sculpy footy bulb mold

Greetings all-
I tried an experiment last night that worked really well for what it’s worth. I made a basswood male mold plug about the right shape for half a footy bulb. Then I took a hunk of this stuff called “Sculpy” which you can get at any Art supply store or craft supply. I put the plug on a piece of cardboard, didn’t even coat it with anything or secure it to the board, and I mushed the sculpy over it to make the female mold. This all only took me like half an hour. It was easy to take the plug out while the stuff was still soft. Now all I gotta do is bake the mold for fifteen minutes at like 275 degrees, and then pur in my buckshot and epoxy afer it cools. Bingo.

This way I can make lots of molds of different sizes to get one the right weight.


I make my molds from Plaster-of-Paris. It shrinks a little with age, as it keeps drying, but I measure the lead before melting it. so I end up with a bulb that’s +/- 1/4 oz.

Idea: why not get some puttty-stuff that, when mized with lead shot, has the same density as cured epoxyw/lead? That way, you just measure the amount you need & shape it. Maybe play-doh? Silly-putty? GOOP?

How do you mount the bulb halves to a fin?

Like the idea of mixing the buckshop with some sort of putty but the epoxy putties I’ve seen might be hard to mold like clay with your fingers. Actually come to think of it, I think there is an epoxy putty that looks like clay.

You could mix it with Plasticine but you’d then need an extra layer of glass cloth and resin to hold it together which would be fine but no better than just pouring it into a mold.

I’ve worked with Plaster a lot. You have to make a box to pour it into and I always make a mess.

I’ll probly just mold a slot into one half or drill screw holes.

Personally, I’d just use some simple mathematics.

As we all know, the volume of the ball of radius r is equal to (1/3)r(πr2); i.e., (4/3)πr3. You might want to choose your best micrometer to measure “r” from your lead shot. Naturally, there may be variations in the lead shot, so I would measure a few - 50 or so - then figure the average.

You can approximate the volume of a bulb of imprecise shape by assuming the volume of several subsections and using a simple length cubed formula (Boyle’s model). Of course you are thinking that the greater the number of partitions - the more accurate your answer. To strike a balance on the number of subsections to calculate, you’ll probably want to apply Clarke’s 2nd law.

Naturally, for the brand of epoxy you use, you will have already logged and analysed the cured mass under different curing conditions. Everybody does this, right? Just refer to your tabulated data of mass, weight and density the mix and curing environment you intend for your bulb. Extrapolate a weight for any given volume. Of course I know some folk like to formularize the results and program the same into their TI-30X IIS scientic calculator, or similar. My 7 year-old thinks this is too old fashioned, and prefers to build a little java-script routine on his PDA. But I like to just graph the output for easy reference during the further calculations below.

So now we have a fair approximation of the volume within our theoretical mold, and we know the average volume of our lead shot. Now the electronic configuration of lead is [Xe]4f145d106s26p2 (the forum doesn’t display “to the power of” but you all know which numbers are which). I know you are thinking that, if we could measure this, we might be able to use it as a means of identifying impurities through the existence of other elements in our lead shot - which of course will impact on the weight of the same. But we need some equipment which is just not found in the typical home laboratory, and I do like to keep things simple - don’t you?

So instead, we’ll just assume that our lead has a density of 11.3 kg/dm3. Feel free to use a different value if you have determined the actual density of your lead shot.

Now I’m assuming you’ve got a handle on the target weight you want to achieve for your finished bulb. Fortunately, as you’ve already calculated the volume of the theoretical bulb above, you can use a similar approach to calculate the surface area using (of course you know this), good old 2(pi r 2) + (2 pi r)* h. [thats “r to the power of 2” in the first set of paratheses]. You can use this to estimate a weight for the paint you want to use on the finished bulb. Why not calculate the wetted surface and displacement while you are at it - just for fun?

Ah ha! You are obviously thinking - “Now when do we get to Keppler’s Problem?” Well you are right - we get to it now. Of course we need to work out how densely we can pack all those little lead shot spheres into our bulb shape.

Don’t be fooled by that sweet 74.048% answer. Despite the fact that you probably heard that evey mathematicial believes, and every physicist knows this to be the answer - personally I have my doubts. If you think you can acheive the close latic spherical packing necessary to achieve this volume ratio, go for it - but I suspect your bulb will turn out somewhat lighter than anticipated. I think I’d be a little more prudent and assume a standard cubic latice (pi/6) and just go with 52%. It’s easier to take weight off the bulb than to add it after all. I knew you’d agree. (OK smartar#, 52.35987755982933 if you must! But don’t you think you are overdoing things? Be realistic. puleeease!)

