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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Waterproofing 3D Printed parts

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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cprobertson1
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Waterproofing 3D Printed parts

Post by cprobertson1 » Wed Jul 18, 2018 8:04 am

I've owned a 3D printer for years and have had varying success waterproofing very thin parts for vases and cups and the like.

With my recent interest in water rockets, I've hit a snag in that 3D Prints don't seem to want to hold water under pressure - so I just wanted to share a few techniques I'm going to try out and hopefully test in the near future.

The first (confirmed) method is to deliberately overextrude by aroudn 30% (setting the extrusion to 130%), at a slightly higher temperature than normal (I use +10 degrees) and 100% infill. The idea being to firmly bond the layers together - at the expense of dimensional accuracy (which can easily be sanded back down to suit!)

I have had success with this method for similar purposes (pneumatic tested to 150 psi with negligable loss of pressure after two hours: hydraulic tested to 600psi with negligable loss of pressure for two hours... um, I borrowed my work's pressure testing facility for this - please don't do that in your garage or anything! Like, the-control-room-is-a-concrete-bunker sort of "please don't try this" :P)

However, even with the overextrusion method, I've hit problems with the occasional leak depending on part geometry (air/water likes to find its way into the tiniest nooks and crannies! Physics is always out to get me!)

My intention is to try two types of coating on the load-bearing surfaces (that is, the surfaces exposed to the compressed fluid).

The first is a simple polyurethane (PU) varnish - the kind used for coating windowsills in my case. What concerns me about this particular method is the increase in skin friction caused by the glossy surface being very slightly tacky, even though it appears smooth.

This is where my other idea comes into play. A colleague (and ultrasonics engineer) mentioned that (very high performance) speedboats use waxes and/or oils on the outside of the hull to decrease skin friction with the water - which gave me the idea of doing something similar using paraffin wax (though any wax should work - take your pick!).

I will coat them tonight and test them at the weekend. the PU varnish can just painted on and raised above some card so that the excess drips off it - a very light sanding before use should smooth out any irregularities.

The wax coating will be a little more involved - using a double boiler (that is a pot of wax floating in a pot of water atop the stove) I need to coat only the inside. I propose to do this by using masking tape to prevent the wax covering the outer surface (don't want to add too much extra mass!) and then dipping it into the wax. While still wet, I will hit it with a hot air gun (on LOW heat!) to blow out any excess wax and to smooth out the remainder.

Alternatively, I can leave it to soak inn the wax (again, while masking the exterior surfaces) - taking care not to warp the print, and then gently drill out any excess (just by turning a drill bit by hand inside it - no need for high speed or torque - it's wax!)

I'll let you know how I get on - and if you have any suggestions, do let me know!



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anachronist
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Re: Waterproofing 3D Printed parts

Post by anachronist » Wed Jul 18, 2018 2:59 pm

I think you're on the right track. If you can get at the surface to rub the wax or oil in, that might be better than simply coating the surface. With a coating, you might run into problems with expansion cracking the wax, because hardened wax won't stretch, unless you're using that soft and pliable dental wax.

I'm curious, what sort of pressure bearing parts are you trying to print?

My 3D printing for rockets is limited to parts like fins, nose cones, and deployment mechanisms, nothing where wetness or pressure matter.



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Re: Waterproofing 3D Printed parts

Post by cprobertson1 » Fri Jul 20, 2018 3:23 am

It varies - the larger rocket I have planned is consisting of 40mm PVC pipe (schedule 40) with 3D printed endcaps: if they fail I have a backup plan for moulded resin endcaps (3D print set in plaster, melted out/burned out with a blowtorch and the void filled with resin)

The schedule 40 PVC (at that diameter) has a max working pressure of just under 200 PSI (and a burst pressure of ~1000PSI). Cylinders are great at holding pressure - the pipe can take a lot of pressure - the end caps cannot. In a plumbing system you are limited by the fittings you use rather than the max pressure of the pipes themselves!

Anyway, so in principle, my rocket can probably run up to 200 PSI assuming the endcaps don't leak.

So far I've tested small 3D printed vessels up to 150 PSI (the max my compressor runs to) using the wax-dip sealing I mentioned above. The paraffin I'm using has some flex in it - though I might reconsider and use beeswax; I'm not too worried about it cracking though.

For 3D printing airtight parts, go slow, use random start points for each layer if possible, and the most important part - increase your temperature and OVEREXTRUDE! :D

Unfortunately I can't really give an idea of the "max pressure" of PLA - as it varies with part geometry.

In the case of my PVC pipe with 3D printed endcaps, the weak point in the system is how the end cap is joined onto the PVC pipe - rather than the printed part itself!

Now here's a question for you... does the water jet from a water rocket erode the nozzle over time? I ask as I'm curious if the water will erode away whatever coatings I apply to the nozzle!



