The RepRap*R*

May sounds a provocation, but I have thought about a new addon:
The RapRapR: Rep Rap Recycler.
This device should triturate PET bottles, ABS recycle parts or other materials, that cn be fed to the extruder.
Trituration could be rough or similar to powder, suiting the need of feeding the extruder.
This approach could add more "social" interest to the RepRap project, and supply of building material could be done at no cost and more easily.

We haven't demonstrated that we can extrude PET yet.

Recycling plastic has been part of the plan from the beginning.

[www.3dreplicators.com]

Basically, you have to have a regrind capability as well as a personal-sized extruder that will put out 3 mm filament with a high degree of dimensional consistency. Considering that a lab machine to do just the filament extrusion costs about $75,000 and requires your whole garage and then some, we've got to do quite a bit of innovation till we have that in desktop size.

To give you a sense of scale, this is as small a filament maker as you can get at any price.

[www.waynemachine.com]

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

I've built a small grinder before that reduced plastic stock to powder and I have built melter/extruder for plastic as well. The grinder cost was around $100 (US) and the melter/extruder was around $100 (US) as well. So it isn't exactly cheap even when made from scrap materials. It also involves having a machine shop to build the grinder. Although, I suppose that if you have enough experience with a drill press you could build it at home.
The grinder design was 2 parts - a shredder then a grinder. The shredder was modeled after an electric pencil sharper - 3 metal blades arranged in a conical offset, driven by a hand drill. From there the "shavings" fell down a funnel into a gear set from an old car transmission. The gear set pulverized the shavings into a powder which fell out through a piece of window screen wire. About once an hour you had to clean the big chunks out of the screen wire. The biggest problems were keeping the blades sharp on the shredder and getting melted plastic out of the gear set.

The melter/extruder was a metal screw type wine press that was placed on top of a coleman camp stove. The "juice" nozzle on the press was replaced with a custom copper nozzle. Once the stove heated the plastic in the press up enough to melt, you just had to put on gloves and crank down on the screw to start extruding. The
biggest problems were getting the plastic too hot, nozzle clogs, and the fumes.

Wow Cris! Blog that puppy! It's important!

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Neat! Hadn't thought of using a wine press.

on thought about recycling plastic,

it seems that getting it long and uniform like our 3mm filament is way tough and perhaps more work than it's worth.

granual extruders are complicated and are difficult to keep from jamming

what about melting plastic into bolts. (either recycled plastic,straight pellets, or more realistically for recycling, a mix. ) say 3 inches long and 5mm =/- .5mm. shouldn't be too hard to orient and feed the stock, and unlike the granual extruder, it's maybe a bit more predictable. and unlike the filament feed, the tolerance maybe won't need to be as tight.

unlike extruding straight from pellets, you can put pressure onto the material while out of the heating zone, keeping the melted plastic away from moving parts.

andres

At a print speed of 12.5 mm/sec you are going to be eating one of those every four minutes with a 0.5 mm orifice. Any idea of how you are going to feed them in to the extruder?

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

i have several, don't you?


in all seriousness though, with a gravity fed hopper, and with pieces of a decent size (what i mentioned above was a quick example) it should be possible to isolate and feed one stick at a time, and like i mentioned above, if you keep the moving parts out of the heated zone, you should be able to avoid some of the issues with a granule extruder.

andres

With a rack gear as a plunger you could push a short rod in slowly, then retract the plunger to clear the breach and let a new rod come in from the side on a spring load... you'd need a long magazine though.

Guy's,
I wonder if this link may give you some ideas to work from. When I was looking into bio fuels a few years back, I though about one of these units.

[www.oilpress.com]

I would have thought that extruding into a cooled tube would maintain dimensional stability. Just a thought.

Oh! That kind of oil. My first reaction was "what the hell is an oil press". Makes sense now. Certainly looks durable enough to handle hot plastic.

I'm happy that my comments has started a productive discussion.
IMHO the ABS powder is the more practical approach.
Could PET or other polymers be treated in the same way ?

I went and tracked down my old grinder and press; and talked with the people that use it. According to those that use it, the grinder works great on hard plastics like acrylic and lexan, but it doesn't turn soft plastics like milk bottles and drink bottles into powder very well. At first they were still grinding the old drink bottles down as well as the grinder would do it and then melting them, but they found that if they controlled the temperature better on the melting pot (heated wine press), that they could just throw whole bottles into the pot, without shredding them first; and then melt them and extrude them.

