Any advice for very big molds ? (Stargate)

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Active Member
Hello everyone,

We’re really close to being ready to mold our Stargate panels and chevrons and wings and glyphs and strips and bumpy rings and whatever you can see on a Stargate. :D

Now, we’ll have to order everything we need for the molds and the casts and it’ll be a HUUUUUUUGE order with many gallons and it’ll cost a lot of $$$$$$ or for us €€€€€€.

But before making any mistake, we’ve got a few questions, because maybe some of you already made really big molds for sets more than for props. I’m thinking of Daleks, or studio scaled models…

I’m searching for the best materials to mold and cast my parts, here is a picture with everything :


/*/ Some details /*/

-The idea is to order to Smooth-On.

-The size of the biggest panel is 2m x 1m.

-We have to make 10 to 20 casts with each mold.

-There are tiny details of 1mm on each panel.

-You can see on the picture above the sizes of the molds : 3 big molds (2m) and 7 little ones (30cm).

-The parts 1 to 4 are hollow, the part 5 is solid and made with clear resin, the part 6 is hollow and made of clear resin and the part 7 to 9 are solid.

-The goal is to make light and durable casts, especially if we hit them because we’ll have transport them a lot.

-The casts will be painted and the clear casts (6) painted in part.

-It’s possible that the final product will be displayed outdoors under the sun and the rain.

-Our budget is not infinite, so we’re aiming for a medium-good quality, not especially the best.

/*/ What I think (and where I need advices) /*/

-For the molds 1 to 3 I would make a brush-on mold so I’m searching for a silicon, maybe a reinforcement cloth if you think it would be useful and a resin for the shell.

Maybe Rebound or Dragon Skin ? / Plasti-Paste or Shell-Shock ?

/The other molds (4 to 9) will be poured so there I just need a silicon.

Mold Star 16 ?

-For the casts, I would make the 1 to 4 with urethane plastic and fiberglass if needed.

Maybe Shell Shock ?

/ the 5 and 6 with the same clear (tinted) and durable resin without reinforcement (we’ve access to a vacuum chamber to avoid bubbles)

Crystal Clear with So-Strong ?

/ the 7 to 9 with a resin made for solid and durable casts.

Smooth-Cast 310 ?

That’s for the materials. But for the process we already attempted to mold one of our panels and the main problem was that one side was not sticking to the jacket as in this drawing :


So I think the best way would be to fill the gap with silicon to avoid the silicon skin to fall, adding chopped old silicon will avoid spending a lot of money just for a bit of mass. One other option would be to make keys that stay in place once the mold is returned, but I really don’t know if it would work…

What do you think ?

I’ll obviously have more questions as we’re still thinking about how to do all these molds, so I hope it’ll be an interesting topic. ;)

Here is the nice answer I received from Smooth-On :

Let me start out by saying your Stargate is very impressive and nicely sculpted.

Because of the size of your larger panels I agree that you want to go with a brush-on rubber. Rebound 25 would be a very good choice for this. You'd want to brush 3-4 layers on and do a thickened layer to smooth out and fill in any undercuts. You can thicken Rebound 25 with Thivex.

For the shell you can go with Plasti-Paste II or Free Form Air. Free Form Air is a bit labor intensive but will give you a lighter shell that you can sculpt smooth and shape better than you can Plasti-Paste II.

Crystal Clear is a good choice for your clear parts. It is UV resistant and very durable. EpoxaCast 690 is also a possibility but it is not as UV resistant as Crystal Clear.

Mold Star 16 is a very good choice for your straight cast parts. It is what we use here when we need to make basic molds for casting. Easy to work with and very durable and long lasting.

Shell Shock will work well and pick up all your detail for your larger panels. You would want to brush in 2-3 coats then back it with glass cloth and Epoxamite. If you want an even stronger piece I'd press in a layer of Free Form Air behind the shell shock and then back that with the glass and Epoxamite resin. That will give your pieces more thickness and added strength while not increasing the weight much. You'd want to press in a layer of Free Form Air that is between .25 and .5" depending on what you prefer.

Smooth-Cast 310 will work for your cast parts if you plan to paint them or you could go with Smooth-Cast 326 which is a clear amber and you could pigment the resin instead of painting. IF you use UVO tints your cast would be opaque and the colors are UV resistant.
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Sr Member
I think you should rebound 40 as there aren't any undercuts and it will be less floppy in the mother mold. For the mother molds use fiberglass, it's a lot cheaper and a lot stronger than shell shock. I just don't think that a huge jacket made of that will last.


Sr Member
I make large molds for a living. To prevent the mold from falling in like that, here's what we do.

