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I am currently designing a simple pen for drawing with different types of liquid varnishes. My current design has a tip at the bottom side of the pen (blue) and a tank at the top side where the varnish is filled in (red). This varnish flows through a 0.7 mm hole in the tip. The drawing works so far. But the problem is, that it flows too quickly such that I have to draw extremely fast to prevent it from dripping. The main goal is now to reduce the flow/pressure in the tip. Unfortunately, the tip is not manufactured by me so that I cannot change it. But everything else is printed with a 3D printer and can be changed.

I thought about two different design changes. Option 1 includes a small channel between the tank and the tip. I printed this option with a channel size of 1mm, but I didn't help. Maybe the channel was too big. Option 2 added cap on the top side of the pen. I hoped that it would reduce the pressure by letting less air in, but again this did not really help. The size was 1mm which is maybe still too big. My question is if these changes help to reduce the pressure on the tip. Are there any other physical tricks to reduce the pressure/flow? Would it help to make the tank wider/narrower?

In addition, I wanted to ask if it is possible and how to compute the pressure on the tip with these designs.


Pen designs

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  • $\begingroup$ The size of gap you need will depend on the viscosity of the fluid. For a fluid similar to water, 1mm diameter sounds far too big. When the pen is not touching anything, the surface tension will probably stop the pen from dripping, so long as the pen barrel is not too long, but when it is actually "drawing" surface tension has no effect and the only thing controlling the flow is viscosity. Think about the size of the pores in the material of a felt-tip marker pen for example - they are much smaller than 1mm. $\endgroup$ – alephzero Aug 20 '18 at 14:05
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you will need some way to apply a constant very slightly negative pressure on the ink reservoir to prevent the pen tip from drooling due to gravity. this is a solved problem in the field of inkjet printhead design. I recommend you obtain a spent HP model 45 printhead, which contains a flexible bag preloaded with a stainless steel spring. when properly filled and primed, this arrangement will produce the required negative backpressure over a discharge volume of almost 40 cc's.

You would use this by first sealing off the nozzle array and then using the fill port as the insertion point for your pen tip.

Contact me if you need more help with this.

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