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perfect model for a hemisphere

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#31 JamesH



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Posted 30 August 2005 - 08:20 PM

Could you do us a favour and make a few pictures of that process and which materials you use?! As a non-passfire member its sometimes hard to get such pieces of information :D

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Here are some photos of my setup: -

This pic shows the mold adapter for connecting the mold to a wet and dry shop vac, 1.75" and 2.25" fiberglass moulds, a batch each of the two size hemis and a piece of lycra swimsuit material as a mould filter. This material is much better than cotton for this purpose because it stretches under vacuum, perfectly conforming to the hemispherical shape of the mould. Because it stretches in use, it relaxes once the vacuum is switched off and helps eject the finished hemi.

This shows the huge amount of 1.5mm holes drilled through a 2.25" mould. A tedious job, but it only has to be done once!

A batch of 2.25" hemis. The only thing left to do is soak them in wood hardener to waterproof them. If this step is ommited the shell will turn to mush during the pasting process and all your hard work will have been a waste of time! Alternatively you could use a layer of masking tape or use any non water-based paint to coat the shell prior to pasting. However, it is less work to use wood hardener. Simply dip the hemi in a bath of the hardener and it will absorb all the way through the hemi with no need to go painting with a brush. Also the wood hardener is acetone based so it will dry very quickly. Remember to dry them away from sources of ignition!

The basics of the process: -
The mould is put on to the wide end of the adaptor, held on by the filter secured by an elastic band. This is then connected to the shop vac and switched on. The mould is then diped into a tank of mulch for a few seconds then removed. The time spent in the tank governs the thickness of the finished mould, so you'll need to try a few times to get it right! To eject the hemi, remove the mould adapter from the vac tube and blow down the adapter, the hemi will pop out without too much trouble.

I use cellulose insulation to make the mulch. This is a fluffy, very finely shredded newspaper. Mix 114g of this with 15 litres of water and a cup of wheat paste.

Mumbles - i have made 1.75" hemis using this method with a wall thickness of about 1/16", it depends on a lot of factors how thick the finished hemis come out i.e. suction rate of vac, number of holes in the mould, how much paper you have in the mulch etc. I am currently making a triple hemi mould for 1.25" hemis to see how small you can actually go while maintaining a wall thickness that isn't too thick. I'll let you know how it goes.

I hope this, along with mumbles post will help explain the process to non-passfire members!

#32 Mumbles


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Posted 30 August 2005 - 10:10 PM

What did you use to drill all the holes, and what material is the form made from. My holes seemed very jaged and not clean. The hemi is so small it is hard to clean .

#33 JamesH



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Posted 30 August 2005 - 10:43 PM

I used a small dremmel type hand-drill with a 1.5mm bit for the 2.25" mould and a 1mm bit for the 1.75". I make my moulds from glassfibre/polyester resin. It's very easy to drill, the swarf comes off as a powder, doesn't clog the drill flutes and doesn't leave any burrs.

#34 Dj Killerboss

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Posted 31 August 2005 - 10:17 AM

Here you have a Template for my hemis...
Posted Image

for all too putridly to make such are :D

Edited by Dj Killerboss, 31 August 2005 - 10:20 AM.

Do you have fire??

#35 curious aardvark

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Posted 08 September 2005 - 02:30 PM

Probably the easiest way to make moulds would be to ask a wood turner to turn both the mould and presser. It's not like you'd need anything tough either, plain pine would do easily. Any competent wood turner could knock out near enough any size mould in well under an hour. Plus most hobby turners will make these things for little more than the cost of the wood (if you happen to have a few fresh logs lying around they'll usually do a swap). There are any number of wood turning clubs around the world.
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#36 JamesH



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Posted 21 September 2005 - 12:13 AM

I finished that triple 1.25" hemi mould - it took a while drilling all those small diameter holes! I'll test it in the morning to see how it performes.

Here's the photo: -

#37 pyrotechnist


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Posted 19 November 2005 - 03:51 PM

Can one use a polysterine hemisphere, wrap it up with polysterine and then put it into fine cement to make a moulde. Then after it is dry take the hemisphere out and then get some lead melt it down and pur it into the cement mould. After this when near to drying put in a handle of some sort so the lead moulds around it causing it to stick? is that possible.
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#38 Amleth



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Posted 31 December 2005 - 02:02 PM

Do you think there are any easier to obtain materials to make the formers from? Not all of us have access to CNC controlled metal forming machines. I have heard some success with lead, and with plaster. I would think plaster would not be the best choice, but it is cheap. If you could perhaps coat the plaster in wax it may be acceptable. I'd be worried about the plaster rewetting. Linseed oil, polystyrene, or even a coat of urethane could waterproof it, and help to prevent sticking. Having access to plastic shell casings woul make forming the molds relativly easy. With the plaster shrinking on drying it would also make room for any of the waterproofers.

Perhaps use fibreglass resin instead of plaster? You can also pour and cast that, and it also shrinks on drying, and doesn't stick to a plastic mould if you coat it lightly with a suitable release like vaseline. It can get pretty damn hot as it cures though so be careful handling it.

Also, you can both increase strength and reduce the cost of the casting by using varous filler materials, the cheapest (but also weakest) being plain old (washed) beach sand. Just ring up a fibreglass shop in your area and ask them about it.

#39 MFX



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Posted 19 January 2006 - 04:27 PM

Maplins (UK) and probably other suppliers sell a low temperature moldable and re-useable Nylon type plastic called Polymorph. You heat it in boiling water and then simply mould it round a form. Because it sets like hard nylon it's fairly non-stick. A hemispherical form could be made using the plaster moulding technique sometimes used in making plaster film and theatre props, get a flat board and put a dowel in it, cut an acurate semicircular form out of thin plywood that can rotate around the dowel, then build up plaster around the dowel (To save plaster you can use a filler material such as paper. Then rotate the plywood form around the plaster while it sets to create a perfect hemisphere. When set cut and sand the dowel flush with the plaster and use to mould the polymorph.
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