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How to fix bad top surface finish?

Sw1tch
(@vidit)
Member

For the last few days I've been getting bad top surface finish for my prints particularly on the lower half of the print.I've never had this problem with the printer before.Any suggestions of how to fix it? From what I've learnt so far is that it could be due to two reasons;

1 : Bad first layer 

2 : Over extrusion/Wrong flow rate

 As far as the first layer is concerned I think it's decent and as far as extrusion is concerned I haven't changed a thing and was having great prints with the same filament and slicer settings before.So, really don't know what could be the issue.I have not changed any settings yet.Just wanted to get an experts advice before doing so.

Attaching a few pictures for reference.

1
3
2

Other details.

Printer : Prusa Mk3s

Layer resolution : 0.2mm

Temp : 210ºC

Material : PLA

No of top solid layers : 5

Quote
Topic starter Posted : 10/12/2020 9:37 am
Luke
 Luke
(@lmf5000)
Expert Moderator

Hi Sw1tch,

It could be a few things. Maybe the filament is slightly thicker in this portion of the roll and is causing you to over-extrude slightly (highly uncommon but could be a contributing factor). Can you check the diameter of the filament and make sure it's got a round cross-section of diameter 1.75mm? You do that by taking two measurements perpendicular to each other, like so:

image

Other, more likely causes:

- The bed is no longer perfectly level. Verify by printing the levelling squares ( https://www.thingiverse.com/thing:2789086 ). Measure their thickness - they should be as close to 0.2mm as possible. If they're not in the range of 0.15-0.25 you may need mechanically re-level the bed and/or adjust your probe offset or bed height. This is most likely because the bottom half of the bed is too close to the nozzle.

- You are over-extruding. 5 top layers is quite a lot and it will be very very critical not to over-extrude. Basically, in the first top layer the extra filament has almost infinite space to go (it just sags into the spaces between the infill lines). On the second top layer, the extra filament goes into the gaps between the lines of the first top layer. On the third top layer, the extra filament doesn't have much place to go because the second top layer is almost entirely sealed. By the 4th and 5th top layer, any extra filament has nowhere to go but up - so it ends up higher than the surrounding layer and then the nozzle ploughsinto it as it comes back to the area later in the print. That's why you get those lines with a v-shape texture that's the inverse shape of the point of the nozzle ;). Then the extra material on the nozzle gets deposited on the print elsewhere and makes an additional mess somewhere else.

So, two things you can do - observe the print and see if the 4th top layer looks better than your final top layer. If it does, consider going down to 4 top layers instead of 5. And tune your extrusion multiplier. The easiest way I've found to do this is to print a 20mm calibration cube and look at the top layer (you could also print just the top half of the cube, or the top 5-10mm to save filament as you only need the top). Reduce the EM drastically until you get underextrusion (you see gaps between neighboring diagonal top layer lines). Then gradually increase EM until you get a print with zero open gaps in the top layer. That should be your ideal EM for that material. 

Here's a visual guide:

image

You might need to increase it by a further 1% for a safety margin on large prints, but under-extruding slightly gives a better visual result than over-extruding slightly so this should in theory give you better results than you're getting now. You can always fine-tune the EM by 1% up or down after every print until you get it perfect.

For what it's worth, with calibrated e-steps my extrusion multiplier for PLA on my CR-10 Mini is 0.97 and I use four top layers of 0.2mm each.

- Possible additional contributing factor - insufficient infill percentage. Try using at least 15% infill. My personal favourite pattern in PrusaSlicer and Cura is gyroid because it provides more even support (i.e. closer-spaced lines right under the first top layer) for the same infill percentage, than many other patterns (like honeycomb or grid).

Hope this helps! Let me know how it goes 🙂

Regards,
Luke

ReplyQuote
Posted : 10/12/2020 10:35 am
Sw1tch
(@vidit)
Member

Hi Luke,

Here is what I've tried so far,

1 : Measured the filament diameter.(Tried this at multiple points. The results were pretty much the same.)

1
2

 

2 : Printed the levelling squares and small first layer test.

3
4
5
6

 

3 : Measured the thickness of the squares.

7

4 : Printed the 20mm cubes

8
9

5 : Printed the same part with same filament and same settings, like I did, before posting the above question and noticed there was a bit of warping even after applying a brim.

10

6 : I cleaned the bed with dish washing soap and with IPA(99%) as well and printed the same part again to check if the warping wasn't happening due to an oily/dirty bed. Still had warping.

I haven't changed my extrusion multiplier yet wanted to show this to you first before i did.Only changes i've made so far is with the live Z value. I have reduced the distance from -0.840 to -0.855. I still, am seeing gaps around the corners and certain parts in the middle of the first layer. But, if i decrease the live z value any further I end up messing the first layer at certain places very badly.

 

PS : My infill before and now was 20% with infill pattern as gyroid.Also, I honestly couldn't make out any difference between the 4th top surface layer and 5th top surface layer.

