General granule extruder design concepts

Re: General granule extruder design concepts

Annirak — Fri, 23 Jan 2009 11:45:43 -0500

Looks like thrust bearings can be had for <$10 from ebay.

Re: General granule extruder design concepts

freds — Thu, 22 Jan 2009 23:58:18 -0500

Annirak Wrote:
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> Does anyone know how much axial force a skate
> bearing can withstand?

No but I stumbled across ceramic bearings with a 180C rating that might strike some idea's here.

[www.shopping.com]

Here's some bearing spec's.

[www.hchbearing.com]

Re: General granule extruder design concepts

Annirak — Thu, 22 Jan 2009 16:11:05 -0500

Does anyone know how much axial force a skate bearing can withstand?

Re: General granule extruder design concepts

freds — Thu, 22 Jan 2009 13:46:29 -0500

Take a look at Forest's captured nut design.

[www.3dreplicators.com]

You could have the motor stacked horizontally on top of the drive piston which could simply be a cap on the threaded rod.

Then use skate bearing like demented did for his extruder on each side of the drive nut.

[builders.reprap.org]

Re: General granule extruder design concepts

Annirak — Thu, 22 Jan 2009 13:22:32 -0500

degroof Wrote:
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> A couple of random thoughts:
>
> 1. The auger designs I've seen have had the auger
> extend into the heated zone. Since the auger tends
> to conduct heat, the thermal break is compromised.
> What would happen if the auger stopped on the cool
> side of the thermal break? Would it still be able
> to push the granules ahead of it or would you tend
> to get packing problems?

I'm not sure whether this would work or not. I think it's got some merit. The worst that can happen is the auger can just spin around instead of pushing. I think I'll try it first.

> 2. The main problem with a piston design is the
> pause while it reloads. What about 2 pistons
> working alternately (1 loading, 1 pushing). Too
> complicated?

I think the real problem here is valving in the hot zone. With the piston, it honestly just makes more sense to have a large enough piston to print one entire layer and reload after each layer.

With either a piston or an auger based design, we need a thrust bearing. Now, as I understand these, they're just two washers with ball bearings between, one attached to the outside, one to the inside.

In the case of the piston, the "inside" is a nut which we crank to move the piston. In the case of the auger, the inside is just the auger shaft.

Re: General granule extruder design concepts

degroof — Thu, 22 Jan 2009 12:57:48 -0500

A couple of random thoughts:

1. The auger designs I've seen have had the auger extend into the heated zone. Since the auger tends to conduct heat, the thermal break is compromised. What would happen if the auger stopped on the cool side of the thermal break? Would it still be able to push the granules ahead of it or would you tend to get packing problems?

2. The main problem with a piston design is the pause while it reloads. What about 2 pistons working alternately (1 loading, 1 pushing). Too complicated?

Re: General granule extruder design concepts

Forrest Higgs — Thu, 22 Jan 2009 12:55:34 -0500

nophead Wrote:
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> I think flourecent lighting has enough UV in it to
> turn ABS yellow.
>
The cases I'm referring to were in my office where only incandescent lighting was used. From what you've said and my own experience, I think the stuff just gets old and fragile on its own with no radiation enhancement. :(

Re: General granule extruder design concepts

nophead — Thu, 22 Jan 2009 12:06:28 -0500

I think flourecent lighting has enough UV in it to turn ABS yellow. I had four little PC speakers on my desk at work for about 6 years. Two that were directly underneath an office light went yellow on top and the other two that were halfway between lights did not.

I also came across a piece of ABS yesterday that had some small incandecent bulbs in it for about 1 year. They will have got warm, but only about 60C I would think. That was very yellow end brittle.

Re: General granule extruder design concepts

Andres — Thu, 22 Jan 2009 11:58:45 -0500

yeah, a full production environment and materials will certainly be much different than an at home fabrication setup.

the trouble for me in a quality job for a medium size manufacturer is that our suppliers would love to use whatever they can get cheaply, and they aren't about to submit an in depth report of the material's entire history.

one thing i do plan on doing is leveraging my experience at work into doing some more material testing over varying parameters (regrind recylcing included) to help optimize whatever we can.

if anyone here is interested in having some strength/yield testing done let me know. besides normal material tests, we can design some test samples that could maybe look at layer adhesion over various temp or z axis step-height parameters as well. i should have time this weekend to design some standard samples that can then be sent back to me and we can start getting some data together for different peoples' machines.

