Smooth bottom on prints?

I have some very small (1/2" or less) pieces I’m trying to print and I’m trying to get the bottom layer (the largest flat surface on the piece against the build plate) to be smooth.

The effect I’m noticing is that if I print a batch (10 or 20 pieces), some will have a perfectly, glass smooth bottom whereas others will show the extrusion lines, usually no gaps, however. If I print one piece, I never get the smooth bottom.

Any tips on how I can get there? I’ve adjusted z-offset, temperature, initial layer flow rate and initial layer speeds.

Some key settings:

Printer: Lulzbot Mini2
Print temp: 206 C
Z offset: -1.19 (factory was -1.09)
Accelerations: 100 mm/s^2
Travel speed: 175 mm/s
Print speed: 10 mm/s
Bottom layer speed: 5 mm/s
Bottom layer flow: 95%
Bottom/Top layer thickness: 1mm
Bottom/Top layer count: 5
Layer Height: 0.1 mm
Using 2.85 mm Polylite PLA
Retraction is enabled

Have you tried setting your bottom layer flow (Initial Layer Flow Rate) to 100% or greater? Are you priming you’re extruder by printing a skirt? What is your bed temperature? What is your “Printing Temperature Initial Layer”? You can probably bump up your Initial Layer Speed to 15mm/s. What is your Initial Layer Height? Nozzle diameter? Line width?

I hadn’t thought about exceeding 100% flow rate, but the bottom layer seems to become its own skirt (die swell?) if I get it too high, so I’ve kept it at the 90% - 95%

Bed temp is 60 deg C, Initial layer is currently at 215, the upper layers I’m dropping back to 205.

I’ll try bumping the initial layer speed.

Nozzle diameter / line width are both 0.5 mm.

I’m also getting slight layer shifting going on - where my sides should be vertical, they have a curvature (shift to one side, then back again).

That is, rather than looking like this: |______|

It looks like this: (_____(

I’ll try to get some pics, but these pieces are so small, it’s hard to get good examples.

Becoming its own skirt? Perhaps you mean Elephant’s foot? A skirt lays down a line (or more) around the perimeter of your object prior to beginning the official print of your object. I think it defaults to 3 lines and/or 250mm. I set mine to 1 line and/or 25mm. That seems to be enough to prime the nozzle.

If these minis are your design you can try adding a chamfer to the bottom edges of your model. All your other settings look good. I set my bed temp to 55c for PLA. A deburring tool may be useful to you for those elephant foot symptoms. Available at Home Depot and Amazon.

I had increased my bed temp to 70c, thinking that would help, but I’ll try dropping it back.

Anyway, yes, elephant’s foot. I’m trying to dial this in so I have as little finishing to do as possible. Right now, about half of the pieces in a print are perfect / require minimal processing. Anything that requires sanding / deburring probably isn’t worth it.

I realize I’m probably asking a lot for this printer and the stock head, but my only real issue here is aesthetic. In a print of 50 pieces, maybe 10 at most have tolerance issues, but half have aesthetic issues where the initial layer under extrudes somewhat. Of those that under extrude, 2-4 do it badly.

I will say, though, increasing the amount of skirt did help a little.

You could also try The option of printing one piece entirely before printing your next piece within the same printing session vs printing all the layers at the same time. See if that improves anything for you. You could also play with the initial horizontal expansion setting in the shell area to combat elephant’s foot syndromes. Another setting you could look at is your layer height based on the pitch of your Z axis drive mechanism. You’ll have to do some research on figuring that out for your belt driven Z-axis. My printer has the lead screw and .04 were my optimal layer height increments. One last thing I would try is use a brand new brass nozzle, some of your problems sounds like you could be experiencing some minor clogging issues and nozzles are cheap enough to replace than messing with trying to clean them back to like new condition. .05 nozzles sometimes are difficult to find so you may have to settle for .04 size, which is probably better for your mini prints.

The layer height / pitch of z-axis point… That’s what’s commonly referred to as the “magic numbers” correct? I was under the impression it didn’t apply to belt-driven machines.

Correct, the “magic number”. You could be right, I don’t have a belt system printer :slightly_smiling_face:, but was aware of it as another “tweak” to make your prints look good.