Bulb Mold

I need some advice for making plaster of paris bulb molds. I have made a couple and they both cracked and fell apart after one or two uses. They were dry. Is there an additive or special plaster? I made the mold an inch or so bigger than the bulb. Maybe that’s not enough? Any help?

Don -

that’s been my experience as well. Seems they are good only for a short time.

I had a thought a while back, :scared: :graduate: but haven’t needed to do a bulb since. Maybe you could try and report back. I have no idea if it will work.

My thought was to purchase “chopped strand” fiberglass like they use in a chopper gun for big boats, cars or bathtubs. I wonder if mixing in the strands would cross link to help eliminate (or reduce) the plaster from cracking. I see where you can now buy wallboard joint compound with the fibers in it to prevent cracking - just wonder if it would work on a bigger/bulkier scale?

Most people have problems casting led with plaster of paris molds because they don’t encase the mold. I assume you make the mold in a wooden box. Right? If so just leave it in the box while casting. Works for me…

ps: some people make molds out of Auto Body Putty (Bondo), but it’s hard to work with because it sets up to fast.

I had that thought. I think there is an additive for concrete that may work. I will report back.
Good idea. It’s not like I’m using the box for something else. Even if it does crack the box will hold it together.
Thanks guys


I suggest that you speak with Lawrie.

He uses a sandbox. He has a replica of the bulb with pouring holes and the fin slot. He presses that in a sandbox. After removing the plug, he does a pour. Then he recovers the sand and repeats the process.

Thats interesting.
We did sand casting in jr.high school.
The sand was red, and smelled very strong like iron.
Even though the sand was dry it would compact, and hold its shape very well. I will have to research it more.

I also remember doing aluminum casting in grade 11-12 metal shop. Actually made a disk sander including tilting table (wish I kept it). Those were the days when schools had shop classes. I checked out the local schools here in my City and they no longer do anything like that, but I guess you could check see if your local high school does it still. We also cast using a yellow powder that when heated formed a hard mould can not remember the name. I will search google for casting sand and see what comes up. I actually believe it had some oil in it as the teacher would add oil to it every once in a while. good luck

I just checked google and the sand is called Perma-bond oil sand