I usually use R&R 965 for my molds, which works great up to about 10 lbs mold weight. I have a wax piece that is essentially a spherical bowl, with a deep cup in the middle, and the size got away from me. I attempted R&R 965, but at 13 lbs the mold was just too big and cracked in the prefire the way larger molds always do with that investment. I've never constructed my own mold out of layers before (except for once in a class a decade ago) and I'm nervous about it, especially since R&R is so reliable and in hundreds of casts I've never had a kiln disaster.
The bowl is round, about 5" sphere, walls 2" thick at their thickest, requiring a little over 3 kg of glass (with a sizable reservoir, this makes the block mold about 8"x8"x10"). Unless someone can recommend an investment for a big block mold that will be sturdy, I'm going to have to build it. I'm planning on using Bullseye investment and grog according to their technical spec sheet.
1.) I'm especially worried about the thick heavy part of the mold that will be the inside of the bowl. Should I do this as I do the outside, or modify it somehow to be lighter so it won't drag down on the mold during dewaxing?
2.) Should I shore it up with a skin of fiberglass between the 1st and 2nd coats? I've met people who swear this makes their large molds incredibly sturdy.
3.) Or is there a different investment that will work for a block mold this big? Or a way to shore up the R&R 965 to be sturdier? Like chickenwire around the outside or something like that?
Thanks in advance for any advice anyone can give me about this!