watertight underwater container able to withstand high pressure

[maurosacchi 0]

I am trying to 3D print a diving canister used to store battery packs that power underwater torches, heating elements, etc. Traditionally these canisters are machined with a lathe using Delrin acetal homopolymer (Polyoxymethylene POM). They are often rated to 200 or even 300 meters (600-900 feet), which means 21 to 31 bars of atmospheric pressure (300-450 psi).

 

This is how a Delrin machined canister would look like:

 

 

And this is one of the 3D model to be used for 3D printing:

 

 

I know POM-C is available from FrontierFila and Uayella as filament for my S5 Ultimaker. Nevertheless I have several questions before commencing this work:
 

1. although POM-C is the preferred material when this item is machined, would it be the preferred solution for 3D printing?
2. once the material is selected, how do I calculate the necessary thickness to withstand the indicated pressure?
3. how do I secure the final result would be watertight?
4. what settings should I use in Cura to execute the printing?

 

It would be great if someone with more experience would guide me through this project! Thanks in advance!

 

[SandervG 1,521]

Good questions! Hopefully you'll find some help here. I'll also share this with our product experts if they have any input. 

[GregValiant 1,251]

These are just some random thoughts...

 

The typical printing materials are fond of water.  I'm guessing that would be a downside so if you can get something with a low affinity for water, that would be a prime consideration.  It may be why Delrin was picked over something like Nylon.  A prototype for initial pressure testing could be made from something cheaper(?)

 

Among other things, "Layer Adhesion" will play a part in water-tightness.  Maybe a coating over the print?

 

I would make the bottom of the vessel a dome.  The print will be weaker than any traditional solid part.  A domed bottom would enhance it's strength in at least that area.  Taking that further, I'd think about an ovoid as well.  Egg shapes bear up well to uniform external pressure.  The inside could still be cylindrical.

 

Put the battery in and fill it with dielectric oil?  I think any infilled volume would still end up near 100% infill.

 

 

[svandestaak 9]

Hi Maurosacchi. Personally, I would be looking at PP with glass/carbon fiber. Extremely strong and PP works well with liquids. Make sure when you print that the z seam alignment is at random. And I would also print it with 100% infill. Final version anyway.

 

If you do want to go for PP, make sure you use the adhesionsheet and/or pp-adhesion from Magigoo. Those are the only adhesion solutions which worked for me when I used PP with GF (30%): XSTRAND™ GF30-PP (Polypropylene + Glass Fiber) (crea3d.com)

 

Sounds like a very interesting project, SandervG forwarded this thread to me. :)

[maurosacchi 0]

On 1/20/2021 at 4:43 PM, GregValiant said:

These are just some random thoughts...

 

The typical printing materials are fond of water.  I'm guessing that would be a downside so if you can get something with a low affinity for water, that would be a prime consideration.  It may be why Delrin was picked over something like Nylon.  A prototype for initial pressure testing could be made from something cheaper(?)

 

Among other things, "Layer Adhesion" will play a part in water-tightness.  Maybe a coating over the print?

 

Thanks for the suggestions. What coating do you have in mind? Epoxy? Is there a specific product and if yes, what filament should I use with it to achieve the end result I am looking for?

 

On 1/20/2021 at 4:43 PM, GregValiant said:

I would make the bottom of the vessel a dome.  The print will be weaker than any traditional solid part.  A domed bottom would enhance it's strength in at least that area.  Taking that further, I'd think about an ovoid as well.  Egg shapes bear up well to uniform external pressure.  The inside could still be cylindrical.

 

Put the battery in and fill it with dielectric oil?  I think any infilled volume would still end up near 100% infill.

 

 

 

Unfortunately I cannot make the whole surface oval. I could make the bottom round though. It would not be possible to use it. This canister is placed on the waste belt of a diver, It would become too inconvenient with that shape.

[maurosacchi 0]

23 hours ago, svandestaak said:

Hi Maurosacchi. Personally, I would be looking at PP with glass/carbon fiber. Extremely strong and PP works well with liquids. Make sure when you print that the z seam alignment is at random. And I would also print it with 100% infill. Final version anyway.

 

If you do want to go for PP, make sure you use the adhesionsheet and/or pp-adhesion from Magigoo. Those are the only adhesion solutions which worked for me when I used PP with GF (30%): XSTRAND™ GF30-PP (Polypropylene + Glass Fiber) (crea3d.com)

 

Sounds like a very interesting project, SandervG forwarded this thread to me. 🙂

 

Thanks! Any specific extruder diameter to be used with this filament?

[GregValiant 1,251]

1 hour ago, maurosacchi said:

 

Thanks for the suggestions. What coating do you have in mind? Epoxy? Is there a specific product and if yes, what filament should I use with it to achieve the end result I am looking for?

 

The coating might be as simple as spray paint.  It is dependant on the material it has to bond to.  Whatever material you decide on will determine the coating used.  It would be best to consult with the manufacturer as to what coatings would be compatible with the product.

 

So the dive belt has a pre-existing carrier that requires a specific diameter to fit?  A domed end avoids the exterior corner and would be stronger.  You might need a cradle to hold them upright when you set them on a table.

