Hi all
So I have my printer up and running (mostly). I have replaced the original X carriage and extruder, and fitted a bed levelling servo, all very worthwhile. The extruder upgrade allows instant changing of filament and jams are a thing of the past. The bed leveling system is worth persevering with and works really well. Main snag I found was that the cura slicer preamble inserts an G90 absolute positioning code which destroys the bed levelling settings in marlin - so I edited the cura definition files to remove it.
But (there always is one

) the performance is poor by my standards, probably aggravated by the use of ABS, needed for its higher temperature stability. The problem is warping of the part caused by the differential expansion of the plastic. The initial layer is stuck onto the bed by a combination of heat, pressure and a certain amount of magic

in the form of ABS juice, PVA, hairspray, or whatever works for you. Bed adhesion does not seem to be a problem using just dilute PVA, and if you add ABS juice to it you will have a very hard time removing the part, however I digress. Once the initial layer is attached the next layer is added. The temperature of the subsequent layer is determined by temperature of 1st layer, surrounding temperature, and radiation/convection of the same. Subsequent layers therefore experience a different temperature depending of the heat losses of the layer, layer size (more time to cool for larger parts) to which to adhere. If the plastic is extruded from the nozzle at 240C, and the temperature at which it hardens is 100C then the surface temperature to which the subsequent layer attaches is very poorly defined. Once removed from the bed the temperature of the layers will becomes equal and warping occurs, because at the same temperature the layers are different sizes. It always seems that the bottom layer becomes convex which implies that the temperature of the plastic when deposted, bonded and hardend is higher than that of the bed, presumably because the heat maintained in the previous layer is greater than the bed temperature. So the upper layers shrink more and warping occurs.
ABS has a glass temperature of around 100C so one would think that provided all the layers were bonded at that temperature warping should be a thing of the past. I have already taken steps to guard the print bed from drafts but this does not seem to have improved things much, despite having an internal temp of ~ 40C.
It occurred to me that it may be possible to solve two problems with one solution. Instead of using a heated bed which adds to the Y axis movement mass, why not heat the whole interior of the machine to 100C using circulated air to even out the temp differential. Could use heat bed controls with fan for circulation. This would eliminate the need for an underglass aluminium heat spreader as the whole interior would be at one elevated temperature. To prevent heat damaging the extruder, belt, motor etc a plastic oven bag or sliding fibreglass/srbp heat seal could be used, attached just above the print head as a possible solution, minimising heat loss.
Still at the thinking stage on this so if anyone has tried this or has any further thoughts on this solution please contribute.
Best Regards
picclock.