Comments on RepRap: Blog: A new wrinkle on CAPA filament production...

I'd suggest being able to fit multiple 'cartridges...

2006-07-23T21:51:39.270+01:00

I'd suggest being able to fit multiple 'cartridges' in the head at once... that way you have much more control over when you refill the thing (you could wait to pop three in the thing when it's almost empty... or you could 'top it off' between parts/stages/at your convience (rather than having to time it around when an entire single piece has fed through enough to fit a new one)

Yvan Roy: That makes all kinds of sense. You cou...

2006-07-22T19:04:43.540+01:00

Yvan Roy: That makes all kinds of sense. You could even make the CAPA piston with a mold rather than extruding it. Putting some holes in the piston would also let the charge of CAPA being extruded outgas properly, too.

Could the CAPA cartridge be a well formed CAPA pis...

2006-07-22T05:29:24.556+01:00

Could the CAPA cartridge be a well formed CAPA piston with a built in preassure sealing fringe and the top end? Push it into a smooth pipe, screw on a cap with a hose attached that feeds pressurized oil/water/air, and as it melts at the bottom end and is extruded, it is pushed down?

To reload, just unscrew the top cap, put in a new CAPA cartridge/piston/seal unit and continue?

Oooorrr, what if you make several cartridges. Each is a simple pipe with a fitting a the bottom, and as a group they are heated, and outgassed, then they are used one by one (cooled, or still hot?) by clipping into the extrussion head just above a heater to melt the CAPA as a pressurized fluid pushes them out?

I see problems with all those suggestions, but...

I just -knew- this idea of Plaasjaapie's was fun! :)

LOL! Does this mean you like the idea, Adrian? :...

2006-07-21T20:40:47.690+01:00

LOL! Does this mean you like the idea, Adrian? :-D

(Me again)^2. Miniaturise the entire device, and ...

2006-07-21T14:20:39.596+01:00

(Me again)^2. Miniaturise the entire device, and make a heated RepRap polymer write head that has a bag-wrappeded shot of polymer in, and is fed by mains-tap-pressurised cold cooking oil. The pressuriser would be elsewhere feeding oil at 4 bar into the head. When the head runs out, just swap in another cartridge.

Exactly what constitutes a "cartridge" may require some thought. For example it might be a length of copper plumbing pipe, a heater, the polymer-in-a-bag, the oil needed to fill it, and the nozzle.

You could be refilling one while your second one is writing.

Oh - we'll need a valve on the end. Bother :)

Or maybe just releasing the pressure would do...

Me again. Outgas the polymer as a separate proces...

2006-07-21T14:01:30.130+01:00

Me again. Outgas the polymer as a separate process, making a lump. Then vacuum-wrap the bag round it cold before placing it in the heating chamber.

Or how about using water instead of air? Domestic...

2006-07-21T13:59:32.216+01:00

Or how about using water instead of air? Domestic tap pressure is about 4 bar. The boiling point will rise with increased pressure, but you don't want to rely on that because a leak will explode. But you can also raise the BP by using a sugar solution.

Or use cooking oil in the heated jacket, pressurised with mains water away from the heating, and with the oil/water interface designed to move up a tube (water underneath) as the polymer extrudes. The pipe conveying the oil from the pressurisation plumbing to the extrusion chamber can be very fine indeed.

Brilliant! I think you need one trivial modificat...

2006-07-21T13:50:45.660+01:00

Brilliant! I think you need one trivial modification - don't push on the polymer with the compressed air direct, have a floating piston that has air one side, polymer on the other.

Or better: put the polymer in a high-temperature plastic bag (one of those designed to go in a microwave oven should do), outgas it as joel says, then just put the bag in a pressure chamber with the air outside and with the nozzle clamped at the mouth of the bag.

You can buy a reservoir-based compressor that can ...

2006-07-20T20:47:33.456+01:00

You can buy a reservoir-based compressor that can run a nail gun that can do the job for about US$25. By the time that you buy a bicycle pump, reservoir and all of the bits you need to do what you are talking about I expect that you'll pay about the same.

