3D printing a QRD N53 diffuser

[Curvature]

in-room measurements show that velocity absorbers work far deeper down than they have any right to

I have been meaning to listen to that interview for a while.

I'd like to clarify a little.

They show more absorption in measurements than the Sabine equation predicts. The difference is largely driven by edge diffraction, but I vaguely remember (from an AES lecture I attended) Sauro mentioning a few other contributing effects.

In-room measurements are fairly consistent from study to study in what I've seen. Sauro's work features better controls for measurements in his facility, which lead to better resolution, particularly in low frequencies because of its size.

 

[-Matt-]

Have you considered printing a fairly thin shell with little or no infill and then turning it upsidown and pouring something else in? Eg, expanding foam, silicone, resin, etc.

 

[kemmler3D]

Have you considered printing a fairly thin shell with little or no infill and then turning it upsidown and pouring something else in? Eg, expanding foam, silicone, resin, etc.

It's doable and not a bad idea, but doesn't save an overwhelming amount of time or material, because most of the plastic goes to printing the outside shell anyway. It would also make assembly more difficult, since my method of attaching the pieces is to glue & clamp them together, and so a really thin and weak piece, or one filled with foam, could crack in the process. Not a total showstopper but it wouldn't make my life much easier.

Also, expanding foam or resin or something would probably not be especially cost-competitive with PLA filament. I was able to get 30kg of decent quality filament for $300 - saving 10kg of filament and replacing it with something else probably won't radically change the budget for this.

 

[Timcognito]

It's doable and not a bad idea, but doesn't save an overwhelming amount of time or material, because most of the plastic goes to printing the outside shell anyway. It would also make assembly more difficult, since my method of attaching the pieces is to glue & clamp them together, and so a really thin and weak piece, or one filled with foam, could crack in the process. Not a total showstopper but it wouldn't make my life much easier.

Also, expanding foam or resin or something would probably not be especially cost-competitive with PLA filament. I was able to get 30kg of decent quality filament for $300 - saving 10kg of filament and replacing it with something else probably won't radically change the budget for this.

I think it would by far the most inexpensive way. They fill whole home exterior walls with that stuff. Print a mold and make many panels from it.
"Their density is generally 2.0 lb/ft3 (32.0 kilograms per cubic meter [kg/m3]).
Low-density, open-cell polyurethane foams (0.5 lb/ft3 [8 kg/m3]) have an R-value of about R-3.6, which doesn’t change over time. These foams are similar to conventional polyurethane foams, but are more flexible. Some low-density varieties use carbon dioxide (CO2) as the foaming agent."

https://www.amazon.com/gp/aw/d/B0CKLYK3SV/

 

[kemmler3D]

I think it would by far the most inexpensive way. They fill whole home exterior walls with that stuff. Print a mold and make many panels from it.
"Their density is generally 2.0 lb/ft3 (32.0 kilograms per cubic meter [kg/m3]).
Low-density, open-cell polyurethane foams (0.5 lb/ft3 [8 kg/m3]) have an R-value of about R-3.6, which doesn’t change over time. These foams are similar to conventional polyurethane foams, but are more flexible. Some low-density varieties use carbon dioxide (CO2) as the foaming agent."

https://www.amazon.com/gp/aw/d/B0CKLYK3SV/

Even if the foam was free I probably wouldn't do it that way, though. It would make assembly more difficult and messy, and would make it harder to have handles / hardware attachment points on the back, which is necessary to hang the thing up. I could add them as separate "lids" to be attached after filling with foam, but then I'm looking at 72 huge print jobs instead of 36 just to save some weight.

Even using light foam and super-thin walls, it's still heavy enough that using cleats for hanging is sensible, which means the internal structure of the print needs to be self-supporting and grip screws well. With 2 walls, I'm already pushing it and will need good tolerances for the screw holes to avoid breakage. With 1 wall, the screws would have a very good likelihood of splitting the plastic instead of setting into it properly.

Doing it with single walls saves roughly 30% of the filament, at the cost of making the part much weaker and prone to print failure and breakage during assembly. Each failed or broken print would cost me a day in the project, which is more annoying than messing up the filament budget.

Filling the print with something sturdier/heavier than spray foam would probably shoot the weight well over 50 kilos or something, so that creates headaches of its own.

You might argue that filling the parts with foam is a good idea regardless of how many walls are used, because it could help kill resonances. This is something I'll think about, but I'm not sure if it's actually even possible to print these parts without creating closed cells within the part either with infill (optional but highly desirable) or bridges (not optional, can't print without them, sometimes seals off wells during the print.)

