Effects of model placement on print bed

[paoletto 3]

Do Ultimakers have a thermal gradient in the build area?
If i have an enlongated part to print, and i align it along the Y axis, should i expect warping in the back? (that's what i'm getting, w/o cooling).
I tried to lower temperature, print outer walls before inner walls, the situation improved, and i believe the difference between front and back became even more apparent, as the front is now perfect and the back gets some warping, and it looks, also from the side, that there's even some curling of the layers.
Side question: might this be also caused/exacerbated by enclosing the printer, like in a non vetilated box or in a bag?

[paoletto 3]

Ok. after the 3 failed attempt i turned  the part and printed it close to the front. It turned out alright.

[geert_2 557]

I once measured the bed temp with an infrared gun thermometer: the edges were ca. 5°C cooler than the center of the bed, I guess due to convection that pulls-in cold air from the sides. But below the nozzle-fans, the bed could be up to 15°C cooler than elsewhere, especially on small models where the nozzle kept blowing on the same small spot. Cooling fans are there to cool... So that could cause warping in those spots.

 

Also, on UM2 printers (which have a less powerfull feeder than newer models), the left-back position is more likely to cause underextrusion, and thus poorer adhesion to the bed, than the front-right position. Because the filament has to take a much tighter curve to get into that spot, and this causes a high friction in the bowden-tube and in the nozzle. High friction means more resistance, and thus less flow. I don't know if and how this affects newer printers.

 

And there could also be dirt or grease in some areas, due to uneven cleaning.