Resin Props and Weight

[loz64]

Hello everyone,

Good to be here officially after lurking for a while. I'm a newbie to prop making and I would like to expand my skill set and create all my favorite nerd junk!

I have some questions about making large resin casts lighter. I recently created a sword about 36" long and cast it solid with Smooth-On's Feather Light resin. I assumed that since the company claims this type of resin floats in water, that I would end up with something fairly lightweight. When I pulled it out of the mold, I found I was terribly mistaken! The sword weighed way too much for my noodly arms and was very unpleasant to carry around a convention.

So I've been researching expandable urethane foam as a filler and I think this may be the solution to my weight problem. My plan is to figure out how I can create a resin shell of my sword and fill it with foam, or find a good self-skinning foam if one exists. Problem is, I can't quite wrap my head around the resin shell part of it. Should I rotocast it and find a way to leave a gap for the foam to be poured into? Do I brush thickened resin into both parts of my mold leaving an escape/pour hole, and then fill it with the foam and clamp the halves shut before it explodes (if I do it this way, how do I address the parting seam if there is one)? Most of the videos I've looked up on YouTube explain how to use expanding foam with open-ended molds, but I'm using a two part mold.

In short, what's the best way to back fill a two-part mold? Or, is there a decent self-skinning foam out there somewhere? Any help would be greatly appreciated!

 

[robstyle]

From the sound of it your best bet is fiberglass. Its cheap, easy to use, light and durable. No sense is spending money for something to lug around only to have it break as would happen with resins/plastics due to the shape of the item itself.

 

[teragon]

If you're looking to make a hollow shell, I'd look into roto-casting resin, like the smooth-on 65D. Then you can fill it with expending foam if you wish to, a liquid formula would work best. You do need a hole somewhere though, to pour the foam and let the surplus escape. The easiest way I see would be to separate the blade from the hilt and fill them separately, then re-assemble them later

 

[bookface]

Smooth on actually have a tutorial on something similar here How to Make Prop Movie or Fantasy Swords out of Flexible Foam and because the whole thing is foam, there'll be no issues with waving it around at said convention.

 

[loz64]

Thanks guys for the responses!

From the sound of it your best bet is fiberglass. Its cheap, easy to use, light and durable. No sense is spending money for something to lug around only to have it break as would happen with resins/plastics due to the shape of the item itself.

I've looked into fiberglass. I'm a little afraid of it because of the dangerous odor (I don't have access to a garage since I'm in an apartment) but I have a respirator and a fan so perhaps I can try it.

If you're looking to make a hollow shell, I'd look into roto-casting resin, like the smooth-on 65D. Then you can fill it with expending foam if you wish to, a liquid formula would work best. You do need a hole somewhere though, to pour the foam and let the surplus escape. The easiest way I see would be to separate the blade from the hilt and fill them separately, then re-assemble them later

I hadn't thought about casting my sword in two pieces; that might work out. If I did that what would be the best way to merge the two halves?

Smooth on actually have a tutorial on something similar here How to Make Prop Movie or Fantasy Swords out of Flexible Foam and because the whole thing is foam, there'll be no issues with waving it around at said convention.

I've seen that tutorial, but there aren't any close up shots of their sword. I would be afraid that since it's a foam there would be nasty little bubbles all over the surface. I can ask Smooth On if their rigid foams are self-skinning.

 

[bookface]

I've seen that tutorial, but there aren't any close up shots of their sword. I would be afraid that since it's a foam there would be nasty little bubbles all over the surface. I can ask Smooth On if their rigid foams are self-skinning.

Their rigid foams aren't, but that isn't a rigid foam in that tutorial, it's the 25lb flexfoam, which is self skinning. I've used it before and it has a nice surface finish. Shoot 'beaker' a PM here, he works for smooth on.

 

[nomuse]

Two random comments here.

One is that I fiberglassed once in a dorm room. Well, sorta. I did it in the elevator well, but left it in the room to cure. But this was the Experimental Floor and we were used to art projects in the rooms (and funny smells).