Bottom line I don't there's any truly appealing way to build something like this, I am using the method that I consider least bad.

 

[Timcognito]

Even if the foam was free I probably wouldn't do it that way, though. It would make assembly more difficult and messy, and would make it harder to have handles / hardware attachment points on the back, which is necessary to hang the thing up. I could add them as separate "lids" to be attached after filling with foam, but then I'm looking at 72 huge print jobs instead of 36 just to save some weight.

Even using light foam and super-thin walls, it's still heavy enough that using cleats for hanging is sensible, which means the structure of the print needs to be self-supporting and grip screws well. A flimsy layer of PLA and spray foam doesn't give me huge confidence in that.

Doing it with single walls saves roughly 30% of the filament, at the cost of making the part much weaker and prone to print failure and breakage during assembly. Each failed or broken print would cost me a day in the project, which is more annoying than messing up the filament budget.

Filling the print with something sturdier/heavier than spray foam would probably shoot the weight well over 50 kilos or something, so that creates headaches of its own.

Bottom line I don't there's any truly appealing way to build something like this, I am using the method that I consider least bad.

Respectfully disagree about weight and attachment. Foam is mostly air and embedding an anchor in it before, during or after it sets would be trivial. BUT, I'm not doing it, you are.
Good Luck, will follow for updates.

 

[kemmler3D]

Respectfully disagree about weight and attachment. Foam is mostly air and embedding an anchor in it before, during or after it sets would be trivial. BUT, I'm not doing it, you are.
Good Luck, will follow for updates.

Well, even with a single walled structure, and assuming the foam weighs zero, the whole thing would still weigh around 20kg, so no matter how you do it, hanging it up is not trivial. I have already trimmed as much weight from the print as I think is safe. Not having worked with spray foam before, I don't have a feel for how much I can expect from it in terms of sturdiness, but I know what I can get away with when it comes to 3D printed stuff.

I have definitely seen people build diffusers by doing thin walls with wood or cut pieces of plastic and fill in the back with foam, but for me in this situation, it would be at least as much effort for a questionable benefit. I would have to do a lot of tweaking to ensure the foam would actually fill all of the cavities, too, the slicer isn't set up for making the whole thing "open cell" by default.

I don't think it's a bad idea in general, just not that practical for me in this project.

 

[Timcognito]

Well, even with a single walled structure, and assuming the foam weighs zero, the whole thing would still weigh around 20kg, so no matter how you do it, hanging it up is not trivial. I have already trimmed as much weight from the print as I think is safe. Not having worked with spray foam before, I don't have a feel for how much I can expect from it in terms of sturdiness, but I know what I can get away with when it comes to 3D printed stuff.

I have definitely seen people build diffusers by doing thin walls with wood or cut pieces of plastic and fill in the back with foam, but for me in this situation, it would be at least as much effort for a questionable benefit. I would have to do a lot of tweaking to ensure the foam would actually fill all of the cavities, too, the slicer isn't set up for making the whole thing "open cell" by default.

I don't think it's a bad idea in general, just not that practical for me in this project.

you had to make calculate it didn't you

In Amazon link I sent there are 12 cans with 27.1 oz ea or 325oz/16oz/lb >20 lb (assumes 1 oz liq = 1 oz wt). Makes 240 sq-ft at 1 inch thick. If the average thickness of the panel is 3 inches > 80 sq-ft of panels or 1 - 2.5 ft X 2.5 ft panel per can weighing 1.7lbs each. The printed part is only a mold used again and again.

 

[kemmler3D]

Update: Finally a bit of progress. I fought a post-wedding hangover to test the new filament, dial in the alignment pegs and do a test print at mini-scale to validate the design. Everything seems to be working as expected, so I am hoping to kick off the first full-size print tonight.

I am still getting some persistent corner bulge, but it will be proportionally less bad on the full-size print. I've tried flow calibration and it doesn't really help, I think it's just because the printer goes very fast with higher pressure on the filament.

 

[fluid]

I am still getting some persistent corner bulge, but it will be proportionally less bad on the full-size print. I've tried flow calibration and it doesn't really help, I think it's just because the printer goes very fast with higher pressure on the filament.

Did you tune pressure advance too?

 

[kemmler3D]

Did you tune pressure advance too?

I need to look into that, I think that setting is an automated thing on this printer, but it does come up as a possible cause.

