Pin-windows, pinholes, starry sky effect.

The root cause of the ‘starry sky’ effect, pinholes or pin-windows is that there is dust/debris on the web before it is metallized and then some time following metallization some of the dust/debris is moved leaving behind the small areas of unmetallized web, the pinholes you see as the ‘starry sky’ effect. 

 

            Corona treatment and other plasma treatments do not remove dust/debris.  It is commonly believed that plasmas can remove any electrostatic charge and the dust/debris will fall off the surface but this is not really true. It is only dust/debris that is very large, probably greater than 50 microns, that has the adhesion force dominated by electrostatic attraction that can be released by discharging using a plasma treatment.   Dust/debris is held on the surface by Van der Waals forces or pressed into the surface or stuck on the surface by residuals carrier in water vapour. This makes the dust/debris hard to remove.  This is why to remove this type of dust/debris it requires a physical technique such as a tacky roll to remove the dust/debris to reduce the size and amount of dust/debris on the surface and hence minimise the pinholes, pin-windows or the starry sky effect.

 

Changes to the number and size of pinholes can sometimes change and then if there have been minor changes to the process they may be linked to these changes.  This correlation is often not real but is simply coincidental.  For example changing the corona treatment is unlikely to affect the amount of dust/debris on the surface as little or no dust/debris will be removed by discharging any electrostatic charge and the plasma will not etch off the dust/debris selectively.  The corona can improve the adhesion which may resist better some sliding of debris that would show up as scratching with a pinhole at the end of the scratching.

 

            Where there are differences between films it could simply be that the starting film is from a different batch and one has more debris on the surface than the other.  Alternatively if there are differences it is worth looking at is the whole winding process in particular the tension, number of front surface rolls and the contact area.   If there are differences in tension or front surface contact rolls and area then it is likely that the higher tension or greater amount of contact will produce more pinholes or stars in the starry sky effect.