So now we have all the variables in hand, or a means by which we can calculate them. It’s obviously a straight-forward matter to work out how many lead shot spheres will fit into our bulb-mold, what they will weigh, what the residual volume of epoxy will weigh (cured) and what your surface paint (if used) will weigh. And the sum is the weight of the bulb.

Simple really.


I asked one of my nerdy-type friends whether he could formularize all of the above. He said…

  1. Get 6 ozs of lead shot (assuming this is the bulb weight you want).
  2. Roll it into a ball with some plasticine, Playdoh, or similar. The plasticine serves as a proxy for the weight of the epoxy. It’s proxy epoxy! Or maybe it’s eproxy.
  3. Remove material from the lead shot/plastercine mix until you get your 6 oz (plus or minus) mix.
  4. Shape it by hand into the desired bulb shape.
  5. Cover it with ceram wrap and use it to make your mold. If it’s too squishy, then use it as the model against which you make a more rigid male mold.
  6. Get some kids to clean the plastercine off the lead shot when finished.

Now I haven’t tried this myself - prefering to use the simple mathematical approach - buy my friend seems to think it’s worth a shot.


What about using boduy filler (Bondo)? I’m sure there are some putty-like materials out there that are stiff enough to keep itsshape long enough to make a mold (in plaster). Using the sculpters clay, you can then bake it hard. I’d make several bulbs of different sizes so the oven can be used efficiently.

BTW,lead is 11.34 g/cc

I was thinking of just pouring some shot into a film canister and putting that in the hull and see where she floats in the sink. The weight of the canister would be negligable. I like the fact that a footy can be test floated in the sink and not the tub because that way its easier to reach. I don’t have to sit on the edge of the tub which is less comfortable.

I digress…

now I have the right amount of buck-shot. Okay let’s see if it fits in my baked hard sculpy mold with some 30 minute epoxy mixed in. Good, now lets see If I can mount the sucker on the fin in the right position. We’ll see

Can you say “Sculpy footy bulb mold” ten times fast?

Don’t forget the mold release!!

Or you’ll have a sculpy mold with some lead shot glued in.

We use small cloth bags with led shot in it as weights to hold plans to the table, weight plywood to stay bent, etc. A small bag is lighter than the film can, and will mold itself to the botom of the hull better. won’t move around,like the film can might…

Maybe a smal zip-lock bag would work too.

On my original BobAbout circa 1984…I made a male form of the ballast with a flat bottom.
I carefully covered the male former in plasticine…then removed the form to leave a delicate plasticine female mould…Then filled the mould with epoxy and lead shot.
When dry simply scrape off the plasticine mould.
Off course these new bake in the oven moulding materials will work also.and will even take the heat from a single lead casting if your careful.

I’ve used this stuff called Dorlands Wax medium which you can get at the Art Supply store too. IT works well as a mold release agent. It’s sortof like Crisco but stiffer. Artists use it to make paints look more buttery. Hey this gives me an Idea: I could mix Crisco with my paints. (I’m an artist)

Good Idea the Zip lock bag.

Like the Plasticine Idea too, but then you can’t re-use the mold.

I’m going to give this a try today.


You gave me an idea: Bees wax is soft if you ewarm it (in boiling water?)or just melt it, so you could mix your lead shot with beeswax and mold it to your shape. It’s hard enough to make your mold when it’s cool. Should be about the same as the epocy lead mix, but try it and let us know!

Maybe I will if I have any left. I use it for the mouthpiece on my didg…

It worked great!! I was initially concerned I might not be able to get it out of the mold but I gave it a tap and the thing fell right out. I like the sculpy mold also because you can mix the resin and buckshot right in the mold without scratching it on the inside. This seems helpful if you’re using good old 30 minute epoxy from the local hardware store which is thicker than fancy West system stuff.

(Psst…I could start a mass production line)

I’ll try and post a picture later.


I suggested elsewhere that somebody could make some molds for different sizes of bulbs, but it’s easy to make your own and share with your friends.

Just don’t tell those hardware store salespeople what you use it for! They never understand, and then just tell you to use something totally unrelated.

Yeah I remember I went into the Hardware store one time years ago and I said “Do you have any servo mounting screws?” and the guy was like “Huh?”

I love hardware stores.

The closest you can get to servo mounting screws is 1/2 inch #6 panhead. Or in the hobby store: #2button head sheet metal screws.

Yeah thanks-
I’ve been using ordinary brass wood screws. At least they have those.


I just take god care ofthe screws I have. After a while, you get many extras.

About everything you could want in miniature fastners right here:

Good prices

My sculpy mold fell apart so I went to the sporting goods store and got a coulpe of lead sinkers. Didn’t know they came in weights all the way up to 20 ounces. Oh well. I guess that’s that

This thread might help?