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Reason for different surface quality on the same print

Posted by drmaestro 
Reason for different surface quality on the same print
May 06, 2017 07:09AM

I am trying to discover why the surface quality changes at different layers on my prints. Here's an example:

If you look at the middle section where the groove is the surface finish quality is quite acceptable and smooth. If you look at other parts, which are smaller in area, there seems to be some overextrusion at the edges, when the hotend changes direction. Also the lines look like they are slightly embossed. I assume this is an extrusion problem and the smaller the part is, the more visible it is.

The E steps is already calibrated. Any other way to fix it?

Re: Reason for different surface quality on the same print
May 06, 2017 08:18AM
Try a different slicer and see if it does the same thing.

I have been seeing some of that type of problem in my last printer. I attribute it to the z axis not returning to the exact same level after a z-lift on retraction. I'm thinking it may be caused by print debris on the Z screws getting between the screw and nut. I need to do some Z axis maintenance.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Reason for different surface quality on the same print
May 07, 2017 03:32AM
Leaky nozzle, I think. Looks like some of my prints. When moving without extraction, my head spits a bit hot plastic, and this is missing when extrusion starts again. I see little holes in your print right after movement, and the "overextraction" signs are visible where movement happens or stops, I assume. With my extruder I couldn't get rid of this completely, no matter what retraction settings I use. More retraction made it slower, and z lift made it even worser, but none did help.
You may try some more and/or faster retract. You may try some extra extruding after retract to get rid of the little holes. Both may reduce the issue. But when there is only movement, without retraction, I notice some excess material during movement, and some material missing after the movement. Best seen during infill.
My nozzle is quite large, 0.5mm. Next thing I'll try is a 0.3mm E3Dv6 clone from a shop nearby with a Wade's gear, and a 0.4mm E3Dv6 china clone with a bowden setup.
Re: Reason for different surface quality on the same print
May 18, 2017 04:58PM
My 2 cents worth.... I had a similar issue and I found it was related to the layer height vs height of the part being printed.

For example a 2mm high platform printed at .2mm layer height would give very smooth final layer. 2mm divided by 0.2 = 10 layers

If the platform height was say 2.3mm and I tried to print at .2mm layer height I would end up with what you're experiencing in the uneven last layer. 2.3mm divided by 0.2 = 11.1 ish.. layers

I now make all my parts finished heights divisible by the layer height I will print it at. Does that make sense to you?
Re: Reason for different surface quality on the same print
May 19, 2017 05:45AM
Has anyone noticed, it seems to be effected by the direction the nozzle is travelling, just an observation smiling smiley
Re: Reason for different surface quality on the same print
May 21, 2017 03:28PM
Thanks for the comments. All of them are probable. Leaky nozzle is definitely a problem and might contribute to the final layer quality.

I tried to verify what psneddon told about layer heights: I created 3 different 10x10 mm cubes with different Z heights (2 mm, 2,1 mm and 2,2 mm) and tried to print it with 0,2 mm layer height. Correct me if I did something wrong but when the object height isn't an exact multiple of the layer height, Simplify3D will disregard it. What I ended up was 2 similar cubes (the 2 mm and 2,1 mm ones), measuring 2 mm, and the 2,2 mm cube measuring 2,2 mm. So the first 2 cubes have been printed using 10 layers, and the last one 11 layers. So in practice, the object height didn't change anything, as the slicing software didn't take the 0,1 mm difference in consideration when using 0,2 mm layer height.

Obewan's observation is also something to consider. A few weeks ago my nozzle crashed into the bed due to a problem with the BLTouch. I didn't pay attention then but it seems that the heat break isprobably very slightly bent after the accident, so the entire nozzle/heater block complex seems to be very slightly tilted. This could be a reason for uneven surface finish depending on travel direction.

Digital_dentist's remarks are also true. While I don't use Z lift on retractions, I do use automatic bed compensation, so the Z axis is constantly moving (as digital_dentist always mentions, it isn't a good idea to use 2 independent Z motors for the Z axis but this is my situation right now and it could contribute to the problem)
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