How does tubing size affect pressure of syringes?

[Ben Rodgers]

Hi,
I am currently working on a project where we use syringes to extrude some viscous materials. I will explain what I am trying to do and I just want to know what size of tubing would be better for me.
I have a 10 cc syringe connected to 3/32" tubing with leur lock and at the very end, it is attached to either a 27 gauge or 30 gauge needle. When I do this, it takes a lot of pressure to push the stuff out but when the stuff comes out, its not a continuous flow. What happens is it takes a lot of force to push but at some point, i will have pushed enough with my hand to make it come out but it comes out very fast and not in a continuous flow.
If I make the tubing diameter larger, would the pressure inside be smaller even if I have the same 27 or 30 gauge needle at the very end. Would I be able to have a more smooth/continuous flow? The reason I use such small needles is because I want to be able to draw a design with precision.
Thanks in advance for your replies

Ben

 

[BvU]

Hello Ben,

You have 6 to 8 $\mu$m inner diameter needles. Your fluid might behave as a non-Newtonian. Check out the Reynolds number too: the pressure drop will be huge -- as you know already

 

[Nidum]

(1) Make the needle very short . Essentially just a nozzle .

(2) If you are using printer type inks there may be a problem of gelling at the point where ink meets air . You may have to dislodge a tiny plug of gel each time to initiate flow .

 

[BvU]

Inkjet printer and paint makers calculate themselves silly with CFD and all kinds of simulations; perhaps you can find some hints there.
Exercising pressure manually may be too uncontrollable for your purposes.

 

[Nidum]

Rotring were one of the first to design viable needle drawing pens .

 

[Ben Rodgers]

The tubing I am using is 24 inches. I can also buy a 20 inch tubing. Which tubing length should I use to help with this?
I actually tried using motors to extrude stuff so its more controlled. What happened was the stuff wouldn't come out when the motor moved but then all of a suddenly, it would come out a lot at a time. I'm thinking the material is so viscous that the pressure buildup inside is huge. When I stop the motors, the material keeps coming out.
What can I do to avoid this?

 

[BvU]

What's the stuff you are transporting ? Physical properties well known ? Does the tubing stretch and act like a balloon ?

 

[Ben Rodgers]

The material I am trying to extrude have a gel nature, specifically it's an alginate gel or collagen. I believe physical properties are well known. The tubing we have does not stretch and act like a balloon. I bought tubing that can withstand high pressures on purpose so it does not expand. I am using PVC Tubing.

 

[anorlunda]

The techniques of ink jet printing might be well suited to your drawing. You may even be able to adapt a cheap inkjet printer and use its software. Or maybe a 3D printer may work.

But the droplets of the gel like substance must coalesce if placed in adjacent spots. That's difficult to predict without knowing the properties of the substance.

 

[Andy SV]

I think you are having a problem of elasticity
the tube swells until pressure has built to the point that flow begins but when the fluid overcame what ever is blocking it the swollen tube squeezes down expelling the extra fluid
If you switched to an inflexible tubing like steel brake line
Then no swelling no excess fluid
PVC still expands some is relatively soft even

 

[Tom.G]

the tube swells until pressure has built to the point that flow begins but when the fluid overcame what ever is blocking it the swollen tube squeezes down expelling the extra fluid

I agree. I was once on a project where we had to dispense micro-liter fluid volumes very consistently for medical diagnostic devices. The biggest trouble was the inconsistent dispense volume. The fluid was about the consistency of water so viscosity wasn't a major concern. We had to add a fraction of a second delay after the positive displacement pump stopped to allow time for the <brand name>, <heavy walled>, Nylon tubing to relax. At the tiny volumes being dispensed, any none-rigid component in the fluid circuit will be quite obvious.