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.