Thick walls without infill. Has this been studied?

[Gary Schwartz]

 

I plan to conduct a series of tests toward being able to design & print "hollow" models with a specified solid wall thickness, and no infill in the walls.

 

For example, a cylinder with OD of 50mm and ID of 44mm, (solid wall thickness of 3mm).  Accomplished with a number of perimeters, and NO infill.

 

I question whether extrusion width can be controlled with the  precision necessary to create this cylinder and provide appropriate clearance for a 43.5mm cylinder to fit inside.  That's what I plan to study.

 

Unless ... it's already been done.

 

I expect significant advantages.  Most solid walls, regardless of shape, would benefit.  As would our printers that could produce the same or better model strength while avoiding the machine-gunning jitters of short infills.  And often consume less filament.

 

Are the answers already known? 

Any ideas on how to proceed?

Inside to outside perimeters vs. outside to inside probably a factor?

 

 

 

 

[Ed Nisley]

> Are the answers already known? 

It's a matter of geometry: how the specified thread width fits into the wall dimensions.

For a straight wall, the only possible wall widths without infill are integer multiples of the thread width. For a 0.4 mm thread width, you can only build walls that are 0.4*n mm wide, where n runs from 1 to whatever; you can print a single-thread wall, but not easily.

If you specify "two threads" along each perimeter, then various slicers give different results for a wall that should be five threads wide: the center thread space may or may not be filled. Walls that are an odd number of threads wide may not fill the way you expect.

At an intersection of two straight walls, geometry says there's no way to lay parallel lines of a specified width into the acute corner, so the wall can't fill properly. You can convince yourself of all this by playing with Popsicle sticks, because they have a fixed width with nice, rounded ends, just like an extruded filament, and show what's possible: if you can't fill a wall's angle with sticks, the extruder can't do it, either.

The various slicers have different techniques of laying parallel threads into acute corners, so different objects produce different failures. For example, walls that should be three threads wide probably won't have a continuous thread between the walls through the corner, because the geometry doesn't work in the corners.

Some pictures of stuff I've done with thin walls:

Single-thread wall cookie cutter:

http://softsolder.com/2011/09/07/tux-cookie-cutter/

Double-thread wall cookie cutters, varying the extrusion multiplier:

http://softsolder.com/2013/09/17/extrusion-multiplier-effects-thereof/

Single- and double-thread fins:

http://softsolder.com/2013/04/28/co2-capsule-fins-16-gram-threaded-edition/

http://softsolder.com/2011/10/21/zombie-apocalypse-preparations/

Double-thread wall with a little fill:

http://softsolder.com/2013/02/24/printing-scale-model-concrete-blocks/

Tiny support structures:

http://softsolder.com/2012/01/30/harbor-freight-bar-clamp-new-handle/

> whether extrusion width can be controlled 

> with the  precision necessary to create this cylinder 

> and provide appropriate clearance for a 43.5mm cylinder to fit inside

That's a separate question that doesn't depend on the presence or absence of infill; if you have the filament diameter and extrusion multiplier set correctly, then the extruder produces pretty nearly exactly the correct width all the time. I use a thin wall open box for that calibration:

http://softsolder.com/2013/04/16/makergear-m2-fundamental-test-object/

The slicer then sets the perimeter thread locations to match the dimensions of the model; in effect, the outer threads are inset into the perimeter of the model. Again, there's a geometric factor in acute corners: you can't get a perfectly square corner with an extruded thread.

The two cylinders must have the same number of facets, so that you don't get a geometric misfit; for a 45 mm cylinder, you can probably stand 1 mm facets and have 140-sided polygons. The Popsicle stick analogy still holds: there must be enough clearance between the cylinders to fit the outside corners of the inner cylinder into the inside corners of the outer cylinder.

A bike helmet mirror mount with several nested cylinders (with a much smaller polygon count!) that have thin, filled walls:

http://softsolder.com/2011/07/01/helmet-mirror-mount-first-light/

The solid model for the mirror mount may be easier to see:

http://softsolder.com/2011/06/29/helmet-mirror-mount-solid-model/

I routinely build walls with two threads and can fit three threads into smooth perimeters, but complex, angular, thin walls simply can't be filled with parallel extruded threads.

But you (and I!) can spend hours finding that out... [grin]

[Gary Schwartz]

Thanks Ed.  What you've given me to feast on will satisfy my appetite for knowledge for a few days.

 

I still have an open issue about infilling thick-walled cylinders; e.g. automotive drive shafts, basement support columns.

 

I conclude from my first reading of your material that a well-calibrated printer can produce "circular infill" that would satisfy specified ID & OD of the walls of a thick-walled cylinder. (If the wall thickness is an integer multiple of thread width).   And, that thread width could be tweaked in special cases.