They also said that when they go to melt plastic drink bottles that they add a few drops of antifreeze to the mix to get a better extrusion. They claim that this is to replace some of the ethylene glycol lost during the melt process. I'll have to trust them on that.

So it sounds like the cheap thing to do is to design a heated screw press with good temperature control and then make a recipe book.
"Cheap" may not be the optimal word since the design will probably involve a rather large piece of cast iron or aluminum...

Where do the air bubbles go in a wine press? Is there a way for it to escape out the top?

The idea of using an additive in the re-cycling process is probably going to be useful, especially during the early times and given i do remember reading about how you need to do something like adding solvent to maintain strength. One big problem with hoppers and big chunks is that they tend to do something called bridging (where the hopper is full, but it empties the lower 1/4 or something and the rest forms a sort of plug/bridge that stops any more from going in. The problem gets worst when you've got large particles that are angular that tend to form these.

Personally, I think that the work by forrest on ABS is probably the way to go in the short term. When I get around to it, I'll try and see if I can design a small scale distilation column that will work for recoving acetone needed for recycling the production of 5-10 people (ie, stick one at some local hack-spot like NYC resistor and people just bring in their old stuff to be recycled). Time to break out my copy of Hysis from the looks of things.

Guys, until we either get a workable granule or powder extruder or a filament maker going, grinding ABS whether with acetone or mechanically, is kind of pointless, isn't it?

Edited 1 time(s). Last edit at 01/16/2009 04:39PM by Forrest Higgs.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

"Where do the air bubbles go in a wine press? Is there a way for it to escape out the top?"

The larger ones get caught in the plastic at the top and due to a rather large head space, they never make it out of the nozzle. The smaller ones, do go out through the nozzle mixed in with the molten plastic. It's not a problem for their application, but it would be a problem for making reprap filament. I guess the heated screw press idea will need a vacuum pump as well. If we try hard enough we can design one that costs more than $75,000 ones.

I think that extruder design is pretty dependant on the type of feedstock. Particle size has a huge effect on hopper design and things like jamming and particle bridging. I'm more used to working with huge hoppers (ton sized ones) but the basic rules all apply.

Besides, having a large amount of work on this will make it all go faster when we do have an extruder designed.

I'll be glad to help design a device to spit out ABS powder, but I need to know what particle size you need. It would also be nice if you could tell me at least one form of ABS supply that will be ripped down to powder.

I'll go on and warn you that a grinding type powder producer is going to be loud and use a lot of power. It probably also require routine blade sharpening.

If you want to go with chemical treatment, I can design that too, but I would prefer not to publish a design that someone would build and then blow their house up with. I have bad memories of an acetone flash fire caused by a visitor with a camera or more specifically a camera with a flash attachment. No one got burned, but it sure did scare a few years off everyones' life.

I've worked with extruders before to some degree... Wouldn't the stock being fed into the extruder be efficient if it was say the size of a gluestick that you'd put into a gluegun? Just curious.

I know this isn't possible (yet), but I was thinking of ways that a granule/powder hopper extruder might work. As I understand it, because the hopper can only hold a limited amount of material, it will eventually run out and the delay in refilling it will cause problems with the build.

However, this problem could be solved with multiple tool heads. Instead of just one hopper head, you have two identical ones and have one being refilled and the other working. When it runs out, then the one that was refilled immediately takes over and the empty tool head goes in to refill.

The amount of material needed in the tool head hopper would be determined by the time it takes to refill an empty tool head hopper.

As RepRap only has one tool head at the moment, I know this is impractical as it stands, but such a system could be achieved with multiple tool heads that can be swapped out.

One of the big problems with grinding things like HDPE and the like is that the grinder operates at a fairly high speed. When it cuts the plastic the plastic gets hot and melts against the metal cutting edges. You see this happening fairly regularly when you drill plastic at too high a speed.

Why not solve the problem for grinding plastic the same way we do in drilling plastic, viz, either lower the speed or dribble a coolant onto the cut... or both?

