Spray Pattern

I was curious about what the spray coming out of the brush looked like.  So I set up an old slide projector as a light source and placed my 155 with a large siphon bottle filled with water in front of it.  With the lights out, I took pictures of the spray as I worked the trigger.  The air pressure was set to 15psi.

In this first picture, the trigger is barely pulled back.  Notice that the spray pattern originates right at the point of the needle.  The reason it doesn't start farther back is due to something called the Coanda effect.  Moving air or liquid flowing next to a smooth surface, tends to cling to that surface.  You can see how close you would have to get to produce a fine line.

In this picture, the trigger is pulled back a little more.  Notice the hairy look around the spray.  This is actually stray droplets being pulled back into the main pattern by venturi action.  I think this shows why some people polish the tip of their needle. Ideally, the liquid would flow smoothly down the slope of the needle and break free at the tip.  However, if there is any roughness, some of it will fly off prematurely and cause overspray.  I notice that Badger is advertising a polished needle in their new Renegade series of brushes.

Even though this is an internal mix airbrush, it's obvious that the paint and air don't really mix until the tip of the needle, which would seem external to me.  However, the name is an industry standard for this type of brush.  I would think that concentric mix would be a more accurate description as the air exits from a circular area around the needle. I guess it doesn't really matter what you call it, but it does mislead a lot of people.  It's not uncommon to read that an internal mix airbrush mixes air and paint inside the brush.  But it just isn't true.

One negative feature of the 155 is the exposure of the needle to damage.  There is no guard.  However, this also means there is less likely to be paint build-up on the tip and it's easier to wipe off if it does.

The 155 gets its wide range of spray volume due to the dual taper on the needle.  From the off position to about half way back of trigger travel, there is fine control because of the shallow taper.  But, then there is a sudden change to the steeper taper which allows you to really blast out a lot of paint.  I find this most useful during cleaning.  I can spray a full paint cup of thinner in nothing flat.  Here is a picture of the needle showing the area that contacts the nozzle when it is fully closed.


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