Monday, February 26, 2007


Paste extruder

I have been working in the background on a paste-extruder for RepRap that is as simple as I can make it. It's based on an idea of Zach's and Forrest's.

It's just passed its first test:

(Slightly surreally, the sound track consists of a musical biography on Radio 3; I was listening, and forgot to turn it off...)

It consists of a 500ml fizzy drink bottle containing a balloon that in turn contains the paste. You blow air into the bottle to pressurise it via a one-way valve, and the pressure then forces out the paste. It goes through a silicone rubber tube that is clamped off by a latching solenoid. Pulse the current one way and it turns on; pulse it the other and it turns off.

Of course I'd rather have used a condom than a balloon, just for max. cred., but they're the wrong size...

Here's a high-res still.

And here's a detail of the valve at A:

The paste being used here is Fine Surface Polyfilla, as proposed as a RepRap material by my student James Low last year. One small advantage that this stuff has is that leaks tend to seal themselves...

Beautiful, simply beautiful.

Every part easy to aquire and easy to replace if damaged during testing. The only parts in contact with the material are a length of tube and a ballon. I was considering a syringe based system but this beat it hands down.

For a chemist type like me this is exactly the type of system I want. One of my largest worries about trying a build was that I needed a system that would allow me to try dozens if not hundreds of different resin types and formulations, without needing to rebuild the extruder or have a dozen extruders available.

I am looking forward to destroying lots of balloons.


The email is a spam trap, ask me if you want a real address.
Cool! Looking forward to some detailed pics and descriptions! :-D
awesome... good to see an idea i dont ever remember giving coming to life =)
I remember suggesting that we extrude CAPA in 3 mm filament that way a while back. Weren't you in on that. Difference is that Adrian has applied the idea to polyfilla.

Looking at what Adrian's done I think there might be a better way to do the job. The key to the idea is Adrian's balloon/condom but there's a better thing to use.

Think of a plastic Colgate toothpaste tube. Empty one of those, screw it into a reprapped extruder trim off the far end and fill it with polyfilla. Put a clamp on the open end to close it and mount the clamp onto the top of your reprapped extruder mount. Force the extrusion thread out by using a solarbotics gearmotor to push a double roller down the tube.

Viola! No compressed air. :-D
It's kind of like Adrian's peristalic pump writ large. :-)
I think you can buy Polyfilla already in a squeezable tube like a giant tube of toothpaste.
This is another very clever idea.

Here's some observations that might be obvious:

-The rate of the flow at the exit must be somewhat related to P1V1=P2V2. To keep the pressure constant throught the process without pumping again, we could use a bigger bottle (2L) maybe (the weight change is negligeble i think).
-That would be cool if we could use the same kind of needle used to pump football balls to build pressure in the bottle. So, what is the easiest way to do a oneway valve? Im thinking of a hole on the bottom of the bottle with and bit of silicone from a caulking gun. Or a glued on valve from a dollar store ball?
-A simple plastic bag (sandwich bag) could be used instead of the balloon, you have to fight the elasticity of the balloon when you fill it. Also, polyethylene is more resistant to chemical than natural rubber. I'm thinking about the ABS-Acetone process submitted on the forum as a good process for this.
-According to some sources on the net, the burst pressure of a 2L bottle is about 150PSI. That would allow us to extrude quite high viscosity fluids. What's the maximum pressure reachable for a bike pump?
I love it. Something I can reprap in the house if it is odor free. And I think you still need the valve whichever way you create pressure to get the contents to come out. Ooh ooh ooh.. how about multi color spritzed out to do the 3 color thing almost like a printer. You could have a mixing chamber or just some premixed colors with enough swappable valves. But be careful not to get your condoms in a knot. Some great possibilities here. Nice work again Adrian.
Well I don't think you want to be tied manually, so how about a pump like this. 12v, where can we get a 12 volt supply? Hmmm.. and it goes up to 250 psi, a healthy margin for being able to blow up our coke bottles.. we're gonna need a safety.
Here's what was cut off s/250-psi-12v-compressor--619093.aspx
I've had one of those inflators. The gauge goes up to 250psi but the compressor tops out at well below 60. I went out and bought a cheap foot-pump instead. With that I could do 100psi, though this completely stuffed the cheap pressure gauge on it.