1. Get a slab of clay, maybe 10"x10". About 1/4" thick. Or thicker, doesn't matter too much. Just make it flat and level.
2. Get a bunch of marbles. Normal glass marbles. Press them about 1/8" into the clay.
3. Block mold those suckers. Pour in enough to cover the marbles well.
4. Remove the marbles from the cured mold. Coat the ever living heck out of it in some kind of barrier. We use any clear spray paint we have available. Coat once, let it dry, coat again, let it dry. Inside the holes, on top of the mold, all over.
5. Cast silicone marbles. Dozens of them.
6. Pop 'em out. This will be easy if you released it well, and hard if you didn't.
7. We usually do like 4 layers of silicone on stuff. In the third layer, we often lay down a layer of nylon mesh fabric. The exact kind is really not important. Coat it with the next layer of silicone.
8. After the last layer, we mix up a little more silicone and thicken it a lot with thixotropic and cabosil (fumed silica). We use it to stick our little silicone marbles to the outside of the mold.
9. Apply the mold jacket as usual. The jacket will now have little spherical holes that the marbles lock into. They're squishy enough that they can pop in and out no problem.

Anyway, that works pretty well for us. Hope it's good info! If anything doesn't make sense, let me know and I'll make some diagrams.


Active Member
Thanks a lot. :D

@Andy : Don't you think it will be harder to demold my cast if I use Rebound 40 because of this ?


If I follow the advice of Zlurpo I think it'd be better to use Rebound 25 to avoid the round keys to break and to let them come out easily.

I totally agree with you for the shell ! I'll use fiberglass, maybe a better brand than the last time. Making the entire shell in Free Form Air or Plasti Paste II would cost me way too much money for a mold that would be even more fragile.

However, for the casts, I know a professionnal who made models for Theme Parks and he used Shell Shock and fiberglass, it resulted in very durable pieces.

@Zlurpo : It seems a really good way to keep the mold in place. Would it be something like this ?


Or just with half spheres ? Because we already tried half spheres and it didn't work as well as we thought :


Which size of marbles do you use, 2 inches ? I would place a marble each 4 inches I think. Is it correct for a big mold like that or would it be better to use more little marbles ?


Sr Member
Yep, exactly like your illustration. Not half marbles, they don't lock. We usually use plaster as a mold jacket, which isn't perfect for detail. If you're going to use fiberglass I would experiment with using like... 60% of a marble, rather than the ~85% my method normally gives you. Fiberglass resin will create a very tight seal around the silicone ball, much more than plaster, so too much of a lock will be hard to take off.

I'd just use the normal size marbles, and use more of them. No need for large ones. Every 4 inches sounds like overkill, but maybe it's the level of precision you'll need.

And if you use fiberglass, remember that unless you use a LOT of layers, it's not rigid so your molds (and therefore casts) can warp while curing if you don't reinforce the fiberglass well.

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Sr Member
Here are a few thoughts on this matter:

the marble idea is good, and will work fine for you, but for this size mold, I would pour a strip of rubber to use as a lock. Meaning build (or buy) a little (1cmX1cm) channel, and pour rubber into it and let cure. Make a dozen of these strips, and after the final coat of rubber is brushed on and still tacky, apply the strips horizontally on those vertical sections to keep them from falling. I think making the strips is simpler than the marbles, and cover more area.

The next thing is the shell. I don't know how many Star Gates you are planning to build, but if it is less than 3, why go to the expense and trouble of a fiberglass jacket?

Gypsum (Plaster) is a cheap and easy shell, but heavy. Your 2m mold could weigh over 100 lbs. If you build you jacket with a burlap-reinforced plaster jacket, with some wooden supports, this will be much faster and cheaper than any type of fiberglass, shell shock, or other polyester material. The drawback is you will need 2 or 3 people (Preferably a forklift) to lift and move around. This should not be too much of a problem if you will pull only one part from the mold.

on the subject of which silicone RTV rubber to use, I have never seen a bad silicone. They all work great. The difference is their tear strength and shore hardness. A small shore hardness is for detailed pieces with a lot of undercuts, and a higher number (harder) is for panels pictured above, where thee is a lot of detail, but not too deep. In the picture above (with the red arrow) shows (what I think) an under cut that is not part of the detail. It is part of the wall section and the mold rubber will (once out of the shell) peel away. So you can easily go with shore 40.


Sr Member
The strips instead of marbles would also work. It holds by friction rather than actually locking on, but based on your diagram it should work.

And with regards to silicone, it will all work fine, but some brands are easier to use than other in the application. The kind we use at my work now is great, because if you add any thixotropic, it basically stops being runny. It goes on where you paint it, and doesn't drip. That's bad for a first coat, but it's wonderful for subsequent layers. The old stuff we used would get thicker, and then just run and drip thickly.

SG Merc

Well-Known Member
Since you are going to be tinting your transparent parts, I would use the Smooth-Cast Colormatch system (325, 326, 327) instead of Crystal Clear. It will cost less and is easier to work with.


Active Member
So, I tried to synthesize all the informations and advices I had until now. I checked all the technical bulletins to be sure.

Here is what we think in the team :

For the mold of the big panels, we will use Rebound 25 for an accurate print coat and then Rebound 40 (with Thivex for the last layer) to get a durable mold. We will also use a layer of nylon mesh fabric and put rectangle strips of silicon on the mold to avoid any part to fall from the jacket.

For the jacket, we will use any kind of cheap resin that works well with glass cloth (I had some troubles last time with a bad resin or bad mat, both was cheap and it became pretty viscous) and a reinforcement with metal tubes to keep its chape. It worked well last time. We will do it in two parts so it will be easier to remove.

