Compressed air supply
Moisture in air agglomerates powder which reduces the uniformity of the film and causes material wastage. Additionally moisture can cause the powder to adhere and build up on the inside walls of the delivery hose and within the gun. When this occurs powder aggregates will be periodically blown from the gun causing an unacceptable level of unevenness in the cured coating.
Air contains approximately 77% nitrogen, 20% oxygen and 1% inert gases. The balance is made up, depending on environmental circumstances, of water vapour, dust and other particles. Also during compression lubricants, with their allied chemical additives and decomposition products thereof, vaporise during the 150-200oC heat of compression. Obviously when the air is compressed and used for transporting powder material it must be treated to remove these contaminants. It is vitally important that the compressed air supply be scrupulously clean as powders are highly susceptible to pick-up of contamination such as oils or moisture from unclean air.
Therefore the first step in moisture and contamination control is to pass the air through an after-cooler which causes the vapour to condense. This removes the bulk of the condensate. Compressed air dryers and/or air line filters are then normally inserted into the compressed air ring main. Also, particularly if the air line is lengthy, draining points are incorporated at regular intervals.
Air should be maintained at a relative humidity of 30-40%. Higher relative humidities could cause the powder to form agglomerates and give rise to ‘clogging’ (or blockages) in powder feed lines and reclaim filters, as well as causing ‘spluttering’ of the powder as it is projected from the gun head.