Casting something inside your resin part?

Maybe a dumb question, but if I had a part and I can't or don't want it rotocast, could I put a core, say balsa wood, inside the mold to cast inside it? I'm thinking I could suspend it inside the mold with pins or something and the resin would just go around it. Is that possible? I might have something I want to cast that I don't want to waste resin making it solid, but still want it structurally sound enough so it wouldn't be crushed.

When I was making some Stargate Naquadah bricks, I poured a layer of resin in an open face mold, then let it cure and placed some MDF cut to fit inside the body on it, then poured the rest of the resin over it so it filled in the sides and then top. The brick was solid, but wasn't solid resin, so it saved a bit on resin. I guess it really depends on the mold and the effort you want to make. I've also seen steel strips cut to fit inside resin swords to make them a little more solid and give them heft.

Absolutely you can do that. Just be careful with whatever you put in that it doesn't have some adverse reaction with the resin. Balsa is good. You also can use some closed cell foams.

TazMan2000

Am I correct in saying that if you use wood to help save on resin, that you need to pour a little bit of resin in first, but add in the piece of wood BEFORE the resin cures, and hold it in place while this happens so it doesn't float to the top?

dropshipbob said:

Am I correct in saying that if you use wood to help save on resin, that you need to pour a little bit of resin in first, but add in the piece of wood BEFORE the resin cures, and hold it in place while this happens so it doesn't float to the top?

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Yes, you would have to use something to hold the wood in place.

TazMan2000

I was thinking I could try to suspend the core in the mold with T-pins or something so it's sitting in the middle of the two part mold. It's a pretty small part, like 3" long and maybe 1 1/2"diameter.

We did this on Ice Pirates, to suspend the electronics into the cast guns, so the electronics were in the middle of the cast part, but the LED lights were at the surface of the casting. This allowed the LED's to be flush with the surface of the cast urethane prop, and be nearly invisible until they lit up. You can use very tiny wire (one strand of multistrand 16 ga. electrical wire), in a "U" shape, hot glue this at the bottom of the "U" to the edge of an item you want to suspend at the surface of the finished part. Stick the overhanging u shaped wire legs into the silicone mold. This will hold the item against the surface of the silicon mold. If you need to you can use multiples of these fine wire "staples". Then cast your part. When you demold the cast part, the two ends of the wires will be sticking out. Grab one of them with pliers and pull the u shaped wire out. Because of the "U" shape, the whole wire should pull out easily.
If you want to suspend something in the middle of you cast, glue at least two lengths of sewing thread, in at least two locations to the object, so there is thread extending from both sides of the item to be suspended. Then make four cuts in the mold seam that align with the two thread ends, two on each side of the mold cavity, so they are deep at the outside edge of the mold, and shallow, or at the surface of the seam, where the mold cavity begins. Use the threads pulled down into these grooves, to suspend you item into the center of the mold cavity. If you need to, you can tie the threads together on the backside of the mold. Then cast your part. One demolded, just cut the threads of the surface of your cast part. We use this technique to suspend stiffening armatures in rubber parts all the time.

Or just brass rod pushed into the wood. They would have to be mounted carefully to be at the seam of one part of the two-part mould. You would have a bit of leakage where they touch but nothing that couldn't be cut, filed or sanded off. Either that or the brass rod can be pushed in far enough so that they would touch each side of the mould and prevent the wood from floating up.

TazMan2000

Thanks for the suggestions! I like both of those ideas.

I was just thinking about this again today for another part and I was wondering if you would have any problem with the wood expanding from heat/cold? I would hate to have a finished part crack from that!

I don't believe you would have any problems with balsa expanding or warping. If you used regular wood, maybe. Depends on the moisture content and the wood.

TazMan2000

One thing to be sure of, is that whatever you use as filler needs to be dry. Moisture can cause the resin to foam.

It would most likely be a piece of balsa because it wouldn't need to be as big. I'm thinking about casting a piece in the barrel section of the Fallout 4 Laser Rifle. In game it seems like it's partly fiberglass so I figure that weight would feel more realistic than a huge chunk of MDF, which I'm making the master out of. So we'll see.

I'll have to go over and ask my dad because he might actually have one of those meters that measures moisture in wood.

I don't believe you have to worry about balsa unless it is fresh cut and green. This stuff is kiln dried to 6% moisture. Wood and paper will actually wick moisture from the air, so in humid environments you will get higher levels.

TazMan2000