The other is that Fermi Approximations are your friend. You had the right track of it by estimating the sword would weigh as much as an equivalent volume of water. Where you slipped up was in not estimating that volume. A possible Fermi for that is to assume the sword has about the cross-section of one of those little plastic juice bottles for kids. Call it six inches tall (they are less, maybe 4") and that means you can multiply how many feet long the sword is by 2x. Now pick up that many bottles (or imagine picking up that many bottles) at once and hold them in an outstretched sword hand. The errors of each estimate roughly cancel each other out and you should be within a factor of 2-3 in how much weight you are looking at.

 

[Darth Lars]

I've looked into fiberglass. I'm a little afraid of it because of the dangerous odor (I don't have access to a garage since I'm in an apartment) but I have a respirator and a fan so perhaps I can try it.

You can use epoxy for fiberglass. Toxic to ingest and not good to get on your skin, but no fumes. The big danger with fiberglass is otherwise not the resin as much as the dust from cutting and sanding.

Anyhow... The classic LARP:er method of making inexpensive one-off swords is to cover open-core soft foam that has been cut into shape with layers of liquid latex where the last layers have been tinted with acrylic metal paint. Rigid foam is best sealed with thickened epoxy.

 

[BartGold]

For a sword, you might want to shove a thin metal rod down the middle. Give it some structure without adding too much weight. Teragon and Bookface are right, you can probably do it by 1: Painting a thin layer of urethane, 2, putting more urethane in and rotating the mold to distrubute it while it dries, and 3: having a hole in the mold to pour foam into the hardened urethane shell.

You can rotate the mold by hand and still spread a decent layer of material around.

A huge majority of the Starship Troopers stuff was made that way. The helmets, for example, would be light on the head with a hard urethane shell to capture the detail.

 

[nukeproof]

hey loz64, something I've done in the past is to suspend other objects in my casts normally like bits of wood or polystyrene. i carve them into shape then suspend them in the mold cavity using pins, you do have to fill the holes but there tiny and its well worth the effort as it will save you weight and resin. rota-casting and fiber glassing would work but there a lot of effort and you cant guarantee success, plus if it breaks it will be a pain in the arse to fix. if you have the skill to do it i really suggest making metal or PVC cavity fillers they get max strength to weight and it means you could make your larger scale props almost hollow.
hope this helps

 

[Duncanator]

I would agree that using epoxy to fiberglass is the best method. You can lay up each half of the sword with your mold split open and let the resin set. Then you run a bead of epoxy around the mating edges of the blade, and close up the mold to cure. The finished part will be rigid, light weight and strong. You could fill it with expanding foam if you want, but it shouldn't be needed. Heck if you wanted to get fancy you could use carbon fibre!
Filled resin just can't compare to fiberglass. I think you would be disappointed with how fragile a filled resin casting would be. It could snap the first time you accidentally whack it on something. The fiberglass cloth imparts a lot of strength especially for a sword shape (or a wing).

 

[loz64]

Thanks everyone for the advice! Good stuff here I can definitely experiment with.

Their rigid foams aren't, but that isn't a rigid foam in that tutorial, it's the 25lb flexfoam, which is self skinning. I've used it before and it has a nice surface finish. Shoot 'beaker' a PM here, he works for smooth on.

Thanks, I'm going to give more thought to the FlexFoam. Good to hear that it's self-skinning. I've sent a support e-mail to Smooth-On but haven't heard back yet.

Two random comments here.

One is that I fiberglassed once in a dorm room. Well, sorta. I did it in the elevator well, but left it in the room to cure. But this was the Experimental Floor and we were used to art projects in the rooms (and funny smells).

The other is that Fermi Approximations are your friend. You had the right track of it by estimating the sword would weigh as much as an equivalent volume of water. Where you slipped up was in not estimating that volume. A possible Fermi for that is to assume the sword has about the cross-section of one of those little plastic juice bottles for kids. Call it six inches tall (they are less, maybe 4") and that means you can multiply how many feet long the sword is by 2x. Now pick up that many bottles (or imagine picking up that many bottles) at once and hold them in an outstretched sword hand. The errors of each estimate roughly cancel each other out and you should be within a factor of 2-3 in how much weight you are looking at.

Oh man, I wonder how hard fiberglassing in an elevator of all places was!! I definitely did not correctly estimate the total final resin weight of my sword and misjudged how heavy the Feather "Lite" resin would really be. Thanks for the suggestion for estimating in the future.

You can use epoxy for fiberglass. Toxic to ingest and not good to get on your skin, but no fumes. The big danger with fiberglass is otherwise not the resin as much as the dust from cutting and sanding.

Anyhow... The classic LARP:er method of making inexpensive one-off swords is to cover open-core soft foam that has been cut into shape with layers of liquid latex where the last layers have been tinted with acrylic metal paint. Rigid foam is best sealed with thickened epoxy.

I'm familiar with Envirotex epoxy; it's a wonderful substance when you can get it to be bubble-free. Smooth-On sells a red epoxy I can look into for coating insulation foam (which I carve my swords from). Will latex (or acrylic paint) work to strengthen insulation foam as well?

For a sword, you might want to shove a thin metal rod down the middle. Give it some structure without adding too much weight. Teragon and Bookface are right, you can probably do it by 1: Painting a thin layer of urethane, 2, putting more urethane in and rotating the mold to distrubute it while it dries, and 3: having a hole in the mold to pour foam into the hardened urethane shell.

You can rotate the mold by hand and still spread a decent layer of material around.

A huge majority of the Starship Troopers stuff was made that way. The helmets, for example, would be light on the head with a hard urethane shell to capture the detail.

I've obtained some threaded aluminum rods to place in for support once I get past the experimental stage. I wonder if I can skip casting foam into the hollow casting if I can get a thick/strong enough plastic shell...

hey loz64, something I've done in the past is to suspend other objects in my casts normally like bits of wood or polystyrene. i carve them into shape then suspend them in the mold cavity using pins, you do have to fill the holes but there tiny and its well worth the effort as it will save you weight and resin. rota-casting and fiber glassing would work but there a lot of effort and you cant guarantee success, plus if it breaks it will be a pain in the arse to fix. if you have the skill to do it i really suggest making metal or PVC cavity fillers they get max strength to weight and it means you could make your larger scale props almost hollow.
hope this helps

Thanks for the advice! I can't work with wood/metal since I'm in an apartment but I can try PVC scraps; I have some bits of piping I can possibly salvage.

I would agree that using epoxy to fiberglass is the best method. You can lay up each half of the sword with your mold split open and let the resin set. Then you run a bead of epoxy around the mating edges of the blade, and close up the mold to cure. The finished part will be rigid, light weight and strong. You could fill it with expanding foam if you want, but it shouldn't be needed. Heck if you wanted to get fancy you could use carbon fibre!
Filled resin just can't compare to fiberglass. I think you would be disappointed with how fragile a filled resin casting would be. It could snap the first time you accidentally whack it on something. The fiberglass cloth imparts a lot of strength especially for a sword shape (or a wing).

Maybe fiberglass isn't so bad after all, haha. I know epoxy is rather thick to begin with and cures very slowly; would I need to keep rotocasting it in the open parts of the mold until it gels or would it stick pretty well to the mold sides?

I will share my progress on the specific sword I'm working on in another thread in the next month or so!

 

[Duncanator]

You don't need to rotocast the epoxy if you are using it with fiberglass cloth. And epoxy comes in many setting times. I've even used 5 minute epoxy for some things! You have to work fast or in small batches with that fast a set time.
If you are using silicone molds you won't need a mold release, but make sure you do if you use a hard mold. Epoxy is an adhesive after all.