Vacuum Web Coating

New machines vs refurbishment?

At the AIMCAL Fall Technical conference it is intended to have a discussion on the above topic. In my time I have heard passionate arguments for both sides of the question.  Some have bought a second-hand machine and refurbished it without problem and have done so at a cost much lower than they could have bought a new machine.  Others have done the same thing but have been less successful with the costs of refurbishment being much higher than predicted and the time taken to obtain working system much longer that it would have taken to specify and have a new machine built.

Where it becomes easier to argue the case is for a completely new process. Here, unless the process is so similar to the existing process, it usually means that any second-hand system is going to be not quite ideal for the process. Anything that is not quite ideal is likely to be a real problem for production. It usually means that the operators are always fighting to make the process run well which is time consuming and costly.  So the trade off here is between the reduced capital cost with higher running costs versus the higher capital cost and reduced running costs.  The difficulty here is to accurately predict the real running costs. This is where experience tells me that is many of the systems I am aware of that have tried to force fit a process into a machine that was not designed for the process have always been significantly worse and most companies, with the benefit of 20/20 hindsight, given a second chance to chose between anew machine and a refurbished machine would opt for a new machine.

Similarly some companies have chosen not to use one of the regular system manufacturers to build their system and this can add to the build risk.  Any company that is building a system for the first time potentially adds to the risk involved. You may have to pay for their learning experience. This may be found as a longer than normal list of small faults in the system or it can be something more major depending on the experience they have and what mistakes they have made.  This all takes time and money and although the initial price for the system may seem attractive the final cost may end higher that buying from the most expensive established supplier.

If you have a metallizer that is getting old and less productive it can make sense to consider progressive updating of the system. There is still a risk that an old system will still have some problems such as fatigue failure of welds that can cause a vacuum leak but the same is true of any system.  In future this upgrading process may happen more frequently, at least for the control systems, because of obsolescence of the electronics. If the metallizer has been in the same company for many years there will be a history of the failures. This may be used to plot the type and frequency of failures. This can be an indicator of what parts of the system could be upgraded and show the best cost benefit. Whatever fails most frequently or with a high frequency and cost a lot to repair or causes most downtime would be high on the list of items to improve or replace.

So as you can see there are plenty of arguments both for improving existing machines as well as for buying new machines.

But that is just my view, from the outside, as a consultant.  What are your views?  Why would you choose to either buy a new machine or buy second-hand or refurbish?  I would like to hear of your experiences of what went well or what went badly that has coloured you view of which route is best to go down.

I will also report back after the AIMCAL Fall Conference in October with a summary of what was presented in the discussions there.

What is an acceptable shelf life?

What about the shelf life? Is there a standard or specific number of months to follow for VM.Is there a reference in the industry that we can follow?  Some say the shelf life of Laminated VM in slit rolls is only 6months, others say 1 1/2 years. Which is the safest shelf life to follow considering that we are in the Philippines?

Answer

As far as I know there is no standard for shelf life.  Again it comes down to storage conditions, control and the type of polymer film.  If the polymer film has few oligomers and additives, has been optimally plasma treated to have high adhesion and then is stored in a controlled atmosphere then I see no reason why it should have degraded much over a year, or more.  If the polymer is the cheapest possible grade that may have a higher oligomer content or a grade that has slip agents added, has had a simple surface treatment and then is stored in an uncontrolled environment where overnight and peak daytime temperatures and also humidity are varying then even 6 months may be a problem for some films.  Similarly the final use may play a part. If the final use is less stringent the tolerance is likely to be greater than a very exacting final application.

It may be that when I post this problem on the Blog that somebody replies with some further information.  I will aim to post this either Friday of this week or first thing next week and so we will see if anybody adds a comment.

So if anybody has some information on any standards that define the Shelf Life of metallized films please write in, I am sure that there are many who would be interested to know of them.

The perennial problem of pinholes.

PLEASE TELL ME HOW TO OVERCOME THE PROBLEM OF PINHOLES DURING METALLISATION

Answer.

There are several possible causes of pinholes.