 

[fluid]

Orcaslicer has built in calibration routines for it, then you store the values in your filament profile. You literally pick which line has the sharpest corner in the test

 

[kemmler3D]

Orcaslicer has built in calibration routines for it, then you store the values in your filament profile. You literally pick which line has the sharpest corner in the test

I did that and Bambu Slicer only did an adjustment for K-value... I'll have to check out Orca.

 

[DoomSlayer]

And it seems to do some kind of resonance testing when it does its initial self test, vibrating the motors at a series of increasing frequencies. Really interesting.

I believe that is for input shaping. It's one thing that makes the newest generation of 3D printers so fast.

 

[kemmler3D]

Update:

Printed the first full-size piece. One down, 35 to go. This print was around 800g of filament, ~16 hours. Next one is 1050g, 21h. (more than a normal roll of filament!)

The surface quality isn't 100% perfect but it's more than acceptable at this stage. Like 99.5% perfect. The last print on this scale I did (on my old printer) looked like absolute garbage in comparison, I had to spend a lot of time chipping "boogers" off the surface of that, this one just has a few tiny ripples here and there.

I got rid of almost all the corner bulge by switching filament presets... somehow this worked better than doing calibration. (shrug) Making minor tweaks to the print settings as I go, but at this point I'm planning to just run the printer as close to 24/7 as I can. I expect to be done with this sometime in late August.


Something I hadn't fully thought through is these don't stack on top of each other well or at all and I don't know where I'm going to keep them before assembly...

 

[MCH]

Update:

Printed the first full-size piece. One down, 35 to go. The surface quality isn't 100% perfect but it's more than acceptable at this stage. I got rid of almost all the corner bulge by switching filament presets... somehow this worked better than doing calibration. (shrug) Making minor tweaks to the print settings as I go, but at this point I'm planning to just run the printer as close to 24/7 as I can. I expect to be done with this sometime in late August.


Something I hadn't fully thought through is these don't stack on top of each other well or at all and I don't know where I'm going to keep them before assembly...

View attachment 382235View attachment 382236View attachment 382237

Looking good!
Suggestion: I see you use small tabs at the base, I guess, to assemble the parts together. Have you thought of adding additional ones at a higher level (i.e. closer to the top of the little towers) to give the assembly more stability?

 

[kemmler3D]

Looking good!
Suggestion: I see you use small tabs at the base, I guess, to assemble the parts together. Have you thought of adding additional ones at a higher level (i.e. closer to the top of the little towers) to give the assembly more stability?

I did think about that, but the tabs are only meant to give the pieces a little resistance to shear, mainly they're to line the pieces up nicely during assembly. I plan to use a 2-part epoxy to glue them together, which IME results in a bond that's stronger than the part itself. (I tested the bond on a past large print by trying to snap the bond over my knee... the part snapped instead.)

For assembly I'm basically going to use long clamps and boards to glue together 6 pieces at a time, then glue all 6 rows together at once... should work?

You can't see it on this piece but I did also add attachment points for wall cleats and carved out some carry handles also, so once it's glued together the hard part should be over...

 

[MCH]

I did think about that, but the tabs are only meant to give the pieces a little resistance to shear, mainly they're to line the pieces up nicely during assembly. I plan to use a 2-part epoxy to glue them together, which IME results in a bond that's stronger than the part itself. (I tested the bond on a past large print by trying to snap the bond over my knee... the part snapped instead.)

For assembly I'm basically going to use long clamps and boards to glue together 6 pieces at a time, then glue all 6 rows together at once... should work?

You can't see it on this piece but I did also add attachment points for wall cleats and carved out some carry handles also, so once it's glued together the hard part should be over...

Ah ok, missed that part. Sounds like a good idea. My only comment to that is that given the parts are white (I forgot if you said you intend to paint them or not) make sure you chose an adhesive that doesn't yellow or it might look not so nice after some time.

 

[goryu]

very cool!

 

[kemmler3D]

Ah ok, missed that part. Sounds like a good idea. My only comment to that is that given the parts are white (I forgot if you said you intend to paint them or not) make sure you chose an adhesive that doesn't yellow or it might look not so nice after some time.

Good call on the adhesive... and I am on the fence about painting. The surface area is a few hundred square feet so it's not a trivial amount of paint, and there's no way I can paint this by hand... not with an amount of effort I consider reasonable, anyway. So it's spray or nothing, but I'm really not sure about getting the paint all the way down into the wells effectively without it having drips everywhere.

Can you really spray paint into a 25x25x250mm deep hole and have it come out good? I have my doubts.

My current idea is to paint the outside faces/perimeter and leave everything else white... might look OK.