Theory on surface

I wondered why reprap nozzles are usually so small that it barely catches the overflow (usually 1-2 mm max nozzle width). I then thought, it isn't flush with the print bed ever, because user manually change this - but since layers build this isn't that case. In fact, the nozzle isn't usually flush with the movement of the XY gantry, so that if you had a big flat nozzle, it would wreck itself.

However, if you could have a very well aligned nozzle, wouldn't this make cleaner prints, both as far as passes and as far as spreading the plastic better? Or would it wreck the part?

I am not sure if I understand exactly what you mean, but I think your idea is to have a much larger hot area around the actual nozzle hole. Is that right? The idea maybe is, that the printed material is swept flat or away so that it is more evenly spread in the print layer.

However, that would not give better quality I think. In fact, the material swept away has to go somewhere. Ideally, for 100% infill, it would exactly fill all gaps between extrusions in the layer, resulting in a massive plastic part without any air inside. However, that is very unlikely. The slicer usually can't take into account the exact geometric form of the extruded material, so there would be small differences at sharp corners, points where the path touches another and so on. Also, the filament is not always the exactly same diameter, and may contain water taken from the air, that evaporates in the nozzle and gives a little uneven flow.

When printing a massive solid body, the total volume extruded has to match the body exactly, otherwise there would be material left. That surplus material would be swept over the printed layer until it settles on the perimeter in some uncontrolled fashion. With a larger nozzle bottom, that would still happen, but maybe even more material sticks to the nozzle and gets displaced.

For that reason, a 90% infill print with a little air inside has more precision on its surfaces. As there is still a little air left, any surplus infill material can replace it instead of being swept to the perimeter where it spoils the final object's surface.

Also, a larger nozzle bottom would keep a larger part of the printed object heated, thus slowing down the cooling, especially for small objects, and thus raising print time.

So the best way to get a clean filled part is to analyse the extusion process and refine the slicer I think.

Good answer - thanks!

I actually tried a big flat nozzle (made from a 3/8 aluminum acorn nut) as part of a design for a no-machining-required extruder. I found that it caused the exact problems [dronus] describes. In addition, it is impossible to print fine details because the larger nozzle transfers enough heat to the part to remelt and distort them. It's also difficult to get the print to stick to the bed because the nozzle tends to peel it up.