In my experience the source of the greatest number of pinholes is from debris on the film before metallization that is moved following metallization leaving behind a pinhole from where the debris was sitting during metallization. If the debris has been rolled away it leaves a normal pinhole but if the debris slides you may also see a scratch from the pinhole.   Debris, as the cause of pinholes, was researched by a postgraduate student at Cambridge University and reported in 'Jamieson
E.H.H. & Windle A.H.   'Structure & oxygen-barrier properties of metallized polymer films'   J. Matls. Sci. 18, 1983  pp 64 - 80'

This debris can be removed by using technology such as tack roll cleaning methods as sold be companies such as Teknek.
Plasma treatment to remove electrostatic charge does little to clean this type of debris from the surface.


A second source of pinholes can be from the metallization process where there are a few contributory factors. The aluminium wire diameter and purity can have an effect as the oxide surface of the wire and any impurities will contribute to the crud that forms on the surface of the molten aluminium pool of metal. This crud flows towards the ends of the molten pool where as the pool changes size it can sometimes be ejected to hit the substrate and cause a pinhole. This type of pinhole can be anywhere from a small loss of metallization through to burning a hole through the polymer film when the ejected material is large and incandescent.  Some of the higher quality wires are cleaned and have a reduced oxide surface and this along with a high purity can help reduce this type of problem.

The third most common problem is pickoff. This is where the filler protruding from the back surface of the web is pressed hard onto the freshly metallized surface and when the roll is unwound the filler picks off the small area of metal that it was in contact with leaving behind a pinhole. This can be affected by the hardness with which the roll was wound as well as the adhesion level of the metallization.  If any pre-treatment such as corona or plasma treatment is optimised to make the adhesion as good as possible and the winding tension is set to a low level this problem will be minimised.


I hope this gives you something to work on to improve the level of pinholes.

Question re. large roll size

1200mm UW Diameter metallising machines has been manufacturing for the years.  Do you think if that much big diameter is useful for vacuum coating?  There will be too many layers and there wasn’t enough air between them so do you think it'll cause sagging or other problems after metallising?  Do you know anybody can use this full diameter for it's whole production period?

Answer.

There have been several machines built with larger diameters some were for metallizing paper that had a thicker substrate and the length was similar to those of polymer films but the roll diameter needed to be larger.

The polymer film lengths have been increasing partly through increasing the roll diameter but also through down gauging giving longer rolls but with only a small increase in roll diameter.  What some manufacturers have found is that the longer deposition time has only a limited benefit in productivity. In some cases the longer time at the metallizing temperature caused a reduced boat lifetime, which is counter intuitive.  There is intrinsically not a problem with increasing the roll diameter, there are options of increasing the core diameter and changing materials to counter any tendency to distort.  If the edges of a roll are becoming proud it may not be because of the roll sagging so much as the edge slitting being done by slightly blunt blades that is raising the edges.

Bear in mind that the paper industry has much larger and heavier rolls and often winding at higher speeds and so the mechanics of high speed wide and large rolls has already been solved.  Also as many of the film manufacturers become involved with metallization I fully expect the widths to increase to half mill roll size and the lengths to increase too. In this way they will continue to obtain the economies of scale.

I hope this answers your question.

A problem of oil backstreaming

The initial question was we think we have a problem of backstreaming, what might be the cause?

Following the first answer the problem was elaborated on and so the second answer was produced.

We have vacuum coating unit for aluminisation of Kapton films by physical vapour deposition. Two numbers of films of 25 microns thickness is pasted on the walls of the chamber and aluminium foils are loaded on the bus bar with tungsten filaments which is introduced in between the films. It consists of a rotary pump, rootes pump and diffusion pump and the vacuum level is 10-5 Torr. The plant is 10 years old. First step is roughing followed by backing again roughing and then high tension plasma treatment for cleaning for 15 minutes. Then diffusion pump is switched on with high vacuum and finally vaporizing aluminium by applying low tension for 30 seconds.  Now the problem is aluminium from the top most portion of the Kapton film in semi circle with some I foot diameter where the vacuum duct is coming, actual size is 2.5m x 2m, is peeling off on tape peel test. We have changed all oils, cleaned all internal parts possible and still the problem is there.