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Re: Waterproofing 3D Printed parts

Post by anachronist » Fri Jul 20, 2018 12:34 pm

I had suggested earlier to try orthodontic wax but I see that beeswax is cheaper.

I know PVC end caps exist, but I don't recall seeing them on larger diameters, except as a combination of a reducer with a threaded plug in it.

You may already know this, but design your end caps as half-spheres. The pressure stress on the part will be more evenly distributed than if you just made flat caps.

If you're concerned about the joint, that can probably be solved by having sufficient overlap of the end cap over the pipe. The cap should be a nice snug fit, like PVC pipe fittings are nice and snug. You might consider using Loctite PL adhesive sealant rather than PVC pipe glue because PL expands as it cures, making for a better seal and making a good mechanical bond against roughened surfaces.

The jet from a water rocket won't erode the plastic, as far as I know, at least not at the pressures you're using. This isn't a water-jet cutting tool after all (and you do have to replace nozzles on those). The water rocket jet erode coatings though.



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Re: Waterproofing 3D Printed parts

Post by cprobertson1 » Mon Jul 23, 2018 2:52 am

The PVC endcaps would in principle suffice for this build - in fact, in some respects they may be stronger than a 3D printed endcap (as they can be solvent welded on).

That said, my upper (nose-side) endcap has a flange on it which is used to attach to the payload bay: I'd need to improvide is using PVC for that! Likewise the lower (nozzle-side) endcap will have the nozzle integrated with it - I'm currently looking at creating a series of different diameter nozzles that can be used with the same launcher (I'll post the design up once I've got it built - I'm still prototyping just now).

Anyway! At the weekend there I ran a few tests:

Polyurethane varnish:
The first coating I tested was PU varnish. I tested five nozzles with this and quite liked the results: three of the nozzles held up under 100 PSI for 60 minutes, one failed with a hairline crack, and the other refused to hold pressure at all.

The nozzle that held no pressure was sealed with three coats of varnish which seems to have done the trick (though I wouldn't trust it as far as I can throw it. Now to be fair, I can throw it a good 10-20 meters on a good day, so your mileage may vary)

Application was easiest to do just by masking the part with some tape and then dipping it in the varnish, holding it to let the excess drip off, and then leaving it to dry on a non-stick surface. Painting would almost certainly work as well - but dipping it is probably easier! Depends on the part you're waterproofing though!



Paraffin wax:
Yuck. I'm sure it works in principle, but it was hard to get right. Again, tested with five nozzles - but applying it was a pest. For one thing, molten wax is hot - and accidentally dunking your finger in it is not a fun experience (though I hear some people like that sort of thing. Not for me!).

I first tried dipping it - but found that it cooled too rapidly and quick clogged the nozzle. Taking this in my stride I drilled out the wax but found it hadn't penetrated into the crags of the print, and thus hadn't sealed it.

Next I tried a short soak in the molten wax (bearing in mind that 3D prints start to become flexible at these sorts of temperatures! Don't want to leave it in too long or your print will deform!). This resulted in a mostly even coat of wax when I pulled it out - and under pressure it seems to have worked.

I also tried painting the molten wax on but ended up with similar results to dipping it.

I personally found the wax difficult to apply by any method - and I'd be worried about part deformation/residual stress if the part is soaked in wax (which was the best method I found).



PVA Glue:
This was the easiest to apply, and seems to have worked well.

Again, testing with five nozzles, I found the easiest way to apply the PVA is to place the part on a flat surface, fill up the nozzle with PVA and then lift it up and let the excess drip out - and then let it set just like with the PU varnish. You can also dip-coat the parts though this tended to be messier.

None of the PVA coated parts had any noticable leaks after an hour at 100 PSI - that's not to say it wasn't a fluke though! HOWEVER - there's a catch - PVA has a tacky feel to it once it is set - which may cause viscosity losses in the nozzle. The PU glue had a gloss finish and wasn't as tacky, so it might be more suitable for waterproofing parts.



Of course, deliberately over extruding the part seems to do a decent job of sealing it in the first place - though I don't seem to have mastered the technique of printing airtight parts yet!



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Re: Waterproofing 3D Printed parts

Post by anachronist » Mon Jul 23, 2018 11:56 am

If a PVC end cap is stronger, why not 3-D print a flange for your payload bay to fit onto the end cap? THAT flange doesn't need to be airtight.

Yeah, with wax you have to be sure the parts are warm first, else the wax will cool too quickly. I suggested orthodontic wax because I suspect it has a lower melting point. Beeswax melts only slightly lower than paraffin.

I wouldn't use PVA glue, even the water resistant kind doesn't do well if submerged. I'd prefer a waterproof sealant.

You might try silicone grease. Very low melting point. Available in any plumbing supply place. You just have to bond your parts before applying it.



