casting a section of a large sphere in polyester resin

Hello, all.  I bought a nice pendant lamp for my house several months back from one of the local big box stores on clearance.  I only got around to putting it up recently, and when I opened it (yes, I already feel stupid for not having opened it right away to check on it...), the main glass lamp shade was completely shattered! Sadly, the lamp was on clearance because it had been discontinued, and none of the stores in Canada have any of them in stock anywhere, so getting replacement is impossible.

I really like the lamp, though, as it matches several of the other lights that my wife and I have purchased, so I figured that I would try my hand at making a replacement shade! 

The shade is shaped as follows: it is a section of a sphere 19.5" across (the diameter of the section, not the of the entire sphere), and 5" deep.  A little math (x^2 + y^2 = 2yr) lets me know that the radius of the sphere that this shade is a section of is 12".

Here is my plan, and I would like to know if I am crazy or if anyone has a better suggestion.  I am going to try to cast a replacement shade from polyester resin, 1/4" to 3/8" thick.  I was going to make a 2-piece mould from plaster, and use my wood router suspended from a gimbal (on a 12" radius arm) to cut the outer mould.  Similarly, I would suspend a plaster blank from the gimbal over my router (on a 11 5/8" radius arm) to cut the inner mould.  Then I would simply coat the two halves with mould release and pour/pump the resin into the gap between the halves.  I'll tint the resin with some dye to get it the amber colour that I want, then lightly sand to get a smoky translucent finish.

My only questions are: 1)  will I be able to get this mould apart without destroying the resin casting, and 2) will polyester resin be strong enough for this kind of casting?  Another option would be to make a wooden mould, but I thought that I might be able to chip away a plaster mould, whereas a wooden mould would be a lot harder to get off of a stuck casting!


PS, of course, if this works, I'll post an instructable of it!

Triclaw3 years ago

it would be easier to make the inside mold and brush resin over it till you get the thickness you desire it will take a while to do but it will work and you will only need one mold . the two haves might get stuck together or form air pockets in the bottom or It could break or chip getting them apart