Question: What limits the extruded plastic width?

(Demetris Stavrou) #1

Question:
What limits the extruded plastic width? I read a comment of @nop_head on his blog (http://hydraraptor.blogspot.gr/2014/06/why-slicers-get-dimensions-wrong.html) saying that “width is limited by the die swell” but I can’t see why. As the plastic is extruded from the nozzle it is then flattened by the extruder head (as it moves) which is a flat surface. Based on this, I would say that the width is limited by the flat surface of the extruder.

I am sure nophead is right and I need to understand why :). Thanks

(Sanjay Mortimer) #2

I have disagreed with @nop_head many times in the past, and have always eventually come around to the fact that he was right and I was wrong.

I think what is being discussed/aluded to is that the total cross sectional area of your extruded ‘road’ should not exceed that of your free air extruded filament (with die swell). However I am not sure that this is the case as I have often printed at wider widths than this, with good results.

(Jason Gullickson) #3

Is it more a matter of the maximum controllable/predictable width?

At some point the extruded plastic will start to have unpredictable behavior at the outside edge as the amount of plastic extruded increases beyond the width of the nozzle (or at least unpredictable by the simple modeling software we use, perhaps if you had some sophisticated fluid modeling or something

The reason is quite simple: If the area of the extruded road is a little less than the die swell then the plastic is being stretched and will span gaps and generally behave. (If it is much less then it will snap when crossing gaps.) When it is bigger than the die swell it will sag over a gap because it comes out smaller than required but has the correct volume, so it has to be too long. When extruded over a surface it is like pushing string. It wants to come out longer and thinner, so you are trying to compress it lengthways. It tends to squirm and produce wiggly lines.

(Whosawhatsis) #5

@nop_head So you wouldn’t have a problem with using thicker, wider threads of plastic on a print that doesn’t need to bridge, or if the slicer allowed you to override the extrusion width for bridging?

I know that Slic3r used to use very narrow extrusions for bridging, though there was some arguing about it and I think @Alessandro_Ranellucc backed off on it.

I mentioned bridging because it illustrates the problem, but you will still have issues when there is no bridging.

Extrusion roads that are supported underneath become wiggly because you are trying to force the plastic to be shorter and wider than the bead coming out of the nozzle. It needs to be always under a little tension, i.e. stretched lengthwise, to ensure it follows the path of the nozzle, rather than meandering.

You can have wide threads (compared to the nozzle) but the layer height must be reduced to keep the area less than the die swell area.

I would post a pair of pictures illustrating this but G+ doesn’t seem to allow pictures in comments.

Two objects printed with the same plastic and g-code. One is printed with a nozzle that is too small for the layer height and is a complete mess of squiggles.

(Demetris Stavrou) #8

@nop_head Can you post the pictures in imgur and post the links here? I think it would be really useful for everyone following the question! I am still a bit puzzled Thanks!

(Whosawhatsis) #10

@nop_head Do you know what the W/T ratio on that print was? A higher W/T ratio seems to make it much more forgiving of packing plastic in this way, but if the ratio was close to 1, it seems obvious that you would have this type of effect.

W/T was 1.5 and T was 0.3mm.

(Whosawhatsis) #12

Yeah, it would probably work better with a higher w/t.

For example, I routinely print my first layer at .35mm from a .35mm with a w/t of 2-2.5. I see the same meandering if the nozzle is too far from the platform, but it looks perfect otherwise. I think the real problem in the picture is that the lower solid layers are sagging, leaving the ones printed on top of them under-compressed, thus leaving gaps between lines and allowing them to meander. Slic3r should actually perform better on this because it now uses bridging settings for the first layer of solid fill over infill. Assuming your bridging settings are good, this should avoid the drooping that results in under-compressed plastic being printed on top.

If we consider a fixed layer height, what I am saying is there is a maximum area, beyond which it becomes wiggly. You are saying the opposite: to stop it being wiggly increase the W/T, which means an even bigger area. Hard to see we can both be right unless there is a band in the middle where it fails.

(Whosawhatsis) #14

That may be the case. Putting out enough plastic to fill in the space (so that the plastic is compressed against what has already been extruded) can fill it in. Of course, this won’t help if the previous layer is so far below where it’s supposed to be (due to sagging when it bridges over infill) that it won’t compress as it leaves the nozzle, which I believe is the case in your example. This hypothesis can be tested by running the same experiment with 100% infill. (actually, anything greater than 50% will probably be sufficient to demonstrate my point).

(Whosawhatsis) #15

Oh, and FWIW, I’ve taken a vase-mode print sliced for a .5mm nozzle with .2mm layers and .7mm extrusion width and run the same code on a machine with a .35mm nozzle without having these problems.

Whilst it is true that it becomes a complete mess on the final layers because the sparse infill below sags, you can also see wiggly outlines of the ears that have no infill due to being too thin. They are all on top of each other but still develop a wiggle when extruded from a small nozzle but not from the bigger one.

How does slic3r manage to do an outline that spans a gap when the W/T is large and the filament is not being stretched? For it not to sag it would need to change the flow rate instantaneously. When the cross section is just a little less than the die swell nothing needs to change while bridging.

(Demetris Stavrou) #17

@nop_head Rereading your post (5th comment of this question) I think I understand it now! I searched for PLA die swell and read that is not much. So should we assume that it equals the nozzle diameter?

(Whosawhatsis) #18

Die swell depends on how fast you extrude. I can at least double the diameter of my extrudate by extruding at a very high speed (into air) vs. a low speed.

(Demetris Stavrou) #19

Is there a formula to calculate that?