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calinb

Useful Cura Trick (Sparse + Solid Infill)

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I print nylon and it's tough or impossible to make solid parts that don't shrink and warp too much, due to the high level of cooling shrinkage. Sparse infill reduces the problem greatly, but sometimes results in a part that's not as strong as I need (often the parts are too easily compressed under load).

Using Cura, I found that I could increase the density of fill all the way to 100% (solid) in areas that require more strength by designing-in thin hollow "cuts" internally in my solid model. In one model, I placed 0.002" parallel cuts 0.050" apart and Cura didn't "low pass" filter them out. :) It obediently attempted to print all the tight parallel walls of the cuts and the result was perfect 100% infill, but only where I needed it (and I used 20% infill elsewhere).

I don't know how to do this with any other slicer and I think it's pretty it's a pretty slick and useful capability of Cura!

 

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You could also try to increase the wall thickness from 0.8 to 1.2 and print 5% infill. This gave great results with PLA (Very light prints, but freakishly sturdy).

 

I'm already using 1.2 mm walls. The center of the part needed to be more resistant to the compression force imparted by a tightened (~40 in-lb) 1/4-36 screw in one area. The walls and sparse fill cross hatch did not provided enough support. Being nylon, the part wasn't even close to breaking, but the distortion, compression, and resulting protruding screw length on the backside of the part did not make for a satisfactory result when I attempted to place the non-reinforced part into service.

 

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Thanks for sharing. Any image of the printing to share? :mrgreen:

 

Sorry. The part is already installed and not readily available for a photo shoot. It's also a cover for a Winchester Model 70 bolt action rifle mag well and some people tend to spin into tizzies when the subject of printing gun stuff come up. I certainly wouldn't want to infringe on anyones "right to not be offended" or frighten anyone by posting a photo of the actual firearm with the installed part. ;)

This part is no longer commercially available and my options were to mill it from metal or CAD it up and print it. Though I'd prefer a metal part and will probably mill one someday (maybe if I ever upgrade to a CNC mill), the nylon part is perfectly functional.

Here are a couple of the solid model CAD screen shots. The slits are 0.002" wide. The slits are covered with a solid CAD extrusion (not shown) so the slits don't come through the top skin, though that might not be a problem, I did not wish to interfere with Cura's normal top skin tool path generation.

sml_gallery_908_1219_39885.jpg

Here is Cura's layer depiction from a mid-section slice from the area of concern. As you can see, Cura lays down walls instead of fill, because of the slits. This results in only this one area being 100% solid plastic.

sml_gallery_908_1219_70417.jpg

 

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Ah, right, that makes sense. If your good with some CAD software, you could make the model hollow, which should force cura not to fill those parts.

You could also try to play with the fix-horrible settings a bit, might be that it removes the weird results due to the slits.

 

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The results are exactly what I wanted! Yes--of course I could make the model hollow, but that's not what I wanted. My goal was to print a part with 20% fill, except around the one screw hole where I wanted to print solid plastic. By placing slits in the model, Cura generated a dense tool path around the screw and the result was 100% fill in just that one place. Printing the part with solid fill everywhere would result in too much shrinkage and warpage in a nylon print. (Nylon is worse than even ABS in shrinkage!)

It's perfect the way it is and I was just sharing a technique rather than posting a request for assistance.

 

Ah, right, that makes sense. If your good with some CAD software, you could make the model hollow, which should force cura not to fill those parts.

You could also try to play with the fix-horrible settings a bit, might be that it removes the weird results due to the slits.

 

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calinb, thanks for the trick, very usefull.

maybe a stupid question but what do you mean by 0.002" ... is it 0.002 inch ?

Is there a relation to the used wall thickness you use? do you need to increase or decrease the cut size and distance based on it? How thick did you make the sollid top/bottom layer?

 

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Yes--I did the CAD in inches, due to the legacy of the part's target design, My 0.002" slits are about 0.050 mm wide. I found that 0.001" slits were ignored by Cura, as I recall, and my strategy was to use the thinnest slits possible. At about 10 times thiner than a line width, the slits don't seem to introduce gaps in the fill. I think every slicer I've ever used produces inside cavity dimensions that print at least a bit too small anyway. (Screw holes are always too tight, for example, if not created oversize in the CAD).

