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.

4 comments:

Zach Smith said...

wow! thats fantastic! when i first started reading i was afraid of the worst. it seems that you actually had quite a bit of success.

i'd love to get all the little modifications you made to the extruder all categorized and put into the main extruder page.

also, you've hit on quite a few things that i'd like to change for v1.1 of the extruder.

good job with this and i'm excited to see your progress when you start printing for real.

nophead said...

Great to see some comparative figures for the extruder performance with HDPE. Looks like you are getting pretty much the same rate as I do, which is reassuring.

Hope your MTBF improves in the new year!

RoundSparrow said...

Steve, I continue to applaud you for very great information. I've run into many of the same problems you have and great to have you ahead providing tips and directions.

HDPE really seems to be pushing things.

Thomas Eastham said...

Steve,
Thanks a million for your notes on the reprap. I've begun working on the repstrap and trying to decide how to handle the extruder. Looks like $90 for the bits to bytes one is probably a good investment. keep up the posts please!!!

Thomas