It is very easy to make changes to the vacuum deposition process but the results of the changes may well be different than what we expected. The reason for this is that we are changing more things than just the one parameter we changed. Sometimes these additional changes are hidden changes and other time we have to intervene and make the additional changes.
Imagine wanting to increase the deposition rate from one resistance heated boat and choosing to increase the current to the boat. This will increase the boat temperature and if there is a stable molten pool present in the boat this will evaporate at a faster deposition rate. However this molten pool will then start to shrink because the wire feed rate no longer is matched to the evaporation rate. Thus intervention is required to increase the wire feed to match the increase in boat temperature and increased evaporation rate. Other changes that are unseen but are also present are the heat load to the substrate will change. The deposition rate will increase, as was desired, which means there will be more material condensing on the web and so the latent heat of condensation will increase. Also the increased temperature of the boat will increase the radiant heat load to the web too. If there is more aluminium vapour being produced the vapour as it deposits will be able to getter more oxygen and so it is also possible the system pressure will change. If the change in temperature is small the change in gettering rate compared to the overall gettering rate of the whole set of deposition sources will probably make the change in system pressure so small that it will not be seen. If all the boats are changed in temperature at the same time and by are large change then a small pressure change may become visible. The increase in radiant heat from the higher temperature source will also increase the heating of the chamber walls and other system furniture and this can increase the rate of water vapour outgassing. Again a small increase in temperature of one boat will be a very small overall change in heat load to the system and so the outgassing rate will be a very small change and again probably be insignificant.
I hope that this example has highlighted how changing one parameter may require some other intervention because of the change and also how other things will also change even if they may be unseen.
In the above example the one unseen change that can be significant is the increase in heat load to the web. As most metallizers are working at their maximum throughput a small increase in heat load can tip the process over the edge and result in a greater chance of wrinkles being produced.
Thus if you are considering process changes it becomes important to consider what effect those changes might produce both seen and unseen. Doing this thinking before making any changes can result in making planned other changes that can counteract the effects of the changes without waiting for the process to go wrong. This might be as simple as increasing the drum cooling capacity to remove more heat from the web and so allowing the boats to be run hotter.
It is always good policy to think first rather than problem solving afterwards.