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

From what I gather, the essential apparatus on the wayne machine is the series of godet rollers. However I can't find any information (outside of google book search polymer extrusion hardcover textbooks) on what those are, beyond rolers moving at different speeds to stretch the filament, how they fit into a system, and why they are so accurate.

Wednesday, March 08, 2006
We appear to have another successful filament extruder concept...

[blog.reprap.org]

Forest Higgs mentions them in his blog post, so if anyone has access to him, could they ask him what a godet machine is and how it factors into a precision extrusion process?

A combination of post-extruder heaters and godet stations are used to finish the raw extruder output to the specified diameter and error rate. As to the details of just how that process is controlled, I have no information.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Hello I found this on instructables and it seens apropriated to an reprap, we could print something to guide an boxcutter's blade and have an fayrilly consisten sting out of a pet bottle.

Temperaturewise will the reprap cope with it? (I'm still waitting for mine from bits form bytes )

If the soda aproach could work it would mean an almost endless suply of material for very cheap wordwide and an increase in PET recyclying

Cheers

I once worked in a factory that blow-molded bottles. The blow-molding machines were fed by extruded molten tube. The extruders were screws with a pitch that grew progressively finer and turned continuously in a heat-zoned barrel that heated the resins but didn't let the plastic get really fluid until it was pushed through a final heat zone with external heater, plus a heated torpedo in the centre. The extrusion screw is a piston as well. The piston stroke is what feeds the molten tube into the mold zone, the mold closes around a sufficiently long hanging tube, pinching off the open end and air is introduced through the neck, inflating the melt into the mold, which is cooled to harden the bottle. While the mold is closed and setting, the extrusion screw is still turning and winding itself up towards the feed hopper, loading melted stock into the piston zone to feed the next molder cycle. Most of these bottles were made from PVC, polypropylene and HDPE, direct from granule to bottle. PET bottles aren't made like that, they're blown from injection molded premolds, not fed from extrusion. I don't know why, it may have to do with the threads for the cap, but it does point to PET not being as compliant as other plastics. The extrusion screws had no provision to exhaust gas bubbles, gas would exhaust itself along the barrel through the feed entry opening. The raw granules are still solid at that point, and as they soften, they're what's pushing the melt down the barrel. Gas can blow back, plastic can't. The variable pitch screw works the plastic against itself, the solid granules in a coarse pitch zone of the screw can't be pushed back up by melt in a fine pitch zone. When production was finished for the day, the screws and barrels were loaded with LDPE, which melts low temp and can be pushed out cleanly by the tougher plastics the next day. If those tougher plastics are the last thing through the barrel, it takes and age to free it all up and restart production. Since we don't normally want to interrupt extrusion, variable speed control on the screw should suffice. If the screw's axially-spring-loaded, a position sensor should control the speed.
These screws were capable of accepting a resin bead of 5mm-6mm and they were about 1.5-2m long to put a lot of heat into the plastic quickly for production. For reprap's 15cm³ per hour, I think scaling it down could easily be done with a pre-heat chamber, avoiding filament extrusion altogether.
The varying-pitch screws weren't like wood or machine screws, they were more like the worms that drive recirculating balls, and the grooves were quite widely spaced. A varying pitch screw of this type can be made by rolling a tapering strip of metal around a bar. Acceleration of pitch is defined by the taper - which can also be a curve, so the screw is non-linear, but can also be geometric or even logarithmic.

Granulation, and I use the term very loosely, can be achieved by cutting up plastics with a drop saw with as fine a blade as you can find. The sawdust is the granule...

Edited 3 time(s). Last edit at 11/01/2009 07:25AM by murd.

Okay, coming into this late, but this topic tickled some ideas in my brain...

As for grinding, what about a very slow cut. I'm thinking a mechanism like a paper shredder. Successive ones, of progressively smaller cut. It likely won't get us down to a dust, but I think it'll get down to small enough to feed into an extruder.


As for the extruder, what about something like they use in candy making? Tapered rollers, set around the extrusion material that feed the plastic (referring to the malleable state of matter, not the material itself), that feed the plastic in, while simultaneously compressing it to the desired shape. Think a cross between spinning yarn and rolling out a play-doh snake. Then the heating hopper element only has to output something 1cm in diameter, and the rollers will stretch that out to a 3mm filament, which is then wound on a spool for later use.

Viable ideas, or am I moron?