It gets worse when the object is a curved surface, as is my AUV hull.  I’ve read that objects can be made watertight by adding outer shells.  That may be true for some objects, but on objects that curve along the z-axis the number of shells exposed to the surface grows as the tangent plane gets closer to parallel to the printer’s build platform—and big holes start to form.

My hull doesn’t actually have to be watertight, it’s a wet hull.  But it has to be airtight in order to hold the bubble of gas that controls the robot’s buoyancy.  Here’s what the airtight hull looks like:

Finished (‽) AUV Hull.

So that’s kind of neat.  I did the job using the following procedure:

1. Clean the surface of each semidemihemisphere with isopropanol or something else that dries clear.
2. Assemble each hemisphere with big wads of epoxy clay filling in the gaps between the four semidemihemispheres.  Then slather the assembled hemisphere in more epoxy clay so that it’s fully covered.  Then leave it until the clay cures.
3. Sand down the epoxy clay until the plastic shows through (I used a 120-220-400-800 grit progression of sandpaper).
4. Coat with lacquer to make it shiny.  I used shellac at first, then switched to polyurethane as it was easier to use (shellac has a very short work time and also discolours in water) and food-safety wasn’t a concern.

I actually did step 2 in reverse because I’m a terrible engineer and didn’t think to seal the gaps between the semidemihemispheres with clay until after I’d done the outer surfaces.

Epoxy clay turned out to be an effective material for smoothing and sealing 3D prints.  I bought 2 ounces of the reddish-brown (“flesh” coloured) stuff on eBay and later supplemented it with a 1 lb. white batch of the unsettlingly spelled Apoxie Sculpt from Sculpture Supply Canada (n.b. what I discovered regarding the difference between Apoxie Clay and Apoxie Sculpt).  The clay doesn’t adhere well to ABS as you’re applying it, I had to keep my fingers lubricated with water so that I could press the clay into the object’s filament ridges without the clay sticking to me.  (The water also made it easier to smooth the clay out so that I could squish it into a thin layer that was quick to sand down.)  Once the clay cures it’s really fricking hard to get it off of whatever it’s on, but it sands and drills cleanly.

The clay can also be used for filling holes and errant curves (e.g. due to the bottom of a print curling up as it cools) and for making good-looking, smooth, heat-resistant surfaces (I made a totally sweet coaster).  It also takes acrylic paint a little better than ABS plastic does, which isn’t saying much.

Another miraculous substance that I’ve discovered is Plumbing Goop.  It’s clear, creates a thin layer, is very easy to apply, and sticks really well to ABS.  It doesn’t peel off like silicone rubber.  It also provides a rubber-like grip, something that bare ABS is sorely lacking.  The downsides: because it’s applied in a thin, clear layer, it’s harder to get a thorough seal with Plumbing Goop than with epoxy clay; it doesn’t make spherical prints look like Jupiter; it doesn’t smooth out the filament ridges; and it’s flexible (comparable to silicone rubber), which might not actually be a downside depending on what you’re doing.

So I went back to square 2 and decided to remake the original brass valve body in lighter ABS plastic using our Makerbot 3D printer.  The beta version looked like this:

Remade underwater solenoid valve with 3D-printed structure.

I re-used the gasket, screws, core, and shaft from the original valve (where the solenoid sits), and replaced the big brass parts with light ABS plastic that is customized to fit onto my hull.  The steel shaft press-fits onto the shaft holder at the top of the photo.  The shaft holder screws into the connector piece, which implements the same idea as the brass valve piece, but instead of the input being a pipe fitting the input is open to the inside of the robot hull.  Note that in the photo I’m holding the valve upside-down, the solenoid shaft will hang down from the top of the robot hull, and the other end of the valve will bolt onto the hull’s top using the existing screw holes, pressing the base of the conical shaft against the hull’s top hole (apparently I’m a terrible photographer too).  When the valve opens, the air inside the hull will flow through the conical shaft out of the hull’s top hole and will be replaced by water flowing in through the bottom hole.

The new valve did in fact work.  The solenoid successfully pulled back the gasket to open the valve when activated, and it almost sealed when the power was disconnected.  The seal wasn’t perfect because it printed over a bump in the 3D printer’s build platform.  The plastic was 277 grams less massive than the original brass, and with a tweak or two the new valve will bolt onto the hull efficiently.

The valve had a few problems.  The steel shaft’s wide screw part was too wide for the hole it was supposed to screw into, so I had to file the hole out until I could press-fit the shaft in.  The support structures on the interface piece are too spindly, as you can see in the photo.  Also the square plate on the interface piece was a little thin, and it sat a little too short.  The corners  of the square hit other structures inside the robot hull, and it needed to be about 1 mm farther away from the nut holders.  In the new version I have tried to rectify these issues:

Updated valve connector structure.

