Q: How Clean Is Clean In Vacuum Coating, Or How Long Is A Piece Of String? Part 3 Of 3

By Dr. Charles A. Bishop  
But what's the source?  
Although this type of examination of the substrate tells a certain amount about the surface cleanliness, it does not identify the source of the contamination. Which leads to the question: Is it necessary to know the composition of the contamination? Polymer film has particulate contamination from the extruder, stenter and slitter – all of which will be the same basic material as the substrate. There will be some additional contamination from airborne particles, but in the vast majority of cases this will be a minority component of the contamination.

Historically when one film manufacturer had a problem of some large-particulate contamination, these were examined in more detail which enabled them to be identified as pollen. The pollen was found to be part of the airborne-particulate mix that had entered the manufacturing building as the outer doors were opened to reduce the temperature to make a better working environment. 
What becomes clear is that there is a gap between the desired cleanliness of substrates and what is purchased. There is a link between cleanliness and cost with increasing cleanliness leading to increased substrate cost. To reduce the contamination during manufacturing would increase the film cost. Cleaning off or coating over the contamination from manufacturing would increase the film cost. As the particle size goes down, the cost of monitoring the particle size over the whole surface and at typical manufacturing speed also increases. Therefore, it is important not to over-specify the substrate cleanliness as producing, monitoring and verifying the cleanliness is costly.