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Vacuum Web Coating

Blogmaster: Dr. Charles A. Bishop

09

How the film is handled can affect how it will run in your vacuum coating system, this may seem an obvious statement but can often be overlooked as the film is shipped or transferred from store.

The aim is to not damage the core or roll in any way as it is moved around.  I have seen rolls of film set down on the floor with the polymer in direct contact with the floor.  This can lead to some roll defects such as direct damage to the film if it is resting on dirt on the floor or if the roll is left there for some time the roll can deform and no longer be concentric to the core with a flat area where it was in contact with the floor.  When this unwinds there will be a disturbance in the tension on the web as the flat region passes round and the web unpeels.  Unless you have a winding system that isolates the unwind tension from the deposition zone tension this tension variation will be seen in the deposition zone which will give rise to a variation in heat transfer coefficient and so the maximum temperature the film will reach.  In an extreme case this could lead to intermittent wrinkles forming with the period with which the wrinkles form corresponding to the change in tension.

Elsewhere I have seen rolls placed on the floor end-on.  This does require the film to be flush with the end of the core, if it is not flush either the film will be directly in contact with the floor or the film will be short of the core end and the danger of this is that the film will slip down to become flush with the core. In slipping down the film may suffer from microscratching particularly if the film does not all slip together.  There is a further risk that when the roll is turned from the horizontal to vertical, either when putting the roll down or lifting it back up, the edges of the roll can be damaged particularly if the roll is tilted against the floor before it is lifted.

Where the roll is lifted using a sling or fork lift truck the skill of the operator when lifting, setting down or transporting between locations becomes key.  Some operators are careless and allow the roll to be bumped against surfaces.   This bumping can damage the roll including damaging the edges where tears can result which may later become responsible for a web break.

Having produced a roll of vacuum coated material it is also important to wrap and box it correctly for shipping.  Again a good film can be easily damaged by poor packaging and so it is important to make sure the roll is secure to prevent it slipping on the core as well as prevent it being damaged by the box being bumped.  Bear in mind that the roll may be subjected to changes of temperature that may cause the film to expand and contract and as the film heats and expands it becomes much easier for the films to slip as the outer layers heat first and so the tension is reduced until the rest of the film also reaches the same temperature.

Where temperature cycling is expected rolls may be constrained using cheek plates.  This will add to the cost of packaging but compared to the loss of film by damage can be argued as a cost benefit.

Training can often be of benefit to make the operators aware of how and when damage can occur to the rolls of film.  In some cases these jobs are low paid and so the turnover of staff is quite high and it becomes important that any change in personnel is covered by the appropriate training of the replacement staff.  Otherwise over a period of time the risk of roll damage will increase as the replacement operators have no knowledge of how or where they might be damaging the rolls of film.

Posted in: Web/Tech

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Blogmaster

Charles Bishop photo

Dr. Charles A. Bishop

Charles is a toolmaker by trade after completing a mechanical engineering apprenticeship. He then entered University and obtained a Bachelors degree in materials engineering with a Diploma in Industrial Studies. During his final year he first started work on vacuum based research, helping develop a process for manufacturing titanium based bone implants for tendon location. He went on to obtain a Masters degree and Doctorate following further research into vacuum deposition processes. During this time and as a postgraduate he also worked as a consultant.

Charles next spent time in industry working for various divisions of ICI including polyesters, nylon, Imagedata, Flex Products Inc., and explosives as well as contributing to other projects. In 1998 he took the opportunity to return to consultancy work and set up his own company.

Charles has more than 30 years experience in vacuum deposition mainly onto flexible webs. He has regularly contributed papers to conferences and recently has edited this blog on behalf of AIMCAL as well as being one of their presenters for various webinars and the more formal Converting School courses.

Charles has also published 2 books, Vacuum deposition onto webs, films and foils and Roll-to-roll vacuum deposition of barrier coatings.