[geert_2 557]

When making tiny filters for a vacuum pump, I had to print them very slowly and in very thin layers. Otherwise, if printed at 0.2 or 0.3mm layers, it had lots of tiny openings, through which tiny jets of water squirted out, similar to out of an injection needle. When printing at 0.06mm layers and 25mm/s in PLA, it was water-tight.

 

But if you are going to print your huge model at such a low speed and layer height, it is going to take forever.

 

So, I would make a *small* test model first, and see how that works under high pressure. Put tap water on it, or even via a high-pressure cleaner, and see how that works out (Use safety glasses and equipment!!!). If it is water-tight and can withstand high pressure from inside-out, which is the weakest, then it will also withstand high external pressure.

 

If you would use paint, try to find one that bonds very well chemically, so it does not delaminate.

 

The filters I printed: the bottom was printed with 70% infill if I remember well, to get the tiny holes for the filtering-effect, good enough for this purpose. Then both halves were glued together around the edges.

 

 

 

[maurosacchi 0]

On 1/25/2021 at 4:02 PM, geert_2 said:

When making tiny filters for a vacuum pump, I had to print them very slowly and in very thin layers. Otherwise, if printed at 0.2 or 0.3mm layers, it had lots of tiny openings, through which tiny jets of water squirted out, similar to out of an injection needle. When printing at 0.06mm layers and 25mm/s in PLA, it was water-tight.

 

But if you are going to print your huge model at such a low speed and layer height, it is going to take forever.

 

So, I would make a *small* test model first, and see how that works under high pressure. Put tap water on it, or even via a high-pressure cleaner, and see how that works out (Use safety glasses and equipment!!!). If it is water-tight and can withstand high pressure from inside-out, which is the weakest, then it will also withstand high external pressure.

 

If you would use paint, try to find one that bonds very well chemically, so it does not delaminate.

 

The filters I printed: the bottom was printed with 70% infill if I remember well, to get the tiny holes for the filtering-effect, good enough for this purpose. Then both halves were glued together around the edges.

 

 

 

Thanks! This is very insightful. I am quite new to 3D printing. If I understood correctly, first of all I need to use a very thin extruder, like 0.06mm. Then I need to 3D print with high density. Considering the size of the item is roughly 10 cm diameter x 30 cm height, how long would it take?

[GregValiant 1,251]

"...very thin extruder, like 0.06mm."  NO.  That would be the layer height after the initial layer.  You are thinking of the nozzle diameter.  A .4 is likely OK so long as it extrudes correctly.  Any under-extrusion and you could develop pinhole leaks.

 

The layer height would be a huge hit on time-to-print.  Likewise the print speed would be a big hit.  The amount of infill, wall thickness, top and bottom thickness all play a part in how long a print will take.  Cura is actually pretty good at time calculation.  Every printer is different though so the real print time will vary.  Slicing a model with different settings will give you a good indication of time though.  If Cura tells you a configuration will take 60 minutes and another configuration gives you 120 minutes, it's a pretty sure thing that the second one will take twice as long.

[geert_2 557]

Exactly: standard 0.4mm nozzle, but 0.06mm layer-height.

 

If you print a model in 5x thinner layers, and print 2x slower, it is going to take 10x longer to complete. And if you then need 10x thicker walls to withstand the pressure...

 

My models are very small, so that is still acceptable. But for big models like yours, it is going to take forever. In that case I would consider post-processing, such as chemical smoothing or painting/covering.

 

I recently did a post on chemical smoothing with dichloromethane here, with photos.

 

Or consider painting with a thick epoxy or other paint for plastics. Car manufacturers often have paints that work well on plastics.

 

Another option would be to print a mould, post-process it to get smooth walls, and then cast polyurethane, epoxy, or reinforced epoxies in it. Search on Youtube for mould making and casting, there are lots of good video tutorials. Then it is for sure water-tight, and you for sure have no issues with layer-bonding. The only thing is that you need to design the mould very carefully, so there are no undercuts, and apply huge amounts of release spray, so the cast does not glue to the mould. Also, use low-exotherm epoxies, otherwise the mould will melt. If I had to make such a model, I probably would consider this method, even though it is a bit messy. Or a combination of methods.

 

Anyway, test all these options thoroughly on a *small* test model first.

 

[jirodriguez72 18]

For watertight lid I would look at putting an o-ring groove into part of it. That way when you screw/clamp the two parts together it compresses the o-ring creating a water tight seal. I would shoot for a 200 series o-ring with approx. 20% compression.

 

Regarding material - my company was prototyping some water tight enclosures for electronics and we settled on Polycarbonate. In submersion tests we kept it underwater (shallow) for 3 months straight and it did not warp, change shape, or change dimensions over time. Polycarbonate also has the advantage of being a very strong material to begin with and if you reinforce it with something like epoxy it will be even stronger. I would suggest using FEA analysis to verify your design - most of the CAD programs have it built in now a days.

[svandestaak 9]

On 1/21/2021 at 8:25 PM, maurosacchi said:

 

Thanks! Any specific extruder diameter to be used with this filament?

If you're going for glass/carbon fiber, be sure to use the .6mm CC core. It's a thick line compared to .4, but it will increase strength a bit. And the CC core will be able to handle these abrasive materials very well. Use a regular AA core and your .4 will be a .6 quite soon....