Printing Too Slow?

[RC-modeler 3]

Note to Moderator: I tried to post this several days ago but it has not shown up, hopefully this is not a duplicate. Please feel free to remove this line.

Hello, this is my first post on this forum. I have been working with 3D printing for 2 years now, using my Ultimaker 2. The above photo (which I hope shows up) covers my main interest, which as my user name shows is R/C modeling. This part gave me an interesting problem that I would like to share.

The parts shown are part of an R/C model fuselage. The top two have an under extrusion problem, the bottom one is where I fixed it. This is the second time I am making this model, the first time was around the first of the year after I finished the design. At that time I planned on sanding, priming and painting the parts after they were assembled, so I thought “print it fast and sand out the ridges”. So it was printed with 0.2mm layers at 60mm/sec with a 0.4mm nozzle, or 4.8 cubic mm/sec. Well, after sanding for a while I decided that PLA was not as easy to sand as I would like, so when I started the second one I decided to go higher res. This time around I went with 0.1mm layers at 50mm/sec still with the 0.4mm nozzle, or 2 cubic mm/sec. The first part printed very nicely, which is the nose section that attaches to part shown above. When I printed this part I was getting significant under extrusion. If you look closely at the middle one you can see places where both the inner and outer wall failed and you can look through it. I cleaned everything including the nozzle, moved the part closer to the front right corner to relax the bend in the Bowden tube and tried again, and got the same results. While the second attempt was printing I was feeling the feed of the filament to make sure it was really moving and realized that it was moving really slow, so I began to wonder if I was going too slow. For the third attempt I reset the speed to 80mm/sec. This time it printed just fine.

An interesting thing to note is that the section ahead of this is completely closed as you go around the perimeter, on this part the print was good on the front section of all the parts where the perimeter is closed. The problem only shows up after it gets to the point where there is an open side.

I have a couple of thought on what is happening. There is not enough pressure in the nozzle to survive the jumps. The plastic is spending too much time melted in the nozzle, and some material changes are happening that can’t be quickly pushed out. You can also see that the first 2 attempts have more stringing and in some cases that lines up with the larger flaws, which makes me wonder if the nozzle was dumping out and could not refill fast enough and for some reason moving faster helped.

All in all somewhat unusual in that the fix was backwards of the normal wisdom of slow down to fix printing problems.

Printing faster did reduce the X-Y smoothness very slightly, but resulted in a usable part.

Steven

[kmanstudios 1,044]

In reference to the sanding; I like to use a bit of light weight filler to get the big dings out.

And, if you are painting, a decent amount of primer, sand and then reapply primer again/repeat will smooth out large form shapes no issue. However, it can bury fine details like door shape insets or protrusions like raised symbols.

[geert_2 370]

If material is leaking out of the nozzle while jumping over gaps (thus causing strings), then indeed you could have underextrusion in the area after this. The software has no knowledge of these leaks, it doesn't supply extra material to compensate. If you print slow, you might try a lower temp, so the material is less liquid: 10°C less or so?

PS: don't leave a PLA printed model in your car in sunny weather, or don't leave it sitting in the sun on a concrete runway: it will warp. I don't know what effect warped wings would have on flight characteristics, but I don't think it will do any good...