Vacuum Web Coating

Winding, edge guidance and slitting.

I have been involved in the purchase of a number of web vacuum roll coaters over the last twenty years and in all that time have never been offered a web guidance system, until now.

This has caused me to ask the question ‘what has changed?’.  The web coaters I have been involved in have varied from slow winding machines that have had a process speed of less than 1m/min but have also included metallizers at speeds of 1000m/min. The widths have also varied from narrow web to wider web of a couple of metres width.  I was always taught that if the distance between rollers was less than the width of the web then the web would not wander.  Also it the rolls were properly aligned the rolls would wind straight.  This has proved to be true and none of the systems had a winding problem.

When I started asking questions it became clear that the reasons for offering the guidance was in fact not to help the web wind straight but was to correct for a problem of poorly slit webs.

I think we will all have had problems of gauge bands in the web causing a problem and film suppliers try to make the impact of the gauge bands less by oscillating the web as they wind it. This does not remove the gauge band but by moving the position of the gauge band either side of the central winding position it spreads out the problem but reduces the hardness of the roll at the gauge band.   Poor slitting can cause a raised lip to be formed on the edge of the web and this can cause a problem during rewinding as the thicker edge increases in diameter faster than the rest of the web.  If the edge diameter increases too much the edge can fold over or the web can be pushed sideways causing slip and micro-scratching.  This is more likely to be seen in vacuum systems where there is no entrained air to reduce this thickness differential.  The sideways slip is more likely to happen if one slit edge is good with no raised edge but the other is poor with a significantly higher raised edge. 

The edge guidance system is used not to wind the roll straight but to deliberately oscillate the web at the re-wind and so spread the position of any raised slit edge approximately 1mm either side of the normal edge position. In this way the poor edge does not raise the diameter or cause the sideways push and so the rolls can be wound with less damage. 

I have come across poor slitting but usually this was spotted before the web went into the vacuum system and the roll was rejected at this point.  I can only recall a few times where rolls had a significant re-wind problem due to a poorly slit edge where some production material was lost.  What has changed here is the type of coatings being deposited and the quality specifications that they have to meet.  It is the increase in the use of vacuum deposited coating for electronics applications where the optical and electronic performance can be affected by micro-scratching. 

I still think that the best solution is to only use well slit rolls rather than correct for a problem that is avoidable. 

Telescoping after metallizing.

Our company experiences telescoping inside the vacuum metallizer but on the rewinding side when the film is already metallized. We observe that telescoping occurs in the inner layers not on the outermost layer.

What could have caused this and how can it be resolved?

Answer

Below are a couple of typical telescoping problems where the telescoping is close to the core.  The top one of the two is caused by the rewind roll not being aligned properly with the unwind roll.  If the core is mounted and the web treaded through and taped onto the core, it is most likely that the web has been taped on to one side of where the true alignment is and so when winding the rest of the web through it re-aligns to its true position. Winding slowly this may not be immediately noticeable but as the web speeds up this will become more noticeable but by then it would be under vacuum.  Sometimes this is easily remedied by making the operators take a little more time to pull a little material through and more carefully checking the alignment before taping to web to the core. 

The lower picture can be produced by either not having the tail of the web aligned properly or by having a problem with the acceleration of the web. If the web is not properly aligned or if the profile of the web is not good the tension on one edge of the web may be different to the other side and so when the web is accelerated the tension difference pulls the web over slightly and gives the telescoping effect. The larger the tension mismatch the greater the telescoping offset will be.

I hope this gives you some ideas as to what you might be seeing in your films.

Question concerning the barrier measurement vs sampling position in a slit roll.

We metallized OPP in Aluminium deposit chamber and I observed a variation in WVTR and OTR data with sampling position in the slit roll :

1. Why barrier seems to be better more deeper in the roll than near the surface ?

2. Where is the best location to take a representative sample : on the roll "out-of-chamber" or on the slit roll ?

Answer from CAB.      

Polymer film as it is wound over rolls will build up a static charge that can attract airborne dust to the surface.  This surface dust will get wound into the roll of film & will be carried through the vacuum coating process. Any dust that is coated may get moved after the coating process & leave behind an uncoated area called a pinhole or pin window.

Now every time the roll is unwound & rewound it can attract more dust each time. Thus from the film line the mill roll may be slit on the production machine into two to make two half mill rolls, each of these will then get unwound & rewound as they are converted from ½ mill rolls into whatever size rolls you are using.   The areas of greatest contamination are likely to be the beginning & end of the rolls as the speed is accelerated & decelerated between stationary & full winding speed. 

This extra contamination at the beginning & end of each roll will be cumulative.  Each machine may well accelerate/decelerate the rolls at different rates & so there is likely to be a different length of higher contamination from each process. 

Thus when it comes to deciding where to take measurements from it is worth going back to each process & estimating how much film is used before the speed is constant and how much film is used on the slow down to stop.  This will give you some idea of the minimum distance you need to be into the roll before you might get consistent, representative results.   It is also worth considering that surface contamination can be transferred from one surface to another & so these acceleration/deceleration distances are a minimum because it is possible that debris from these regions can be spread further into the roll during winding.

