Tribology question

[spiridij]

I have a problem with a paste that I am attempting to extrude through a glass syringe. The paste is about the consistency of toothpaste and has very small (under 150 micron) particles suspended in it. When I attempt to apply a constant load to the teflon pusher of a glass syringe, I do not get a constant flow rate. When I lubricate the system with a PFPE lubricant (which is not soluble in the paste or any of its components) it seems to get a little better, but I still don't get a constant flow rate. Does anyone have any experience with extruding pastes and or tribology that can help me take this problem apart. One interesting thing that I noticed is that the paste seems to stick very well to stainless steel, so I'm wondering if it is sticking to the glass as well, causing the system to bind.

 

[racookpe1978]

1. How large is the syringe, and long is the plunger? (Dia and length).

What max pressure can you withstand? (Hand only?)

2. What length needle, and how is the needle detailed? What ID, what OD?

Double opening, single cut-off tip, slanted end?

 

[spiridij]

1. The plunger diameter is 0.41" and the length of the syringe is approximately 2.4". I'd like to stay below 10 psi.

2. I'm not using a needle on the syringe, there is an opening on the end of the syringe that is 0.074" in diameter.

This is merely a test setup to determine the flowrate vs force relationship of different formulations of the paste and at different temperatures.

Does that help?

 

[LuisFelipeMS]

When you apply a constant load to the teflon pusher of the glass syringe, does it run smoothly or does it stuck sometimes? If it runs smoothly, maybe when you admit that paste onto the glass syringe you are also getting air which may create air bubbles inside the syringe leading to a non constant flow rate. Have you checked if you are also expelling air?

 

[spiridij]

Thanks for the tip. I've used a centrifuge to remove the entrapped air, so I'm pretty sure there isn't any air in it. When I apply the constant load it is difficult to see what's happening since I'm targeting a flow rate that would move the plunger approximately 0.02mm/min.

 

[Compositepro]

If you use a centrifuge to remove air bubbles, what do you think it is doing to your glass beads?

 

[spiridij]

The suspension doesn't appear to be separating so my guess is that the particles (they're not glass by the way), remain in suspension and hopefully are remaining evenly distributed.

 

[IRstuff]

What is the glass load in the mixture? You may have created a thixotropic material. Too much glass most likely makes the material not readily extrudable. Glass on glass does not work well under pressure.

Did it ever work the way you intended?

TTFN
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[racookpe1978]

With no glass beads, how does it extrude?

 

[spiridij]

I think there is some confusion. The particles are not glass beads. It isn't possible to test the material without the particles since the particles are in suspension in a very high concentration and it would give very different results. I have tested a high viscosity silicone oil (20,000 cP) and was able to obtain repeatable results.

 

[IRstuff]

"particles since the particles are in suspension in a very high concentration and it would give very different results. I have tested a high viscosity silicone oil (20,000 cP) and was able to obtain repeatable results."

I think that was essentially my point. Your "particles" are relatively large, and you seem to be saying that the particle loading is high, both of which are not conducive to smooth flow. You should at least try a run with your mixture with a lower load, or a wider orifice, and see if that changes things.

TTFN
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[Tmoose]

"... there is an opening on the end of the syringe that is 0.074" in diameter...."

150 um ~ 0.006" so the "opening" is about 12 particles across.

http://assets.nydailynews.com/polop.../derivatives/article_970/fixaflat6n-1-web.jpg

What is the detail of the transistion connecting the "opening" and the end of the syringe?
Is the "opening" part of a separate component screwed on to the end of the syringe?
Are there any shoulders or steps in the transition?

Depending what you are starting with, Perhaps a gentler smoother profile would help buy some consistency.

For gravity fed systems the transition is very important to prevent a variety of problems.
"first in, first out, bridging, rat-holing"

https://www.youtube.com/watch?v=PmTnUOcAnuY

https://www.youtube.com/watch?v=H6llvgfTkJE

http://jenike.com/case-study-search-results/