Really weird problem

Just out of curiosity i drilled one of my J-head nozzles out to .8mm to see how it would print. Once i had the extruder tuned back in it prints great at 30mm/sec, but if i try printing above 30mm/sec on X and Y speed the printer cant keep up with itself. I can print easily at 100mm/sec with a .35 nozzle. When printing a corner it will take off aroud the corner about 1/4" before the actual corner. And same thing on infill, X and Y dont run in sync with eachother. If i slow it down everything is fine but if i speed it up it just goes bonkers.

In Sprinter i set the max E speed to 100mm to make sure my extruder wasnt getting limited by the firmware. Made no difference. I am using 1.75mm filament and the extruder runs at an extremely fast pace printing at 30mm/sec.

I am slicing with Slic3r 9.8.

I cant figure this out.

have you checked the gcode to make sure it is slicing correctly?

To run at the same federate, a larger nozzle needs to melt much more plastic. I think what you are running into is he melt rate limit of your hotend. A bigger nozzle probably needs a longer and possibly higher power (depending on the power head room you have) heating zone.

What may be also happening is the filament cannot be squished as thin as the 0.35mm nozzle.

The biggest backpressure for the 0.8mm nozzle would be from the pressure against the surface the plastic comes out from. If this is the case raise your layer height to .4mm or .6mm. feedstock should push thru very easily as the nozzle hole has a lot less pressure than the 0.8mm.

On another note, it is now safer to raise the temp higher to compensate for the increased volume of plastic leaving the nozzle as long as the temp is still within specification of your hot end. The 0.35mm nozzle had time to heat the plastic and reach melting temp. your nozzle may be able to keep current temp, but by the time the plastic goes thru the nozzle it may not have heated enough.

I think you guys are right about the hot end just not heating the filament up fast enough. I checked G-code and it looked fine. I am running it at .6 layer height, man does it build a part up fast. Even running it at 30mm/sec it cut build time by about 40% but obviously at much lower resolution.

I think the hang up is that even if i add another resistor or run the hot end alot hotter the plastic is passing through the hot end at such a high rate it just isnt in there long enough to melt completely. I am going to do some experimenting when i can to see if running up the temp alone will be enough to make the difference. Right now its running so good i dont want to mess with it.

It may also be a good compromise to get a 0.6mm nozzle. I used that size for a bit, and although I would not recommend it for gears, it looks good and runs fast for about anything else. It also allows walls to be a bit thinner and feedstock won’t disappear so quickly.