1^st  ANSWER

There can be a few causes of oil backstreaming. One is that the roughing pump has stopped working as efficiently as previously. In this case it is worth checking the oil level and also the water content of the oil, it may be that the rotary pump needs to be ballasted.   If the oil in the rotary pump looks to be a milky and opaque it needs to be ballasted to raise the temperature and encourage the water to be evaporated out of the oil.  If you have changed the oil this would solve this problem, unless you have an internal water leak that is contaminating the rotary pump oil rapidly. So even after changing the oil it is worth checking the colour of the oil and making sure it remains a transparent liquid of a light straw colour.

The other common problem is that the system has been left pumping for a prolonged period of time at the base pressure of the system. It is often believed that the best way to remove the water from a vacuum system is to pump down to a very low base pressure. This is not so. Water is removed from the surface by gas bombardment or some other technique for adding some energy such as heat or ultra-violet light.  What was found in the semi-conductor industry was that it was better to feed gas in at quite a high vacuum pressure and restricts the falling pressure so that the gas bombardment of the surfaces is maintained. This has proved to be more effective at removing the water. This larger gas flow makes it harder for the diffusion pump to start backstreaming because of the continued gas load.

Another common problem is that the backing pump does not pump the vacuum vessel down far enough to safely change to the diffusion pump and the diffusion pump stalls as the gas pressure in the diffusion pump rises so high that the oil vapour collides with so much gas that it never reaches the cooled walls to condense and drain back down to the heater of the diffusion pump to be re-vaporised.  Instead the oil vapour collides and is redirected up into the vacuum system. 

I would suggest a close review of the system including how the system is operated paying particular attention to the order and pressures used for the opening of valves and switching pumps.

2^nd ANSWER – following the additional information.

Going back to the problem and working through the possibilities.

The starting point with any problem is to firstly check that we are trying to solve the right problem.

Firstly I would check that the failure is really at the interface and no a cohesive failure within the substrate.  This is unlikely but it is worth checking as if the failure is cohesive then we could spend months trying to solve the wrong problem.

If you have access to surface analytical machines then checking a fresh failure to see what elements are on each surface will tell you if the failure is at the interface or within one of the layers.

Assuming the failure is at the interface there are several possibilities.

From your description of the position and shape of the area of failure I would agree that it looks as if it is a failure related to the geometry within the vacuum coating system and not a contamination that has been brought into the system on the film.

There are several possibilities. You mention using a plasma clean. This may not be optimised.  The plasma density will be affected by the pressure and positioning of the film within the plasma.  The ideal would be to have a regular shaped plasma that all the film sees uniformly for an equal amount of time.  Your geometry does not lend itself to this.  There can be problems with plasma cleaning.  Because some plasma treatment is good thee is temptation to think that more will be better. This is not always true.  The plasma treatment can remove any low molecular weight contamination (assuming there is some oxygen gas available) and may also do some chain scission to enable direct metal bonding to the polymer.  How ever if the plasma treatment does not contain any oxygen the organics may not be removed and the chain scission may be too great creating a powdery surface, that is often referred to as a weak boundary layer, which will fail.  If the surface energy is plotted the surface energy will increase with an increase in plasma treatment time up to a plateau and beyond this time the surface energy will remain constant for some time before it finally declines.  If the adhesion is measured this is not the same, the adhesion will initially rise but as the surface energy reaches the plateau the adhesion will only peak and then decline even though the surface energy remains high.

Thus optimising the plasma treatment needs to be done but if there is a difficult geometry then this may be hard to do for the whole surface.

Oil contamination.

It is possible to get oil backstreaming from roughing pumps as well as from diffusion pumps.  It is not good policy to run pumps at the low end of their pressure range for long periods of time.  

There are two aspects to pumping down a vacuum chamber, one is to reduce the system contaminants and the other is to produce a low pressure to give a long mean free path so that the evaporating material is not scattered between leaving the deposition source and reaching the substrate. 

