It is difficult to describe with words what's happening, so take a look at the picture. Somehow PrusaSlicer decides to move and print in the air (blue line), where instead it can continue going from outwards to inward. I understand it wants to print first the outer layer but in this case it is obvious it will not hold that layer.
The final position is shown in the second image.
The final step of that layer:
Any idea how to configure PrusaSlicer to avoid this situation?
You'll want to turn on supports, which will create a series of low density pillars in the "floating" area and provide a foundation for the layer that is printing mid-air in your second image.
Another option is to create a hollow support cylinder perhaps a half millimeter larger diameter than the hole in the floating layer. This creates the support for the inner diameter and allows the printer to create bridges.
The former method will use more filament and take longer to print but provide a better under-surface than the latter method.
The inability to do this is not fundamental, but it's currently a limitation in slicing software, and not likely to change soon.
You can see from this demo that the kind of printing you want is possible. However, it's currently only being done with software to assist in hand-crafting gcode to make the printer do what you want it to do. The hard part is making the slicer automatically figure out that that's what it's supposed to do.
For your needs, just printing from outer loop inward here is not going to suffice. There needs to be overlap, computation to compensate for the fact that extrusion lines will not squeeze out to their nominal width because there's nothing below them to extrude against, and using the overlap in the XY direction to achieve that instead. This is all new stuff slicers have just not been programmed to do.
So for now, you need supports. If you're willing to use Cura, Conical Supports or Tree Supports will let you support this with less waste material and make removal easier. Alternatively you could design your own support into the model and put it just under the hole in the middle, letting the flat part bridge. Or you could make a sacrificial bridge layer so that the first layer is solid bridge all the way across, and cut/drill it out after the print is done.
There is one way assuming the picture is complete, and that is to print the object upside down from it's current orientation. This would negate the need for any supports or worrying about printing in thin air.
If thats not possible then as Freds answer says, you need supports. If you're printing a run of the same objects you can make a support and insert it into the object at the correct layer and then re-use it for subsequent copies rather than print supports each time.
It isn't universally applicable but I have had success avoiding undesirable sagging in some situations by printing a part in two pieces, one inverted, and gluing the two "bed" surfaces together.
The software I use (Repetier 2.2.4) includes a setting "Cut Off Object Bottom" and accepts a dimension. For some objects it's possible to choose a cutoff point that will produce two pieces without overhangs that can be joined. The image shows a simple example.
Using the cut off option makes it possible to do this without making changes to the model.
How to configure PrusaSlicer to prevent printing in the air
You need either support or change the print orientation.
You cannot print into the air, you can however print lines on top of a prior layer with a certain overlap, or bridge a certain gap. E.g. (screw) holes are often covered with 2 layers over the screw hole so that you use bridging to print (the cutout shows that the hole has a bottom):
After printing you need to punch through the layers to open the hole.
Starting and stopping extruding in thin air will result in deposition of the extruded material on the location the nozzle crosses a prior layer (often seen when support structure pillars have fallen over).
What is used frequently together with support in Ultimaker Cura is the option called Enable support roof, this will create a dense top layer on top of the support interface which allows for pretty good bottom surface of the overhang.
This e.g. results in the following slice:
When applied to a recently printed flanged tube you see that the bottom surface is pretty good (note the concentric lined top surface of the support structure with roof enabled, as seen in the second image; note, the , first image shows the product with support, second image, on the left a view of the bottom above support layer and right the support with roof layer):
How to configure PrusaSlicer to prevent printing in the air
In PrusaSlicer you should be able to something pretty similar with Interface pattern spacing set to zero:
Slightly annoying fix and might not be exactly what you are looking for but you could try adding the slightest angle to the overhang. For my application, a 5° slant with a 10 mm layer height solves the issue. I guess the angle helps the slicer start with the layer closest to the object. Note that I believe the layer height you are using determines the smallest angle you can use and still get the slicer to behave as desired. See pictures for reference.
Before adding a slight angle:
After adding slight angle:
Again it is a context-dependent solution but in my experience, you can get away with a 5° slant to flat surfaces without having too much functionality or visual differentiation of the final part.
Hope this helps!!
