Printing at 100mm/s on a Chinese i3 clone using Matterhackers ProPLA with extruder at 220C. Getting under-extrusion on some(?) of the support structure.
The support structure is printed from left to right, and the straight runs mean printer can get up to full speed. That means more plastic passing through the nozzle, removing heat, and cooling the nozzle. This likely means the plastic at the end of the run is much cooler than the desired temperature. Perhaps cool enough to cause the observed under-extrusion.
The cooler nozzle temperature will eventually soak through the metal and reach the thermistor. The software will then pump more energy into the heater, and the new heat will soak through the metal and reach the nozzle.
Seems to me the timescale for cooling is much faster than the heating response. Different extruder designs (Igus?) might reduce this some.
I can (and did) raise the extruder temperature (to 240C), which did reduce the under-extrusion seen here. But hotter plastic may not be what you want on slower (more detailed) sections of the print. You might (mostly) be able to counter-act the over-hot plastic with a good strong part cooling fan (with good airflow near but not on the nozzle).
Bunch of guess-work above…
At slower print speeds, the reactive temperature control loop can more-or-less keep up well enough. At higher print speeds, we are going to need predictive algorithms that pump heat into the extruder in advance of faster movement.
https://photos.app.goo.gl/gBiq4B8Gm3xVGibo2
100 mm/s travel speed or print sped?
@Adam_Steinmark Print speed. Intentionally pushing the printer to limits.
Yeah that’s definitely past the limits, you’re pretty much asking for things to go wrong. The i3 construction isn’t meant to move at such speeds.
@Adam_Steinmark While I agree the moving-table design is not well-suited for higher speeds, Prusa is claiming 200mm/s with the mk3. I view Prusa as fairly credible.
That said, my Chinese i3 clone is not built as well as a Prusa.
But in this case I suspect the problem is thermal, not mechanical.
@Preston_Bannister The MK3 only uses speeds at that level for travel speeds only. I think print speed is limited limited to 80 mm/s. The MK2 which is probably similarly contrstucted as your Chinese machine uses 120 mm/s for travel and 60 mm/s for max print speed. Your immediate issue may be thermal but those older style hotends weren’t meant to print that fast, you may want to try an all metal one. I wouldn’t recommend printing this fast for extended periods of time, you’re likely to damage parts of the frame.
" With all new Y-axis MK3’s printing speed is 200+ mm/s, which is twice of MK2. "
Top of the page:
And mentioned in Prusa’s blog post:
I believe this is possible. Even with my Chinese i3 clone I have got usable draft quality prints at 150mm/s for some parts. I saw some under-extrusion, and could tell there was a pattern, but not the cause, then. (Insert a bit of skepticism as to whether the printer was actually moving at 150mm/s, though it was clearly moving faster.)
With the higher quality components throughout, I can easily believe the mk3 could do well at 200mm/s. I do wonder what the thermal limit is for different extruders. Also the moving bed design is pretty much a non-starter for higher speeds - especially with larger beds and tall prints.
Yes that’s because they use aluminum extrusions directly screwed into a steel frame. You’re machine isn’t constructed that way and is significantly less rigid. A closer comparison would be the MK2, as I mentioned above. Pushing a standard i3 printer at 100+ mm/s is roughly equivalent to running your car at 6500 rpm, it will work for a while but cause significant wear on the machine.
The particular i3 clone in question - a Monoprice Maker Select Plus (a re-branded Wanhao Duplicator i3 Plus) does in fact have a decent, rigid bent-sheet-metal frame, and good reviews. This is a $400 printer (not a $200 printer), and I got what I paid.
The quality of all the other components are … decent, but a step less than a real Prusa in almost every respect.
Ah my bad. I couldn’t tell from the short video be it looked kinda like it used an acrylic frame. Yeah the hotend/extruder is gonna be your main issue with print fast. They’re still using the horrible Makerbot style combo where the hotend is clamped to a large aluminum block and the heatsink has very little surface contact with the block. This leads to heat creep and your drive gear might be stripping away filament which produces underextrusion.
When I said “Chinese i3 clone”, you made a reasonable assumption. 
Looking at the MK10 extruder on my printer, and looking at a E3D Aero (meant for a printer I am building), do not immediately see a huge difference in the thermal design. (There is of course a huge difference in quality of fabrication.) That said, one of the times I had the extruder apart, realized the thermal design is not very efficient with the fins on the heat sink horizontal. Though as someone else in the group noted, there should not be too much heat getting past the heat break.
There is an extreme difference in the design. The Aero has a thinner heat break and the unibody heat sink functions at a completely different level than the massive aluminum bar and cheap heatsink that Monoprice uses. The aluminum bar has too much thermal mass, the heatsink has very little surface contact with it, and the Aero has a larger, more powerful fan. E3D took the concept of a hybrid hotend/extruder combo and made it actually usable with significantly better cooling. The drive gear also doesn’t push the filament directly into the heat break it pushes through an injection molded channel so there’s additional space before the drive gear to further prevent heat creep.