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For some reason the only the top layers keep failing or underextruding on my prints, for no clear reason. Here are my settings:

  • Slicer: Simplify3D
  • Filament: 1.75 mm Black PLA from Filamentive
  • Resolution: 0.1 mm on a 0.4 mm nozzle
  • Temperature: 200 °C nozzle and 50 °C bed
  • Speed: 45 mm/s and 50% outline speed
  • Infill: 15%

The prints were going perfectly fine on a 0.2 mm resolution and only seemed to fail when I changed to 0.1 mm; which is strange considering the rest of the print goes fine, apart from the top layers (which I have 3 of).

My only thought is that it could be a bridging issue and is somehow underextruding, and getting caught on the infill as that it where it is centred around. I wouldn't know how to fix this.

If anyone could give me any information or tips I would be very appreciative, thanks.

The pictures are taken after some light sanding.

enter image description here

enter image description here

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If you decrease layer thickness, you should increase bottom and top layer amount, or set it to a fixed shell thickness. The thinner the layers the more difficult to span over the infill (there is much less filament extruded).

You could try extra part cooling, higher percentage infill, reduced hotend temperature and slower top layer printing. But, best results are reached with more top layers and higher infill percentage.

See e.g. this answer by user "dnewman":

That said, with very low layer heights (e.g., 0.1 mm), there's a tendency to use very sparse infill to speed up printing time. However, very low layer heights bridge over voids very, very poorly. So poorly that you can have the print nearly finished only to find that the top won't close up. Thus, don't make the infill too sparse when doing low layer heights. MOREVER, you definitely need more top layers to get the final finished top to look acceptable. The thin layer heights will take many more layers (many more in physical height, not just layer count) to give a nice top. At issue (again) is how poorly low layer heights will bridge voids. With 0.2 mm on up, you generally get a nice, thin strand extruded which can stretch across voids. But at 0.1 mm layer heights the printer is just doing tiny, discrete squirts of plastic which it spreads like cake icing across the lower layer. There's not a single, fine strand extruded and instead tiny little beads. When there's a solid layer below, these squirts have something to be spread against by the extruder nozzle. But when there's a void, the squirts just build up on the nozzle and then come off in a big blob when the nozzle next brushes over a non-void space.

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The holes in your part tops are the result of a combination of poor bridging and too few top layers.

What's happening is that when the printer tries to lay down the first layer of the top/roof, it has to "bridge" over the top layer of the infill. If you are trying to lay down that layer too quickly, with too hot a filament temperature, over too sparse an infill, the lines won't go down properly, and then the next layer over that has to try to bridge this same gap with very little support (and very often with up-curled broken strands of filament in the way).

The printer can eventually lay down a smooth layer, if you give it enough tries to smooth over the rougher, broken layers underneath. How many you need depends on how badly the first one failed to bridge the infill gaps. If you're only printing 2 layers, try 4. If you're printing 4, try 6.

Also, look at your infill percentage and pattern. The top lines have to be drawn over the top of whatever the layer of infill looks like as of when the slicer calculates that the first layer of top needs to go down. Virtually all slicers provide a "preview" of the sliced layers, with some like Cura allowing you to trace through the extruder's "tool path" over each layer. This can be a very useful tool to diagnose potential issues with what the slicer will expect the printer to do.

As for how to change it, it really depends on what you need out of the part in terms of material strength (different patterns have advantages and disadvantages in how much force can be applied in what directions), weight (more infill means heavier), and print time (higher percentages and more complex infill patterns increase print time). Cubic infill has the best overall strength-for-weight, and it slices and prints pretty fast as each layer is just straight lines. However, it's among the worst in terms of the grid it provides under the top layer. Isolinear/Triangle fill provides one of the better support structures, and is near the top in Z-axis compressive strength, but compressive strength other than in the Z-axis tends to be low. Gyroid infill has among the best combinations of strength, weight and required bridging distances, but slicing and printing the complex curved structure takes more time, and some slicers don't offer it (I'm still kicking around with an old MakerBot that's only fully supported by MakerBot Desktop, which doesn't offer any "fully 3D" fills like cubic or gyroid at all).

Lastly, if you absolutely need the model at a light weight or fast print time that precludes increasing the infill or slowing print head speed, try printing a bridging test, especially something like a "temperature tower" that allows you to see the effects of varying extrusion temperature all in one model, and make sure you're printing at the exact temperature giving you the best results for the other configuration settings you're using. Your model may not require externally-visible bridging or overhangs, but the ability to draw a long line of filament literally into thin air without sagging or snapping is a useful trick even in "solid" object prints, because as you've found out, these solid shapes are up to 90% air inside.

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