geometric limits of printability without support material

Hi folks,
can anyone tell me (or point me at a document that tells me) what the geometric constraints are for making printable objects on a basic reprap type system with no support material?

I guess there's a limit to how steeply you can print "outwards" from layer to layer but what sort of angle is allowed?

Do the various slicing software options warn you if you go over these limits?

Also how on earth does the top layer on the whistle in this video get printed sucessfully when it has nothing to rest on as far as I can see: [www.youtube.com]

Is it just relying on getting the speed right so the filament stretches over the void? If so what are the limits of what can be achieved with this technique?

Many thanks,
Robin.

How much overhang is possible really depends on what sort of shape you are making. If you are making a round overhang like a wine glass you can probably get it to work at a much shallower (more horizontal) angle than if you were making something with square overhangs. I've seen some people even claim to get 90 degree overhangs (completely horizontal) if they stretch the plastic right and make round shapes. That way the plastic sticks to the neighboring layer sideways instead of the (nonexistant) one below.

As far as general purpose overhangs I think a good rule of thumb is to keep the overhanging slope greater than 60 degrees from the horizontal with a layer height of 0.4 mm.

For the whistle what happens to make the top face is called bridging. The print head moves faster and extrudes less over the gap to stretch the plastic across. THe result is a first layer with a bunch of holes in it but it makes a solid enough surface for the subsequent layers to stick to and make solid.

I think it really depends on what object you are making, some shapes are more amenable to overhanging than others.

Thanks for the info!

It's good to know that people are overcoming the limitations ... hopefully some of the more esoteric techniques (like getting 90 degree overhangs by looping out over the gap) will get incorporated into the Gcode generators so that things "just work".

One day I'll get a machine together and get to experiment myself I quite like the idea of having a solenoid actuated oil brush or similar on the print head that could be used to selectively stop layers sticking so that support structures can be made from the main printing material.

In fact it just occured to me that if the "oil" on the brush were more like sugar syrup (or other soluble adhesive of some sort) then the support structures could be made to fall apart into handy granules after use so the waste can be fed back into a home extruder to make more filament again.

Cheers,
Robin.

The same material break away support actually works quite well with good settings.

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