Pin holes, sometimes also referred as pin windows, are the areas where no metal has been deposited. These can be a big problem for quality Metallizing especially in yarn grade metalizing where a pin hole can cause the thread to break.
Reasons for formation of pin holes and how to avoid them-
1) If there was a particle on film that got removed after metallization, it will leave a pin hole. This can be easily identified by careful examination as this pin hole will also have an area of thinner metallization around it caused by ‘shadow’ of particle in the path of metal for deposition.
Pin Holes in Metallizing (Image 1)
Solution: Storing the film roll properly, away from dust goes a long way in solving this problem. For this, vertical stacking of rolls in custom made wooden pallets with core ends jutting out for handling of roll is the best possible way.
2) Spitting from boats may also cause pin holes; this is easily inferred by heat damage to film around pin hole.
Solution: Controlling the wire feed and adjusting current to boats can solve this problem. For details on the subject of correct wire feed, refer to my earlier blog post here.
3) Excessive tension while winding of film will also cause pin holes. This can be inferred by examining the film. Here, pin holes will be elongated in the direction of pull.
Pin Holes in Film Metallizing (Image 2)
Solution: Avoiding this problem requires adjustment of winding tension to the appropriate value.
4) Residue of some solvent or masking oil on film can also cause pin holes. These pin holes are clean and are also called pick off. These are formed by removal of Metal from film because the solvent did not let metal adhere to the film.
Pin Holes in Roll to Roll Metallizing (Image 3)
Solution: Storing the film where solvent vapors are present should be avoided. Also, if in any application masking oil has to be put on the film; care should be taken for its proper removal as well.
Hence, cleaning and properly storing the film solves this problem.
5) Aluminium wire that is oxidized on surface is another reason of pin holes as Aluminium oxide will not deposit properly. This is evident by a uniform pattern of pin holes.
Solution: Use of fresh Aluminium wire or ultrasonic cleaning of wire if wire is old, goes a long way in solving this problem.
Pin holes on metallized film can be studied effectively by placing metallized film on a flat surface below which a strong light source has been placed on a diffuser to spread the light uniformly. A large area of film can be quickly scanned in this way.