Question on waste molds

zoal · May 9, 2016

So for my next sculpt (likely a batgirl cowl) I was wanting to do a waste mold so i could cast the whole thing in resin for more controlled sanding/filling. What is typically the recommending method? I assume i'll still need a mother mold for a thin silicone waste mold as I can't think of a way to get a good resin cast without the structure. This sound right?

rundown · May 9, 2016

Alginate is used allot as a wastemold. It's cheaper then sillicone.

zoal · May 9, 2016

Would alginate be a good surface to cure a urethane resin on?

pitchforknumb · May 9, 2016

My typical waste mold is 3-4 layers of silicone or urethane rubber backed with plaster bandages. Usually does the trick.

clonesix · May 9, 2016

Urethane HATES moisture, so alginate might not work as well as hoped.

If there are no undercuts areas that would lock, a stone mold will work. It needs to be released well before casting.

zoal · May 10, 2016

Thanks guys. I think the 3-4 layer silicone and plaster bandages sounds perfect. Appreciate it.

mfxreplicas · May 14, 2016

I just use plaster for waste moulds. Make a two piece mould without any strengthening such as scrim or hemp fibres. Im usually casting fibreglass out of this kind of mould so I seal the plaster well with PVA release and then fibreglass the two halves and join them. Once cured I literally just drop the mould, the plaster smashes and falls away, and there you have your master ready to sand and fill. Should also work fine when casting PU.

pitchforknumb · May 16, 2016

mfxreplicas said:
I just use plaster for waste moulds. Make a two piece mould without any strengthening such as scrim or hemp fibres. Im usually casting fibreglass out of this kind of mould so I seal the plaster well with PVA release and then fibreglass the two halves and join them. Once cured I literally just drop the mould, the plaster smashes and falls away, and there you have your master ready to sand and fill. Should also work fine when casting PU.

I think I'll have to give this method a go sometime. Do you ever have trouble with the plaster falling apart when you demold it from your sculpt? Also how thick to you make your plaster?

Scote · May 16, 2016

Everything that is Resin need a latex or a silicon layer to sit on. Resin HATE moisture. so Plaster and Alginate will produce very weird chemical result

I useful do 3-4 layers of latex or silicon then finish with the solid shell of plaster

Mr Mold Maker · May 16, 2016

A few thin layers of silicone is cheap enough, and will cause you much less headaches in the long run.

As mentioned above, Urethane Resins typically hate moisture. If you're planning a waste mold, you may as well plan so all the products play nice together.

1630 or FC54 if you're over the pond generally do better with moisture than other products. If you absolutely HAVE to cast a urethane piece out of an alginate mold, I'd recommend one of those two resins. I'd perhaps do a quick swish of acetone around the mold to wick off the surface moisture, and then seal it with a wax.. I like Epoxy Parfilm personally.. I believe there is a product called J-Waxx over the pond which is popular.

udog · May 17, 2016

Hi
I also use cheap normal plaster for waste molds. But I normaly cast polyster+fiberglass in them.
Why are you thinking of a waste mold?. You want to be able to sand and fill on a hard surface and then make a silicone mold (or other) of the sanded piece?.
If that´s the purpose personaly I would go for a plaster waste mold, and cast polyester+fiberglass in it.
Also, it´s true PU resins don´t like moisture. But you can minimize the problem by sealing the mold surface with a fast setting paint spray (the kind used for cars etc...) and then wax as a release for the plaster waste mold.
And in alginate molds, you can dry out a little with a hair dryer (careful, not too much or it will shrink), swirl some acetone in it (it draws out moisture),and then apply a pair of coats of the spray paint mentioned above.
I´ve got fairly good castings this way, some foaming once in a while, but nothing relevant.
Also using a quicker setting resin will help to limit the moisture draw. Smooth on has Smooth cast 300Q(quick) I recall.

- - - Updated - - -

#10
Sorry, didn´t see you already mentioned the acetone thing .

- - - Updated - - -

pitchforknumb said:
I think I'll have to give this method a go sometime. Do you ever have trouble with the plaster falling apart when you demold it from your sculpt? Also how thick to you make your plaster?

If the mold is correctly done, it shouldn´t happen.
I like them to be 2cm thick so they´re easy to break down. 3cm at the most.

zoal · May 19, 2016

udog said:
Hi
Why are you thinking of a waste mold?. You want to be able to sand and fill on a hard surface and then make a silicone mold (or other) of the sanded piece?.
If that´s the purpose personaly I would go for a plaster waste mold, and cast polyester+fiberglass in it.

That's exactly the reason. I've never worked with polyester+fiberglass before so I may try this in the future, but for this one i'll probably stick with a thinner silicone application and a plaster shell for a urethane resin casting.

About the polyester+fiberglass, do you just buy the stuff from a home improvement store or is there something better out there i should be looking at?

udog · May 19, 2016

"About the polyester+fiberglass, do you just buy the stuff from a home improvement store or is there something better out there"

Most probably, no. But I don´t know what they have in a home improvement store where you live.
I buy it in specialized suppliers in 25kg cans or similar. Search for laminating products suppliers, if not some Fx products suppliers carry it.
Other crafts like boat making, or insulation suppliers sometimes have it, as it´s used.

To use it for molding casting you will need gel coat (first registration coat), or prepare it yourself with cab-o--sil (wear a respirator, very tpxic) and fillers.
Some people prepare gelcoat with a mix of bondo (automotive reapir paste) and polyeste resin.

It´s a toxic product,s o make sure to work in ventilated areas and the correct filter respirator masks.
Threre are alternatives like epoxy, acrylic resins (not the ame result)...but more expensive.

pitchforknumb · May 19, 2016

zoal said:
That's exactly the reason. I've never worked with polyester+fiberglass before so I may try this in the future, but for this one i'll probably stick with a thinner silicone application and a plaster shell for a urethane resin casting.

About the polyester+fiberglass, do you just buy the stuff from a home improvement store or is there something better out there i should be looking at?

I usually get the bondo and fiberglass resin from Autozone, or any place like that. For the gel coat I always mix the bondo up with the fiberglass resin, and use both hardeners. Works great.

Question on waste molds

zoal

Member

rundown

Sr Member

zoal

Member

pitchforknumb

Sr Member

clonesix

Master Member

zoal

Member

mfxreplicas

Official Licensee

pitchforknumb

Sr Member

Scote

Active Member

Mr Mold Maker

Master Member

udog

Well-Known Member

zoal

Member

udog

Well-Known Member

pitchforknumb

Sr Member

Similar threads