The main contamination for any vacuum system tends to be water vapour that is absorbed on the surfaces and which takes some time to desorb.  Often systems are pumped to try to achieve a very low base pressure in the belief that this is the best way of removing the moisture. This is not so. Most water is removed by gas bombardment of the surfaces and so as the pressure falls the amount of gas is reduced and so the number of collisions with the surfaces goes down and the effective rate of removing the water also falls.  Thus it is better to add a dry gas during the pumpdown to increase this surface bombardment, which would be seen as limiting the pressure the system is pumped down to. Also adding some other energy can be beneficial in removing the moisture. This can be as simple as using a UV lamp or by using a plasma which also has a very high UV content.

Thus I would review how you pump the system down and where you have the crossover points for changing from roughing to pumping with the diffusion pump.   If you look to the semiconductor industry they have also looked at the problem of pumping and contamination. They use this higher pressure type of operation and get cleaner coatings than when they try to pump directly to a very low base pressure. 

A simple test to see if you are getting backstreaming is to add to the system a cooled plate where the adhesion to the substrate is poor. With this cooled plate installed, run the process but without igniting the plasma cleaning and without depositing the aluminium. So the plate is pumped for the same lengths of time as the substrate would normally see.  When the system is vented check the surface of the plate for any oil contamination. If there is any oil contamination you may need to repeat the process just as far as the end of the plasma clean time, but again without the plasma, just as a method of checking to see if it is the roughing line that is backstreaming or the diffusion pump that is causing the problem.

If it is the roughing pump then you need to revise the pressure and how you achieve the plasma cleaning.  Either you need to switch to the diffusion pump and use an input gas to provide the correct operational pressure or you need to increase the roughing pump gas load to reduce the backstreaming or you need to consider adding a cooled baffle into t he pumping throat to collect the backstreaming oil. These cooled are commonly used but be aware that adding one will constrict the pumping area and so the pumpdown time will be much longer and also it does not stop the backstreaming but only limits the amount of oil that gets past the baffle.

I hope this gives you something to try to solve the problem.  Please let me know when you have some success. It is always difficult for me to judge if the suggestions I have made are relevant or not and it is only if people let me know afterwards that I can judge if I have helped or not.

Transparent metallizing & what are Zones?

ANSWER

Transparent metallizing can refer to a number of different products. This includes depositing aluminium metallizing that is subsequently oxidised to form transparent aluminium oxide such as by the process patented by Amcor (Camvac) a number of years ago.  It can also refer to the deposition of zinc sulphide (ZnS) that is a transparent high refractive index material that is used for transparent holograms where the high index ZnS on the low index polymer gives rise to some reflectance at the interface. Moving away from the resistance heated boat technology and using electron beam deposition technology it is possible to reactively deposit many materials such as silica, titania or alumina of which the alumina and silica are used for transparent barrier applications.

Zones.  A zone is just an area or volume. Typically metallizers are separated into the 'winding zone' and the 'deposition zone' and the pressures maintained within these zones are allowed to be different. The zones have to be separated by plates but as the web has to pass from one zone to the other and back again there has to be small gaps known as conductance gaps. The gas in the higher-pressure zone will leak into the lower pressure zone through these conductance gaps.   Generally in the winding zone there will be the roughing pumps and cryogenic (Polycold or equivalent) pumping. This is because most of the water vapour can be expected to be released into the vacuum system at the point the web is unwound.  However experiments have shown that it is worth including some of any cryopanel in the low-pressure deposition zone. Typically the pressure difference will be one to two orders of magnitude difference between zones.  Where a larger pressure difference is required an intermediate buffer zone may be included.

Zones are also used where multilayer deposition is used around the same deposition drum. The zones may be used to keep apart the deposition of pure metals from compounds such as oxides where leakage of the oxygen from the oxide can contaminate the metal deposition.