The square plate is now half again as thick, and the nut holders (the bits sticking out near the top of the conical shaft) are 1 mm farther away from the plate.  The supports are a lot thicker, too: they’re 4 mm wide instead of 2 mm, and the six supports that don’t connect to the nut holders flare outward stylishly.

There’s one other issue that I foresee.  The heavy copper solenoid and the steel shaft/magnetic core sit pretty high in the hull, and raise the robot’s centre of mass.  This is a potential problem because if the centre of mass is too high the robot will roll too much in the water.  I can add extra ballast to lower the centre of mass, which may or may not be fine, depending on what kind of buoyancy the rest of the hull has.  I might actually have to do some engineering to figure that out.  Or, I can try it and see what happens.

Pictures and video follow:

A semidemihemisphere with overhang supports attached (but not the walls that the cutaway rests on).

Cutting away an overhang support.

The full AUV hull. Neat!

Here are two videos of a semidemihemisphere being printed. Part 1:

Part 2 (the cool part):

Tweaked version of the AUV model.

I made a few interesting changes to the cutaway portion though:

The cutaway is needed to hold up the hull while it prints.  Without the cutaway, the plastic, heated to its melting point by the Makerbot, will droop and hang or fall.  The cutaway gives the hull something to sit on as the plastic cools.  Here is the cutaway portion, viewed from the side:

AUV hull cutaway pieces from the side.

A – I added a bunch of little bits on the bottom layer of the model.  This is a hack to get the printer to print a larger raft.  (The raft is a thick layer of plastic that the model sits on as it prints, intended to let the model stick well to the build platform.)  I had to reduce the raft margin so that it would fit on the build platform, but with the smaller raft the pieces were susceptible to being knocked around by the toolhead as it goes.  These pieces get raft added around them, which merges with the raft of the actual model so that the real pieces get a nice stable raft.

B – This bit connects the cutaway material to the hull.  The hull is getting pretty tall at this point, so if it gets hit by the toolhead there’s a force applied that is strong enough to knock around the hull and perhaps misalign the layer.  This part (and its equivalent on the other side of the model) help hold the hull steady against the cutaway shells as they get close to being joined together.

C – This bar connects the cutaway shells together to keep them steady.  In a previous model (shown in the last log entry) I made this part too wide, and it took so long to print the horizontal bar that the hull cooled too much and split a little.  It wasn’t anything a little plastic welder couldn’t fix, but the supports don’t need to be very wide to fulfill their purpose.  After shrinking them to 1 mm wide the printer could print one bar layer with just two quick strands of plastic.

AUV hull cutaway pieces from the top.

D – This is the support for the upper tab that hangs freely over the semidemihemisphere.  You can see it from the side at the top of the cutaway side image above.  This is a pretty cool piece that’s anchored vertically on the cutaway shell, and ends up a horizontal flat surface at the upper tab bottom.  It is topped by a 0.5 mm shell that the tab sits on (see E).  I did this piece with a loft from a rectangle on the vertical plane to a rectangle on the horizontal plane.

E – The lattice pattern holds up the hull’s overhang.  The lattice rests on a 2-layer thick platform.  The bottom layer of the platform is printed over open space, and droops badly.  The second layer of the platform is printed on top of that and is pretty flat.  Then the lattice is printed on the nice flat layer, and the actual hull is printed over the lattice.  The lattice is sparse enough that it can be separated from the hull easily with a sharp utility knife (same as the shell in D).  The squares in the lattice are 5 mm to a side (I originally used a 10 mm lattice, but it was too sparse and the hull overhang drooped too much) and 0.5 mm thick, i.e. the width of one strand of plastic.  Note that the top-most (innermost) hull overhang is narrow enough that it doesn’t need to be supported by a lattice.

F – The circular patterns support the circular holes in the hull.  Without these the Makerbot would try to print a circle over the lattice, and it would just collapse into the open spaces.  The square support on the upper lattice supports the receptacle that accepts the upper assembly tab.  Without it, the printer would again try to print the square over the lattice and the first layer would fall into the holes.  With the support the first hull overhang layer is a full surface, and the receptacle square sits on that full layer instead of the lattice.

Here is an old version of the cutaway that had a bunch of problems:

An early, problematic cutaway version.

Problem 1: The bottom layer had these wedges to hold everything together and provide a solid base.  After I started using a raft these were no longer necessary; in fact, they were a detriment because they took a long time to print and the first couple hull layers cooled too much and came apart.

Problem 2: As stated before (C) the cutaway support bar is too wide and takes too long to print.

Problem 3: The lattice is radial instead of square.  The printer couldn’t print the off-angle 0.5 mm lines.  The lattice would be printed near the edge of the lattice pattern, but there wouldn’t be anything in the middle, and the hull overhang would not get its support.

Problem 4: The circle supports in the lattice are too thin (0.5 mm) and they don’t get printed properly.

Problem 5: The innermost cutaway support is thicker than the others.  Once again, there was no benefit to this and it took significantly longer to print than the thinner version in the later versions.

Here is my current design in full, including the support structures: