Advice needed for casting large open model parts

userd1402

Sr Member
RPF PREMIUM MEMBER
I hope this is the best place to ask for help with a new project I'm just getting into. I am building a master model for a studio-scale item from a film (that's as clear as I can be for the moment) and I need to replicate it several times but it's pretty big.

The overal size of the completed part will be 600mm high x 450mm wide and about 180mm deep and it's got to have an open back for the installation of electrics and other things. I have thought of three ways I could make copies of it; A fiberglass mold made with matt and resin, which would be nice and rigid but would probably require I make only fiberglass copies (not sure as I have never used fiberglass before). Another method would be to get vacuum-formed copies made in rigid plastic by a commercial thermo-forming company but there may be a large set up and minimum order quantity problems with that idea (and my mdf and car-body filler model might not be quite good enough too).

The last method I'm thinking about is a silicone/rubber mold and then poured resin copies. This method is probably the most suitable for someone like me to do in my own workshop at home so I'm looking into it seriously but nobody seems to be able to tell me how to pour resin into open molds and achieve large parts like this;


SW-Lamp-01.jpg




Everyone I have spoken to says they either make solid resin casts (which are obviously small) or if they need hollow casts they use a roto-cast frame but this piece will be a BIG wooden box (for rigidity and alignment) filled with rubber and resin which I estamate will weight over 10 kilos or they make a two-part mold with an 'inside' surface and an 'outside' surface and pour resin in between the two but I don't know if I will be able to create my pattern again as a smaller 'inside' part.

So, can I pour resin into a large open mould and achieve a 'skin' somehow?
Can you 'paint' resin onto the inside of a mold? If so, how?

Help and advice please.
 

JPH

Sr Member
There is a guy here, name is Thorisil or something lie that, garden gnome for a pic, and he makes YUGE warcraft armor sets.

You can slush cast, but holding a 20 lb mold with resin ans dancingb around is gonna make for a great workout video.

If it has fine details, I would make your master prop. Brush on a light layer of Rebound 25, constantly working it, constantly working it. After the first layer, add a little Thivex, like ONE DROP per4 oz. And apply. Keep adding layers of Rebound with Thivex until 1/4 to 3/8 inch thick.
For final layet of Rebound, cutbout foam pieces and coat them in Rebound 25.

Let Rebound dry/cure.

Coat Renound in mold release.

Next, get a hoola hoop or something like that, with a circumference that spans the open/back part of your cast. Use FreeForm air or Plaster o Paris to make a hard outer mold in ATLEAST 2 detachable parts.

You dont want to make a hard outer mold that you cannot pop off!!

Cover the hula hoop with plastic wrap or cheapo plastic bag and only surround it half way around with FreeForm or Plaster.

If you can, make the center of the hard mold round.

After plaster or FreForm cures/hardens yiu are ready to pour.

Pour and use hula hoop and round center to move the piece around.

Viola!
 

userd1402

Sr Member
RPF PREMIUM MEMBER
If it has fine details, I would make your master prop. Brush on a light layer of Rebound 25, constantly working it, constantly working it. After the first layer, add a little Thivex, like ONE DROP per4 oz. And apply. Keep adding layers of Rebound with Thivex until 1/4 to 3/8 inch thick.

Mate this was excellent information. Looking up those Rebound products I see a way to make a two-part silicone mold which I can simply fill with resin.

I can use your technique with the Rebound brushable rubber inside my first silicone mold until I have a nice thick layer then add mold release and fill the void with silicone to give me the second half of my two-part mold. When the Rebound layer is removed from between them the gap between the two will be my shell and it means I can make my finished casts just the thickness I want by adjusting the thickness of the Rebound layer.

Thanks for the advice everyone. I will put some pics up when I make my mold.

Cheers. D.
 
Last edited:

swgeek

Sr Member
You can make a rubber open mold and cast your part in fiberglass. Then your part will be thin and strong. You can also make a two part mold as mentioned. If your pattern is solid or one sided and you need a consistent wall thickness, you can mold the hero side of your pattern, then take out the pattern. Before you pour in rubber for the second half you can lay in sheet wax to get a consistent thickness.
 

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