Thus the use of the word zone is not necessarily very specific.  I would take the term double zone metallizer to indicate the winding is separated from the deposition area. The intention being to minimise the contamination of the depositing aluminium with any water vapour which could cause unwanted oxidation or the formation of some hydrate on the aluminium metal surface.  The lower the contamination level of the aluminium will give a greater reflectivity and would be expected to improve the barrier performance and electrical conductivity.  So it is not necessarily for a special product but just for a higher quality product.

Air-to-air vacuum coating machines - design issues

This was the question asked.

I have a job interview coming up. I am a mechanical engineer and would be
designing the mechanical equipment for the web. I am comfortable with this
part however I have never pulled a web through a vacuum chamber before. I would
a least like to study up on the mechanisms of sealing a web passing into and
out of a vacuum. Do you know of a place on the WWW where I could see some
pictures and pros and cons explaining these mechanisms. Perhaps some links
to manufacturers of seals might do for now. This is a semi-conductor
conversion for solar cells on .005 thick stainless steel. I would be
grateful for any other information you think might be helpful. Thank you in
advance for helping.

The answer.

There are some papers that it would be worth checking out.

Hartwig E.K. et al  ‘Air-to-air metallizer – suitability & profitability.’

            Proc. 34^th Ann. Tech. Conf. SVC  1991   pp 152 – 161

Taguchi T et al  ‘Air-to-air metallizer: design & operational data.’

            Proc. 35^th Ann. Tech. Conf. SVC  1992   pp 135 – 140

Taguchi T et al  ‘Air-to-air metallizer.’

            Proc. 35^th Ann. Tech. Conf. SVC  1992   pp 424 – 426

There are also some patents, The one I have to hand is a UK patent assigned to Hitachi.  GB 2084264.

As I recall Mitsubishi Heavy Industry & Hitachi both had patents on air-to-air systems for vacuum metallization.

Neither appears to have been a great success.  Their target was aluminium metallization which needs to be done at 600m/min or greater.

They wanted to take polymer film directly from the film line through an in-lime metallizer.  Unfortunately the metal deposition efficiency was poor and so the stray deposition collected by the shielding inside became the limiting part of the process. This was reduced by using heated shields but this then increased the heat load on the polymer and so the coating speed had to be reduced thus becoming the bottleneck to the film production.  Net result was that using batch coaters was more effective.

Thus most systems that have been built have been batch coaters.  ECD would be one of the suppliers of PV deposition systems and there are many pictures of these machines.  Currently with the huge current interest in photvoltaics most of the machine manufacturers have been approached to build systems. Most sell machines that are to a functional specification but do not offer a process.  ECD are different in that they also offer a complete process.

Problems that need to be looked at carefully are contamination and surface scratching due to the high number of surface contact rolls.  The conductance gap between chambers needs to be small to minimise the number of chambers used between air and the vacuum required for the process. If the gap is close fewer chambers will be required but the risks of surface damage due to the web catching the edges of the gap increase. As the air is drawn in through these gaps there can be occasions where the air howls in and sets up a flutter on the web (polymer or metal) these oscillations can be large particularly if a resonance is set up. This flutter can make the gap that works well at atmospheric pressure appear too narrow when used in action.

Any debris brought into the system on the surface of the web will have to pass through the nip rolls. This can cause dents and/or scratches to the surface. Thus the process does not start with the machine but with the quality of the incoming material.

Generally the metal webs are relatively narrow and so some suppliers think the winding system is easier. However the stiffness of the web means that the issues of having a good quality winding system do not disappear. Similarly the quality of the profile of the incoming material is critical to good winding.

Second-hand metallizers & retrofitting

Here is a question that was posed any my reply along with some other generic comments about second-hand equipment and retrofitting.

The Question.

Hi,

I would like to know if its possible to retrofit an older vacuum metalizer ( made in the early 80’s) with a electron gun for the deposition onto glass / lenses.   If so have you come across any company’s that sell this equipment?

Regards

The answer.

The simple answer is ‘yes, it can be done’.  This is usually followed by a BUT.