Andres

Re: General granule extruder design concepts

Forrest Higgs — Thu, 22 Jan 2009 11:17:33 -0500

nophead Wrote:
-------------------------------------------------------
> Maybe that is partly because ABS seems to got
> brittle with age so recycled ABS is already more
> brittle before you melt it.

I've wondered about that. A lot of ABS is used for PC peripheral housings. I've noticed that the ABS goes yellow after 4-5 years even if it is not exposed to sunlight. That argues strongly for the case you make, imo.

Re: General granule extruder design concepts

Forrest Higgs — Thu, 22 Jan 2009 11:15:56 -0500

Annirak Wrote:
-------------------------------------------------------
> Thanks for the link, Forrest. Do you know whether
> this is a global polymer property or whether it is
> specific to the class of materials that they ran
> the test with?

Wysiwyg with that article, Annirak. A year or two ago, I ran across an Asian article that indicated that you could extrude, grind, extrude HDPE 5-6 times without noticeable problems of material change. That study, of course, was done in the lab with the handling of the regrind material closely monitored.

In practice, HDPE is probably the most recycled plastics by weight out there. This link on recycling practice is quite useful

[cwc.org]

I've always pushed using HDPE rather than ABS simply because it is readily available and we know a lot about how to handle it. What we DON'T know, at the present time, is how to get past the warping problem. I'm hoping that the turkey bag approach will help with that. It's a matter of somebody doing the work, though.

Tommelise 2.0 is proving so valuable as a milling machine that I'm seriously considering simply building a next generation reprap machine for actual printing rather than switching T2 back and forth between printing and milling. Mercifully, the buffered, off-line printing mode that I developed for T2 means that I can run several machines off of the same PC.

Re: General granule extruder design concepts

nophead — Thu, 22 Jan 2009 11:08:14 -0500

Maybe that is partly because ABS seems to got brittle with age so recycled ABS is already more brittle before you melt it.

Re: General granule extruder design concepts

Andres — Thu, 22 Jan 2009 10:29:42 -0500

What i can say about regrind is from my professional life (corporate quality engineer)

we tend to limit the regrind allowed for our suppliers (chinese and otherwise) to about 20%. it lets us ignore the heating cycle questions for older material since most of it is guaranteed new. as the regrind goes up on a material like ABS the part becomes more and more brittle, which as you can imagine tends to effect mold stresses in an injection molded part and therfore weakness or break lines. how it would effect a reprapped part is a bit more difficult, probably just a stiffer/weaker part, though depending on the function, may not be that bad of an issue.

but yeah, once recycling is a bit more feasible i will personally aim to mix old regrind with new pellets, depending on the material i need.

andres.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 22:08:50 -0500

Thanks for the link, Forrest. Do you know whether this is a global polymer property or whether it is specific to the class of materials that they ran the test with?

Re: General granule extruder design concepts

Forrest Higgs — Wed, 21 Jan 2009 19:45:17 -0500

On the regrind/remelt question, I ran across an interesting little article a few weeks ago. An important aspect of the article was this reference...

--------------------------------------------------------------------
Temperature or heat history is commonly believed to be the biggest issue in polymer degradation. However a study done by Sue Janicki et al. Antec 1992 pgs 1201-1204;

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 19:25:09 -0500

Yes there are pros and cons. My machine RepStrap is about the same footprint as Darwin but only 1/4 the flat build area. Having said that I haven't found it too limiting so far.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 19:15:28 -0500

Nophead,
I think that static heads or z-axis heads have a lot to recommend them. If for no other reason than that they make head-exchange really simple: you just move the table under the next head.

There are two problems, though. The footprint of a static-head rapid prototyper is a fair bit bigger than the reprap. There is a limit to how many g's you can give the table before finer designs start falling over. Not sure how important this is, but I think it's worth taking into account.

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 19:08:30 -0500

Somebody posted a while back that remelting polymers degrades them. That means we are already one heat cycle more than injection moulding from virgin resin when we use welding rod. If we make our own filament from recycled plastic then that adds one more.

If we extrude direct from granules we save a cycle.

Once the extruder gets heavier than the object you are building plus the bed you can switch from moving head to moving table. The head weight is then of little consequence because you can counter balance it and it only moves slowly.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 18:06:49 -0500

Does plastic degrade with repeated meltings? How do conductive polymers respond to being melted repeatedly?

