Vacuum Web Coating

CAB going AWOL

For those of you who have checked out the AIMCAL website you will be aware that coming up in a weeks time is the AIMCAL Annual Fall Technical Conference at Myrtle Beach SC.  Thus I am packing my bags & travelling out to the US to attend this event. 

            The good news is that I shall be listening to some good papers & will report in to the Blog when I’m back on what I’ve found interesting.  I also hope to meet with some people I can recruit as additional contributing authors & so you should start to get some more varied input to the Blog than just my thoughts.

            The down side of this is that the site might not get updated for a couple of weeks depending upon the time I have whilst on the road.

            For any of you asking questions I will get round to them all although there might be a bit of a delay.

            For those of you attending the Short Course on the Sunday before the Conference I look forward to meeting you & for those of you attending the Conference feel free to introduce yourselves to me & tell me what you think of the Blog & what you would like to see included.

            

            See you soon.

CAB

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.