Molding Material for big pieces

ChalkyChalkson

New Member
Hey there!
I am trying to make a safe to play with replica of an antique saber of the Moghul period, to complement my kit.
I thought I'd just do it the how lots of people make stunt props, by just making a silicone mold and filling it with PU foam and a carbon or glass fiber core. I've done similar castings of smaller things before, so how hard could it be, right? So I headed on the the web and tried to find 30l of silicone. Until then I did not realise that this amount of silicone would cost me €1000+.
Does anyone have a suggestion how I could do this? I have thought about plaster, but I don't think the mold will turn out that well. I've also look at the silicone caulk, but it doesn't seem fit for this purpose.
I am quite new to making bigger things from plastics, this will actually be my first foam casting, so please tell me if I am being stupid with my assumptions :)

Greetings from the other side of the pond,
Chalky
 
The truth is, good silicone is not cheap. Not sure what you guys pay in the US or UK, but here in Australia, silicone starts at $65/KG. When calculating the volume needed, you do it the same as if it were water. So the bigger the part, the more silicone is needed.

In a typical box mold, take the length in CM x width in CM x height in CM divide by 1000 to find how many KG you will need to fill that cavity, less the volume of the item you are molding. When I mold my "All Spark Cube", I will build a special box with removable inserts. I only calculate the volume of the cavity around the box.

With really big or long items like a sword, you might want to go the glove and jacket approach. This uses far less (usually about 1/4) silicone but does need other materials like fibreglass.

You still set up your part in clay, but rather than boxing it in and flooding the box with silicone, you only use the amount to make a glove to fit the part. The wall thickness might only be 1/4 all over Vs the box method where a wall could be 2" thick.

There are heap of videos on this method. I've only ever done one open back or one part mold and where I used D65 plastic and cloth for the jacket. The mold used 1.2KG vs 4.5KG if it were boxed. I've included pictures, so hope this helps.

Since doing that mold, I've learned a better way to ensure more consistent wall thickness and an over all better mold which involved layering clay over the part and making the jacket first. This can be a little more complex because you will registration point for both the jacket and the master part.

Image 1 is the pulled test part. It came out good, though I did not have enough product on hand to make it full depth.

Image 2 is layering up of the plastic and cloth to make the jacket. I used pet mesh because it is fibreglass reinforced mesh. The resin filled in the hoiles and this makes the resin very strong. You can also use fibre glass cloth for this. The jacket needs to be as thick as required to be self supporting. D65 resin is about $40/KG here and so is fibreglass. The difference is, there is no itch from D65 resin and why I used it.

Image 3 is the inside the glove. So this is 1.2KG instead of 4.5KG that I would have needed in a box.

$65 x 1.2 = $78 Vs $65 x 4.5 = 292.50 Big difference. And I used a bit of over a KG of resin. Say another $60 to make the jacket

Image 4 is the master part. because this was made originally from laser cut MDF, I used epoxy resin to seal the MDF and then sanded it to 800 grit to ensure a smooth surface with a matt finish.

Image 5 is the layering of silicone onto the master part. Even though this part was symmetrical and had hollows to act as as registration points, I still added a silicone dome (you can see one in the picture) to ensure I knew which way was the front.

You might think open pouring like this is wasteful, and yes some does flow off the edges. The point being, this first bit is only for the "print coat". It has to be poured in one layer and it has to be completely covering the part. Once that has gelled, you can go around your piece and scoop up and overflow and tip that back on top. I also mixed a filler called Cansosol into the silicone to help it layer up better and stick to the vertical walls of the cutouts. You just keep adding until you have a desired thickness.

In the end, the glove in this case is only 3mm or 1/8th of an inch thick. It is supported in the jacket and when set up, does not warp.

For your sword, you can do a similar thing, but you will need registration points so that when you flip it to make the side, everything lines up.

Good luck.
 

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Hi,
Thanks for the answer!
I am not 100% sure I fully understood what you were saying, so let me paraphrase and correct me where I am wrong:
You took your masterpart, and covered it with fiberglass, then you covered it with a very thin layer of silicone. So when you are casting you will put the thin siicone mold into the jacket of fiberglass.
This seems like a really good method, but the (probably wrong) way I read your comment, the silicone glove wouldn't fit into the jacket, since both are modeled around the same object.
The covering in resin and sanding part is quite smart, too. I was going to make my masterpart from styrodur and cover it with wax, but I think your way might result in a better mold.

Thanks again and I hope to hear from you again.
(your casting looks very nice btw, what did you use it for?)
 