Good idea about depressurising the extruder, though. That notion has a lot of merit.

One more quick idea: what about a bicycle pump, a ...

2006-07-20T19:53:13.066+01:00

One more quick idea: what about a bicycle pump, a reservoir, and a pressure regulator? That way, you could just pump up the reservoir, set the regulator, and forget it.

If you already have pressure fittings, you may wan...

2006-07-20T19:49:54.883+01:00

If you already have pressure fittings, you may want to revisit the idea of using vacuum to degas the polymer. Just pump it down to a reasonable fraction of an atm a few times, and all the bubbles will be gone.

***Maybe that's why your reply sounded... ugh, tes...

2006-07-20T15:45:01.910+01:00

***Maybe that's why your reply sounded... ugh, testy!***

The testiness wasn't about you. I was just remembering what happened one time when a pressurised hot water line broke at a joint on a project that I worked on in Sweden many years ago. It was fortunate that I wasn't standing any closer than I was.

***I think I'd better warn you...***

Thanks for the warning, Tom! I really appreciate it.

I think I'd better warn you - I used one of those ...

2006-07-20T09:57:54.513+01:00

I think I'd better warn you - I used one of those exact models of compressor in my early furnace burner experiments, and I'm afraid they're worthless for continuous running. They overheat in about ten minutes and mine wore its cheap bearings into a massive ovoid in about an hour.

I bought a second one and retrofitted it with a scrap of phosphor bronze I salvaged from the department workshops, but gave up on it and eventually switched to a salvaged fridge compressor - those things are bulletproof.

Oh, Plaasjaapie, just caught something, I think my...

2006-07-20T07:15:10.720+01:00

Oh, Plaasjaapie, just caught something, I think my first sentence on my first post "Okay, well that sounds like fun..." might have sounded sarcastic. It wasn't, it was literal (I'm a very literal person). Maybe that's why your reply sounded... ugh, testy! ;)

Just something to keep in mind.Yup, I know a...

2006-07-20T06:42:36.233+01:00

***Just something to keep in mind.***

Yup, I know all to well about how much energy can get stored in compressed air. I don't think that you've thought about what happens when a fluid like water heated to 160 degrees under 3 atm pressure is suddenly depressurized. I'll stay with air, thank you so very much. :-s

***How rapidly will the molten CAPA settle in the cylinder?***

From my toaster oven experiments with a 20 mm piece of brass pipe as the extruder barrel it took 30-45 minutes to settle and cook out most of the bubbles.

***Even a 13cm^3 volume is going to be useful in the mid-term timescale.***

There is nothing to stop anybody from scaling this down to something like you describe. IMS makes all kinds of sleeve heaters. In fact, iirc, Adrian bought a couple of small ones some time ago when we were working on the auger bit extruder project. I bet that he'd send you one.

How rapidly will the molten CAPA settle in the cyl...

2006-07-20T06:25:09.670+01:00

How rapidly will the molten CAPA settle in the cylinder? Force it out more rapidly than it settles, and you'll end up with a hole blown through and CAPA stuck to the sides of the chamber.

On a related angle, I'm producing good joins on short lengths of 3mm CAPA, so stringing lengths a metre long together is not going to be a tricky task. Even a 13cm^3 volume is going to be useful in the mid-term timescale.

Vik :v)

Okay, well that sounds like fun...I would just poi...

2006-07-20T05:32:00.716+01:00

Okay, well that sounds like fun...

I would just point out that pressurized air is not like pressurized water (aka plumbing pipes). When there is a breach in a pressurized air system, there is an explosive release of energy, very unlike the more mild mannered event that occurs when a plumbing pipe bursts.

Just something to keep in mind. That, and hot CAPA at 160˚c can cause much pain and cursing. (It sticks on contact, unlike hot water which politely drips away, minimizing thermal transfer!)

In any case, you probably had thought of that, so I'm just going to wait for experimental results! :)