Prop Casting Question

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Blue Leader

Sr Member
Greetings. I have a question regarding making a mold of a prop that I made, and then making some resin casts of it.

I've been watching videos and reading guides about making this because I've never done anything like this before, and while I think that I understand how to do it, before I order the silicone and resin I just wanted to make sure that what I'm understanding is correct. I'm just a little paranoid, I guess. Heh heh...

The chestbox pictured below is what I'm going to be making a mold of. Well, one of the things. I'm also working on a helmet (which is fairly small, actually) and there will be various greeblies, too, though I figured that the helmet mold could be made about the same way as the chestbox mold, and the greeblies would just have silicone poured over the top of them as a one-piece mold.


To make the mold, I was watching the video here from Tested, and it seemed like that would be a good way to do the chestbox and helmet. Following the video, I'd put clay halfway up the chestbox, line the clay with box walls of some sort, spray on some clear coat, and then pour the silicone on. After the silicone was dry, I'd rip apart the box, pull away the clay, flip the chestbox over so that the silicone side was on the table, rebuild the box, more clear coat, and pour on the other side of silicone. Does that sound correct? Also, I was wondering what type of silicone would be best for this? In the video they used Mold Star 20T from Smooth-On but I can't find that on the Smooth-On website... I also don't have access to a vacuum machine, or really any other type of machine besides rotary tools and small things like that, so that would be out of the question, though I don't know if it would be needed or not. I saw the OOMOO silicone that says it doesn't require vacuum degassing... Would that be a good silicone for me to purchase and use for this project? If so, which one would be best, OOMOO 25 or OOMOO 30? If not, which would be a better silicone for me to use?

To cast the props I was watching some videos, including this one, and thought that it looked simple enough. I thought that it would probably work fine for what I'm trying to cast, though I'd have a big block of silicone rubber mold rather than one more shaped to the piece... For the resin, like in the video, I'd use Smooth-On Smooth-Cast 65D... Pour some in to the open end of the mold, rotate it around for a while, let it set, pour in some more, rinse and repeat a few times until the thickness of the resin looks good. Does that sound about right? Is the Smooth-Cast 65D what I would want for this? I also noticed that for most of the Smooth-Cast resins the Smooth-On website says that a scale isn't required for mixing ratios of part A and B, but for Smooth-Cast 65D it doesn't say that... Is a scale required for getting the correct A and B ratios or is that just a fluke on the website?

And lastly, should I use a mold release before pouring the resin into the silicone mold? I know that it's usually a good idea for molding most objects, but when it comes to rotocasting (albeit by hand) I wasn't sure if I would still use a mold release or not, because I wasn't sure if the resin would be able to adhere well enough to the mold with it to take the shape of the prop or if the mold release would just make the resin pool at the bottom?

So, yeah, I think I get the gist of how to make these but I just wanted to make sure one more time before buying the resin and silicone. Any advice would be appreciated. Thank you! :)
 

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Kevin Gossett

Master Member
RPF PREMIUM MEMBER
Is there detail on the back of the chest box that you're trying to capture? If not, an open face mold will work just fine for that as well. Always use a mold release. It will not only help with demolding, but also help extend the life of your mold.
 

Blue Leader

Sr Member
There are slots on the back for webbing straps, but that's about it. I'd thought of using a one-piece mold but that would make a solid hunk of resin, which would be pretty heavy hanging off of a flak vest. Rotocasting, in theory, should give me a hollow cast which would be much lighter.
 

funboy1013xx

Sr Member
HELLO Portland! measurement's of the chest box and pic's of the back would help, you have TAP plastic's locally in Portland for mold supply's! :thumbsup
 

Blue Leader

Sr Member
Honestly, it's cheaper to buy the stuff from Smooth-On, even with shipping. And if I do use something like the Smooth-Cast 65D, TAP doesn't have any resin similar to that. Not to mention that I just don't like TAP, I find their employees, at least at my local store, to be extremely rude. Their customer service is terrible.

As for a picture of the back, here's a picture of the slots on the back...


But really, I'm just curious if the products I listed and the methods I mentioned to make the chestbox, helmet and so on sound about right, or if there's some other type of rotocast resin I should use, or what. I got some deposits from people to get the materials, but before I ordered them, just because I'm being paranoid and it's fairly expensive, I just wanted to make sure one more time before I spent the money that these would work for me... In theory it seems like it would work fine, but I just wanted to make sure of it, or if some other resin/silicone/whatever would be better, or something...
 

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Fierfek

Sr Member
I think you're on the right track. However, I would recommend using platinum cure silicone like Mold Star 30 and Rebound 25. If you plan on making a block mold, then use Mold Star and if you go with a brush on mold use Rebound. Either one will yield great results and will last longer than tin cure silicone molds, but keep in mind that these are more expensive. The trade offs are worth it in my opinion especially if you don't have a degassing chamber. As for the resin, I would go with Smooth Cast 320 over 65D, either one will work though.
 

Blue Leader

Sr Member
I think you're on the right track. However, I would recommend using platinum cure silicone like Mold Star 30 and Rebound 25. If you plan on making a block mold, then use Mold Star and if you go with a brush on mold use Rebound. Either one will yield great results and will last longer than tin cure silicone molds, but keep in mind that these are more expensive. The trade offs are worth it in my opinion especially if you don't have a degassing chamber. As for the resin, I would go with Smooth Cast 320 over 65D, either one will work though.
This is what you're talking about, correct? http://www.smooth-on.com/Silicone-Rubber-an/c2_1115_1341/index.html?catdepth=1 Hmm, I'll look into those, too... So the Mold Star 30 is stronger or something than the OOMOO? It can be poured as one big "block", too? (Though I suppose I could always use a brush-on rubber, too... I suppose it doesn't really matter much to me in the long run. I just figured pouring it on would be faster and easier.)

As for the resin, this is what you're talking about? http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1210/index.html?catdepth=1 That works fine for hand rotocasting as well? I wanted something that dried hard and solid enough that I wouldn't have to line with fiberglass material, but could still be worn (like the helmet) without breaking. And why would you use Smooth Cast 320 instead of 65D? I'm just trying to understand it all. :)

Also, I know that you mentioned that you rotocast some of your helmets by hand... Do you use a mold release on them? Is there any particular kind of mold release that works better for rotocasting, or would any mold release work alright?

I was wondering, too, if I did go with something like Mold Star 30 or OOMOO or something if it would be possible to brush on a layer or two of it to make sure that it gets any finer details, and after it dries, pour on the rest of it to make the "block" mold?
 
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