Vik :v)
I just used a couple of shots from a bicycle pump and it was good to go. The one-way valve is a flap with an O-ring under it and a smear of silicone grease.

The pressure drop in operation is not a problem. The filled balloon is about 50 ml, so with the 500 ml bottle and using (bf5man's)x Boyle's Law, the pressure drop is only 10% from start to finish.
PS. One problem is, though, that the balloon deflates and then blocks the exit hole before the paste is exhausted.

But I'm pretty sure that I can fix that by putting a small-diameter RP tube up the middle of the balloon with holes in it for the paste to flow out through.
We could also use a strip of plastic (cut from another PET bottle and about 5mm x 100mm), folded in two and inserted in the balloon. The strip would be disposable with the balloon. Less clean up to make.

What was the tip of your bicycle pump that was compatible with your oneway valve?
I'm been thinking about the ways to pressurize the bottle.

-The first one is pneumatic from the action of a pump.

-The other one is by squeezing the bottle with a clamp or a worm-drive hose clamp.

-By a chemical reaction happening in the bottle like a mixture of sodium bicarbonate and vinegar releasing gases.

Squeezing the bottle with a worm-drive hose clamp is interesting because you don't have to modify the bottle, and it could be driven by a geared motor of the reprap machine.
I used some more of the silicone tube between the pump and the valve - it's very elastic.

We could have lots of fancy ways to pressurise the bottle, but given that just using a simple pump is pretty quick (much quicker that refilling the balloon with the paste), I'm not sure I see the point.
So would this be the pump for making silicone things? Or is that to thick?
I have visions of a terrible and sticky accident caused by a balloon full of RTV...

Vik :v)
//But I'm pretty sure that I can fix that by putting a small-diameter RP tube up the middle of the balloon with holes in it for the paste to flow out through.//

How about just running a popsicle stick up the middle?
You'd probably want to use silicone straight from the original packaging. Letting in some moisture will cause it to cure in place (If you've left the cap open, you've seen what I mean)...the alternative would be transferring the goop inside some kind of well-dessicated glovebag or something.
Some baloonatic thoughts:

1. On the collapsing balloon problem: is there room in the base for a thin rod with a clip on top to hold the balloon upright?

2. How will the balloon hold up to more reactive compuonds like RTV silcone or the electrolyte pastes that fab@home uses to make batteries? Also, how critical is viscosity? I don't know how chewy polyfilla is but RTV silicone can be hard to get out of the tube sometimes.

3. I am reminded of the recipe that starts out "obtain 1 camel, one sheep..." How do you get your paste into the balloon? I like plaasjaapie's roller idea because lots of things already come in squeeze tubes. It seems a shame to empty a tube into a bowl and then spatula it into a balloon with the attendant fuss and mess when you could just screw the tube into the extruder head, slap the rollers in place and go. Also do you have problems with air bubbles in your matierial after the transfer?
Stuffing ballons:

Put balloon in bottle, end of balloon spread over mouth of bottle. Insert nozzle of injector gizmo into balloon and fill away.

If you're squirting relatively small quantities of something solventy or corrosive, a neoprene glove finger might be a better idea.

Vik :v)
As Vik deduced, I filled the balloon with a 10 ml hypodermic syringe (£0.50; minus needle - strange looks from the chemist shop assistant). The trick is to put a slight positive pressure in the bottle first; that excludes all the air from the balloon. Once the first 10-ml-syringe-full is in, you can pop the valve and return the bottle to atmospheric for the rest (so it doesn't all come out again when you've finished...)

I'm pretty sure it would work for polydimethylsiloxane type stuff; that's less viscous than the Polyfilla. Once the bottle is pumped up the flow rate is pretty constant as the pressure doesn't change much from full balloon to empty. It seems to be quite simple to alter the flow rate just by counting in different numbers of pump strokes.
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