Skip to main content

Experiments with polyurethane foam for injection molding a HAB capsule

Back when I worked on GAGA-1 I used a polystyrene container for the capsule. This worked well but it was quite large and polystyrene is annoying to cut.

So, I've been experimenting with expanding polyurethane foam to injection mold a HAB capsule.

My first experiment was to work with a big block of foam that I sprayed onto a sheet of baking parchment. The foam doesn't stick to the parchment and it can be easily peeled off. The set foam can be easily cut with both a saw (a bit messy since small parts break off) or with a Stanley knife. The Stanley knife gave clean cuts. It was also possible to hollow out a space inside the foam where the electronics could be stored.

One test I did was to see how strong the cured foam is by hitting it with a hammer. Here you can see the hollowed out capsule as I hit it with a geological hammer. It seemed strong enough to me for a HAB landing.

I then tested various glues and paints on the surface. Here I've painted it with spray paint (yellow) that is solvent based, again with the acrylic paint I used for the GAGA-1 capsule, then drawn on it with permanent markers, stuck on some yellow duct tape, glued on with spray adhesive some space blanket and used epoxy to stick some more space blanket in place.

The solvent based paint did not attack the foam, but the coverage wasn't good and once again the acrylic paint put on with a brush gave the best yellow colour (at the top of the photograph). The epoxy glue stuck well and didn't damage the space blanket or foam (the spray glue didn't adhere well). The markers didn't damage the foam but the outside of the foam can't take much pressure from the marker tip.

The duct tape stuck well, but I wouldn't use it as the only means of keeping the capsule sealed. Next I left the capsule in the freezer (at -18C) overnight to see if the foam became brittle.

Didn't notice any ill effect from this temperature, but still need to do a proper test of the insulating properties.

Next I moved onto the injection molding experiment. I made a mold from a thin sheet of cardboard that came from a dry cleaner (used to hold folded shirts) coated with baking parchment. This was rolled up and taped and a piece of parchment used to seal one end.

Then I made a small mold for the space where the electronics will go from an old business card holder covered in baking parchment and some tape.

Then I suspended it inside the tube. This was a mistake because it really need to be suspended better because as the foam expanded later the box moved and I had to manually push it back into the appropriate position before the foam set (wear gloves!).

Then I sprayed the inside with water to help the reaction and sprayed in the foam from a can. After letting it set over night the mold looked like this:

I cut off the bottom with the Stanley knife with ease to reveal a nice strong pattern of foam inside.

And the parchment was pretty easy to remove to reveal the capsule.

Here's the trimmed capsule when removed from the mold.

And a slice through revealed the mold inside.

With a bit of pulling the bottom half was off revealing the internal mold. I had to cut through the cord to remove it and then pull the cord out completely.

And finally the completed capsule in two parts.

And shown next to the GAGA-1 capsule to show its size for comparison.

The GAGA-1 capsule weighed over 200g, this one weighs 25g. Switching from Arduino to Arduino Pro saves another 18g.

So, I think I could put together a HAB project whose goal would be to go for maximum altitude with a total capsule weight in the 100s of grams (including a very lightweight video camera this time).

Will have to see if I have time to work on GAGA-2: Baby Gaga.


Edward said…
The expansion of the urethane foam will vary based on temperature. The warmer the foam and the air, the greater the expansion. This might save a few more grams :-)

I used expanding urethane foam on my hovercraft project. Our local astronomy club and ham radio operators used polystyrene containers for their high altitude balloons.

Spokane, Washington, USA
That foam-in-a-can stuff is very weak. For a better alternative (light, and so hard you can barely pierce it with your finger nail) see:!=Serie/c1122/index.html

Nicer to work with too.

Popular posts from this blog

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it: Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name. There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly. What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message: Our system is unable to process last names that contain non-letters, please replace them with spaces. Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your

All the symmetrical watch faces (and code to generate them)

If you ever look at pictures of clocks and watches in advertising they are set to roughly 10:10 which is meant to be the most attractive (smiling!) position for the hands . They are actually set to 10:09.14 if the hands are truly symmetrical. CC BY 2.0 image by Shinji I wanted to know what all the possible symmetrical watch faces are and so I wrote some code using Processing. Here's the output (there's one watch face missing, 00:00 or 12:00, because it's very boring): The key to writing this is to figure out the relationship between the hour and minute hands when the watch face is symmetrical. In an hour the minute hand moves through 360° and the hour hand moves through 30° (12 hours are shown on the watch face and 360/12 = 30). The core loop inside the program is this:   for (int h = 0; h <= 12; h++) {     float m = (360-30*float(h))*2/13;     int s = round(60*(m-floor(m)));     int col = h%6;     int row = floor(h/6);     draw_clock((r+f)*(2*col+1), (r+f)*(row*2+1),

The Elevator Button Problem

User interface design is hard. It's hard because people perceive apparently simple things very differently. For example, take a look at this interface to an elevator: From flickr Now imagine the following situation. You are on the third floor of this building and you wish to go to the tenth. The elevator is on the fifth floor and there's an indicator that tells you where it is. Which button do you press? Most people probably say: "press up" since they want to go up. Not long ago I watched someone do the opposite and questioned them about their behavior. They said: "well the elevator is on the fifth floor and I am on the third, so I want it to come down to me". Much can be learnt about the design of user interfaces by considering this, apparently, simple interface. If you think about the elevator button problem you'll find that something so simple has hidden depths. How do people learn about elevator calling? What's the right amount of