Prints caving in

I've just got my Prusa I3B together and have been doing lots of small prints to try and find the right settings. I printed a hollow cube which turned out decently except for the top which caved in. Any sugggestions on how to stop this?

Thanks

Might just be that it is printing too fast for layers to cool sufficiently. Try printing two or three at a time and see what happens, or turn on the print fan if you have one (this is different from the hot end cooling fan). Also you could try messing with the bridging settings. Each layer needs to cool to solidity before the next layer prints on it so small prints can have saging/distortion if the bottom layers are squishy when the next layers are printing on top. That is why you have delay settings (minimum layer time) in most slicers for cooling when printing narrow tall objects.

Bryan

I'd avoid a hollow box and unsupported printing. Use infill and also several layers for skin or top layers.
stretching out a road across unsupported air is a Varsity level of stunt.
You won't get there easily, until you've paid some dues.

That being said, it's likely one of these reasons:

too hot. lower temp and practice extrude until it just barely sticks to itself when it falls into a pile.
move faster across the unsupported area.
too much volume of extrusion versus speed.
maybe need to recalibrate extruder (RichRap has a good article somewhere).
need a fan blowing on PLA to solidify it soon after it's put down.
More fan is always handy for stretching across air.

Just my guesses.

Dave

Bryan (or Aimee!) is right also, about too much heat and moving too quickly.
printing speeds up on peaks and parts with pointed tops.
the part will sag, like on the top of a traffic cone. it's a hot mess at times.
you need to have a minimum layer time, so the fan can cool the finishing layer, before the next layer is applied.

Edited 3 time(s). Last edit at 08/31/2015 06:28PM by davew_tx.

Thanks for the replies! I'll be sure to test your suggestions out.
I have another question, the sides of my print are very stringy with a few very small holes. Any suggestions on how to fix it?