It is but natural that during the process, material being coated on substrate will fall off in the vacuum chamber. For example, Aluminum heated and evaporated to be coated on BOPP film (film metallizing) or on some object (object metallizing) will fall off on vacuum chamber and shields. Being in molten state this will weld to the chamber surface. (PHOTO-1)
Metallizing in the Vacuum Chamber (Photo-1)
After a number of work cycles there will be a buildup of this welded metal which will have large gaps capable of trapping a lot of air (PHOTO-2).
As it is evident the porosity holding air will be zigzag as it has been formed randomly. Pumping system will find it very difficult to remove this trapped air. Hence ultimate vacuum will drop and vacuum time will increase. It is important therefore that this build up of molten metal is removed from time to time so that trapped air inside vacuum chamber remains to minimum level.
Naturally a question comes to mind that why don’t plant manufacturers provide shields all around the target so that the extra metal dropping and welding to vacuum chamber is eliminated. The problem is that excessive shielding will in itself become an air trap and will result in what is called a virtual leak (details at https://supervacoils.com/leakage-detection-and-prevention-in-vacuum-systems/). This virtual leak itself is capable of slowing down the vacuum time considerably. Therefore this is impractical.
Production engineers therefore are left with no choice but to clean the vacuum chamber periodically.
Various means of achieving this cleaning are discussed below———————-
- Boron Nitride Suspension: –
In this method Boron Nitride Suspension/Paste is applied in vacuum chamber on parts where stray metal can fall. Boron Nitride Suspension adheres to applied surface and forms a protective coating. This coating acts as a physical barrier and prevents welding of stray metal to vacuum chamber. At the time of cleaning it is very easy to remove stray metal that has fallen in the chamber; it is just swept off!
Alternatively some users mix Boron Nitride Powder with water and then apply it to the required surface.
Advantages: – Advantage of Boron Nitride Suspension/Paste is that it is easy to use, neat in use, has very good lubricity and is therefore very effective (Fig.- a). It has good adhesion properties so it easily sticks to the applied surface. Boron Nitride Suspension is also inert and is very easy to remove (normally removed with a wet cloth Fig.- b).
Disadvantages: – Boron Nitride Suspension is expensive and mildly hygroscopic which causes it to absorb moisture.
Application of graphite suspension/paste is very similar to BN suspension/paste. In this method Graphite Suspension/Paste is applied in vacuum chamber on parts where stray metal can fall. Just like BN suspension, it adheres to the applied surface and forms a protective coating. This coating acts as a physical barrier and prevents welding of stray metal to the vacuum chamber. At the time of cleaning it is very easy to remove the stray metal that has fallen in the chamber, it is just swept off!
Advantages:- Advantage of Graphite Suspension are that it is easy to use, has very good lubricity and is therefore very effective; has good an adhesion properties so it easily sticks to the applied surface (Fig.-c). Graphite Suspension is inexpensive and is very easy to remove (normally removed with a wet cloth Fig.-d).
Disadvantages: – Graphite Suspension/Paste is somewhat messy to use and mildly hygroscopic which causes it to absorb moisture.
- Caustic Soda (NaOH):-
In this method, after vacuum is broken, the chamber is opened and Aqueous Caustic soda is applied to remove deposited Aluminum. Caustic soda reacts with aluminum and therefore removes it from chamber surface by chemical reaction.
2Al + 2NaOH + 2H2O ————> 2NaAlO2 + 3H2
Advantages: – Main advantage of caustic soda is that it is applied after operation is over and machine is idle. So it does not interfere in the vacuum process.
Disadvantages: – Caustic soda is polluting and a very hazardous chemical and needs to be handled very carefully.
Reaction of aluminum with caustic soda is exothermic and produces a lot of heat. Cleaning with caustic soda requires chemical reaction so it is slow. Cost of cleaning is also high as special facility needs to be created to store and dispose of caustic soda.
- Removing by Abrasives: –
In this method, a small hand held grinding machine is employed to remove deposited metal by abrasion. Care must be exercised to polish/ buff the ground surface afterwards otherwise rough edges can trap air and moisture.
Advantages: – Main advantage of this process is that it is applied after operation is over and machine is idle. So it does not interfere in the vacuum process.
Disadvantages: – Disadvantages are many. Fine abrasive particles fly during the process and can contaminate the chamber. This process is time consuming and requires expertise on the part of operator. Excessive grinding because of repetitive use can weaken the walls of chamber and cause vacuum leakages. Also it is impossible to be able to remove deposited metal cleanly with this process.
- Hammer and chisel method: –
Crudest of all methods, can damage plant in a matter of days. Must not be attempted.
In my opinion, application of Boron Nitride Suspension/Paste continues to be the most effective and practical way of keeping the vacuum chamber clean of the deposited metal. If cost is a major consideration, for example, if the surface to be protected is very large then Graphite Suspension/Paste can be used as the next best thing. For comparison between Boron Nitride Suspension and Graphite Suspension detailed blog on the subject https://supervacoils.com/graphite-suspension-vs-boron-nitride-suspension/ can be read.