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emilio2

Printing gears: how to prevent skirt at bottom layers?

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When priting any part, the bottom layers are always slightly larger than the rest. There is some sort of auto-skirt. I can live with it for normal parts where the bottom layer is not the most important.

But, when printing gears this problem is crucial because the part does not fit other parts. If I want to fit the gear in place I need to clip the excess plastic around bottom layers.

Which is, if there is one, the best method to minimize this effect and get fitted bottom layers?. (other than changing the cad model)

emilio.

 

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In the advance tab, there is a first layer line width. Reducing that might help some but it reduces it for the entire layer so infill may not completely "touch".

Precise leveling calibration is critical to not having the elephant foot. It's also important to have the nozzle primed to the right pressure. Change the skirt count to something like 5 so the pressure can equalize before the actual part is printed.

Also, I've been play with gears a lot. You can get a better result by putting a small bevel on the teeth so the first few layers aren't actually part of the teeth so that the "elephant foot" has a little room to spread before the teeth would meet the other gear.

Finally note is that this may not really be "elephant foot". My understanding is that EF occurs when the bed is too hot causing the layers to "run" outwards because the material is too hot for too long. Instead, you might be over extruding due to slightly off leveling and printing a 0.3 mm layer into a space of 0.2mm or 0.15mm etc or because the nozzle was primed just before printing and has a higher than normal pressure build up. However, the results are the same.

 

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use the chamfer, use fresh pla/glue and really push how little squish you need to get the part to stick to the bed. When I am printing small inter meshing parts I hardly squish at all and go for a lower initial head temp - some failures, but no flashing. (.e. all the things you would normally do - but for special parts.

do the chamfer first - it really works.

 

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What the other guys said is the best approach when it's possible.

But since it's not in this case, well... it's going to be tricky. You're going to want to dial in the bed height very precisely so that the parts have just barely enough "stick power" without the first layer being smooshed. It might also help to print without any heat on the bed as well (but this also makes it harder for stuff to stick). You can also experiment with the first layer thickness. Maybe going with a very thin first layer makes the excess thin enough that it can be worn away during the first few turns?

In short, you're going to have to experiment and swear a lot about parts coming unstuck until you get it right.

 

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The standard advice for 1st layer is to reduce speed and increase bed and extruder temps. I never increase the extruder temp. As you cannot change 1st layer extruder temp in Cura I suspect that most users use their print temp. for the 1st layer. So if your optimal print settings are say 210c at 40mm/s then if you print the first layer at 20mm/s you will get over extrusion, inevitable. If it is important to me I reduce the dimensions of the first layer by 0.1mm i.e. say it was a cylinder in vertical orientation that would be 0.1mm off the diameter. I do though think that using a chamfer is possibly a better solution, maybe.

And yes Emilio it would be nice if the slicer compensated for you :)

 

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... So if your optimal print settings are say 210c at 40mm/s then if you print the first layer at 20mm/s you will get over extrusion, inevitable...

 

Well, not really :) The fact that the plastic will ooze more at higher temperatures is one thing. On the other hand, while the printer is extruding (no matter which layer), the volume of the extruded plastic is controlled by any other parameters but temperature. It's true however that increasing the temperature can allow the plastic to slightly "expand" over expected shape and therefore producing an aspect similar to overextrusion.

 

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Yes I agree, mainly. Certainly temperature is not (I think) a parameter that affects the pressure created for extrusion. But I have seen over-extrusion in a print where extruder temp is too high - which in context is the point on layer one. Now if the result is due to expansion rather than over extrusion then I stand corrected :). But the end result is the same, more filament than you need/want.

 

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Well, if one maintains bed temperature above glass temperature (Tg) after the first layer, then there are great chances that "elephant foot" will enlarge due to lower layers being slightly pressed under the part weight. That's why, if for adhesion reasons bed temperature is set above Tg for the first layer, it should be reduced to something below Tg for the rest.

 

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