Friday, July 15, 2005


Closeup of Turntable

Here's a shot of the levelling mechanism and gearing used on the latest Meccano turntable. By adjusting the long bolts under the turntable we can ensure the turntable remains level while it rotates, and adjustment of the long bolts on the large blue pulley allows us to ensure the axis rotates vertically.

Once we've modifed the Polymorph saddles, we'll probably add a cam mechanism to move the carriage as the turntable rotates. This should allow the deposition of shapes other than round bits of tube, without having to develop the electronics and software first. Hopefully we'll have some of the mechanical principles debugged and ready for the other bits.

Vik :v)

In this construction your
are using a driver belt.
But I have been wundering if it is posible to make a
chain gear on a rapid prototyping system ?

So can we use a reprap to print a working chain gear ?
At the moment we are toying with using strings and weights though the belt is installed. The string and weights are working remarkably well.

Vik :v)
Looking at your design an idea for the second generation occurs to me. Picture a 5gal bucket with the bottom cut out and a gasket installed around the cut. The bottom then becomes the top of the turn table The bucket is full of a ground plastic HDPE being a good choice. A rigid tube of adjustable length enters from the side of the bucket. Acetone or some other welding solvent is pumped through the tube as the turntable rotates and the tube indexes in and out. Sort of an r, Theta based system rather than adjusting the position of the turntable. The lower viscosity means the stream can be controled better. Surface finish and layer adhesion would be better, composites easier to make, and you could substitute almost any catayst/resin system depending on local availablity. I wonder if you could also add a second tube to inject wire. Or for that matter a concentrated salt gel as a substitute for metal. I need to pull out my reference and see what the conductivity differences might be. The tube index only has to travel in one direction, out, since you can reset by hand after each run, and it would only require a simple gear assembly rather and the rail and drive system the carriage needs.

Just a few thoughts.

Where does the solvent go?

What solvents exist for HDPE?

Vik :v)
Some possible HDPE solvents:
carbon tetrachloride
methylene chloride
I think we can probably rule out benzene, carbon tetrachloride, and
chloroform as being restricted and/or dangerous substances.

Methylene chloride, xylene and trichloroethylene are being phased out.

That leaves toluene. I suspect if you continuously squirt toluene into a bucket of stirred plastic, you'll re-invent the cement mixer in polymer form!

Vik :v)
you'd need to get it such that the evaporation rate and the deposition rate are matched - apart from that, it shouldn't be horrible.
I'm wondering, myself, whether it'd be more effective to extrude a rod through a hole, or to wrap a filament around a heated roller? it does bring up some issues with direction, but that could probably be managed.

As for getting that filament, it's fairly easy, just a matter of melting plastic in a container (in my case, some (poorly done) slightly-reforged copper pipe), and then pulling it out of a hole that's somewhat larger than the filament you want to produce. Pull consistently, and you ought to have relatively consistent filament.

Alternately, I've heard that carbon dioxide lases surprisingly easily - no experience, but that's what I've heard. So do the powder stuff, but replace the solvent with heat. But that's also more dangerous.
Your selection of solvent working of HDPE strikes me as a bit strange. Certainly, almost any solvent has health and safety issues. If you are seriously planning on this technology being generally available I'd worry about it needing solvents.

Some years ago I did some work with powdered polythene and heat. I don't see what the danger is that you seem to be worried about. HDPE doesn't burn very robustly and certainly at low enough temperatures to melt it it's not likely to ignite. The only problem I ran into was the tendency of the surfaces of the objects I created to oxidize a bit. That made recycling the thermoplastic a bother.
The danger I was contemplating wasn't inherently from doing heat work with polyethelene - I mean, the stuff is practically very large-molecule paraffin - but rather the idea of doing it with a carbon dioxide laser with a potentially minimally trained individual who might do somethng stupid with such a device.

Potentially versatile, but also potentially quite nasty.

Probably significantly easier just to use extrusion or perhaps filament-fusing mechanisms instead.
I really like the idea of using HDPE as a material. Seems like half the stuff I throw in the recycling bin is made of it. It'd be nice if I could cut out the middle-man and recycle it myself.

