3D Printing Printing Tips for Fins and Other Structural Components

OverTheTop said:

https://formlabs.com/materials/engineering/

It was printed in the "rigid" resin. They are not very forthcoming with details of what is in the resin, but there is a datasheet that has the material properties on that web page.

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It's hard to directly compare properties. (It appears that different methods are used for different types of materials.) The tensile strength of gray resin appears to be better than "plywood" (although I'm not sure what kind).
https://www.matweb.com/search/datasheet_print.aspx?matguid=bd6620450973496ea2578c283e9fb807
The high end for "plywood" is 35MPa compared to 61MPa.

"Aircraft plywood" appears to be "MIL-P-6070 B". Unfortunately it's a scanned document so hard to search, but the only thing I saw on quick perusal is a shear strength of 250PSI. (No shear strength is listed on the FormLabs data sheet.)
file:///Users/john/Downloads/MIL-P-6070.PDF

This is great stuff! Now I'm getting excited about SLA printing. Only big reservation is build volume.

JohnCoker said:

This is great stuff! Now I'm getting excited about SLA printing. Only big reservation is build volume.

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What? You aren't in the mood to park $10k for the Form 3L?

when printing fails, 20 hours out of 24, the rest of the 4 hours was a rats nest

BDB said:

What? You aren't in the mood to park $10k for the Form 3L?

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That would be more palatable if the technology wasn't changing so fast. It's a lot of money though, and I imagine I'd be eyeing the next incremental improvement a year or two later.

I've been thinking about doing some destructive testing on a 3D printed fin can to compare it against more traditional techniques. But I've struggled coming up with a protocol that would not require days worth of printing.

Based on the unitary 29mm fin cans I've printed, they certainly 'seem' stronger than a typical surface mounted fin on a cardboard or phenolic tube. I haven't tried yet but I suspect a layer of even 2 oz fiberglass tip to tip would create a structure that could withstand Mach speeds for a short time. (If the leading edges are protected from heat.)

On the down side, one of the guys in our group has a brother who works for a manufacturer that bought a mid 6 figure 3D sintered metal printer. After a year it was still not up and running correctly. Even my Prusa has had some issues that were a bear to resolve, including filament stuck in the PTFE tube. So as has been mentioned, still definitely a tinkerers platform. But clearly one that will gain greater acceptance and time and materials move on.


Tony

I stumbled across this video the other day:



It is very rare to find any actual engineering data on as-printed materials. All printers are a little different, but this should be a useful resource for estimating material strengths.

TurbulentSphere said:

I stumbled across this video the other day:

It is very rare to find any actual engineering data on as-printed materials. All printers are a little different, but this should be a useful resource for estimating material strengths.

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In that video, his comments, from 15:00 - 15:15, seem to sum up the situation with 3D printing.

Dave F.

Ez2cDave said:

In that video, his comments, from 15:00 - 15:15, seem to sum up the situation with 3D printing.

Dave F.

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The same could be said for other fabrication methods if done incorrectly. I've seen epoxy melt in a car before; that doesn't mean all epoxy is unsuitable for rockets, just that you should use something better than 5 minute hardware store epoxy. If you are worried about the strength of your prints, tension testing isn't that hard; some rope and weights can work for basic characterization. The main point I took away from this is the layer adhesion strength, which I had not found hard numbers on before. Most of the parts in my rockets are well under 1 ksi, so it is not difficult to stay within the material limits of even poor printing.

TurbulentSphere said:

The same could be said for other fabrication methods if done incorrectly. I've seen epoxy melt in a car before; that doesn't mean all epoxy is unsuitable for rockets, just that you should use something better than 5 minute hardware store epoxy. If you are worried about the strength of your prints, tension testing isn't that hard; some rope and weights can work for basic characterization. The main point I took away from this is the layer adhesion strength, which I had not found hard numbers on before. Most of the parts in my rockets are well under 1 ksi, so it is not difficult to stay within the material limits of even poor printing.

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OK . . . How does the "Average Joe" learn to do this "correctly", on an inexpensive 3D printer, for highly-stressed fins on maximum-performance HPR rockets ?

What are the "correct" methods, materials, techniques, and designs for 3-D printing, in the "non-commercial-grade" world of typical home 3-D printers ?

Who teaches him ?

How can one verify, at home, that they have "done it correctly" ?

Pulling apart little "test pieces" is not the same thing as breaking Mach with 3-D printed fins, since there are multiple variables in play ( angle of attack, drag, flexion, turbulence, flutter, shear forces, aerodynamic heating, etc, etc, etc. )

Could you keep 3D printed fins intact on the rocket in the video below ? ( Watch at 29 seconds into the video )

Those are "real world" questions . . . Plywood doesn't melt.

