Wednesday, August 25, 2010

Two Designs

Final piece of the puzzle, the extruder.

Against all advice, I decided to roll my own here too... I don't mind failure as a way to learning, and I feel that I've read enough over the past months to have a decent chance of getting this to work.

I decided to use salvaged nylon gears out of an old printer. They fit the old Nema 14 stepper I had (perhaps they were connected in the printer) and gave me using two stages, a reasonable gear ratio.

Pictured below is a first draft of the design, I ended up using a full 60x60x4 mm angle, and attaching the motor mount/front bulkhead with small (4-40) csunk screws.

Pictured also is my all metal isolation, as per nophead's no comprimise extruder.
I used a MIG welding tip cut short (which I believe caused me problems, more later), in the bottom of my aluminum heater block. Two resistors giving me about 15 watts, and a thermistor. The heat-sink is out of an old power supply.

This design also came to me. I am not a fan of supporting the heater through M6 thread in PTFE, as it is soft and many people have reported stripping the threads while learning to use their printer. I also ordered a brass barrel with .5 mm hole in it in case I couldn't work out the bugs in my all metal design.

Here is design one hooked up temporarily for testing. The motor and gears have a LOT of torque. As I discovered, it will take a large bite (1/4 of the diameter) out of the filament when it jams without even noticing...

A closer view of the Hot-end. I left space between the heat-sink and aluminum chasis to minimize the transfer of heat. I'm using high temp gasket sealant to glue the etched PTFE 1mm plate onto the block. I'm holding it with SS locking wire as a strap. I was still waiting for kapton tape to arrive in the post. I've stuck a bit of PTFE in there to keep the thermistor wires off the strap.

It worked very well.. during 2 or 3 hours of testing, the top of the Stainless isolator and chasis never got above 30 degrees C. It was cold to the touch if I ran the fan... at 5V. 12V was overkill.

My 15 watts could keep up, but it felt under-powered. It took a couple minutes to heat up without the fan, and with the fan going perhaps 3-4 minutes. I recorded the duty cycle on my digital scope, and the heater was on for 60% of the time when not extruding, gradually increasing to 85% as I increased extrusion speed.

I intend to set up a grinder to carefully thin down the SS barrel above the resistor block, thus preventing some of the heat from escaping up to my heat-sink.

My MIG tip has a 0.8mm hole in it so I was pushing quit a bit of plastic out. I increased the speed until I found a limit, where I couldn't melt the plastic fast enough for it's speed. The extruded filament speed still felt slow at this limit.

The heater was still only on for just over 80% of the time when this limit was reached. Stripping of the filament drive started happening regularly as I ran the un-melted feed-stock into the heater end.

I believe I have discovered that one can have too short a melt zone.

One thing about having easy backup options, is you perhaps don't spend as much time working on a problem as you might otherwise...

I had set out to cut another MIG tip longer, and try to decrease the hole size to .5mm or less.
During my attempts to melt the end of the nozzle with my TIG welder (so I could re-drill), I oxidized the top badly and decided to give that idea up as a bad job. I will perhaps re-visit stretching the tip by rolling it, which should shrink the center hole in the narrow neck, and I can then cut it off at the narrow point.

In the mean time I built a more conventional PTFE isolated hot-end with the purchased brass nozzle, based on the above design... as pictured below.

I also built another hot-end, this time with the same resister that nophead uses in his.. I run it at 12V and get 20 watts of heat in my little block.. it heats up very quickly indeed.

Over the course of an hour of testing, I felt the isolator was keeping the heat away from my chassis very well. while the outside of the PTFE gets quite warm to the touch, the chassis is only slightly warm. I still have the fan mounted, and if I ran it, would keep all heat out of the top-end completely.

I had hoped that the exposed aluminum holding the nozzle into the PTFE would help cool it down and prevent melt above the nozzle, but that is not the case.. twice I got jams after having it 200 degrees for 7 or 8 minutes... extruding slowly or not.

When I took it apart, I saw that I had a plug above the nozzle, jamming it. Tomorrow I will do a better job of making the nozzle meet exactly with the PTFE inside. I'll be shopping for an M6 bottoming tap.

I also spent some time figuring out how to live broadcast using ustream. I hope to use this facility to get help tuning up my system when I start Printing, hopefully tomorrow or the next day. I have recorded a little silent walk-around of my extruder on my ustream feed:

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