3D Printing Layer adhesion, strength, and other questions, FDM vs. SLA

[neil_w]

Disclaimer: if I sound like I know what I'm talking about, it's an illusion. I don't own a printer, others will be doing the printing for me.

So I'm looking to print the dark orange part labeled "the slide" in this picture:

This is a slide mechanism for an Estes Mini A Heli. The orange slide is glued to the bottom tube. At ejection the top tube is propelled upwards until the stops hit the top part of the slide. Simple. Also note that this is a 13mm ejection charge, and the top of the rocket is vented, so the mechanism will only see a part of the force of the charge.

I have a friend with an Elegoo SLA printer who is willing to print for me in resin, or else I can get it printed from a number of sources using FDM (probably PETG).

1) I'm trying to figure out if I need to worry about layer adhesion here. The most obvious way to print the slide would be exactly in the orientation in the picture, which means the layers would be horizontal, and the force of the top tube hitting the stop would be absorbed primary by layer adhesion. Given the relatively small forces involved here, is that likely to be a problem? Those verticals are about 3mm wide, 1.15mm thick. Do I even need to worry about that here? My sense would be that resin would have better layer adhesion, but maybe PETG is just fine as well. I guess it's ABS that I've heard is most prone to layer adhesion problems.

2) Is 1.15mm sufficiently thick for reasonable structural integrity? Again, any significant different between resin and PETG? From my reading it seems resin might be more brittle, but again not sure if it is likely to matter here. I could make them slightly thicker if need be, but only a little. They should mainly be subject to longitudinal forces, since an internal coupler will keep the two body tubes in line.

3) Anything else here that would drive you to choose resin vs. PETG? Does this look like it is reasonably printable?

BTW, I'm hoping to print the little green "stops" as well, but those seem non-critical since they're completely affixed to the body tube and should be very strong.

Thanks for any advice.

 

[Michael L]

Link to Resin vs SLA strength tests

Some resins do pretty well but SLA seems to be the stronger of the two depending on which filament you choose

 

[neil_w]

Thanks, good article. It confirmed for me that resin prints are isotropic, which I sort of hoped for but wasn't sure. Given that I'd be comparing against the weakest direction of an FDM print, maybe it wins, I don't know. Although I can't tell if the brittleness would make a difference for me.

I think I'm going to adjust the dimensions a bit; it occurs to me that I might have made the slide pieces a bit smaller (and hence more delicate than I needed to.

 

[HHaase]

Truth be told it's not a great fit for FDM printing. I think that an FDM printer will struggle with strength and dimensional accuracy to print the 'slide' component. With the cross section available, no matter the material, you're going to have issues with the layers holding when the charge hits. To make it easier to print you'd need some kind of structure to close the 'windows' but easy to break out once printed.

Maybe with a resin printer, tilt it 45 degrees to get the layer orientation oriented for less chance of failure, but you'd need to use an engineering resin instead of the standard material to get the strength you need. The materials are out there to make this work but you'll have to make sure to pick the correct one. There's a lot of them out there branded as 'tough' resins. This is what you need to look for.

-Hans

 

[neil_w]

Truth be told it's not a great fit for FDM printing. I think that an FDM printer will struggle with strength and dimensional accuracy to print the 'slide' component. With the cross section available, no matter the material, you're going to have issues with the layers holding when the charge hits. To make it easier to print you'd need some kind of structure to close the 'windows' but easy to break out once printed.

I presume there would be supports in there for printing... but yeah, they're pretty fine parts, certainly finer than anything I've printed before (again "I've printed" meaning "farmed out to a service").

Maybe with a resin printer, tilt it 45 degrees to get the layer orientation oriented for less chance of failure, but you'd need to use an engineering resin instead of the standard material to get the strength you need. The materials are out there to make this work but you'll have to make sure to pick the correct one. There's a lot of them out there branded as 'tough' resins. This is what you need to look for.

Unfortunately, since my friend doesn't print mechanical components, he doesn't have tough resins, and I can't really ask him to buy it just for this little thing.

I've been trying to figure out if there's a reasonable way to construct this with paper centering rings and strips of wood... Maybe I should try that first. Small strips of 1/16" basswood has a ton of tensile strength; the hard part would be securely gluing the strips to the rings. I guess I have no excuse not to give that a try.

But I'll probably get a print as well, just to see if it could work.

Hmm.

 

[HHaase]

rather than building up pieces of wood and cardboard, can you take a body tube of the correct size and just cut the windows out? Then use a layup epoxy or soak it in CA to give it a bit more strength.

-Hans

 

[KilroySmith]

Do you have control over the shape of the dark-green stops?

If so, consider making them narrower so the verticals can be wider. The stops will be remarkably rugged, as they're strictly in compression, so they don't need to be very large. If you print them, put some holes in them (either printed or drilled) to provide some "glue wells" to reinforce the glue line against shear.
Also consider making them circular, fitting into a semi-circular recess in the slider. This will reduce the stress riser that occurs right at the inside corner of the slider.
You could also consider putting a "bumper" on the inside of the slider; just a thin piece of plastic that S-curves about 1-2 mm away from the upper part of the slider. The purpose being to be a shock absorber - when the stop hits, it'll hit the bumper first, which will eventually end up flattened against the flat part of the slider but which will significantly reduce the shock loading.

If you don't have control over the stops, print it and give it a go. Worst case is it fails and you have to try something else; best case is that everything you're building is very small and very lightweight so it works great. You'll probably need to print several times, changing the ID slightly, to get the fit that you want. Especially with FDM, IDs tend to be small and ODs tend to be large by something like 0.1-0.2 mm. I don't have any experience with SLA, so can't comment on it.

From a practical perspective, adding supports for the "window" is pretty easy; just means you need to spend 5 minutes with a file to smooth things out once they get printed. The bridging distance may be short enough that you don't need the supports; it'll really depend on the printer and material. If you try without supports, print it upside down from the orientation shown so the "bridged" part is what the stop rests on during launch. Actually, if you print the stops, making them fully circular means that both sides of the slider windows are semi-circular, which means that you can print it pretty easily without supports.

 

[neil_w]

Thank you for the input. I thought of making the stops smaller, but ultimately decided that at 4mm wide the verticals were wide enough to work... and I was too lazy to make the change.

Fellow forum member @Michael L was kind enough to fire off a couple of prints for me in PC blend, and they seem to have come out quite nice. I'll find out about fit later; I oversized the ID slightly in the design file but if I need to sand out a bit more than so be it.

Putting some sort of soft bumper in there is something I also thought about. I didn't include any such thing in the print, but I'll try to find an appropriate piece of material to affix to the bumpers.

Ultimately, I expect the forces involved to be fairly small, given that there are significant vents at the top of the rocket, and it's only a 13mm motor. But we'll see.

I'll report on all my experiences over in the Rocket Boosted Gliders forum.

 

[Michael L]

I PM'd you but - PETG printed ok on one but not right and the second one failed. I didn't use supports. So, I corrected that and will print 2 PETG and 2 PC Blend. I really like the PC Blend from Prusa. There was some slight stringing without supports at the top. I feel like it's probably weak there so I'm reprinting all of it. The parts are small and they print quickly. Initially I used a brim but they print fine without it so I left it off.

 

[vcp]

Regular SLA resin is often mixed with ~20% flexible resin to reduce brittleness.