This question should be easy to answer, but that is because we are not asking the right question. A better question but impossible to answer is: “What don’t I know about my process?”
My reason for commenting on this is because a number of questions I have recently answered were about processes that had been run without problems for some time but then had suddenly produced coated film that failed in some way further downstream. All had commented that the process they were running was identical to when the coated film was good and so were questioning what had gone wrong with the process.
The simple answer is that something has changed and, if everything they controlled was identical, it must be something they do not control and do not appear to be aware of.
No list is complete
When operators or supervisors are asked to write a list of all the things that might affect the process, there will usually be more things on the list than variables that are recorded during the process. So, there are some things that are believed, rightly or wrongly, to affect the process that are being ignored. Beyond this there may well be additional things that have not even been thought of and so are not even on the list.
In previous columns, I have commented that the film to be coated should be regarded as a variable and not a constant because the film surface ages, and this can be accelerated with temperature. There can be effects that relate to the temperature and humidity at the time the film was manufactured. This may be a small effect, and it may only be when every film throughout a whole year, or over multiple years, are compared that this variable becomes identifiable. This is often more pronounced in countries where there is a monsoon season. I used to get a sudden spike in questions about product changes about a week after the monsoon season started.
Similarly, it is unusual for anyone to have a measure of air quality in the facility where the roll of film was manufactured, or the facility where it was slit or pre-treated. Each time the web is wound or rewound, there is the triboelectric charge that will attract airborne particles onto the surface. If there are changes in the amount of airborne particles over time, the number of pinholes in the deposited coating could be expected to change, which also would affect barrier performance of the final product.