I’ve been doing a little research on 3D printing for an automotive application. It appears that the ~225C melting point of ABS should be high enough to deal with the environment it will be in, but I’m not sure what kind of burst strength I can achieve.
Could I conceivably make a box out of ABS plastic that would be 100% air tight at 150C / 300F holding pressure like a tire at say 15 psi? 20 psi? 25 psi? 30 psi? Would this be a matter of sufficient bracing / material thickness in the design or will be I butting up against a fundamental limitation of the material?
TIA
-Dave
I think you will find that most burst pressure for automotive is far above 30 psi. Check burst specs for automotive tubing. Some of it is well over 1000.
I don’t need 1000psi of burst pressure or I would have asked for it as a specification. 30psi of burst pressure would be double maximum working pressure.
thanks,
-Dave
Dave,
I’m basically speculating here. No real experience with 3D printing.
Depending on the volume of the container, I’d expect a higher burst spec—more like 4x working pressure. If it’s something that will be repeatedly filled and emptied, you’ll have to do derating to handle fatigue strength.
I’m no 3D printing expert, but I’d expect an airtight box to be pretty difficult just because of the inherent filament nature and “porosity” of the construction. If it’s for mass production, I’d expect some serious liability-type requirements that would ensure adequate durability. I’d also expect 3D printing to be much more expensive and time-consuming than some other method, just as injection molding or blow molding.
If you’re just talking about a small production run, I’d guess that it’s possible, but I’d expect it to be massively overbuilt to prevent leaks and to last a long time. You might be able to use a flexible bladder of some sort for airtightness, and just use the 3D part to support the bladder.
I’d expect the mechanical attachments and especially the pressure fittings (connections) to be a problem. The “filament nature” will also cause “stress risers” that will kill your fatigue strength. If you imagine a hole in a flat plate, the edges of the hole can have 3 times the stress of the bulk part of the plate. If you then imagine an elliptical hole, the multiplier can be more than 3. If you imagine a very long, very skinny elliptical hole, say formed by two adjacent 3D printing “filaments”, the “hole” appears to be a crack, and the stress multiplier can be (as a first approximation), something like the length-to-width ratio of the crack—so 100 times the “flat plate” bulk stress is possible, and even likely, and designing around that is going to be tough. You’d have to be sure enough of your printing process to be certain that those long, skinny types of pass-to-pass gaps don’t exist, or are just completely overwhelmed by criss-crossing filament passes.
I’m going to go out on a limb and say that probably no one will accept a purely analytical design of something like this. You’re going to have to prove it empirically, by building them and testing them in a representative manner (and probably in an “way overkill” manner as well, just to be able to say you’ve done it). You’d probably also have to do some process testing to figure out how to minimize the pass-to-pass gaps and ensure a good solid part. I’m assuming this would involve measuring and maintaining the temperatures of the part as well as the “hot glue gun” like print head (Is that what it’s called?).
Matthew
This is for me so spec can go to hell if it works. 
I’m sure that there will be quite a bit of trial and error involved.
D
I would be careful of using 3d printed parts in service. Although my the abs Melts around 220c,the plastic will begin to soften much sooner than that, and has a service temperature of around 75c,above that the abs will begin to lose strength rapidly. Try to find a tensile strength vs temp plot for abs and it should give you a good idea.
Additionally the printed parts fatigue and Crack very easily between layers due to weak fusion between layers and high stresses from thermal expansion and contraction.
That being said, if you design your part correctly and use a surface smoothing solution (acetone vapor for example) you should be able to significantly increase the strength and life of the part. Build with margin, wear safety glasses, and don't modify mission critical components when driving on the road and you should be fine. You could also use the Shapiro, or pla burnout process and cast an aluminum part for additional reliability
Depending on the size of the thing… would it be feasable to print it (strong enough) and then seal the inside to make it airtight? Maybe paint something like epoxy on the interior to seal any leaks in the filament walls plus adding the strength of lamination? (or fiberglass?)
I like to believe there’s more than one way to solve a problem.
Depending on the size and shape, could you modify a piece of that automotive tubing that was mentioned, to hold/do whatever this project is?
I would like to echo many of the earlier comments. Pressure vessels are nothing to joke about. Sure low pressure (5 psi) is not to bad, but once you get into higher pressures the forces get large quick. Larger vessels can have tremendous forces acting on them (that’s why you don’t see huge high pressure tanks, at least not light ones). Just do some quick math checks before you build anything large, and walk up very slowly to your test conditions. Put a LARGE safety factor into everything. Once you solve the leakage problem you basically have a small bomb waiting to fail.
Not trying to be a downer, just hate to see anyone get hurt. 
,
Keep making.
Thanks,
Bradman