Using an old metallizer to give you a cheap, large vessel with a pumpdown set can be cost effective if you get it cheap enough and you know the cost of the changes you want to do before you start.   In many instances the whole project does not get the costs estimated well and it ends up more expensive than buying new equipment or a readymade second-hand glass / lens coating machine.

Old machines that have seen a lot of work can have problems, or increased risk, of leaks due to fatigue failures of welds from the vacuum cycling.  So make sure it is leak-tight and can be pumped down to a low base pressure, if at all possible.

Using an old metallizer may mean the vacuum pumps are not sited well for getting good uniformity if you are doing reactive deposition.  This does depend on the type of process you are using. If you are removing the winding system and replacing it with a planetary motion or some similar complex moving jig to average out the deposition you may be able to average out any inherent non-uniformity from having non-uniform pumping.

The installation of electron beam guns is generally not a problem. There are plenty of suppliers of electron beam sources with single crucible, multiple crucible or rotating crucible designs.   It only requires space and sufficient leadthroughs to be able to install one or more into the system and if you are removing the standard resistance heated boats there will usually be plenty of space. 

If you want to deposit multilayer coatings there are some choices to make such as depositing from different guns or depositing from a single gun with multiple crucibles. So of this decision may be governed by the quantity of material that needs to be evaporated throughout a single run and it may be that a multi-crucible e-beam gun does not have the capacity unless the crucibles can also be fed.  If separate e-beam guns are used this again impacts the deposition uniformity.  Also the monitoring of each source and/or each deposited layer to make sure the total optical performance is delivered from the multilayer coating needs some careful thought.

Hence the design of your retrofit system will be critical to the success of your process and often the cost of correcting minor problems can tip the balance away from retrofitting an old system to buying either a new well proven system or a know second-hand system.

Companies such as Angstrom Sciences Inc,  Leybold Optics GmbH, and Soleras Ltd all offer re-built systems and so probably would help retrofit your machine. Alternatively there are usually smaller more local vacuum/engineering shops that would be capable of doing the job with suitable expert supervision from someone who understands the process you wish to end up with. 

General comments on retrofitting.

Retrofitting is often considered to extend the lifetime of a vacuum system but in some cases this has proven to be a false economy.  Upgrading the on-line monitoring system on a metallizer may give better information about the coating thickness but unless the control of the boat temperature and wire feed is upgraded too it may well be that all the upgrade does is to improve the information about how variable the process is.  If you then upgrade the control of power to the boats and the wire feed then for an old machine the cost of taking the machine out of production, the cost of the items that are often customised to fit that particular machine as well as the cost of recomissioning may be a significant proportion of buying a new machine.  When one considers that a new machine will also include a newer, more up-to-date winding system and any other developments that have taken place since the original machine was built it is likely that the increased productivity of the new machine would more than compensate for the difference in cost between the new machine and the old but refurbished machine.

Most retrofitting projects underestimate the effort, time, inconvenience and hence true cost.

Latest question re. Static charge and surface energy

Query.

Could you please explain what is the difference between static charge & surface energy in polyester film? Generally it is clear that surface energy is inherent property of film but static charge that develops also because of the modification of surface structure although is vanish rapidly when comes in contact to a ground medium.

Surface Energy

All materials have an energy associated with the surface. 

The analogy has been made that the energy is related to the excess energy present from not having the next layer of atoms present and so having some bonds unmade.

The precise energy is then a function of the atoms present at the surface thus for polymers there are different amounts of Hydrogen, Carbon and Oxygen.  Of these the oxygen will produce more energy on the surface than carbon which produces more than hydrogen which is better than fluorine.  Thus to produce an increase in surface energy it is common to use an oxidising plasma to reduce the proportion of hydrogen and carbon and replace them with oxygen.  If a hydrophobic surface is required then the surface is fluorinated to remove any oxygen and increase the fluorine content of the surface.

In general the higher the surface energy the easier coatings will wet the surface this is because the coating will have a surface energy (known as surface tension) and the system will naturally produce the lowest combination of surface energies. Thus the substrate surface energy plus the substrate/coating interfacial energy will be have to be lower than the coating/air surface energy for the coating to wet.  If the combined energy is higher the coating will not wet and the coating will want to form beads on the surface.

