Sunday, December 30, 2007

Complications


My goal for this weekend was to extrude enough HDPE to represent the volume needed to make the minimug object. The host app says it's 2955.9 mm3, which translates to 418mm of 3mm filament. So the idea was to mark a point on the filament where it's entering the extruder and another point 500mm up from that. If I could get the extruder to run continuously between those two points, it should eat (and therefore extrude) enough plastic to create the piece. Here's how that went:

Extruded a few cm. Cable/drivescrew connection broke. Tried soldering it. No luck. Cleaned it up. applied JB Weld, left it to cure for 15 hours. Baked for 2 hours at 200C according to nophead's suggestion.

Reassembled and started again. Extruded a few cm. PTFE barrel slipped out of clamp. Backed up the filament, put the barrel back in clamp, tightened it up again. Clamp broke. Drilled a couple holes through the extruder body and PTFE barrel, one on either side of the hole (i.e. cutting a chord through the barrel). Sank two drywall screws through the holes. There, that's not going to budge. Extruder is now starting to look like Frankenstruder's ugly brother. Broke yet another thermistor. Replaced thermistor.


Reassembled and tried again. Extruded a few cm. Despite having a hose clamp around it, heater barrel slipped out of PTFE barrel. Internal threads are stripped. Had a spare PTFE barrel. Replaced barrel and tightened hose clamp with power driver. Terminal on gear motor broke. Resoldered and used a zip tie as a strain relief.

Reassembled and ran without the nozzle to clean out the gunk. Took the opportunity to mod the nozzle according to Vik's suggestion. Screwed the nozzle back on, waited for it to heat up.

Extruded a few cm. Nothing fell apart!

Marked the filament for a 500mm segment. Set the extrude speed to 50 (about 1/5 full power). Extruded a full 500mm in 33 minutes. That's about 1.8 cubic mm / sec.

Measured the extruded output: 3170mm. That works out to a diameter of 1.2mm. Extrusion speed works out to 1.6mm/sec.

Cranked the extruder speed to maximum. Marked 50mm. Took 84 sec to extrude 320mm. That's 4.2 cubic mm / sec. Diameter consistent at 1.2mm. Extrusion speed works out to 3.8mm/sec.

If I've got all the numbers right, that'd work out to a 3-day extrusion time for a RepRap kit. Not too shabby. The volume rate is good but I'm using a large nozzle, so the linear rate is low. I might have to run some tests with a smaller nozzle.

Lessons learned:
- Baking JB Weld helps a lot.
- The clamp on the extruder body doesn't seem to be enough to hold things in place.
- When using a hose clamp, tighten the crap out of it, then retighten when it heats up.
- Always use strain reliefs on soldered connections.
- Grinding down the inside of the nozzle helps a lot.
- Clean out the heater barrel now and then.

Thursday, December 27, 2007

Things Get Affected By Things

The new extruder firmware was doing odd things. I was using the heat profiler and noticed that the temperature leveled off at about 77C. I looked at the heater LED and noticed that it was alternating between flickering and being off. About 6 seconds flickering; about 6 seconds off.

Troubleshooting mode...
- Maybe it's the heat profiler. Try the extruder exerciser. Same symptoms.
- Remove the thermistor from the heater barrel and let it cool down. Symptoms go away.
- OK, so, it's an interaction between the temperature sensor and the heater control.
- Replace the thermistor with a variable resistor.
- Start at 10k and decrease. Heater cuts out at 8.8k (temperature reads 77C).
- I've got C3 at 20nF. Try 10nF. Heater cuts out at 17.5k (temperature reads 89C).
- Try 100nF. Up to 130C.
- 200nF? 156C. Close enough.

I'm not entirely sure why it's cutting out like this. I'm guessing something in the extruder firmware is acting up, causing the safety cutoff to kick in at a lower than intended temperature. Whatever the cause, increasing the value of C3 seems to be a viable workaround.