Dave F.

Ez2cDave said:

OK . . . How does the "Average Joe" learn to do this "correctly", on an inexpensive 3D printer, for highly-stressed fins on maximum-performance HPR rockets ?

What are the "correct" methods, materials, techniques, and designs for 3-D printing, in the "non-commercial-grade" world of typical home 3-D printers ?

Who teaches him ?

How can one verify, at home, that they have "done it correctly" ?

Pulling apart little "test pieces" is not the same thing as breaking Mach with 3-D printed fins, since there are multiple variables in play ( angle of attack, drag, flexion, turbulence, flutter, shear forces, aerodynamic heating, etc, etc, etc. )

Could you keep 3D printed fins intact on the rocket in the video below ? ( Watch at 29 seconds into the video )

Those are "real world" questions . . . Plywood doesn't melt.

Dave F.

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I didn't say anything about supersonic flights. In the same way you wouldn't use unreinforced balsa on a mach buster, you probably shouldn't use printed parts for that (although people have); every material has it's place. As for who teaches people, you can say the same for any other material. How does the average Joe learn how to make structurally sound carbon fiber composites? They learn for experience, from asking people who have used it before, by doing testing, joining a club, by reading threads on this forum, etc. That is what this thread was originally made for. Not for using the edge cases to say a construction method shouldn't be used, but to try to collect and record the information and data to help people build 3D printed rockets safely.

TurbulentSphere said:

I didn't say anything about supersonic flights. In the same way you wouldn't use unreinforced balsa on a mach buster, you probably shouldn't use printed parts for that (although people have); every material has it's place. As for who teaches people, you can say the same for any other material. How does the average Joe learn how to make structurally sound carbon fiber composites? They learn for experience, from asking people who have used it before, by doing testing, joining a club, by reading threads on this forum, etc. That is what this thread was originally made for. Not for using the edge cases to say a construction method shouldn't be used, but to try to collect and record the information and data to help people build 3-D printed rockets safely.

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So, where does the suitability of 3-D printed fins end ? ( Those printed by Newbies, on entry-level 3D printers, without others to help them, because the technology is so new ).

The threads on this forum argue that "those with the knowledge", who "do it properly", have no problems doing anything. Of course, many of those people doing 3-D printing, professionally, on six-figure, commercial printers.

The rest are people who saved up a few hundred bucks, or a little more, and plunked down their money, waiting until an entry-level unit is delivered to their door. Then, they unpack, assemble, and download files to print . . . Presto "instant 3-D printing Rocketeers" ( not quite, in reality ), with ZERO experience !

There is a huge learning curve on the route to rocketry-application 3-D printing, especially fins on HPR rockets . . . It's not "plug & play" and there are no How-To Tutorial "Cookbooks" out there to teach the needed skills !

Dave F.

Ez2cDave said:

So, where does the suitability of 3-D printed fins end ? ( Those printed by Newbies, on entry-level 3D printers, without others to help them, because the technology is so new ).

The threads on this forum argue that "those with the knowledge", who "do it properly", have no problems doing anything. Of course, many of those people doing it for a living, on six-figure, commercial printers.

The rest are people who saved up a few hundred bucks, or a little more, and plunked down their money, waiting until an entry-level unit is delivered to their door. Then, they unpack, assemble, and download files to print . . . Presto "instant 3-D printing Rocketeers" ( not quite, in reality ), with ZERO experience !

There is a huge learning curve on the route to rocketry-application 3-D printing, especially fins on HPR rockets . . . It's not "plug & play" and there are no How-To Tutorial "Cookbooks" out there to teach the needed skills !

Dave F.

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Again, this thread is trying to be a "cookbook" for rocket specific printing information. The limits of fins, as with all materials, depend on the size, thickness, material, flight regime, etc. With the data I have, I would say mid subsonic (up to mach ~0.6) is easily doable. I have gone up to ~450 mph with a root-to-tip span of about 5 inches printed in PETG. The fin thickness was 0.25" with 4 solid perimeters that took up half of the thickness, and a ~1/8" thick core at 50% infill. Each fin had fillet-like flange on the base that allowed them to be bolted to the motor mount with 8 #6-32 screws each.
My printer is a Prusa I3 Mk2S (~$700), but I know some people with cheaper printers (Ender 3) that get similar quality. I have not met any people using commercial printers for hobby rockets (I wish I did, Ultem is amazing and I would love to have access to it...).