As there are a number of moving parts inside any roll to roll vacuum coater it is important that bearings are lubricated. Similarly of you are using rotary vane and diffusion pumps an essential part of the pumping system is the oil. However from the standpoint of adhesion of any coating oil is always seen as a potential problem, where the slightest amount of contamination, i.e. one molecular layer and above, results in a loss of adhesion. Thus it becomes important that any possible source of contamination is minimised.
Bearings may be supplied with grease that is not really vacuum compatible and so often bearings start be being cleaned of existing grease and a small amount of vacuum grease added instead. Where sealed bearings are used the grease added may be more than the minimum as the seals will be replaced and so the grease contained.
With pumps there is the basic careful use of how the pumps are used such as not trying to pump beyond the pumps capability. Pumping to a low pressure can give more time for oil backstreaming. A low backstreaming rate is good but if the system is left at the low pressure for a long time the accumulation of oil even at a low rate becomes much more noticeable. Hence it is always good policy not to hold systems at a low pressure for too long. If the system has to idle for some time it is better and cleaner to give the system some gas load to pump against where this continuous gas flow in and pumping out sweeps the system clean.
Included in the good practice for oil based pumps is maintenance of the oil. Rotary pumps can easily take up any moisture and the oil can change from a transparent pale yellow to a translucent cream liquid where the water has been emulsified and the light scattering changes the liquid from transparent to translucent. This only requires the pump to have the gas ballast used periodically to keep the oil free of moisture.
Pumps also work best if the oil level is maintained between the minimum and maximum levels. I have seen a diffusion pumped system that was still operating with only a fraction of the required oil. The pump down time was slower than it should have been and the tolerance to gas load variations was poor but it would still pump the system down and allow production. Once the level dropped further the pump then failed to pump the system down at all at which point after various checks it was discovered the it was simple a lack of routine maintenance and checking of oil levels that was the source of the lower performance and then failure to pump. The loss of production time to troubleshoot the problem was greater than the time needed in routine maintenance to check the oil level and keep it within the required tolerance.
The choice of oil can also become important. Oil can be recycled which is can be good practice if the recycling is done well. However some oils end up mixed with other oils and the final recycled oil may have more than just the required oil present. What this means in practice is that when the oil is heated, particularly for the first time, some of the lower boiling point oils can be driven off and can cause a backstreaming problem. Once they have all been boiled off the remaining oil may well act in the same way as good quality new oil but the inside of the system may well have been contaminated by then. Thus if you are tempted to use lower cost recycled oils it is worth checking the quality and reproducibility of them to make sure you are not introducing a contamination problem to your process.
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