How to bend 4 mm miniature aluminium pipe correctly into a circle?

The fill the tube with sand method can work. At least it works on hydraulic tubing that is also heated with a torch. The question of annealing has come up, if the material works hardens you may find yourself having to anneal more than once.

Im sitting here trying to understand this cooling ring. Im assuming the jets would point to the center and that this would require a predrilled tube. If so it might be a problem to keep what ever you fill the tube with in the tube.

By the way whenever possible make a proper form to bend the tubing around. Even better consider making a a bending machine that keeps the stress constant on the material as it wraps the die. A proper tool will keep an even draw on the tube reducing the possibility of cracks.

Some if the handheld tube benders out there will allow you to get close to 360 degrees if you release and get a second grip on the tube. Close might be 300 degrees but it is better than nothing.

Somebody mentioned air flow above and the starvation of ports farthest from the source. This can be a real problem so the tube size and the port sizes must be chosen carefully. This makes me wonder how much space you have because an alternative here is commercial spray nozzles which come in different widths and patterns.

Another option would be a single port with a baffle that forces the air stream to circle around the tip. Sort of like a squirrel cage fan in reverse.

The holes are drilled after the bend is done, as the link shows the holes blow onto a nozzle that deposits filament, its a hotend from a 3d printer, the pipe is connected to a mini air pump that lows enough air around the nozzle to cool the filament, as the blower can be adjusted its air force is enough that it does its job. This is really a one-off job, not something that I will need 100s of, so making a machined form bender is beyond cost-effective.

http://www.themakerhive.com/shop/ccdata/images/full3_6_45.jpg

Maybe 3D print a ring and then drill the holes?

If it is one off and that small id turn one on a lathe. It would be a square ring upon which you would solder (glue or whatever) a closing ring. You could have it done before you even get half way with the tubing.

Simon Edwards said:

The holes are drilled after the bend is done, as the link shows the holes blow onto a nozzle that deposits filament, its a hotend from a 3d printer, the pipe is connected to a mini air pump that lows enough air around the nozzle to cool the filament, as the blower can be adjusted its air force is enough that it does its job. This is really a one-off job, not something that I will need 100s of, so making a machined form bender is beyond cost-effective.

http://www.themakerhive.com/shop/ccdata/images/full3_6_45.jpg

Click to expand...

Why not just 3D print it instead? The cooling nozzle that came with my printer wasn't the best but one of my first prints was an improved design that worked far better. As long as you've got something to start with (so you can start printing), you can get the printer to upgrade itself. For a printer, the less weight on the print head the better so plastic would be my preferred material.