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I was thinking about this kickstarter project where they print circuit boards, and later they show it printing solder paste. I've seen other liquid or paste dispensing systems like this too, maybe for medical or chemical uses.

My question is how do you make a system that dispenses a known amount of paste like that? I get that I can run a motor and put pressure on top of a syringe and goo will come out of it. Maybe a stepper with some gearing?

Is there a feedback mechanism? It seems too small to have some kind of flow meter there. Are these kinds of things just open loop and you calculate the volumes ahead of time and just crank down the motor accordingly?

You must have to get it to some baseline first though, like a home position where paste is at the bottom of the nozzle.

Just curious how these things work.

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I do this type of things quite a lot at work, and yes they are typically syringe-actuated device, where you either apply a pneumatic pressure inside the syringe barrel for a certain amount of time (such as this glue dispensing machine) or move the piston inside the syringe by a certain amount at a certain speed, using some kind of electrical drive (stepper motor + gearing) and a lead screw type mechanism (like this syringe pump).

Typically, there is no feedback mechanism, they operate open-loop, but are calibrated beforehand, by dispensing onto a balance and using gravimetry to work out the volume dispensed per unit of movement of the plunger.

As you say, you have to make sure, the system is primed before use so you would typically dispense a little bit to waste before using it for its designed purpose.

The challenge with viscous liquids (and therefore pastes and assorted goos), is you have to control the speed of the plunger quite carefully, otherwise you may over-dispense, and you may cavitate the pump if you refill too quickly. You would typically dispense and refill quite slowly in these cases.

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