All size round shell mold

[Givat]

Hi all,

I wan't to share with you this easy to make method for round shell molds.

Some times its hard to find the exact size ball to use to make a shell mold, so after some thinking I came to the idea of using rubber balloon filled with water - AND IT WAS SO EASY TO MAKE TO ANY SIZE!

You just need to decide the mold diameter and use the "ball volume formula":http://upload.wikimedia.org/math/c/e/e/cee10af73cbd198af616d9a19fc9a09b.png

We know that water weight 1 gram per 1 c"c, so you fill the balloon with the amount of water you've just calculated and you will get a balloon with the diameter you wanted.

 

Now just make some cement/gypsum for the mold:

http://i1111.photobucket.com/albums/h479/questl/Shells/P1010830.jpg

http://i1111.photobucket.com/albums/h479/questl/Shells/P1010834.jpg

http://i1111.photobucket.com/albums/h479/questl/Shells/P1010838.jpg

 

This method will be great for people with unusual size mortar like I have.

[dagabu]

Givat,

 

That is wonderful! Thanks for sharing this method with us.

 

-dag

[VintageRacer]

This is an interesting approach. Have you tried this method with something a little easier to work such as plaster?

[gelamex]

Thank you very much for a well thought out solution to what can be a rather frustrating problem. I have just tried this with plaster of paris with no problems encountered. This solution to round molds has uses well beyond pyro applications as well.

[NightHawkInLight]

Looks great! Now make a positive and you could use it to quickly press hemi's out of blended paper mache rather than laying out strips one by one. Coat it up with a little wax and I bet they'd pop right out. Edited January 12, 2012 by NightHawkInLight

[Givat]

I'm glad you all liked it.

The reason I used concrete and not plaster is that I wanted to use it in a press and thought the concrete is less prone to break from pressure.

NightHawk, I took your advice and started making today the positive mold - I used again a balloon and 90 gram/m^2 fibreglass with epoxy, after it will dry I'm going to fill it with concrete too.

 

I'm not so sure it will work with paper mache, but crossing my fingers that you are right.

[gelamex]

Interesting. I have gone about making the 'hard' molds in a somewhat different fashion. Having used the balloon with plaster, I then varnished and waxed the resulting mold. This was then given a gellcoat of marine grade epoxy and then filled with marine epoxy and sand mix. Epoxy is expensive on this side of the pond, however this was a batch that was reaching its use by date. I have previously found that fine beach sand makes a good and cheap filler for epoxy, also, when cured marine epoxy shrinks by very little.

The resulting epoxy hemi on a stick was given several coats of kraft and then painted and waxed. I then used this as the hemi to cast a female epoxy mold again using sand and marine grade. The result is a mold and former that will take huge pressure and is exactly the size I want.

Edited January 29, 2012 by gelamex

[Givat]

Gelamex, sound like you made yourself a tool for life. please post some photos for us to see.

 

as for my mold, I filled the female mold made from fibreglass with plaster and glued it to a wood plate as base. I don't need to use my press to make the hemmis, just use about 12 layers of pasted newspaper and press by hand.

here the result:

[dan999ification]

excellent work,

i saw this ages ago and want to give it a try now only problem is..maths is not my strong point, could someone please simplify the formula in laymen terms/5th grade symbols:http://upload.wikimedia.org/math/c/e/e/cee10af73cbd198af616d9a19fc9a09b.png

i completely understand in no way whatsoever

this is what i see volume=4 [what] . [what] 3.142 .r3 over 3

i was going to forget the equation and make it too small building up to reach the size/s i need but if its worth doing its worth doing right.

any help would be greatly appreciated.

 

dan.

[californiapyro]

pi (3.14ish) and radius cubed

[Col]

Try this Dan My link

Use centimeters for the radius and the volume will translate directly to grams of water

Edited February 3, 2012 by Col

[JFeve81]

So for example purposes if you wanted to make a 3" ball the equation would be:

 

V = (4 x 3.14 x 1.5³) / 3

 

So

 

V = (42.39) / 3

 

 

V = 14.13

 

So the balloon would be filled with 14.13 grams of water?

 

 

Am I correct on this or missing something?

[Col]

Assuming its 70mm in diameter (2.75") i make it 179.6g of water. Enter 3.5 (cm) as the radius in the calculator link above.

[JFeve81]

Assuming its 70mm in diameter (2.75") i make it 179.6g of water. Enter 3.5 (cm) as the radius in the calculator link above.

 

 

I was just speaking hypothetically. Mainly just to make sure I got the equation right. I entered 1.5 in the link posted and it came out to 14.137166941154 so I guess I got that part right.

 

In Givat's post he states: "We know that water weight 1 gram per 1 c"c, so you fill the balloon with the amount of water you've just calculated and you will get a balloon with the diameter you wanted."

 

 

So would I just put 14.13 grams of water to get the right size or does that need a different calculation?

[Mumbles]

You did the calculation right, but don't have the units right. When you used 1.5, that is in inches. Your answer gives 14.14 cubic inches. 1 cubic inch of water weighs significantly more than 1 gram. If you use centimeters as the length measurement, then there is a direct conversion between volume and mass of water. 1 cubic centimeter = 1 milliliter = 1gram of water.

 

To get the right answer either use 3.81cm for the radius, or multiply your answer by 16.387. That conversion will convert cubic inches into cubic centimeters.

[JFeve81]

You did the calculation right, but don't have the units right. When you used 1.5, that is in inches. Your answer gives 14.14 cubic inches. 1 cubic inch of water weighs significantly more than 1 gram. If you use centimeters as the length measurement, then there is a direct conversion between volume and mass of water. 1 cubic centimeter = 1 milliliter = 1gram of water.

 

To get the right answer either use 3.81cm for the radius, or multiply your answer by 16.387. That conversion will convert cubic inches into cubic centimeters.

 

 

OK. That makes more sense. Thanks. I learned something new today.

[dan999ification]

thanks for all the help guys and the link col, should have some free hemis soon.

 

 

dan.