I'm looking for any idea of what could cause this problem. I'm printing (1.75 mm PLA @ 220 °C) a 14 x 14 cm box, sliced with Simplify3D. Relevant settings are 3 bottom layers, 3 top layers, 3 outline/perimeter shells, and 15 % orthogonal infill every other layer.

The first 3 layers print fine. Here's the first (bottom) layer after removing the print and turning it over: bottom layer

The infill (layers 4-10) also prints beautifully (see left side of photo below).

But the moment it starts printing the next layer (layer 11, the top layer of the bottom of the box), which should be solid fill exactly like layers 1-3, it starts underextruding and generally looking like crap:

Not Good

The first time this happened, I figured the nozzle got clogged or the extruder gear started slipping. This is not the freshest PLA, so maybe it was a PLA quality problem. But the same thing happened at the same layer when I tried it again. And again. And again. As soon as I cancel the print I can have it extrude 5 cm of filament and it's fine, no clogs...and if I immediately start another print it again perfectly prints layers 1-10. So it doesn't seem to be the extruder, the nozzle, or PLA quality. I can't imagine the "stress" of laying 7 layers of infill could screw up the next layer...

I just can't figure out how layers 2 and 3 could be basically perfect, but layer 11 is consistently a disaster, when they should be almost the exact same G-code (only a milimeter apart). I looked at the G-code and it's basically identical for layers 3 and 11, including same feedrate (G1 F2250).

This is on a DeltaMaker printer with a new E3D Lite6 hot end. Can add more details if needed, but basically I'm just looking for an idea of what could cause this.

Update: Just tried a different roll of PLA and got the same results.

  • $\begingroup$ This is completely normal, your first layer over an infill is never solid as there is no support under the layer, due to this lack of support and the heat it sags out. Increasing infill percentage (e.g. gradual infill at top), more cooling at this layer might improve the first layer above the infill, but the quality will never be as good as the final top layer. $\endgroup$
    – 0scar
    Jan 20 at 8:18
  • $\begingroup$ Well 7 years and 3 months of printing experience later and this is a pretty embarrassing question! As you say it’s completely normal, and these “flaws“ should be covered up by any following top layers - each layer improving on the next. I tried everything except letting the print finish! $\endgroup$ Jan 21 at 18:05
  • $\begingroup$ There are no dumb questions, not everyone knows this, you find it out when you stop the print too early, as you found out. Infill is usually printed fast, so the filament is hot, stretched and unsupported, then you get such layers. I have put down the comment because I felt the answer addressing "why" (as you stated in your question) the layer looks as such are receiving less votes than "what" you might do about it. $\endgroup$
    – 0scar
    Jan 21 at 21:45

3 Answers 3


The infill percentage is a bit low.

Even with infill percentages that are higher than 15% (I use 25%), a single layer on top of infill will not create a good top. It takes at least two and looks really good with three. The first layer is often patchy looking!

If you're stopping your print after one layer, and have more layers to go, maybe let it continue and see if you are more satisfied with it after the second or third layer.

If you only have one top full coverage layer in your model, then I believe you will need to configure one or two more.

Sometimes too high of heat will cause the layers after the infill to looks sparse and sag, so temperature (as always) is a factor for covering infill. :+)


I doubt this is a printer or filament issue. Rather, I suspect it might be related to how your slicer handles bridging scenarios (which basically is what laying down layers over infill is).

You could try to:

  1. Increase the infill percentage and/or use an infill pattern that provides better support for the top layer (so that it isn't allowed to sag as much). You will probably still se some sag, but that is usually covered up by the next layer. Adding print cooling might also help here.

  2. Adjust the bridging settings in your slicer software. If under extrusion is the main issue, perhaps you could try to increase extrusion for bridges?

I am no expert with Simplify3D, and I suspect that the default settings actually might be pretty good. I would therefore try option 1. first. Perhaps someone else here can give a better answer related to your slicer. :-)

  • 1
    $\begingroup$ Does S3D allow for other infill patterns (you stated "orthogonal")? I suspect other patterns give better support at the same percent infill. $\endgroup$ Oct 12, 2016 at 13:45
  • $\begingroup$ @CarlWitthoft, You are probably right that another pattern would give better support. I found this video on how to set the pattern type. $\endgroup$ Oct 12, 2016 at 13:49
  • 1
    $\begingroup$ Spot on. The low infill had worked for me on some other designs, so I assumed it always worked...but with this PLA at the higher temp I decided to run as part of a previous troubleshooting experiment - nope. 30% works perfectly. $\endgroup$ Oct 12, 2016 at 17:26

Two things. First for a first layer from infill this is not unexpect. This is why you do 3-4 shells with such a sparse infill.

Most of the time we also have the first layer at a slower speed. Often we will get to this point and the difference in flow can be contributed to the temp to speed ratio. Try increasing your temp, or decreasing your solid layer speeds.

I recommend starting with a basic calibration STL. You can find them on thingiverse. Start with single wall, upgrade to a square, hollow square, etc.

Also increasing either number of top shells and or infill percent will also help.

3D printing is all about balancing 20 settings. I think of it like a performer spinning multiple plates. That said all about baby steps.

But to answer your question. Ignoring that this is exactly what I would expect even from a well calibrated printer on the first layer post infill. layer 1-3 is that the first layer is very slow. The extruder has built up a backlog of melted plastic. Assuming you are running too cold or too slow for some different types of printing (infill, solid, bridging, solid infill) you will slowly run out. Then suddenly you hit this solid layer and it needs a LOT fast. But it can only melt plastic 90% of what you need. Then less will come out. A good sign this is happening is if there is a lot of plastic shavings by the geared extruder. (if it can't melt then it stalls and the extruder shaves the plastic)

  • $\begingroup$ Thanks and +1 for "3D printing is all about balancing 20 settings. I think of it like a performer spinning multiple plates." That's a pretty good description. :-) $\endgroup$ Oct 13, 2016 at 23:51

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