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Back to Basics, Part 1: Corrugatingas appeared in Flexo & Gravure Asia Pacific January, 2001

by Patrick Mafrici, Consultant / Technical Specialist for Brently Engineering

 

 At the beginning of a new millennium it is important to look back at what the corrugator industry has achieduring the last sixty years. The quality of board has improved dramatically. Total grammage has been redu

even though ECT and FCT values have been increased. Printing has reached a level where sometimes i

impossible to recognise whether it is a litho-laminated product. In this ever-changing world, nothing remathe same for long. New profiles for corrugator rolls, faster speeds, large format, process printing, etc. All of

may be acceptable to the hardened experienced manager and operators; unfortunately, this industry hasfairly substantial human resources turnaround.

Training in many cases has been neglected or is just passed on in a SIB fashion that (Sitting by Nellie} or watch the other worker and learn from him. With this in mind, we feel that it is about time that we get...

Back to basics.

Let’s start looking at what is the heart of a corrugator factory. The answer, of course, is always PEOPLE, b just for this exercise let’s leaves the human factor out and concentrate purely on Equipment and its functio

The heart of every factory is the corrugator and if we focus closer, the single facer is the main focus and wthe single facer the corrugator rolls are the most important factor if we want to produce board of good quali

and strength. Over the years, corrugator rolls have come a long way from the scraped, uneven, paper-wasting, short lived type, to the modem precise long lasting and most importantly paper saving types.

Why are the corrugator rolls very important? Because, together with paper, steam, starch, they begin theimportant facet of creating the single-faced web: Whatever quality we produce here will influence the qualitthe finished product, that is the shape, strength and printability of the board.

'Let's have a closer look at how each of these factors participate in creating the corrugator board and somethe problems associated with them.

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Of course, a well-built solid and well-engineered single facer will help the production of a good quality boarwith minimal breakdowns, very little vibration and precise adjustments. However, in many cases we have to do with what we have and improve it to make it work to the best of itscapabilities. The biggest factor in the single facer is the corrugator rolls. There are various types, however most utilized are: nitride or induction hardened, chrome, tungsten coated. From past experience, we know the wearing factors are as follows:

Remember that the area where the rolls wear off the most is the tip, due to the fact that the medium istraveling over the flute at a higher speed than the rolls, thus creating friction. What should we know aboutcorrugator rolls? The first and most important factory here is the profile.

Why is this important?-The profile will dictate the strength, and most importantly the TAKE-UP Factor. Whathe take-up factor? That is the relation of medium used to the liner. For example, if we look at one meter ofboard produced in B-flute.

 Any changes in the profile will change the take-up factor, thus in many cases costing us big money in extrapaper (medium) being given away to the customers at no extra cost. If we consider the wearing factor of course the long lasting rollers will offer two advantages: I. Consistent board caliper -strength 2. Saving in

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medium used

Typically, when we order corrugator rolls we take into consideration the wearing factor and we build that faon top of the minimum height required. For example, let's say our minimum requirement for a typical B-flute2.50. When the rolls are manufactured (chrome) they will be manufactured with a flute height of 2.65.

That means that when the rollers are new we are giving away extra medium paper to the customer andgenerally after three refurbishments we throwaway the roller since the diameter has been reduced to anunusable level.The other option available now is tungsten coating, due to the low level of wear on the tip -0.02 to 0.04. 

This allows us to manufacture the rolls with a lower flute height.

 This of course minimises the usage of medium and offers us very consistent caliper strength. A very imporfactor of the profile is the tension factor. By minimising the tension factor we are not stressing the medium

allowing the paper to retain its strength.

Some machine manufacturers over the years have spent lots of time and money investigating how to reducthis tension. For example, some manufacturers have used a larger diameter upper roller over a smaller diameter bottom roller in order to reduce the labyrinth thus reducing tension build-up in the medium.

 Another important factor to consider is the fact that in many cases we don't always utilise the full size of themachine.

For example, if we have an 1800 mm wide machine, most of the time we will be running paper between 11mm to 1500 mm. This of course will wear off the rollers quicker in the centre, giving us two problems: 1. Wwe try to run the full size (1800), we find that the line is cut at the edges due to the extra pressure; 2. We ahave to increase the amount of starch being used to compensate for the gap between the corrugator rolls athe starch applicator roll.

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This influences the shape of the board (WARP) as the more starch we apply; the more water goes into thepaper.Starch on average is 80% water, and MORE WATER = MORE WARP.

The Pressure Roll:

The pressure roll is one item that, even though it is a very integral part of running a single facer, most peopforget about. There are very few things that can go wrong with the pressure roll but one item is the key tomaking sure that the equation works. That is CROWN. What is a crown and why do we need it in some

machines? Depending on the type of equipment and type of rolls, some have a crown. The centrecircumference is slightly bigger than the edges, so that when we apply pressure at the edges we end up wpressure right across.

The reverse could happen if no crown was present. (Not every type of machine uses CROWN.)

The other important factor is to make sure that the steam condensation system is working and the righttemperature is available. The temperature should be within 170° to 190° Celsius.

In many cases, the best option is to replace or regrind the pressure roll to suit the corrugator rolls.

Starch Applicator Roll:

This item in many cases is really the forgotten one. Yet this is the item that will apply the starch to the tip offlutes, so it is a very vital part of the system. In many cases, this roll has been in contact with the corrugatoand damage has occurred, especially at the end. This of course means that the starch application is uneveand of course, this spells WARP. This starch roll should of course run within 0.005 to 0.010 of TIR totalindicated run out.Instead in many cases the bearings are worn or the journals are worn, again applying uneven amount of starch, which translates into warp.

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Paper:

There are many different types of paper available: Kraft, top Kraft, SP, recycled, semi-chemical, clay coate

etc. However, in this case, let's have a look at this effect the single facer and the corrugator rolls.

The biggest problem faced by most operators on the single facers in Asia are wet streaks, un- even moistuuneven tension, loose edge. Of course, if we could provide paper with consistent moisture levels, eventension, and no wet streaks, the job of producing good board would be made simpler.

However, the reality is that in many cases, we try to combine liners with 6-8 percent moisture variation andthen we wonder why we create warped board, when we really should be working with no more than 1 percvariation.One important factor here is to remember that Kraft paper generally needs more heat and recycled paper requires less heat and is more absorbent. Sometimes, depending on the moisture content in the medium, wneed to introduce steam on the medium to plasticise the paper in order to facilitate the forming of the flutes

In many cases operators refuse steam purely because the system does not have controls to shut the steamin case the single facer stops. Of course, if the single facer stops and steam keeps flowing, the result is thathe medium breaks off, which in turn means more work for the operator to re-feed the medium into the singfacer. Nobody likes more work. So in many cases those showers have not been in operation for years.

 

Have a look at our Back to Basics Part 11: Corrugated Basics, Flexo & Gravure, March 2001, the first of outechnical articles by Patrick Mafrici.

 About the Author...

Patrick Mafrici is Consultant for Brently Engineering Pty. Ltd., Sydney, Australia. Patrick travels extensivelyand together with Don Daisley are responsible for most Brently talks and seminars across the Asia Pacific.

 

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