After bumping the C3 to 200nF, my heater profile came out like this:


That gave me some numbers so I could adjust the hm value. It was pretty close at 1.1 but I dropped it to .9, just to make sure I got a decent heater temperature. I cranked up the heat, loaded some filament and tried extruding some HDPE.

I managed to extrude 2770mm (109") of .8mm (1/32") plastic. That works out to about 1.5cc.


Near the end, I pushed the extruder speed to maximum. The JB Weld joint between the drive screw and the wire cable snapped. That's a lot of torque. To be fair, I probably messed up that joint. I think that was the first thing I glued using JB Weld and I'm not sure I mixed it properly.

I'm now on thermistor number four, having fried or mangled the previous three. This one came from a cheap indoor outdoor thermometer, similar to this one. There are two thermistors in it. I used the outdoor one, setting aside the other for later.

Wednesday, December 26, 2007

Frankenstruder

This is the current sate of the extruder. It has:
- sandpaper to keep the PTFE barrel from slipping
- a hose clamp to keep the heater barrel from slipping
- a terminal block glued to the hose clamp to connect to the nichrome wire
- PTFE tape wrapped around the heater
- a thermistor cannibalized from a thermometer, embedded in JB Weld and strapped to the barrel

And, yes, "it's alive". Well, pretty much. It doesn't fall apart, melt or catch fire, which is a big step up.

Funny thing, though: The firmware is being difficult. The old firmware (v1.0) runs the heater properly but flakes out on the temperature readings. The new firmware (20071219) seems to do better at temperature readings but flakes out on heater control over about 80C.

I wonder if there's some sort of interplay between the RC factor of the thermistor input and the duty cycle of the heater output.

Sunday, December 23, 2007

Extrusion

I managed to get the extruder to put out some plastic without falling apart. After trying various things, two mods made the most difference:
1. Zach's suggestion of using a hose clamp on the Teflon / heater barrel connection worked extremely well.
2. The other end of the Teflon barrel kept slipping as well, so I wrapped it in some doubled-over sandpaper.


I cranked up the heat and pushed some filament through. Here's the result. When stretched out, it's about 14".


Unfortunately, I seem to have destroyed another thermistor in the process. On my next order, I might have to stock up on a few.

Friday, December 21, 2007

Extruder Barrel #2


I built a new extruder barrel. This time the nozzle can be removed easily. I did a simple test of heating it up and manually pushing HDPE filament through. Worked fairly well. After I got some plastic pushed through the nozzle, I let it cool down and disassembled it. Here's what I found:
These images show the motor end and nozzle end, respectively. On the motor end, there's a small amount of pooling. I'm guessing the gap is due to the shape of the drill bit. The nozzle end has quite a bit more pooling. The gap there is caused by the shape of the acorn nut.

What I'm wondering is, does this create extra resistance for the filament? If so, is there any way to prevent it? My first thought is to fill in the gaps with JB Weld. Maybe even turn the nozzle cavity into a taper.

Thursday, December 20, 2007

Turning Up The Heat

Figured out what was wrong with the heater. When it heated up, the electrical connections came loose. It'd get up to about 135C and start to cool down again. Nichrome doesn't solder well. I cobbled together half-assed crimp connections for it and, for good measure, wrapped the barrel in insulation.

This time, the temperature got up to 160C fairly quickly. I turned on the motor and let the filament feed in.

I'm not entirely sure what happened because the insulation blocked my view. Somehow, though, the heater barrel came out of the Teflon spacer. It doesn't look like the threads are stripped. There was a large blob of melted HDPE in the gap but none came out of the nozzle. I'm guessing the nozzle is clogged.

As it sits now, the extruder barrel is in pieces. The connections for the thermistor tore loose when the barrel came out.




Next steps:
1. Try to reposition the thermistor onto the barrel instead of the nozzle so that the nozzle can be easily removed for cleaning.
2. Remove the nozzle, heat it up and clean it out.
3. Reassemble and try again.

