Mendel? Who’s Mendel?

I previously wrote about my start on building a Mendel RepRap.  I recently purchased the bearings and stepper motors, and have had the shafts and threaded rod at the ready for several months.  But on Monday, while scanning the local tools section on Craigslist, one of the titles jumped out at me: “Stratasys FDM 1600 Machine”.  Stratasys…  I could scarcely believe that someone was selling an actual rapid prototyping machine on Craigslist.  Not just a rapid prototyper, but a fused deposition modeler (FDM) – the very same type as a RepRap.  For anyone wondering just what in the world I’m talking about, imagine a CNC controlled hot glue gun – a FDM rapid prototyper builds up an object out of many thin layers of plastic extruded out of a tiny nozzle.  This particular machine has two nozzles – one for the plastic (ABS to be specific) and one for a support material (a very brittle plastic that supports overhangs and is broken off once the part is complete).

The asking price was simply too good to be true, and I called the seller immediately, who confirmed that it was still available, and that it was in fine working condition.  I told him I’d stop by around 6pm to have a look at it.  Well, perhaps my eagerness was far more apparent, as I told the fellow to consider it sold (I was actually shaking from excitement).  The size of the unit was probably going to be too big for my truck, so I called my dad to see if I could possibly borrow his.  As generally happens, I wound up borrowing dad as well (which is always a comfort when buying and moving machinery – something about decades of prior experience to keep me from doing something boneheaded).

Sure enough, the machine was just like the ad indicated.  The seller gave me a crash course in using the machine and the software, though I knew I’d have to do a lot of reading as well – running a rapid prototyper isn’t something you can really do with only 10 minutes of fragmented instruction.  The three of us managed to wrestle the unit and accompanying stand into dad’s truck and we were off.  Dad helped me unload the unit at work, where I hoped to be able to keep it indefinitely (as we’d probably find it useful to have for the engineering department – I couldn’t imagine having enough to run on it personally to keep the machine very busy).

It took a few days to get the unit situated in the office area (an office environment is needed as the machine needs reasonably controlled temperature and humidity), but I was finally ready to attempt actual use of the machine.  I had read enough of the manual to understand how to load the plastic filament (about 0.070″ in diameter, fed from a reel) into the extrusion head, so I started with this simple procedure.  I managed to get the gray support material extruding out of the nozzle, and then the red ABS, but when I switched back to the support material, there must have been a jam and no more came out.  I powered down the machine, let the head cool, and cleared the backflow from the head.  I tried it again, but once more I could get the support material to extrude only at first, but not after having switched to the ABS.  Close inspection of the head indicated that the gripper wheels weren’t getting proper traction on the filament (perhaps it was getting a touch too soft from the heat).

A few days later I felt ready to try actually making a part, even if the support material extrusion gave me issues.  I also powered on the air conditioner on the back of the unit for the first time, hoping that the cool air blown onto the backside of the head unit would improve my luck with the support material (this cold air flow seems to be critical in keeping the filament from melting before it actually enters the heating chamber).  Again, I was unable to get the support material to extrude after switching to ABS, so I decided to try just running without support material at all.  I was going to just try making one of the .stl files for Mendel, and then I remembered that the traditional first part made on a RepRap is the shot glass (or ‘minimug‘) to allow you to make a toast to your own creation.  While the FDM had likely made hundreds of parts already, I figured the minimug was the perfect inaugural part for me to try.  I drew up my own version with zero overhang and generous draft angles, not knowing how well things would actually work – best to start simple.

The file I sent to the FDM included about 1/16″ worth of support material as a base – since the support extruder wasn’t cooperating, I was hoping the ABS would ‘fluff’ itself up to form a reasonable support for the build (much like a basket of clothes dumped on the floor would have a height greater than if the items were folded and stacked).

This actually worked quite well, but about halfway up, disaster struck when the part lifted from the foam base.  After making a presentation in my experimentation class many years ago (detailing a blower and heatsink and subsequent failure of the test heating element when we applied too much power for the airflow to effectively remove the thermal energy), my professor implored me and my lab partner that there was no such thing as a failed experiment.  He was so emphatic about this point that it has stuck with me ever since, and I recall it whenever I am disheartened by a setback.  I suppose the old maxim “every cloud has a silver lining” is an approximation, though I feel the Dilbert-esque phrase of ‘learning experience’ is closer to capturing the sentiment of retrieving useful knowledge from mishaps.

While the top portion of the minimug looked pretty good, the underside showed a rather loose fill – the lines of ABS should be much straighter.  There is also very little adhesion of the ABS to the glass foam base.  I knew that I had to remove the dead space that should have been filled with support material, or increase the surface area of the part bottom.  I wound up doing both.  I figured out how to remove the support layer from the part (Stratasys Quickslice 6.4 is not exactly intuitive, dating to the previous millenium), and I added on a hefty plate to the bottom that would hopefully improve adhesion to the foam.  These two changes worked beautifully.

To make things interesting, I added some holes and flanges to the bottom flange of the part.  The very first layer was close enough to the glass foam base that the plastic was injected into the pores.  This worked almost too well to secure the part, and I needed a putty knife to pop it off of the substrate.

I then decided to give the support material one more try.  The ABS had absorbed a good deal of moisture – the manual instructs to test for ‘droop’ by extruding some ABS, wiping the nozzle, and then seeing how much leaks out over the next 10 minutes.  The more moisture the ABS contains, the more the material will ‘droop’ – an acceptable level of moisture is to have 0.5″ or less of ABS dripping out of the nozzle after 10 minutes.  I was seeing several inches after 5 minutes.  I guessed that maybe just running through a foot or so of support material through the head might finally get to a drier section of material (the spools of plastic filament are kept in a ‘dry box’ on the machine with desiccant bags).  Wonder of wonders, this actually worked (or perhaps I was simply just lucky this time) – after feeding support material through the head, I was able to switch to ABS and back again and be able to extrude at will on both nozzles.  I loaded up a small Mendel part and gave it a go.

This support (which appears to be called the ‘raft’ among RepRap users, though they currently have no separate support material) looked to be adhering to the substrate very well.

Once the ABS started being extruded, however, I could see that all was not well – the support material should be perfectly centered between the two uprights.  Still, things were progressing smoothly, so I let the part complete.

I’ll need to calibrate the head so that the support material gets extruded properly in line with the ABS, but I’m extremely pleased with the results thus far.  In short, I’ve leapfrogged well beyond where I would be if I actually had a working Mendel.  Rather than trying to figure out how to make the machine work, I can now decide what I actually want to make.