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Re: Waterproofing 3D Printed parts

Post by cprobertson1 » Tue Jul 24, 2018 4:20 am

anachronist wrote:
Mon Jul 23, 2018 11:56 am
If a PVC end cap is stronger, why not 3-D print a flange for your payload bay to fit onto the end cap? THAT flange doesn't need to be airtight.

Yeah, with wax you have to be sure the parts are warm first, else the wax will cool too quickly. I suggested orthodontic wax because I suspect it has a lower melting point. Beeswax melts only slightly lower than paraffin.

I wouldn't use PVA glue, even the water resistant kind doesn't do well if submerged. I'd prefer a waterproof sealant.

You might try silicone grease. Very low melting point. Available in any plumbing supply place. You just have to bond your parts before applying it.
Not a bad idea for the upper- surface actually. In fact that's quite a good idea... Look at me overcomplicating things! The lower cap will probably need to be 3D printed though - but I can still make the upper cap strong! In fact, it might even be lighter than a 3D printed equivalent!

Good shout on the PVA - it's water RESISTANT - not waterproof - but can tolerate a few cycles of soaking/drying. I still think the PU varnish may be the best bet of the materials I've tried - it's really easy to apply and seems to work well (It is slightly flexible once set - which suggests it may be subject to creep under pressure though - I've not exactly tested it mechanically! Worst case scenario it needs a new coat every so often.

I'm going to experiment with adding baffles into the inside of the nozzle - the idea being that when I dunk the wax into it, it fills the space between the baffles, rather than restricting the nozzle itself - think of tapping out the throat of the nozzle and filling the threads with wax. Will see how that turns out - though I still suspect varnishing may the be the best compromise between usability and functionality.



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Re: Waterproofing 3D Printed parts

Post by sebswift » Fri Aug 24, 2018 5:03 pm

I've been using PETG filament to print a lot of parts lately and they do leak slowly but I've found that they hold pressure long enough for a launch (10 min without much loss of pressure). When I rub them down with soapy water (leak detection) and pressurize, they will foam slowly but not that bad. I didn't have good luck with PVA. I also tried ABS but found PETG worked very well. I had to crank the temp on my nozzle up by 10-15 deg to get better layer adhesion. I have not experimented with over-extruding - good idea - I may play with that. I have a 3D printed multi-stage clamp which holds the 2nd stage with three cams - all 3D printed in PETG (even the hinge pin is PETG) and it holds 90-100 psi fine.



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Re: Waterproofing 3D Printed parts

Post by cprobertson1 » Wed Sep 05, 2018 4:09 am

sebswift wrote:
Fri Aug 24, 2018 5:03 pm
I've been using PETG filament to print a lot of parts lately and they do leak slowly but I've found that they hold pressure long enough for a launch (10 min without much loss of pressure). When I rub them down with soapy water (leak detection) and pressurize, they will foam slowly but not that bad. I didn't have good luck with PVA. I also tried ABS but found PETG worked very well. I had to crank the temp on my nozzle up by 10-15 deg to get better layer adhesion. I have not experimented with over-extruding - good idea - I may play with that. I have a 3D printed multi-stage clamp which holds the 2nd stage with three cams - all 3D printed in PETG (even the hinge pin is PETG) and it holds 90-100 psi fine.
Very nice :D

I've discovered that overextrusion is perhaps the easiest way to get the parts waterproofed from the get-go - and I've upped the layer-thickness to 0.3 and 0.4mm (if you consider that each layer has a small chance of leaking at the interface, then you reduce the potential for leakage by reducing the number of interfaces by reducing the layer count: fewer layers = fewer possible places where it might potentially leak!).

I've also experimented with putting a layer of 2-part epoxy resin on the inside of the part and then pressurising it to force it into any cracks or defects, holding it there for a third of the curing time, and then "launching" the rocket to clear the nozzle of epoxy before it sets hard. In my case, I had it strapped down to my testing jig - but you could probably get away with just doing a purely pneumatic launch if you are so inclined!

Note, purely pneumatic launches make a fairly loud "bop" when you release them - might want to wear ear protection just in case. I also had a "whistler" recently where the airstream was split over some surface and made an abhorrible shrieking noise like some demonic flute... ended up with all the dogs in the neighbourhood barking at me!



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Re: Waterproofing 3D Printed parts

Post by cprobertson1 » Wed Sep 05, 2018 4:12 am

Oooh! I meant to say, I also tried using a "spiral vase" printing method (usually used for single-layer vases) to print a nosecone - worked brilliantly - super-light, yet fairly sturdy!

Should give it a go! Mine were designed to detach at the zenith and provide drag, but you could also fill it with expanding foam if you want something a bit sturdier, or keep it hollow for a payload bay :)



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