ReplyQuote
Topic starter Posted : 12/12/2020 6:26 pm
Luke
 Luke
(@lmf5000)
Expert Moderator

Hi Sw1tch,

Sorry for the late reply. Filament diameter is fine - mine are all around 1.70-1.72mm despite the label saying 1.75mm 😆.

What's your first layer height? If it's 0.2mm, it seems that your levelling is a tad too high and you're compensating for that excessive gap between the nozzle and the bed by over-extruding. That way you'll get a print that adheres to the plate (just barely) but will have a tendency to peel up in critical areas (like edges) and will give undesirable artifacts elsewhere in the print.

So, to get the printer set up correctly, firstly you need to redo the levelling and use a thin piece of receipt paper to verify the height. You want the paper to slide smoothly between the nozzle and the bed at Z=0.1mm, but be held firmly in place by the nozzle at Z=0.0mm. If you do the test with the printer as it is now, you'll most likely find that the paper still slides easily under the nozzle at Z=0.0mm.

Since your levelling square is 0.24mm thick and your overall levelling seems to be quite consistent over the whole bed, you could avoid the above process and take a shortcut and just lower the nozzle height by 0.02-0.04mm. Print the levelling squares again and lower the extrusion multiplier as necessary while printing them to get a good flat top surface (you can tune flowrate from the LCD menu). Then re-measure the new squares and see how close you can get them to 0.20mm thick. If you measure just the outer perimeters of the levelling squares (not the middle) you can usually get a pretty good measurement of nozzle height that's relatively independent of extrusion multiplier (the extra plastic tends to be in the middle portion that's printed in a zig-zag motion).

Your 20mm cubes are showing quite a bit of overextrusion, so after you've sorted the levelling and reset the LCD flowrate back to 100%, try lowering the extrusion multiplier in the slicer by 3-5% to start with. For future testing, you will be able to gauge it clearer if you print one of the cubes with a "z" on the top surface rather than a flat top surface - something like this one: https://www.thingiverse.com/thing:1278865 . There's no need to print the whole cube, in most slicers you can just drag it below the level of the bed and print the last couple of milimeters, like so -

image

Could you upload your print profile too? Either export and attach the file, or post screenshots. I'd like to check for any other potential issues that won't be so easy to discover from the pictures.

Looking forward to your feedback 🙂

Regards,
Luke

ReplyQuote
Posted : 13/12/2020 11:29 pm
Sw1tch
(@vidit)
Member

Hi Luke,

Even my goto filament brand has diameter similar to yours (1.71mm-1.73mm) 😆 

I haven't tweaked other settings yet.I'll give them a go today and keep you posted.

In the mean time here is my print profile for that print.

Do let me know if i need to change anything else and i'll make sure to do that.

Thanks for your advice like always.

Cheers,

Sw1tch.

 

 

 

ReplyQuote
Topic starter Posted : 15/12/2020 7:14 am
Luke
 Luke
(@lmf5000)
Expert Moderator

Hi Sw1tch,

Print Settings

For the slicing quality, I'd set them like this to make sure you have extra support where needed:

image

For horizontal shells, my settings are 4 top and 3 bottom. You might not see a huge benefit from changing these, but it will save you time and material (at the expense of marginally decreased strength) and will go some way to improving top layer quality if you're slightly over-extruding.

Solid infill - raise threshhold area to 5mm, will save you time and material and it won't try forcing infill into little gaps:

image

Also consider ticking "only retract when crossing perimeters" - it means less retractions and more speed because it won't bother retracting over infill (as the ooze will be inside the part there).

Acceleration control - set to 500mm/s2 for perimeters, first layer and default. 1000 is quite high and will cost you quality.

Wipe tower - why is yours enabled if you only have a single extruder? Unless there's something wrong with the config file or something I'm missing 😉

Extrusion width - I have all of mine set to "0" - this way it will automatically compensate when I load a different nozzle and just enter the new nozzle diameter in printer settings (no need to come back here and adjust everything) -

image

Filament Settings

Your extrusion multiplier is 1. You should decrease that. Mine is 0.97, start with that and then fine-tune up or down based on results.

Cooling - disable "keep fan always on" and set disable fan for the first 3 layers, not just 1. You don't want to blow cold air anywhere near your delicate first layer as that will cause the corners to lift 😉

Advanced - max volumetric speed - set to 0 to disable it. Dunno if this affects your problem but it can't hurt to turn it off and see how it goes.

Printer settings

Custom gcode - comment out the G92 in the "before layer change" gcode. This shouldn't be required, and it might be messing up extrusion distances for you.

Retraction - lift z - you can go down to 0.2mm instead of 0.6 if you want. This saves time and wear-and-tear on your leadscrew

Retract on layer change - untick, this shouldn't be needed.

Retraction speed - consider raising it to 70-90 (will speed up your prints)

 

Hope the above improve your printing experience somewhat! :). Let me know how it goes!

Regards,
Luke

ReplyQuote
Posted : 16/12/2020 4:41 pm
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