[KwartzLab] improving our 3D print quality

[doug moen]

I attended the hacklab.to 3D printer symposium last night, and
gathered information on how to get better quality 3D prints.  What I
got was not an authoritative diagnosis of our printer's problems, but
rather, a set of avenues for us to explore.  The two main hacklab
experts that I talked to were Geordie and Rob.

Our prints have suffered from drooling, blobbing, and stringiness.
They couldn't help much with the stringiness, but they had plenty of
experience with and suggestions regarding the drooling and blobbiness.

The two most popular materials for printing are PLA and ABS.  We use
PLA.  The green bracelet on the 3D printer desk at Kwartzlab is ABS.
PLA is more transparent, and more brittle.  ABS is more opaque,
tougher and stronger than PLA, and generally has nicer mechanical
properties, unless you want springiness.  Legos are injection molded
ABS.

PLA drools more than ABS.  ABS warps more than PLA: early in the
print, the first few layers of a large object tend to curl up.
However, this warping is mitigated by using a heated platform.  At
HackLab, ABS is the material of choice. With their heated platform and
MakerGear extruder, they find ABS less finicky than PLA.  However,
extruder design is a black art.  The MakerGear extruder seems to
prefer ABS, whereas the Ultimaker people are accomplishing miracles
using PLA.

If we wish to try ABS, we need to clean out the PLA from our extruder
first, using solvent or a blowtorch, or the extruder could get
plugged.

The usual recommended temperature for PLA is 190 C, but we have been
printing at higher temperatures than this, which makes drooling worse.
So we should try lower termperatures.

The blobbiness we see is likely caused by the extruder.  If the
extruder is working properly, it will extrude at a perfectly uniform
rate. The plastic should start coming out when the motor starts, and
stop when the motor stops [1].

One form of extruder pathology is that the temperature is not held
uniform, but instead cycles up and down.  This causes the viscosity to
cycle up and down, leading to non-uniform extrusion, and blobs. We
could test for this by attaching a thermocouple test instrument to our
extruder, and watching the temperature during the extrusion of a long
bead.  There are PID parameters in the firmware that might need
adjusting, if we have this problem.

Another extruder pathology is that the pinch wheel is not feeding
uniformly, but sometimes slips.  Geordie noticed some play in the
gears on our Wade's extruder, which he said could indicate a problem.

[1] Note that some drool after the motor stops is inevitable, and
there are GCODE techniques for mitigating this, which can be
configured in Skeinforge: the extruder motor can be stopped early, and
the motor can be reversed to pull up the filament at the end of a
bead.  Since fancy GCODE generation is used to work around hardware
limitations, we should play with SkeinForge, or rather, with SFact,
which is a redesign of SkeinForge with an easier to use user
interface.  SkeinForge has a number of cryptic parameters which
require computation and special formulas to derive, while SFact
performs these computations for you.  [There is unfortunately work
required to massage Skeinforge output to be compatible with our
ancient hardware and firmware.]

More generally, 'play' in the various parts of a Mendel is a cause of
printing problems.  For many people, registration and alignment of the
Z-axis is an ongoing problem.  The fundamental Z-axis design, with
those threaded rods, is not a good one.  The rods need to be straight,
high quality, and greasing can help.

Extruders are finicky, and performance is very sensitive to the
design.  The geometry of the melt chamber and nozzle are critical.
Temperature gradients in the melt chamber must be just right.  The
melt chamber must be properly insulated from the rest of the print
head.  Somebody at Maker Gear said that you want a sharp transition
from solid to melted plastic, and that an extended plastic zone is not
good.  I speculate that an extended plastic zone leads to "mushy"
response when the controller starts feeding plastic at the start of a
bead, and retracts the plastic at the end of a bead.

Our extruder dates from an era when extruders were rubbish.
Currently, the best extruders are MakerBot and MakerGear (in Rob and
Geordies experience). Ultimaker has fantastic print quality, so they
must have a good extruder as well.  It is possible to mount a MakerBot
or MakerGear extruder in our Mendel.  Greg has designed a milled
mounting block for attaching a MakerBot plastruder mk 5 to a mendel
without sacrificing too much vertical space (this was Kevin's
concern).

Which 3D printer is best?  At the low end, Geordi endorses
printrbot.com, because it is very cheap, very easy to build, and good
enough for most of the small print jobs that people typically run at
hacklab and elsewhere. (They are still in startup mode, and haven't
shipped any kits yet.)  At the high end, the MakerBot ThingAMatic and
Ultimaker are the best open source printers.  MakerBot is more mature,
with more add-ons available (dual print heads, heated platform,
frosting extruder), plus a larger community with more units shipped,
while Ultimaker has a fundamentally better design: better
architecture, high quality components, much faster and much higher
resolution than is possible with the MakerBot design.  Ultimaker has a
great future ahead of it, with lots of add-ons planned for the future
(and the current system is designed to support these add ons).