I mostly used the Repetier-Host gcode editor and viewer to determine how the slit spacing affected the tool path. If I increased the spacing, the "fill" (to Cura it's actually walls) became less than 100% and I could increase the spacing distance above 0.050" (1.27mm) to obtain less than 100% fill, if desired.

My Cura Shell Thickness setting was 1.2 mm and my Nozzle size was 0.5 mm so Cura lays down two lines to make a wall. Thus it makes sense that Cura lays down two lines between every slit, which fills the gap very well between the slits.

The slit width is about 8% of the line width so one might expect the "fill" to be slightly sparse, but it's not the case in the actual part. It appears to be just as dense as 100% normal fill and, as I said, I think Cura and all the other slicers tend to generate gcode that results in over-extrusion around cavities. Normally I find this behavior to be really annoying, because I make mechanical parts that must hold reasonable dimensional tolerance, but in this case, I think I've leveraged this typically undesirable quality to do something useful.

In general, to obtain a solid region within a part, I recommend using the same size slits I used (0.050 mm) but space them apart by your Shell Thickness. You might have to tweak the spacing a little here or there to obtain a nice, even toolpath. The Cura and Rep-Host gcode viewers can help to evaluate the results.

 

calinb, thanks for the trick, very usefull.

maybe a stupid question but what do you mean by 0.002" ... is it 0.002 inch ?

Is there a relation to the used wall thickness you use? do you need to increase or decrease the cut size and distance based on it? How thick did you make the sollid top/bottom layer?

 

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Sorry. The part is already installed and not readily available for a photo shoot. It's also a cover for a Winchester Model 70 bolt action rifle mag well and some people tend to spin into tizzies[...]

sml_gallery_908_1219_70417.jpg

Ha -- "trigger warning"?

That's a nice trick. That said, it's not the shape I would have expected; it is a pre-1964?

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Ha -- "trigger warning"?

That's a nice trick.  That said, it's not the shape I would have expected; it is a pre-1964?

Sadly, no. It is for a recent model that has a cheap plastic stock and a blind magazine. It enables the use of one of the better stocks that are inlet for two-piece bottom metal, like the Hogue overmolded stock made for FNH or the Coyote Outback laminated wood stock. The blind magazine box is retained along with the original trigger guard. The printed part covers the hole where the two piece bottom metal door and frame usually rests.

I have a 243 WSSM and the stiff Hogue works much better with a sling than the original stock. It's actually the most accurate rifle I own. The rifle shoots  under 1/2 MOA out to 400 yards+ (the longest range I've tested) with my best handloads. Who says chrome lined barrels can't shoot! ;)Winchester put chrome lined barrels on all the WSSMs, because they were worried they would be barrel burners. I remember Winchester graphs of their testing showing that a non-chrome lined WSSM barrel that lasted for 2000 rounds would last for 4000 rounds with chrome lining. Of course they didn't specify what constituted a worn out barrel.

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Ha -- "trigger warning"?

That's a nice trick.  That said, it's not the shape I would have expected; it is a pre-1964?

Sadly, no. It is for a recent model that has a cheap plastic stock and a blind magazine. It enables the use of one of the better stocks that are inlet for two-piece bottom metal, like the Hogue overmolded stock made for FNH or the Coyote Outback laminated wood stock. The blind magazine box is retained along with the original trigger guard. The printed part covers the hole where the two piece bottom metal door and frame usually rests.

I have a 243 WSSM and the stiff Hogue works much better with a sling than the original stock. It's actually the most accurate rifle I own. The rifle shoots  under 1/2 MOA out to 400 yards+ (the longest range I've tested) with my best handloads. Who says chrome lined barrels can't shoot! ;)Winchester put chrome lined barrels on all the WSSMs, because they were worried they would be barrel burners. I remember Winchester graphs of their testing showing that a non-chrome lined WSSM barrel that lasted for 2000 rounds would last for 4000 rounds with chrome lining. Of course they didn't specify what constituted a worn out barrel.

I've since used this trick on another arm--a slingshot!

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