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mark10970

Wavy Lines??

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Hello all,

I am printing a thin walled mechanical item <http://www.pbase.com/mark10970/image/151291990>. I have less problems at higher temperatures (230C) than at lower ones, but the problem still persists. I also have less problems at .1 layers than at .2.

The walls are about 1.6mm thick, though they vary in some areas. I am using .4mm wall thickness, .4mm fill, 100% fill density, but I have varied all of these parameters with little improvement.

Any help, suggestions or comments would be greatly appreciated. I am very new to all of this, so please be patient with my ignorance.

Mark

 

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The thin patches in the walls are due to under-extrusion, which creates an un-even layer of plastic. Subsequent layers then find it harder to stick to that uneven surface, and you get a progressing pattern of partial extrusion.

What speed are you printing at?

 

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The fact that it gets better with hotter and thinner makes me think that it's a throughput issue. There's a limit of somewhere around 8-10mm³/s that you can push out of the nozzle. (See my blog for some research I did, albeit with a larger-than-normal nozzle).

100mm/s at 0.2mm layer height is quite fast - it's 100 x 0.2 x 0.4 (nozzle width) = 8mm³/s - which getting up towards the limits that I've seen people have with the standard nozzle. High throughput can lead to pressure build-up, which causes slipping and under-extrusion.

 

Try slowing down and see if that helps. Try printing at 50mm/s instead, while keeping the temp up at 230. Also check that your extruder drive bolt is clean, and that the spring is at the right tension (it should be about 11mm long when the filament is loaded, so that it's not fully compressed, and has some room to move in both directions).

 

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You are near the limit of volume for you nozzle but I've printed faster...

Your feeder is probably fine but test it anyway - it should be able to pull about 22 pounds with no load (no pressure in the head).

Whenever I print this fast (100mm/sec with .2mm layers) I keep the temp at 240C so it flows like honey. Even so I still get a little bit of underextrusion in the walls (as you did).

If the layer height is .1mm then 150mm/sec is no problem at all. It's not the speed, it's the volume.

If I want a quality print I keep the volume below 4 cubic mm/sec and keep the temp low (180-200C) which typically means I print at 50mm/sec.

 

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There's a lot of useful reference points here, and it's good to see that they generally follow the print profiles I've been developing as I do more prints.

I wasn't aware that the volume limit was near that though - My most aggressive print so far has been 10mm^3/s (80mm/s speed * 0.25mm layers * 0.5mm nozzle) and it came out with great density and adhesion. (This was even using the tar-like Ultimaker Black PLA. ugh.) What is the max volume flow people are seeing?

 

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Nick - see the blog post that I linked to above - the limit is ultimately determined by pressure in the head, and as such it is a function of three things... the size of the nozzle opening, the ability of the extruder drive to push the plastic, and ability of the head to melt it quickly enough.

Since you're nozzle opening is 0.5mm, rather than 0.4, the opening is about 56% larger - so, all things being equal, you'd expect a limit somewhere around 15-17 mm³/s.

In practice the limit probably varies by filament type, and it's definitely very sensitive to the print temperature.

 

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Great info. One thing I've noticed when turning things by hand is that higher speeds/volumes also greatly increase the turbulence of the flow in the nozzle, causing much more erratic behavior of the plastic as it exits. Notably, the Ultimaker Black PLA had this lovely tendency to hook to the right as soon as it left the nozzle when fed at higher speeds/volumes - and when immediately switching to Printbl PLA, the problem disappeared. This was something that was definitely affecting my higher speed prints, particularly on getting good first layer adhesion.

Anyway, it seems like one of the most readily solvable problems in this equation is the extrusion system - have you experimented with any improved drives?

 

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