I don't know the answers to these questions, but I think they're important to consider.

Right now, I just need to extrude granules so that I can test conductive polymers in real applications. Once I have done those tests, I'll look at retooling for filament production.

Re: General granule extruder design concepts

Anonymous User — Wed, 21 Jan 2009 17:41:03 -0500

Robert really has a point - it's much, much better to build a separate filament-making device than to try turning the extruder into the proverbial Jack of all trades, master of none. If you're still not convinced, just consider the fact that the more weight you add to the extruder (that means metal augers, pipes, powerful motors, granule storage etc.), the more it will overshoot when changing directions, limiting the maximum printing speed (probably to the point of being unbearably slow) and increasing wear on the motors (higher holding torque required) and motor drivers (higher holding current). The filament extruder, on the other hand, can be extremely light and compact, which is a Good Thing. The granule-fed clunker can as well sit aside and make the filament for later use in a more stationary manner. Or, if you're really desperately into using granules while printing, put the granule extruder on a high shelf above the RepRap, feed the fresh filament to its extruder and make extrusion rates match.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 13:42:27 -0500

bobt Wrote:
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> It appears that Zach may have fixed the problem of
> the extruder on the machine and what is needed is
> a way to create the 3mm material that that machine
> can use.

If a 3mm filament extruder is truly what is needed, then a continuous feed auger design is the way to go. I think it would require two augers to work perfectly, but that's doable.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 13:39:34 -0500

I don't think the mobile hopper is necessary.

I think that putting the hopper over the inlet would work, but there's a little bit of a challenge in it.

It's not terribly complex, but there needs to be a mechanism to keep the granules from coming back out the inlet when we decouple from the hopper and when we start applying pressure with the piston. It might be as simple as a T fitting, but I'm not sure about that.

Brendan

Re: General granule extruder design concepts

bobt — Wed, 21 Jan 2009 13:38:50 -0500

Guys - The design that I was talking about is used to generate 3mm material. Why change what works. It appears that Zach may have fixed the problem of the extruder on the machine and what is needed is a way to create the 3mm material that that machine can use. The system that I built for my friend has a 10 CU Yard bin on top and when the system is turned on the piston powered by a hydraulic pump causes the piston to move back an forth. Each trip of the piston is loaded by gravity from the hopper. The hopper is loaded from the dust collection system. There is a second hopper that the wood pellets squirt into. These pellets then are used in a several pellet stove to heat his shop, house and cutting shed.

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 13:22:48 -0500

Don't you simply place a hopper over the inlet?

Possibly a larger stationary hopper to fill a smaller mobile one.

Re: General granule extruder design concepts

Annirak — Wed, 21 Jan 2009 12:31:55 -0500

I think it's relevant to point out that we only see a plain piston working here. While it does work, it requires manual intervention to refill the piston. I know that there are likely easy workarounds to this, but I still think it is relevant to note the difference between a working manual intervention piston and a working automated piston.

Brendan

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 12:07:30 -0500

[blog.reprap.org]

Re: General granule extruder design concepts

freds — Wed, 21 Jan 2009 12:02:46 -0500

Hi nophead

Where can we see this?

nophead Wrote:
-------------------------------------------------------
> I think Adrian has already shown this design
> working. The piston is not in contact with the
> melt, just the granules that have not melted. So
> there is no problem pulling it back and as his
> version is mounted horizontally so the granules
> just fall in the inlet when the piston clears it.

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 07:03:17 -0500

I think Adrian has already shown this design working. The piston is not in contact with the melt, just the granules that have not melted. So there is no problem pulling it back and as his version is mounted horizontally so the granules just fall in the inlet when the piston clears it.

Re: General granule extruder design concepts

Joules — Wed, 21 Jan 2009 06:49:51 -0500

Guy's,
I just don't see the piston idea working. You need that inlet to stay clear, how is it going to prime? Will you have a non return valve in the nozzle so the piston doesn't just pull air, why is the plastic going to enter the chamber and displace air than sit around congealed to the inlet. I think the current solution and possible a miniature screw feed is a better solution.

Joules

Re: General granule extruder design concepts

nophead — Wed, 21 Jan 2009 04:43:00 -0500

A moderate sized piston could easily hold enough for one layer of an object so that would be a good time to reload. I think that was the basis of Dr Bowyer's design.