Electrostatic charge.

When dissimilar materials are brought together and separated there is an electrostatic charge produced. If both materials are electrically conducting this will not have any significant effect. However, if one of the materials is an insulator a surface electrical charge will be produced. The rate of which this charge will be formed depends on both materials. There farther apart they are in the triboelectric series the faster the charge will be generated. If there is any conductivity at all the charge will leak away with time, the better the conductivity the faster the leak rate. Thus for Polyester Terephthalate (PET) which is used as an electrical insulator and usually has a low moisture content there is no appreciable electrical conductivity and so the surface will charge rapidly and will retain the charge for a long time.  In cases like this the mechanism for discharging the surface will relate to the ambient humidity making the air conductive. This is not very efficient and so it is still possible to charge the surface up to very high voltages often to the point where the surface will arc to the atmosphere as a method of discharging the surface.  This generally will only happen once the voltage is around 10,000V or greater. 

As the charged surface approaches a conducting roll the charge will reduce and may disappear whilst in intimate contact with the roll but as it leaves the roll the charge will reappear as the two surfaces separate. In very bad cases sparks can be seen between the approaching and departing web. Again it depends on the polymer conductivity and the maximum charge that appears on the web.

As the polymer is an insulator it is also possible to have charges of opposite polarity on the same side of the web that are not mobile enough to cancel each other out.

The surface charge can be tested using a hand held meter. This is held in close proximity to the surface and the average charge will be measured. In this way it is possible to follow the web around a winding system and chose the best site for any neutralisation system to be fitted.  Note that changing the polymer type will change the rate of charging and the capacity for retaining the charge and so an electrostatic audit needs to be performed for each different polymer.  It is also worth noting that the static charge will form on both sides of the web and it may be of the same polarity on both sides, it does not have to be of the opposite polarity. If it is of the opposite polarity there is again no guarantee that the charges will link around the edge of the web and neutralise each other.

It is also worth pointing out that holding a meter close to a fast winding web, possibly with a large electric charge on it that can discharge to the person, is a potentially hazardous measurement and so care does need to be taken.  Sticking the meter to the machine and measuring the voltages remotely has some advantages.

In coating machines run at atmospheric pressure it is typical to prefer to have a low ambient relative humidity to produce high quality coatings. This in turn can produce much higher electrostatic surface charges and so often where the humidity is controlled there will be a compromise in the set level.

It is also common to then see electrostatic neutralisation methods employed to reduce the winding and coating problems that static charge can produce.  This can be as simple as using a conducting metallised foil ‘tinsel’ in close proximity or even touching the web to provide multiple leakage paths to earth.  This can cause surface scratching in soft coatings or can cause micro-arcs that can also be seen as slight surface damage through to larger Litchenberg figures type damage.  This has led to powered electrostatic neutralisation bars where a charge of the opposite polarity is produced near to the charged polymer web and the air acts as a medium for charges to cancel each other out.  Using electrostatic neutralisation and a gas flow can direct the charge to the surface and can also help remove some larger debris from the surface.

Electrostatic charging does also occur in vacuum. It does not require air to charge the surface up. Thus in vacuum it is one of the hidden benefits of using a plasma cleaning process that the plasma also neutralises any electrostatic charge on the web surface.  As the plasma will fill a greater part of the vacuum system and not just the bright area of plasma that is easily visible this plasma can usually reduce the charge through much of the winding system.

Charles A. Bishop

C.A.Bishop Consulting Ltd.        www.cabuk1.co.uk

Dust levels in metallizer & factory.

The following comments/questions were made about the dust levels and I suspect this will be common to many metallizers.

1.Many efforts are made to keep the factory dust free but still lot of dust is seen in the factory.

2.In a metallizer itself lot of dust in powder form is seen.

3.What should be the acceptable dust / particle size in a factory where metallizing is done which will not affect the barrier properties of metallized film?

4.What is the economical way of cleaning the film before metallizing?