Remote Paste Extrusion Oozing Problem

I've been trying to build a pressure-based paste extruder and would like to understand why longer tubing from a remote syringe barrel results in longer duration of oozing.

The paste extruder works great when extruding directly out of the syringe. Since the syringe is large, I'd rather mount it off of the printhead, and have tubing go from the syringe all the way to the printhead, kind of like how Structur3d's and ORD Solution's paste extruder work, except with air pressure instead of the screw/plunger -based extrusion.

I noticed that the longer the tubing is, the more ooze I get. In addition to ooze, there is a delay from when I apply pressure to when the paste (peanut butter) actually starts extruding full speed. For instance, for a 2.5 foot 3/16" ID PVC tube, I got maybe 10 seconds of oozing, and also a few second delay after turning on the solenoid. I improved it a bit by using stiffer tubing and shorter length (and verified very few air bubbles), but its still very unsatisfactory. I'd like to understand what exactly causes this issue. Is it bubbles in the liquid? Is the tubing expanding and contracting with the increases and decreases of pressure?

With no tubing, there is zero oozing and no extrusion delay.

YUM,
nice subject

My best guess is that other than fluids, paste is a compressible medium.
You´d have to put a valve at the nozzle to keep it from oozing and also keep the pressure up.
-Olaf

Pastes can't be that compressible to cause it to ooze for 10 seconds would it?

I guess removing the solenoid, and keeping the pressure constantly on the paste, and adding an electrically controllable valve near the tip of the long tube could be a solution. But I'd still like to understand why oozing occurs with long tubing, and if it can be fixed without using some kind of retraction.

It probably takes a lot of pressure to move peanut butter through the extruder and tube. If the nozzle diameter is smaller than the tube, you probably have pressure building in the tube causing it to stretch. Once the pressure is released, the tube tries to return to its normal size by squeezing the goo out the nozzle.

Try measuring the diameter of the tube at multiple points with a caliper, then turn on the extruder and apply pressure and while it is extruding, measure the tube diameter again.

I'll give the measuring of the tube OD a try. But I don't think thats the problem since the tube is relatively stiff and I'd expect it to relax quickly (rather than over several seconds) once the pressure is removed, but ya never know. In a separate test, I'll put the peanut butter in a closed syringe (with no air bubbles) and apply very high pressure, and see if the peanut butter is actually compressible. I assume the tube is not expanding, and that the PB is not compressible, but we'll see.

how viscous the the material? If it's thin it may actually be vacuum. Think about using a hose to syphon gas out of a car, once you get some to start down the tube it creates a vacuum behind it that then pulls more out, as long as the seal between the walls of the hose and the material is made the reaction keeps going.

It's either that or like others have mentioned, pressure.

You don't post a photo of your setup but if your syringe is pointed down, why not try pointing it upward so the material must go up before it can go down, just thinking this might reduce the possibility of it leaking out.

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jessicabrenner
I'll give the measuring of the tube OD a try. But I don't think thats the problem since the tube is relatively stiff and I'd expect it to relax quickly (rather than over several seconds) once the pressure is removed, but ya never know. In a separate test, I'll put the peanut butter in a closed syringe (with no air bubbles) and apply very high pressure, and see if the peanut butter is actually compressible. I assume the tube is not expanding, and that the PB is not compressible, but we'll see.

If the tube is flexible, then it's expandable, weather it's relatively stiff or not. The amount of spring force the tube can store increases geometrically with the length of the tube, so if it is long, then it can absorb and release a lot of energy without expanding more than a few hundredths of a millimeter.

This issue is being caused by lag in the build up of pressure (for start delay), or residual pressure in the tube (ongoing oozing).

I would suspect there is some compression of the paste occuring, or tube expansion/contraction, or both, or something else causing the pressure delay in the system.

Residual pressure will cause oozing so if there is someway to release the pressure instantly that will help. But then you have the issue of delayed pressure build up, so a valve on or close to nozzle tip might help further.

A screw based system might have a much shorter pressure build up time, and i suspect is more controllable. Could you use the screw instead of air pressure with the longer reach hose? That might eliminate any lag due to hose flex.

IIRC, there is a rule in physics, that says:

P * V * T= constant

P= pressure
V= volume
T= temperature

This simply means, when you apply pressure to your medium, it will get warm(er).
This might be another reason, why it expands ( oozes ).
-Olaf

I believe you are attempting to refer to the ideal gas law (PV=nRT) which applies to compressible gasses, not to peanut butter, which is behaves more like an incompressible fluid than a gas.

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the_digital_dentist
I believe you are attempting to refer to the ideal gas law (PV=nRT) which applies to compressible gasses, not to peanut butter, which is behaves more like an incompressible fluid than a gas.

Yup, I believe it was gas.
The question still is, when the PB paste is compressible, will this rule also apply?
-Olaf

When peanut butter starts behaving like a gas, then it will apply.

Maybe when the oceans boil and the rocks melt...

Then you wouldn´t consider "whipped cream" being compressible?
I don´t say PB is compressible like gas, but there is a small amount of air in the paste.
-Olaf

I thought we were talking about peanut butter...