Making a mould for the dome.

[fox]

I have decided to make a mould to form my 30" dome.
I am planning to make the dome in four separate components & also make the entry as a separate component too.
I have bought a 30" exercise ball and another 34" one. I will use these balls to form a fiberglass mould ... wish me luck...!
I have made the base from concrete, I formed the top in one piece allowing a 5" deep recess for the insulated vermiculite cement mix...
More pics in a few days as I progress

[UtahBeehiver]

I would suggest you work or contact "David S", he is our expert on casting ovens, He can give you some insight of do's and don't. Also there was a very nice cast oven done by Iron Pony who used a fiberglass mold. BTW, like your bonsai plants in the background of one of your pics.

[fox]

Yeah I love my trees!
I did have a look at ironponys thread but my mould will be very different as I intend to make a 3D mould than can be filled and vibrated.

[david s]

Making inside and outside moulds is time consuming and not worth the trouble for a single casting IMO, but you've already started down that road so you may as well continue. Be extremely careful in the design of your moulds (it appears that you have gone a little deeper than a hemisphere on the exercise ball) and not forgetting to use a release agent, you don't want to be in the situation where the castings can't be removed without damage to either the castings or the moulds. The first casting from a mould is always harder to remove than subsequent ones. A sand castle mould in situ, apart from having no problems with mould removal also negates the need to move the casting, because it's already in position. What materials are you using for your castable mix? Have you considered either or both burn out fibres and stainless steel needle reinforcement?

[fox]

Hi David, I am relying on your support to get this right.
I have decided to go down this route due to two friends also wanting to build a similar oven, they are helping finance the mould.
I can purchase pre mixed dense refactary cement and ss fibres although it won't be cheap to import, I have found a helpfull UK supplier.
In lots of ways I would like to use home brew but that involves importing clay and river sand and s/s pins so it would cost much the same for me as dense castable!
To be honest I have not got a steadfast plan as to how the components will be taken apart or how many parts to cast but I am confident I can make it work.
At this stage I will get on with casting the inner and outer domes in fibreglass and then consider the options of how to get a clean removal of the cast cement.
Typically it raining today ... not productive for working with glassfibre.

[fox]

We have been suffering a very bad spell of weather and it is a Bank Holiday weekend!
However I did learn something about Gym balls... after watching youtube vids about inflating the balls it became obvious that I need not have purchased two different size balls.
It seems you can pump them up pretty big without fear of bursting.

[fox]

I have managed to produce a few components, I decided to make the dome in five parts of roughly the same mass.
I plan to make a one piece mould for the entrance & one more for the base.
The next issue will be making removable pieces for the lap joins in the dome....

[deejayoh]

Very nice. You can paste those pics right into the post. hit the "Upload" button and then paste the address from the "Direct" box in the "share this photo" section of photobucket.

I say this because Photobucket is a spammy, ad-filled piece of cr@p and I never want to hit those links! High likelihood of malware sharing with that one.

[fox]

OK, I am of an older generation & I don't fully understand what you are saying.. what link do you need to hit it order to see the picture?
I have posted hundreds of photo bucket pictures on numerous forums, very often that is the preferred method, never had anyone mention it before!
However, of course if my photo is an issue then perhaps a mod can sort it out?

[deejayoh]

Well that's funny. Where this morning I only got links when I opened your post - now there are pictures. It was the clicking over to Dropbucket that was the issue. Not sure if something changed or it's just working right now. But there is a way to load your pics direct to this site, you don't need dropbucket. (FWIW, I'm not the youngest guy here either)

[fox]

Wow making this mould has proved to be extremely time consuming & pretty difficult!

The main issue has been that the inflated balls are not actually uniformly round, in fact they are quite deformed. What that means is that when the inner and out sections were placed together the gap between them is all over the place! I have put over 70 hours into the mould so far and it is far from finished.
However I have ordered my dense casting mix + S/S fibres & ceramic blankets - enough to build 3 domes so it is not an option to slow down now.
I have the day off work tomorrow, I hope that time spent working on the mould will see some positive progress....

[david s]

Told ya. 4
probably too late to do a U turn now, press on.

[fox]

Yes you did LOL..... I will get there though...

[fox]

I found a few spare hours today & I think I have it pretty accurate now... lots of filler, a few gussets & a lot of sanding....
I had planed to separate the pieces with a half lap joint but I don't know how necessary this would be? Can I just make straight joints?
If the balls have given me a perfectly symmetrical cast then I could of made lap joint quite easily but now it would involve much more work & time!
Certainly it would be easier to cast the dome in one piece with a separate key stone & entry but the main dome section would be very heavy & maybe more prone to cracking?
I guesse I could cast it in one piece and just cut it into sections with a 9" cutter?!

[fox]

I have answered most of my own questions about the mould & I have made positive progress today
Also my order of Refractory cement mix arrived today, it looks like fire brick grog & cement but it also has other additives that looks like black sand? The needles are really needles, quite large too.
I wonder if I could of just collected some S/S swarf from any of our local machinist?