Following a problem with my Harder and Steenbeck CR Plus (0.4 mm nozzle) I rang everythingairbrush for advice (they are always very helpful). What they said could apply to other ABs. I wrote down the conversation (it is surprising how writing down forces you understand it) and thought it might be helpful for others. He gave me two main reasons for backflow (there are several others):
Please see my sketch. (The aircap is bigger than it should be, its thread fits into the mouth of the AB):
[ATTACH]79549.IPB[/ATTACH]
The nozzle fits into the hole in the aircap and protrudes a short distance forward. Surrounding the outer surface of the nozzle is an annular (think of a car inner tube) space, between the outer surface of the nozzle and the hole in the aircap. (A) That is where the air comes out. The nozzle must not actually touch the aircap because that would block the air. As the air comes out, its speed increases (due to the restriction) and by Bernouilli's Principle (see Wikipedia) its pressure drops (which is surprising but that's science for you). It is this drop in pressure that sucks the paint from the nozzle and mixes it with the air to produce the fine spray.
Therefore the position of the nozzle in the aircap must be very accurately maintained so that the annular space is just right. When the nozzle and aircap are assembled into the AB, the aircap "O" ring © presses against the end of the AB. At the same time, washer at the back end of the nozzle (B) is pressed back by the aircap towards the AB, and held firmly in the correct forward/back position. The position of the nozzle is therefore set by:
1. The thickness of the aircap's "O" ring © (which determines the position of the aircap in a front-to-back position); and
2. The thickness of the nozzle washer (B).
(and of course by the length of the nozzle but forget that because it doesn't wear).
First reason: If the aircap "O" ring © becomes thinner, either with wear or by a habit of over-tightening, the aircap will become positioned nearer the AB and the nozzle might be pushed too far forward into the annular space (A), reducing the size of the annular space and therefore reducing the air flow. Worse, the nozzle might be distorted by actual pressure from having being rammed into the aircap hole. It could split along its length. To check for that, look at the nozzle with a magnifying glass for a crack running from front to back.
Second reason: if, instead, the nozzle washer (B) becomes compressed or worn, that could make space for the nozzle to move forward or back. If forward, this causes air to pass from the aircap, round the outer surface of the nozzle and then via the defective washer into the flow of paint and up into the colour cup. If so, try flipping the washer back to front (or fit a new one).
So, the advice is don't overtighten the aircap, and if you get leaks examine the "O" ring and the nozzle's washer.
A variation of the second reason for backflow is caused by paint particles adhering to the nozzle washer (B), preventing an airtight seal. So, clean and inspect the parts thoroughly before putting the AB away.
Please see my sketch. (The aircap is bigger than it should be, its thread fits into the mouth of the AB):
[ATTACH]79549.IPB[/ATTACH]
The nozzle fits into the hole in the aircap and protrudes a short distance forward. Surrounding the outer surface of the nozzle is an annular (think of a car inner tube) space, between the outer surface of the nozzle and the hole in the aircap. (A) That is where the air comes out. The nozzle must not actually touch the aircap because that would block the air. As the air comes out, its speed increases (due to the restriction) and by Bernouilli's Principle (see Wikipedia) its pressure drops (which is surprising but that's science for you). It is this drop in pressure that sucks the paint from the nozzle and mixes it with the air to produce the fine spray.
Therefore the position of the nozzle in the aircap must be very accurately maintained so that the annular space is just right. When the nozzle and aircap are assembled into the AB, the aircap "O" ring © presses against the end of the AB. At the same time, washer at the back end of the nozzle (B) is pressed back by the aircap towards the AB, and held firmly in the correct forward/back position. The position of the nozzle is therefore set by:
1. The thickness of the aircap's "O" ring © (which determines the position of the aircap in a front-to-back position); and
2. The thickness of the nozzle washer (B).
(and of course by the length of the nozzle but forget that because it doesn't wear).
First reason: If the aircap "O" ring © becomes thinner, either with wear or by a habit of over-tightening, the aircap will become positioned nearer the AB and the nozzle might be pushed too far forward into the annular space (A), reducing the size of the annular space and therefore reducing the air flow. Worse, the nozzle might be distorted by actual pressure from having being rammed into the aircap hole. It could split along its length. To check for that, look at the nozzle with a magnifying glass for a crack running from front to back.
Second reason: if, instead, the nozzle washer (B) becomes compressed or worn, that could make space for the nozzle to move forward or back. If forward, this causes air to pass from the aircap, round the outer surface of the nozzle and then via the defective washer into the flow of paint and up into the colour cup. If so, try flipping the washer back to front (or fit a new one).
So, the advice is don't overtighten the aircap, and if you get leaks examine the "O" ring and the nozzle's washer.
A variation of the second reason for backflow is caused by paint particles adhering to the nozzle washer (B), preventing an airtight seal. So, clean and inspect the parts thoroughly before putting the AB away.
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