For the casts of the big panels :

We will follow the advices from smooth-on and use Shell Shock and Epoxamite to fix the glass cloth on it. Which glass cloth would you use? I think 3.5 oz (100g/m2) would be enough as it will stand on a metal frame and it will also be usable for the jacket.

For the little molds we will use mold star 15 and cast the pieces with Smooth-Cast 310 or 326. Do you know which one would be the best as we would like to tint it black AND THEN paint it?

For the clear casts we will use Smooth-Cast 326, but are you sure that it will be durable enough compared to crystal clear ? Our parts are from 1/4 in (0.5) to 1 in (2.5 cm) in thickness.

I think it should work well like that. Do you see anything that could go wrong?

Again I also asked Smooth-On :

You can use Epoxamite with glass cloth for your shell. It is very much like fiberglass resin but it is much easier to work with and has a lot less of a smell to it.


The glass cloth you use is entirely up to you. The mesh you indicate sounds like a good choice to me but it is really your preference.

Smooth-Cast 326 is a clear amber so will that the black SO-Strong much better than 310 which is white and will only give you a deep gray if you add black.

Smooth-Cast 326 is clear amber so if you make casts with it they will have a slight yellow/amber cast. Crystal Clear is water clear. If you are okay with the yellow/amber cast either material will work for you. SC-326 is 73D while Crystal Clear is 80 to 85D depending upon which you choose to cast with Crystal Clear 200, 202, 204, 220 or 221. Crystal Clear is also our best material for UV resistance and will yellow much slower over time if exposed to direct sunlight than SC-326 will. Of the two materials Crystal Clear is the better one to use but I believe either could work for you.

Ah, no matter how well you plan lots can go wrong. Material wise though everything sounds good. My suggestion to you would be to make sure you have a clean work area and that it is as close to 73°F as possible. Also mix your materials very well making sure to scrape both the sides and bottom of the buckets and pre-mix each part separately before combining and mixing them together. If you go with Crystal Clear make sure to follow its technical bulletin, heat your mold and do a post cure.

Make sure you seal your wooden original before you make a mold of it.
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Active Member
Hey guys,

We finally received our order of materials for the mold and for now we did the silicon part and only a half of the shell.

We had 15% from KauPo (SmoothOn seller's in Deutschland) on the order as we asked for a huge amount of silicon and resin. Enough to mold and cast the entire Stargate.

We paid more than 12'000$.

Here is how we did the mold :

- We made some big keys with a mod allowing to make two at a time. As it was too thick I then cut each one in half.

- We placed a few pieces of wood screwed to the table to keep the panel in place as it was not rigid enough to stay in place.

- We the used some non sulfur clay to seal all the sides and then used some vaselin on the table to avoid the silicon and the resin to stick on it.

- We made a first print coat of rebound 25, maybe a bit too thin. The whole thing is made only with R25. We used a vacuum pump to avoid air bubbles.

- We made a second layer of silicon with a bit of blue to be sure to see were we put it. Again vacuum pump.

- We then wanted to place a glass net but forgot to put a bit of thixotoped silicon on the edges before... so we had to remove the net... and one piece was already stuck in the silicon so we had to let it there.

- Putting back the net after the silicon was cured was really painful, but we managed to do it anyway "massaging" the silicon thru the net. We could have made an other layer to stick the net on it but the mold could have been too thick, it's already really thick as we already used something like 32Kg on this mold.

- As the net didn't stick to every place but well enough, we cut the bubles, filled it with thixotroped silicon then replaced a patch of net on it.

- We then applied a full layer of thixotroped silicon in which one we placed the keys.

- To avoid the keys to be cut by the shell, we waited for the thixo to be cure and then added a border of thixotroped silicon at the base of the keys.

- We then made a final coat of a nice red-bubblegum silicon on top of that to smooth everything.

It took like 4 days to do it as we were four people only for the first two layers then we were only two.

Now for the shell, that a bit more complicated as we don't even know if we will not destroy what we just made.

- I cut the edges to be sure it fit well in the shell, I made the corners round to avoid to tear it appart.

- We made a wall using what we had, a piece of MDF and four batten.

- We used some clay to fill the hole under the wall and to make some keys.

- Then we applied a release on the silicon mold and some more vaselin on the wall (I should have put more on the clay itself).

- Here is the first layer of an acryl resin (not from smooth-on) it's made from a liquid and a powder. It's non-toxic but I'm affraid as it could be too fragile, that's why I could destroy it and do it again with a normal polyester shell. I'm waiting from some advice from the store.

- The second layer.

- Then a glass net soaked in the resin.

- We removed the wall and the clay, if everyrhing is ok we will make the other part this weekend and use batten as reinforcement.

And that's it... it took 4 to 6 days to acheive this.

We made some videos of the process but it could take a bit of time before we put it together. We'll also do it for all the molds we'll make, it should be easier as it will be the same process on smaller pieces.

If you have any advice, question or comment, please don't hesitate to say what you think.
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