Thank you!
Now I understand the technique cavx described before :)
I am not too sure whether it is possible to start with an open pour to secure the rod when using foam tough. But I guess I will figure this part out... Since my sword (for safety reasons) will be a lot thicker, I guess I will be able to suspend the rod in the closed mold.
Why did this guy powder the mold with aluminium though? It can't be for the metallic effect, since alu powder is pitch black, can it?
 
Hi,
Thanks for the answer!
I am not 100% sure I fully understood what you were saying, so let me paraphrase and correct me where I am wrong:
You took your masterpart, and covered it with fiberglass, then you covered it with a very thin layer of silicone. So when you are casting you will put the thin siicone mold into the jacket of fiberglass.
This seems like a really good method, but the (probably wrong) way I read your comment, the silicone glove wouldn't fit into the jacket, since both are modeled around the same object.
The covering in resin and sanding part is quite smart, too. I was going to make my masterpart from styrodur and cover it with wax, but I think your way might result in a better mold.

Thanks again and I hope to hear from you again.
(your casting looks very nice btw, what did you use it for?)

Covered in Silicone first, then resin/matting. If I was to do this again, I would lay clear cling wrap over the master, then layer clay, then make the jacket from fibreglass. Once done (and this is where those registration points come into play) remove the clay, place the jacket back over the master and fill the cavity with silicone. This ensures a more consistent glove thickness.
 
Hi Chalky,
That's my blog. I had to do the open pour to secure the rod because when I did a closed pour the pressure of the plastic burst the edges of the mold and it leaked. This probably shouldn't be a problem if you're using foam, and if you've a thicker piece the mold edge should be less likely to buckle.

Dusting the mold with aluminium does create the metallic effect. The powder is grey, and when the piece is buffed and polished is comes out looking like shiny metal.
 
Not sure where you're getting your silicone from, but I get mine from mbfg.co.uk, it's about £80 for 5kg, which I think would be about €100? Still not cheap but lots cheaper than your original figure!
 
Not sure where you're getting your silicone from, but I get mine from mbfg.co.uk, it's about £80 for 5kg, which I think would be about €100? Still not cheap but lots cheaper than your original figure!
The sword would need about a 3x10x0.5 dm (decimeters are awesome for volume calc :p) mold that is 15l which is about 15kg, so about 300 bucks. My original figure came from using the cheapest silicone I could find in GER for 1000€ / 40kg and estimating the mold with 4x10x1
 
Mbfg.co.uk is a good site, I get my aluminium powder from there. I get my silicone from http://www.benam.co.uk/. With the fibreglass matrix mold, I used 2-3 4.9Kg buckets of Mold Max 40 to mold one of my swords, so somewhere between €200-300.

To polish the aluminium powder, I first buff it with 0000 steel wool, then rub Autosol Metal Polish over it and polish it with a rag.
 
That is still quite expensive especially when factoring in the glass/carbon fiber matts and the resin I will need in addition to that (might resort to cloth, depending on the prices). I think I will need to find other people who are interested in a casting of this to afford that.

Thanks non the less,
Chalky
 
That is still quite expensive especially when factoring in the glass/carbon fiber matts and the resin I will need in addition to that (might resort to cloth, depending on the prices). I think I will need to find other people who are interested in a casting of this to afford that.

Thanks non the less,
Chalky

Yep, it's an expensive business alright! You may find it helpful to try making a mould of something smaller like a knife for a first project. That way you have a better idea of how the project will go when you try to do something bigger, and you don't find yourself throwing away €300 of silicone or something because of a stupid mistake.
 
That is still quite expensive especially when factoring in the glass/carbon fiber matts and the resin I will need in addition to that (might resort to cloth, depending on the prices). I think I will need to find other people who are interested in a casting of this to afford that.

Thanks non the less,
Chalky

You can make the support mold with plaster, I've used the craft type that kids use to make hand prints in. For a large mold reinforce it with burlap.
 
You can make the support mold with plaster, I've used the craft type that kids use to make hand prints in. For a large mold reinforce it with burlap.
Thought about that, too.
I think I am going to go with old cloth and plaster, or, if i manage to get next to free cheapo resin, with resin and cloth

- - - Updated - - -

Yep, it's an expensive business alright! You may find it helpful to try making a mould of something smaller like a knife for a first project. That way you have a better idea of how the project will go when you try to do something bigger, and you don't find yourself throwing away €300 of silicone or something because of a stupid mistake.
It surely is.
If I buy 5 gallons of silicone and 1 gallon of foam (I always calc with 1 gallon = 5 litres) I should have enough to make one or two smaller test pieces
 
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