? If you know of it, please don’t hesitate to comment on this post.

I also sometimes have the tendency to measure the upper glue machine by accident, hitting the emergency red mushroom with my fat ass 🙂 If this happens twice in 20 minutes then the operators have the tendency not to smile at me anymore and are starting to say dirty words in their communication headsets…

The doctor roll needs to stay intact and smooth as glass, otherwise uneven glue application will be a fact.  Again it maybe not be a big thing on big flutes but on N or G flute, where we are maybe talking about 80 microns of glue, then 10 microns more or less is a big difference. This is sometimes one of the reasons why people have problems running E flute or lower.

You can also understand that the scale of the metering device is important. In some places I see people use one from 0 to 500 microns or even from 0 to 1000 microns.  You can give it to your kids and let them play with it; the scale is just too big to read small differences. Use one from 0 to 200 or 250, that’s definitely working out the best.

This type of parallelism I described you can have as well in doublebacker glue machines as in the SF glue units.

Glue splashing in glue machines is another issue where one is adding unneeded glue in the valleys of the flutes that is absolutely of no use what so ever.

glue problemPlease have a look at the picture, this is what happens if maintenance did not do a good job… you can see a glue machine running without paper, it helped to see clearly where the problem  was. Guess where the board constantly opened up?

Last but not least and sometimes forgotten, the best way to check your glue application is still to make a iodine image.  The iodine reacts with the starch and gives you a pretty nice image of how the glue is applied on the flute tips and on the liners.

Please feel free to put comments or add if I forgot some things.

It all starts with parallelism.


The ideal we want to get in corrugating are  flat sheets, no delamination, no wash-boarding and no honeycomb.

To obtain above one has to have a corrugator which is in a perfect state. And this has nothing to do with the age of a corrugator but all to do with maintenance.

People spend millions on a 2.8 or 3.3 meter corrugator, but seem to have no money left to spend on some simple and basic tools to maintain the state of their equipment. For instance, when I’m visiting corrugating plants, I often need to improvise to be able to make an iodine image. I some cases I even have to bring or buy my own iodine. A simple setup can make a big difference and allow you to save multiple thousands of Euros a month. Other affordable but indispensable tools around a corrugator are: a wet film glue metering gauche, and infrared thermometer, a device to measure absolute humidity, a tachometer, a digital microscope, etc.

In this article, and probably many times in the future, i will repeat the importance parallelism. With ‘parallelism’ I refer to both mechanical parallelism (such as reelstand splicers, incoming rollers, glue machine preheaters, etc.), thermal and glue application evenness.

Today there is the tendency to run lower board grammages compared to the past and in addition we run them on wider corrugators. This may lead to creases while unwinding.

Sometimes these creases can be invoked from the paper, either the creases are already in the reels, or the humidity and thickness profiles may cause the crease happening while unwiding. Do not forget that a mechanical chock on a reel, which makes the reel dancing (bouncing), can also be the cause of irregular tension resulting in.

thickness profile paper humidity profile paperGood profile (left: thickness, right: humidity)

A mill expert will be able to tell you based on the profiles if the paper is causing the creases. This can always be the case of course, but in most mills the paper has been wound and often been rewound, so the operators would have noticed if some PROFILES were bad. Winding and rewinding speeds in the mills are at least three times faster than on a corrugator….. the webs are also wider than on the corrugator.

Imagine you are the liner of the singlefaced C  that will end up as BC double wall board. At the start you will be unwound on the UNWINDING STAND that is furthest away of the take-off. Already here you have the option to be unwound on TWO DIFFERENT UNWINDERS.  PLENTY OF ROLLERS will guide towards the singlefacer. Some will be FORMED SPECIALLY to avoid wrinkles or creases (banana reels). You get PREHEATED a couple of time and then you MEET the C flute. Depending on the type of singlefacer you will be PRESSED to the C flute and in between there will be starch applied through the singlefacer GLUEMACHINE. Then you are pulled upstairs on the bridge and they fanfold you. Then you are put on the bridge after being fanfolded, awaiting the next stage in the process.

In the imaginary story above I placed the words in CAPITAL of the areas where parallelism is important.

I think we all agree that if you have two different UNWINDERS (let’s call them unwinder 1 and 2),   there should be no difference between the two. In reality I often see lots of difference. And when i say ‘lots’ then I refer to the fact that I see three corrugators a week by average and this for about 40 weeks (for a period of about 9 years now).  One corrugator a day keeps the doctor away 🙂

If you place your hand on a singlefacer, you will feel the vibration. This vibration is going on every second the machine is running and in the long run there is a risk that parts reposition and mess up parallelism.

The same thing happens to your equipment as to the glass of wine in the video. Centerlining or checking parallelism should be done every second year especially if you are running low grammages.

Other causes to destroy parallelism may be small accidents. For instance if one hits a reelstand with a reel truck or so. Things like this happen more often than they are revealed.

One easy trick if you have issues unwinding is to first unwind the reel on stand 1,  check where the creases are happening (OS or DS),  change the reel to stand 2 and check again.  When you change the reel to stand 2, make sure to follow the unwinding logic.  If the creases disappear it could be that there is something wrong with unwinder 1. If the creases remain in the same relative position toward the reel, then the chance that there is something wrong with the profile of the reel is big. If the creases are still there but at the same side, like when the reel was on reel unwinder 1, then most probably you need to look a bit further in the process to find the cause of the problem.

In this article I concentrated on the singlefacer and reel stands, but parallelism is important in many other area’s which I will discuss in future blogs.

It’s time for my sauna now.