Wednesday, January 31, 2007


Tidying up the Mk 2.1

The Mk 2.1 seems to be a very different beast from the Mk 2. Yesterday before I got things working properly with the Mk 2.1, Vik mentioned that he often put a clamp on his Mk 2 for a bit of extra pressure on the polymer pump. I grabbed a c-clamp and tried that, applying just enough pressure to stall the GM3 (I don't worry about damage in that it has an internal slip-clutch) and then backed off a touch.

That's when the Mk 2.1 started pumping like crazy. I had the GM3 seated on #40 studding bolts not locked down. I noticed that the motor coupling was riding up pushing the GM3 up on its studding bolts. That didn't hurt anything and I've had the motor coupling, made of a cut coupling nut and nut with a lock washer between the two, come loose before, so I didn't worry too much about it. After I finished running the 400 mm of HDPE, I had a rush of good sense to the brain and decided to take the Mk 2.1 polymer pump apart before I tried running HPP.

That was rare good sense. When I disassembled it I discovered that the motor coupling hadn't broken loose at all. Instead, the thrust collar that I'd made of two nuts with a lock washer between had broken loose letting the whole threaded polymer pump try to work its way out of the pump.

One thing wants noting at this point. I use a thrust collar to absorb the tendency of the threaded drive rod to try to come out of the pump rather than the two bushings that the Mk 2 has. I have the a brass plate with a hole through it to admit the threaded rod at the top of the pump before you get to the motor coupling. The previously mentioned thrust collar butts against that. Basically, I used the same idea that Vik did with his drawing that is in the Mk 2 documentation on the reprap site.

Where the bottom bushing is in the Mk 2 I have a steel bushing that is the same diameter as the threaded thrust rod. This lets me avoid having to mill down the thrust rod since it seats right against the threads.

Back to the story. The threaded rod's thrust collar had come undone and let the threaded rod ride up in the pump till the bottom of the rod had slipped completely off of the bottom bushing.

That's when the pump started working properly.

What that meant was that with the configuration I was using all the bottom pressure springs in the polymer pump were doing was adding friction to the device for no good reason. The springs at the top where I was applying a touch of extra pressure with the c-clamp were effectively the only ones that mattered.

For the HPP I reseated the threaded rod in the polymer pump, really tightened the thrust collar and applied a bit of pressure to the top of the pump with the c-clamp in addition to what the springs could offer.

HPP pumped just fine with this arrangement.

This morning I sloped down to the hardware store and bought a more powerful pair of springs for the top of the pump in the hope that I could get rid of the c-clamp. That worked. You can see the Mk 2.1 sans c-clamp operating this morning in this pic.

The picture shows you something else interesting that is happening. The BBQ paint that I used to tack down the insulated nichrome 80 heating wire is perishing at the top of the heater coil. This lets it delaminate from the extruder barrel and then causes the insulation to overheat and begin to perish as well. The extruder barrel is still working, but I can see how this is going to go.

My first try at a fix for this problem is to use more BBQ paint coats to tack down the heater coil. I've got some other options if that doesn't work.

Regarding that failing BBQ paint, have you run across that high tempurature silicone stuff? I've got a little red tube of the Permatex stuff here that claims immunity up to 345°C, or 650°F.

Not sure if that matches your heat requirements though...
The bbq paint handles 600 F for prolonged periods and 1200 F for brief periods.
Interesting, Briand. I haven't used sodium or potassium silicate since I was a child building my own rockets. I used sodium silicate (water glass) to keep back flash from the rocket engines from burning the fins off of my rockets.

Except for the muffler bandage kit, which contains methyl ethyl ketone which would be driven off when the extruder barrel heated and which is not something you want to encounter in anything less than a well-ventilated environment, the products you have there are essentially potassium or silicon pastes (water based) filled with metal dust.

They are water soluable and vibration sensitive. For muffler patches you can't but consider them as temporary until you can save the money to replace the muffler.

For a heater barrel I wouldn't want a metal power filler, since we are surrounding an electrical heater coil, but sodium or potassium silicate mixed with something like marble dust might just do the trick.

As a safety issue, both of those compounds are relatively strong bases (think of lye but not quite so strong).

Before refrigeration, people used to preserve eggs by putting them in their shells in big jars filled with a liquid made of sodium silicate and water. When I was a child you could still buy this stuff in chemists' shops and some hardware stores. Mind, when I was a child you could still buy Ford Model T spark coils for $5, too. I learned a lot about high voltage electricity from one of those.
As you say, interesting. I hadn't chased down the ingredent lists, I just remembered that the paste version could indeed survive on an exhuast manifold, and i couldn't picture anything hotter than that in a human habitation. As you say, water glass is interesting stuff, perhaps filled (mixed) with chopped fiberglass to make it less fragile and a layer of something to keep away water. Or perhaps soak fiberglass mats in this and wrap the nozzle in a few layers. Alternate with layers of metal screen (to wick off some heat) and you can go back to bbq paint on the outside to seal out water, pets, etc. Just sitting here kicking out ideas until I can get a garage that my wife will let me build DANGEROUS THINGS(tm) in.
I saw that stuff in the store too, but it seems to be in the format of a fiber reinforced square.

If sodium silicate can hang on to a muffler for a while, how long could it hang on to an extrusion nozzle? I think the nozzle has contraction/expansion cycle that is different though, and it's a much smaller diameter.

That thing about the eggs is cool, apparently they would keep for up to nine months!
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