This whole part of the design has been bothering me for a while.
If we want to cool the plastic we just laid down, we should have a tool for that.
We have a tool known to the G-code interpreter as T0, which is the first extruder. If we have a second extruder, physically offset from the first (in this case by about 75mm), it is known as T1.
Why not have another tool, let's call it T2, which is physically offset from T0 by 37.5mm (putting it halfway between T0 and T1 on dual-extruder machines). This tool is a dedicated fan with a nozzle on it. The nozzle directs cool air to the target area, which is [37.5, 0, 0] relative to the T0 nozzle. The firmware knows how to turn the fan on when needed.
After the extruder nozzle(s) have finished with one layer, tool T2 is enabled. This tool is a very broad brush compared to an extruder nozzle, so it can quickly sweep the entire layer. A simple raster scan of the layer is done, then T2 is turned off and we go to the next layer.
The slicer software would need to be modified for this. We can't reasonably expect the slicer gurus to put this feature in when nobody's hardware supports it. So somebody needs to build the hardware first. We can do this.
We could take advantage of such hardware without needing any help from the slicer. We just create a G-code fragment that turns the T2 tool on, does an X-Y raster scan of the entire platform, and then turns T2 off. All that is needed then is a simple program that reads in the G-code for your print job and inserts this code fragment after every layer. It will take longer than the smart slicer would, but would allow us to use this tool while we wait for the slicers to catch up.
For those with single-extruder Bukobots, there is one complication. The design suggested above would miss a strip 37.5mm wide at the left edge of the platform. This could be fixed by a dual-nozzle design, where one air nozzle directs a second stream to the other side of the extruder and aims it at a point 37.5mm to the left of the extruder nozzle. The slicer software would need to be modified to take advantage of this (but not the simple G-code fragment mentioned above).
The Azteeg controller already has everything needed to support all this. One would need to buy one additional fan, some screws and nuts, and some wire (with connector) to hook it up to the Azteeg. Everything else we can print.