 

I believe that adding this as a user-selected feature in slicing software would be a valuable step toward simplifying 3D printing.  "What would we ever use this for?"  My reply is the same as when I put an interesting piece of hardware in my "Stuff" box ...

 

I'm awfully uncomfortable when I see my M2 hammering away at X-Y infill.  I expect to minimize that (for me) with what I've learned today.  I think the future needs something that automatically recognizes when circular infill can be applied.

 

      Gary
Make Better Things
Make Things Better

 

 

 

 

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[Ed Nisley]

The gotcha is that there's no such thing as "circular" in an STL file, because (by definition) STL files consist entirely of triangles. As a result, all circular features are really polygons, each with a vertex count determined by how "circular" you want it to appear, and the geometry of those angular joints still works against continuous infill. Eventually, we'll be using both a file format that encodes true circular features and firmware that supports true circular arcs, but that seems rather far in the future.

Given that getting "perfect" parallel-thread fill depends on having a wall thickness that's a precise multiple of the thread width and a joint geometry that can be properly filled, I think there's no good way to automate this for arbitrary wall sizes and geometries. In other words, would you prefer that the slicer automatically meddle with your model's wall dimensions, thread width, or geometry to make the fill come out right? I'd rather handle that tradeoff manually, because I know which parameter I can compromise in a given situation.

If you tell the slicer to retract the filament only when the nozzle crosses a perimeter, then the only "hammering" comes from XY acceleration. That's what the printer does for a living: start and stop and change direction on command. There's no additional mechanical wear due to short infill segments, so you won't improve anything by eliminating them; if the acceleration doesn't exceed the ability of the motors to move the masses, it's all good.

As long as you have threadlocker on those pulley setscrews, of course! [grin]

> infilling thick-walled cylinders

If there's room for ordinary infill, then I think you get better results by specifying a high percentage of infill, rather than trying for solid parallel infill, because ordinary infill automatically handles all wall sizes. It's not clear that a "solid plastic" cylinder (or shell) has better mechanical properties than, say, 75% hexagonal infill, at least in practical terms; the strength of fused filament objects isn't particularly high or consistent.

I'd love to see some well-done mechanical tests...

[Gary Schwartz]

On Fri, Sep 20, 2013 at 11:51 AM, Ed Nisley <ed.n...@gmail.com> wrote:

The gotcha is that there's no such thing as "circular" in an STL file, because (by definition) STL files consist entirely of triangles. As a result, all circular features are really polygons, each with a vertex count determined by how "circular" you want it to appear, and the geometry of those angular joints still works against continuous infill. Eventually, we'll be using both a file format that encodes true circular features and firmware that supports true circular arcs, but that seems rather far in the future.

​The circle approximations that Simplify 3D Creator generates are excellent, and circular enough for my work​.

 

 "... would you prefer that the slicer automatically meddle with your model's wall dimensions, thread width, or geometry ..."

 

Nope.  But I would like the software to determine the thread width required to satisfy my ID & OD.

 

"If you tell the slicer to retract the filament only when the nozzle crosses a perimeter, then the only "hammering" comes from XY acceleration. That's what the printer does for a living: start and stop and change direction on command. There's no additional mechanical wear due to short infill segments, so you won't improve anything by eliminating them; if the acceleration doesn't exceed the ability of the motors to move the masses, it's all good."

 

I don't buy "all good".   I also have a problem when the acceleration shakes loose a tall print in its fifth hour of printing.

 

"If there's room for ordinary infill, then I think you get better results by specifying a high percentage of infill, rather than trying for solid parallel infill, because ordinary infill automatically handles all wall sizes. It's not clear that a "solid plastic" cylinder (or shell) has better mechanical properties than, say, 75% hexagonal infill, at least in practical terms; the strength of fused filament objects isn't particularly high or consistent."

 

This is an area that would benefit from study.  I'm not sold on the superiority of hexagonal infill in round bodies.  In a large double-wall cylinder, there will be areas where long threads are parallel to a tangent, and other areas where the nice "X"s occur.  Seems like a variation in strength is a possibility.

 

Another concern is situations where I want low % infill in part of the model.

 

 

Maybe we should just go back to whatever we were doing/thinking two days ago.

[Ed Nisley]

> software to determine the thread width

Come to think of it, Slic3r now has a Concentric Infill options for the top and bottom external layers, as well as all internal layers. I don't know how (or whether) Concentric Infill interacts with Slic3r's automatically computed thread width.

I know Concentric infill doesn't work with preset thread widths, but maybe Slic3r already has something close to what you want.

> go back to whatever we were doing/thinking two days ago

Verily, there's nothing like a good new problem to take your mind off all your old problems! [grin]