Any ideas on mechanisms to grind HDPE into a powder? I tried cutting it up into pieces a running it through a blender but that didn't seem to produce powder so much as it did noise.
I tried a number of strategies to powder polythene and never ran across one that worked satisfactorily. The closest I got was to use a carbide grinder surface. That produced a nice powder, but if you didn't watch the feed rate through it the cutting surface clogged with partially melted polythene powder and had to be hand cleaned.
Acteone is generally viewed as a very safe solvent, highly flammable but extremely cheap and easy to handle. It is excellent at dissolving plastics like HDPE, so only very small amounts would be necessary to weld the plastic particles together, and any amount of airflow causes it to evaprate. Frankly gasoline could also be used as the weldling solvent.

It is common to substitute inert fillers for some or all of the plastic shell binder uses acteone as a welding solvent and gets about 500psi with only 2% binder and 98% sand.

If the solvent bothers you I would suggest using a commerical water based polyurethane. No harmful solvents and rather than welding the particles you would be glueing them together. The weakly bonded particles could then be baked at low tempature for an hour or so to cause the HDPE particles to soften and weld together. Sort of like firing clay. The result is recycleable but needs a constant supply of polyurethane. Hide based glues could be used, but would need more careful handling before being baked I suspect. Depending on the strengths you needed in the final form you might be able to get away with using some percentage of inert filler rather than a plastic. The Foundry industry uses 100% sand for heat resistance and because it is extrememly cheap. We get 200psi out of only 1% polyurethane binder.

Another suggestion would be to use Furfuryl alcohol and inject it onto plastic particles that are coated in an acid catayst. This would glue the particles together at the same time as producing heat enough to weld them together. FA is produced from recycled agracultural waste. It is cheap, but like most organic solvents not a good idea to smell all the time. Also if the catayst and the FA get mixed in pure form they react violently.

Another suggestion would be take the plastic and coat it with Sodium Hydroxide and then inject Hydrochloric acid using the nozzle. Instant heat production in a very small and highly controlled area. No fumes other than steam, but the more heat that is needed the higher concentration you would need. I also dont know what the water contamination would do to the plastic.

"Any ideas on mechanisms to grind HDPE into a powder? I tried cutting it up into pieces a running it through a blender but that didn't seem to produce powder so much as it did noise."

Did you try putting them into the blender with water? I just got what I'd call a reasonable pulp of HDPE by doing that - the water helps to keep the HDPE from tangling up, and pulls it down into the blades. The same sort of thing happens when making paper.

Also, it seems to me that a potentially viable device for making pellet-type things might be a meat grinder with nichrome wound around it to heat it up.

Or, you know, make or aquire (I think there might be something appropriate that's normally used for making sausages) an attachment to extrude the polyethelene run through the grinder into a rod......
A ball mill is the easiest way to fine grind HDPE. They are realitivly easy to make, I will if see if I can find a set of instructions. People that make fireworks as a hobby have several designs that they are comfortable with that dont take more than a drill a coffee can and a handful of ball bearings. They are slow but they grind almost anything in to powder. That assumes I am remembering correctly but I will see what I can find.

Another way would be to add an excess of acetone, make a slurry and then drip the slurry into water to form the powder. the finer the drops the finer the powder.

"A ball mill is the easiest way to fine grind HDPE."

So, something like this? Reminds me of those rock tumblers kids used to have.

"Another way would be to add an excess of acetone, make a slurry and then drip the slurry into water to form the powder. the finer the drops the finer the powder."

I bet that would work a lot faster than the ball mill. Any way of recovering the acetone afterward?


Harmful by inhalation, ingestion or skin absorption. Irritant. Liquid may cause permanent eye damage (corneal clouding). Contact with skin may cause defatting, leading to irritation. Long-term exposure may cause liver damage. Typical TLV 750 ppm.
Acetone can be reclaimed by seperatory funnel since it will not mix with the water. Or simply skimmed off the top since a little water in the solvent shouldnt effect the next batch.

Acetone is very safe relative to other solvents. You still need to treat it with as much respect as any other solvent. Gasoline is every bit as dangerous as acetone and has more long term health effects because it contains benzene etc.

Whichever you use, wear gloves and use in good ventilation.
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