Problems with print progressively 'collapsing' as height increases

Hey All,
I have managed to get 'a result' from my reprap i3 but I am wondering whether anyone can help me diagnose the issues I am having...and hopefully suggest some get-outs. I am trying to get things working using this model - a stack of 5mm cubes. I do not have a cooling fan. In a nut shell:

[1] Cube layers start at 5mm high (as they should be) and get progressively smaller as print proceeds. The nozzle head seem to be burying itself in already laid plastic at the end
[2] The vertical walls bulge out at the top of the stack. This seems to be related to the 1st issue
[3] Plastic drapes over bridges - this has repercussion on levelness of upper layers

I have attached 3 photos which I hope demonstrate the issues. I am printing with 3mm PLA at 180C extruder temp with a 0.5mm nozzle. Slice heights are 0.4mm.

This looks like a classic cooling issue to me. Its more of a problem as the layers get smaller and smaller because, quite simply, the hot end never moves away.

Are you doing anything to cool? The first step for most people is to adjust slicer settings to slow down on small layers. To me this has mixed results because by slowing things down, you are actually keeping the head nearby longer. A fan, especially one that is g code controlled, can make a big difference on something like this. So can printing multiple copies of the object because it gives the head somewhere else to go.

You may want to lower your temperature a bit, the hot end burrying itself in plastic also suggests to me that you may be extruding too much, decrease the extrusion multiiplier. A better(more systematic) approach would be to calibrate it.

> Its more of a problem as the layers get smaller and smaller because, quite simply, the hot end never moves away.

Actually conduction from the hot end into the material is very low, most heat is transferred in the extruded material itself. Small parts simply dont have the time to cool. This is why slic3r only has a slowdown before layer time and no 'idle away from the print' the potential for ooze isnt worth it.

However if the nozzle gets to swim in the plastic, thats a different story.

well actually it depends on how far the nozzle sticks out from the heater block, on my hot end it only sticks out a few mm, so if I try to print something thats about a 1cm a cross, even if there are a bunch of them, the part is always melty by the time the print is done. this is with abs though, pla needs to be cooled by a fan, and even abs can be cooled to increase precision and speed.

Hey,
Thanks to all for responding. After playing around with temperatures etc etc I found I had a more fundamental issue with my z-axis missing steps here, there and everywhere. Slowing things down, heat sinks on my drivers and turning down the current seems to have fixed things.

A new (previously hidden) issue seems to have arisen now though. I have calibrated the extruder feed rate...so 100mm means 100mm(ish) - I end up with a number of aound 415mm/step for the E-value. I have measured my filament diameter and put that in slic3r. However when I go to print there seems to be serious overfilling. If I put the value down to 300 using the command M92 e300 in Pronterface things seems to behave much better.

Does anyone know what might be causing such overfill, given that my calibration pre-print seems to be okay?

Thanks for any help.