Throw away mold making?

GBrittelleJr

Well-Known Member
Hello, I've read thru the site about making a throw away mold. I'm sculpting a helmet and it needs to be very smooth. So I know all about the silicone caulk and soapy water method for the mold itself, but what of the throw away helmet? I don't really want to use my smooth cast 300 for something that isn't the final piece, but I am going to (obviously) need a material that can be sanded and tooled and possibly sculpted upon further. Any suggestions, or should I just suck it up and use resin? Thanks!!
 

PepMaster

Sr Member
I've seen people that have used wax, 2 part expanding foam, and I've even seen good results from people who have used plaster.
 

GBrittelleJr

Well-Known Member
The wax sounds interesting, more firm than the clay I'm using I'd suppose. And I have ample amounts of hydrocal that I could use. I just didn't want to use plaster because it tends to chip, and I don't want a 30 lb buck.
 

the_batdemon

Sr Member
I've seen people use Rondo (Bondo thinned with resin) to run a slush cast. They've also only used plaster as the throw away mold, since it's cheaper and absolute perfection in detail wasn't necessary due to the inevitable clean up and smoothing.
 
Personally, I'm not a fan of using plaster for any kind of casting. I would like to hear more about this Rondo thing though. Anyone have any pictures of results with it?
 

the_batdemon

Sr Member
The Rondo formula I was taught is 50/50 Bondo (or any other brand body filler) and polyester resin. The body filler is basically thickened polyester resin anyhow. I was also taught to use both of the hardeners for the body filler an resin, but have seen online people just using the MEK for the resin in double the normal amount.

The resin thins down the body filler. You can adjust your ratio for desired viscosity.
 

GBrittelleJr

Well-Known Member
It's not the mold material that I'm concerned with, it's what I'm going to cast the helmet in itself. The Rondo sounds promising, but how strong is it? Would I just better off biting the bullet and casting in resin?
 
It's not the mold material that I'm concerned with, it's what I'm going to cast the helmet in itself. The Rondo sounds promising, but how strong is it? Would I just better off biting the bullet and casting in resin?
From what I've heard rondo is not as strong as fiberglass/resin. This is because from what I understand body filler isn't as strong as fiberglass/resin at the same thickness. If you're going to refine it further why not use something that saves you a bit of money? I haven't analyzed the cost of fiberglass/resin vs rondo so I'm not certain on which one is better from a cost perspective. However, if you're going by just weight/cost ratio then go with fiberglass/resin. If you want the surface to be sandable go with rondo.
 

the_batdemon

Sr Member
While still sandable, Rondo is stronger than body filler by itself. One advantage over 'glass and resin is you can make a slush mold and not have to worry about laying up the cloth or mat in the tight confines of a helmet mold.
Is it stronger than the same thickness of a 2-part plastic resin? Anything less than 1/4", probably not.
Is it a cheaper alternative? It might be, I haven't priced materials in awhile.
As far as weight goes, it's a throw away cast and won't be worn. Personally I wouldn't be concerned. For a final cast, I'd personally use a product from Smooth-on over a glass/resin combo.
 

GBrittelleJr

Well-Known Member
So the rondo would be strong enough to hold its shape and not crack when I'm working on it? It sounds very promising, considering I already have bondo and resin.
 

the_batdemon

Sr Member
So the rondo would be strong enough to hold its shape and not crack when I'm working on it? It sounds very promising, considering I already have bondo and resin.
Definitely. If you're concerned about strength, just make a thick casting, say 1/2". As long as you use the appropriate amounts of hardener.
 

Ozymandius

Sr Member
RPF PREMIUM MEMBER
For pouring up a hard master for fine tuning and sanding, I just go straight to Smooth On's Onyx mixed with Urefil 7. Far easier and less toxic than Rondo, plus much cheaper since you have to use so little of it.
 
Top