The greater the number of time the roll is wound the more surface contamination will be generated & the further into the roll it can have moved.

Thus, it is always better to minimise the amount a roll is handled (rewound) wherever possible.

Question following on from ‘Tramlines’ question.

Today, I read your answer in Aimcal's Q&A regarding curvature/flatness. (Tramlines)

I have been working in polyester film industry. Polyester film has both the curvature

and the flatness problems due to many causes.

But I do not have experience in other films industry such as BOPP, BOPA and the like.

Are there the same problems in other films? Which film is most serious?

My answer was as follows

Most, if not all, films have some problems that are a result of the method of manufacture. The films that are produced by extruding a polymer & then stretching the cast film to get bi-axial oriented film, like PET, will have similar problems. The quality of the film is likely to be a reflection of the expertise of the supplier & where the supplier wants to be ranked in the level of quality of the films they produce.  Some of the problems can be exaggerated in the vacuum system and these may be worse for the likes of polyethylene than for polyester simply because of the difference in things like tensile performance.

Other films such as cast cellulose will also have some problems but the source of these will be slightly different because of the different manufacturing process.

The thing they all have in common is that it is very difficult to produce a uniform thickness film to within better than 1% & this will give uneven tension on the web with the thickest part of the web taking more tension than the rest & if this happens to be near one edge it is likely that the web will form wrinkles somewhere in the winding process.

A question relating to tramlines

Question. - Reasons of Tramlines in polyester film during metallisation
and actions taken to avoid them?

Answer.

Tramlines are a heat related problem.  As soon as the polymer web touches the deposition drum it starts to contract as the temperature is reduced from ambient to the temperature of the cooled drum. At there is tension on the web the frictional force between the web & the drum are high enough to stop the web moving & so it will be in tension. Then the web reaches the deposition zone where there is a high heat load and all of this is reversed. The web wants to expand quickly & soon passes through ambient temperature & gets hotter still. Again the web tension is high & the friction stops the web moving over the surface & thus the web will be under a high compressive load. If the heat load is too high this compressive load gets too great & the web will buckle off the drum surface. Where the web is then unconstrained the surface a rea will be greater than if it stayed in contact with the deposition drum & so the same amount of coating is spread over a greater surface area & so is actually thinner & this appears as a tramline.

So there are several possible solutions. One is to use a slightly higher tension on the web around the deposition drum. This requires some care as if the tension is too high the web can neck if the elastic limit is exceeded. This is possible as the elastic limit falls as the temperature rises.

Another possible solution is to use a spreader roll as the polymer web is laid onto the deposition drum. This will prestretch the polymer web & put it into tension that will offset some of the compressive force produced during the deposition process.

A third possibility is to use a gas injection between the drum & the polymer web. This works in two ways. One is that it will improve the heat transfer coefficient & so lower the polymer temperature reducing the stress. It also floats the polymer web and reduces the frictional force allowing the polymer to change dimensions more easily & also reducing the tendency to balloon off the drum surface.

Incidentally some recent work has also shown that debris trapped between the web & drum will also lift the polymer off the surface & cause tramlines &/or ballooning. Thus using clean polymer film & paying particular attention to cleaning the system is also helpful.

Winding in vacuum – Asking for suggestions for books that specifically address the problems of winding in vacuum

Below is a copy of some correspondence between myself & Dr David Roisum who is the Editor of the AIMCAL Web handling / Converting Blog  @ http://www.webhandlingblog.com/  who is a recognised world expert on the subject.

Q

David,  which books would you recommend for those wanting to learn more about winding in vacuum?

Winding in vacuum systems have the added problems of not having any entrained air to help roll conformation, (they are always hard) there is the added problem of the deposition zone where the web is cooled, often to sub-zero temperatures, then heated to around 100 deg.C, re-cooled to a little above ambient. This all happens inside a box that is inaccessible when it is under vacuum.

Most books I have seen do not have any reference to what special requirements there are for winding in vacuum.

Any suggestions?

A

Charles,

There are thousands of articles and a half dozen books on the subject of winding.  However, I do not recall ever seeing even a brief mention of winding in a vacuum.  Perhaps someone in the cybersphere knows of something.  There are, however, a couple of recent articles on the subject of heat wrinkles on the metallizer.

David Roisum, Ph.D.
Finishing Technologies, Inc.
920-725-7671
drroisum@aol.com
roisum.com

Comment by CAB

David, thanks for the reply..

I am aware of the papers by Dilwyn Jones & Mike McCann on heat & wrinkles & the earlier work they refer to.

I had been hoping that somewhere there was a book with at least a chapter devoted to winding in vacuum.

It looks like a hole in the market if you want to write another book

best regards

CAB

Throwing the question out to the world –

Does anyone know of such a Book that specifically includes a chapter on Winding in vacuum?