Tuesday, December 18, 2007

Moving Forward

I didn't like the profile of the drive screw. It was too high overall and one end stuck up more than the other. I used a heat gun to heat up the CAPA and a pair of pliers to push it down.

Once it cooled down a bit, I assembled the extruder without the barrel. I used the longest cap screws in the kit to clamp the two halves together, using the tubing as springs. I fed some HDPE through it, just to see if it'd push filament. Did this first with the motor hooked up to 2 AA cells, then switched to the extruder board.

Ran into problems with the RepRap host app. Serial errors. Turns out that the extruder board (or at least the firmware I'm using) gets very unhappy if a) the thermistor cap comes loose or b) you plug the heater into the thermistor connector.

Once I got that working, I determined which way the motor leads should hook up so that the extruder pushes the filament in "forward".

Since I had everything hooked up anyway, I added the heater barrel to the extruder. The clamp wouldn't tighten down quite enough so I wrapped the end of the barrel in electrical tape.

I ran a piece a filament into the barrel until it started to complain and then fired up the heater. Couldn't get the temperature above 140C, though. May need to re-do the heater barrel. Or maybe it just needs a bit of insulation wrapped around it.

Anyway, here's some video of the drive screw pushing HDPE filament.

video

Monday, December 17, 2007

Getting My Bearings


I went ahead and used the bearings from RRRF, following Zach's instructions. I used a Dremel with a cutoff wheel to cut the bearings in half. Then I heated up some CAPA (using hot water in a coffee cup) and stuffed it into the two indentations and shoved the half-bearings into the blobs. I used the drive screw to line up and sink the half-bearings into the CAPA.



After letting it cool, I trimmed the excess CAPA with a razor. I removed the half-bearings, put some JB Weld on and re-seated them, using the drive screw to line them up again. The main thing I wanted to ensure was that, when I pushed the drive screw toward the motor, it sat on both bearings. This would be its normal position when it's pushing filament.

Sunday, December 16, 2007

Extruder Kit

I started on the extruder head kit (thanks Zach). The body is made from molded parts based on the RP'd extruder. Things I've noticed so far:

On the plastic parts, some holes weren't completely clear. Easy enough to drill out, though. One of the motor mounting holes was missing entirely. I temporarily mounted the motor using the one hole, lined it up and drilled the second hole using the motor as a template.

The feeder screw assembly refused to solder. I ended up using JB Weld instead. Currently curing.

I realized a bit late that I was supposed to cut down the braided wire to fit the distance between the motor and screw. Fortunately, I figured this out before the JB Weld had hardened. Note to self: Test-fit everything first.

I've avoided soldering the wires on the motor for now. Not exactly sure which direction the filament will go under power, so I've just twisted the wires on until I'm ready to test.

Not sure what I'm going to do about the bearings. The ones that came with it have fairly thin walls. I'm wondering if they'll slip out of place under axial load. Options: 1) Use the supplied bearings. 2) Use Vik's washer configuration. 3) Grind down a piece of brass stock to fit. I'm leaning toward option 3 since it'd provide the most support. Maybe I'm overestimating the forces involved here, though.

The bolt to tighten the barrel clamp is a bit too long. It butts up against the extruder body. Options: 1) Cut a chunk out of the extruder body. 2) Cut down the bolt. 3) Find a shorter bolt. 4) Stack a bunch of washers under the head.

I can see a Home Depot safari in my future.

Thursday, December 13, 2007

Adrian Bowyer Lecture

A lot of people ask me what RepRap really is. I don't think I explain it very well, to be honest. Here's an explanation from the man himself.

Wednesday, December 12, 2007

Holding Pattern

I currently have an extruder head kit on order from RRRF. BitsFromBytes is now selling a plastic parts kit made from molds of RP Darwin parts. RRRF should be getting these in stock at some point, so I'll probably wait until they do, rather than do overseas shipping.

Sunday, December 2, 2007

Extruder on Order

I somehow managed to completely miss the fact that Zach had extruder kits available at the RRRF store. It contains some parts I've already got but it doesn't hurt to have spares.