Hollow mold help

[knibs7]

Can any of y'all help me? I want to mold a blowpipe I have. I was originally going to do a two-part mold, but then realized this would only make a solid casting. This is a problem because the blowpipe has a hollow metal rod in the middle for the darts to be shot out. Can anyone tell me how I could go about making this mold, so that there is still the hollow tube in the middle for it to function properly?

I'd really appreciate it.

 

[Gattaca]

I assume your blow pipe is a hollow tube? You can mold the pipe as you said, a two piece mold, however you need to block the inside of the pipe with something to keep it from filling up with rubber and casting material. I would suggest fitting a dowel into the blow hole so it fits snug and extends out each end of the pipe a few inches. Try to use something non porous like steel or plastic. Hopefully the interior of the pipe is round and not tapered. Once you have the pipe filled you can form a two part silicone mold around the blow pipe. I would use a stiff rubber (high shore strength) provided your blow pipe does not have a lot of fine detail. Also make sure your mold has some thickness to it as the mold will need to be stiff to maintain it's shape and not collapse in on it self. The bar could be contained fully in the mold (sticking out of the original blow pipe about one inch on each end, but still within the rubber mold, this way the bar will fit snugly in it's place when the mold is formed around it)
You'll have to figure out where your filler and air vent holes will be, probably at the end and use the mold vertically. Once the mold has cured you can cast your blow pipe by placing the steel or plastic rod back in the empty mold, this is essentially the 3rd part of the mold, the interior of the pipe, be sure to apply mold release to be sure it can be removed from the cast. Now place the two halves of the mold together, and pour you casting material. When cured, open the mold, remove you cast blow pipe, and slide the steel or plastic bar out of the cast. I hope this helps, I'll watch this thread if you have any questions or need more info.
Good Luck!

 

[Gattaca]

I made this crude picture to help explain:

 

[knibs7]

Thank you so much for your response!
Here is what the blowpipe looks like. The yellow lines show how it tapers. It starts out with the blowpipe material being about 3/4" off of the metal rod. It tapers all the way down to where there the blowpipe is barely a millimeter thick off of the metal rod. You can see where the metal rod and blowpipe meet where the green arrow is. Hopefully this will give you an idea of what I'm trying to do.

 

[Gattaca]

I see, well my idea might still work with inserting a rod into the tube part of the blow pipe. Where it tapers inward, like a funnel, you could fill that section with some kind of hard filler so it is part of the inner rod. Make sure it's removable, you don't want to ruin the blow pipe by having something like an epoxy putty stick to it. The taper of the outside does not matter as your rubber mold will take care of that. Essentially you need to have something solid taking up the space inside the blow pipe to create like I said the 3rd part of your mold.

 

[knibs7]

Imagine the red line is the filler rod, where would I put the fill hole and air vent? (also, what does the air vent do?) sorry i'm rather new to this.

 

[Gattaca]

You've picked a tricky project to begin your journey into molding.
The air vents allow trapped air a way out of the mold when casting. You can use wax dowels about 1/8" diameter, and the filler hole is just a larger funnel shape air vent to allow you to pour the casting material into the mold. You can build your mold horizontally but you will need to cast it vertically.


The long picture shows where the vents could go, the smaller picture is an end view of what your mold will look like.
Maybe you could attach a picture of the full blow pipe. One problem that may come up is if the hole through the blow pipe is not even from one end to the other, but you indicated that the centre of the blow pipe is a metal pipe, if so then you should be ok.

 

[Gattaca]

A better way would be to make your mold like this:

This image shows one half of your mold with the filling of the cast from the bottom, which pushes air out the top. I explain this a bit on my blog
Gattaca Models and Props: Molding and Casting small parts

The post is for small parts but the theory is the same.

 

[knibs7]

So I got to thinking... what if I plugged the openings on each end and made a solid cast and then drilled the middle canal? Would this be easier or should I try the original method?

 

[Gattaca]

That would work, not sure if it would be easier, just different. Make sure you choose a casting material that is easy to drill. You may want to build some kind of jig that will help you drill through the cast without wandering and drilling out the side. Something that will firmly hold the cast in place while a drill does the work.

 

[knibs7]

On a side note, do you know what kind of resin could be used to resemble the following picture (minus scratches)?
If you're at all familiar with the "Sugar babies" or "Sugar Daddy" candy, it looks very caramel-like. Would I have to add something to the resin? and if so, What resin should I use to achieve this look?



BTW, I cannot tell you how much I appreciate how much you have helped me so far.

 

[Gattaca]

Well, polyester resin, which is the same typical resin used when doing fiberglass work can be coloured using polyester pigments. I've also mixed dolomite powder (ground stone) with the resin to give it some bulk, making it thicker than the normally fluid resin. Doing this also allows you to cast larger volumes of resin without it cracking, overheating or discolouring. I've used this method to pour a block about the size of a big mac burger, if you did that without the powder it would not work, it would get hot and weird. You can usually buy the resin in hardware stores, especially ones that have a good automotive section. The pigments might be harder to find unless you have a plastics supply or online. Make sure you use the right pigments, others might not blend well with the resin. You can mix the powder with the resin until you are happy with the colour and thickness, then add the hardener according to the instructions.
What are you making?

 

[knibs7]

If you're familiar with Temple of Doom, I'm making a Sankara stone. I've made a few in the past that were pretty spot on, but I want to try a different approach (I made the others from clay) so that it looks more like a polished river rock

 

[Gattaca]

not sure how you would form the rock, maybe make a mold of one made from clay, then cast a polyester resin one. The polyester is pretty tough and would take a polish real nice. Work your way with finer and finer sandpaper, then some kind of polishing compound. I've always used the Novus products, but I'm sure auto polish would work too.

 

[knibs7]

Ya, I already have one shaped with clay, just need to mold and cast it. So polyester resin?

 

[Gattaca]

On second thought you might want to try this stuff:

Smooth-Cast® 325 Colormatch® Product Information | Smooth-On

You might have better luck with this as a beginner. The polyester resin could give you grief if you've never worked with it before.

 

[knibs7]

Ok thanks! For helping me so much, I'll send you one free of charge if you collect Indy props.