# Casting Hollow objects?



## JustJimAZ (Aug 19, 2010)

Anyone out there want to point me in the right direction or offer some tips?
If I want to mold and cast a solid a pumpkin or a skull I know how to do that, but... What if I want the pumpkin or skull to be hollow?
What's the Secret there? One example I am thinking of is Funkins. They are hollow on the inside.

It's no secret that I love to make steampunk Jack o Lanterns, and I'd like to sculpt and mold some. They have to be hollow though.

So, any help out there?
http://api.ning.com/files/Nu9dRPSyrQdE1zq5Y0t9tByfZH-1lKWad5go4j4UqFSQ0vOMpstbPkMP227dUY6Lld2DfQXmP-xHvdXhgqdSIu20ynAVgdfK/steampunkin11.jpg?width=737&height=552


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## bourno (Jan 21, 2006)

Were your other pieces that were solid filled, done with Great stuff, or did you use 2 part urethane foam? For some hollow pieces, I like the 6 pound, 2 part urethane foam to slush around in a mold. The 2 pound stuff, expands too much for my liking for that.


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## RoxyBlue (Oct 6, 2008)

The technique you may be thinking of is called rotational casting, and it uses a resin. Here is an example from YouTube of the concept:






Here's a less sophisticated hand method using a specific casting medium:






And here's a homemade $20 rotational casting machine:


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## corey872 (Jan 10, 2010)

Not cast per se, but hollow pumpkins via paper mache. I think on some later models, he used a plastic bag, stuffed with crumpled newspaper and ran tape around it to simulate the 'ribs' of the pumpkin.

http://www.stolloween.com/?page_id=4017

With casting, you could make a bunch of identical shapes pretty quick....once you invest in the mold and other parts to make it work. The paper mache works better for low volume / unique parts as each one will be slightly different by default.


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## JustJimAZ (Aug 19, 2010)

Thanks for the replies. 
Bourno - I did not think slush casting would work with 2 part foam.

Roxy - I am interested in the rotational casting, and I found a local guy who seems to know something about it. I am going to check out these videos and see if i can make some small steampunkins to start with. Thanks!

Corey - I did make some paper mache pumpkins after reading Stolloween's site, but what I am looking for is the ability to sculpt one and make multiple copies inexpensively. Paper mache, for me, takes even longer to make than my current process.


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## fontgeek (Jul 24, 2006)

You might want to talk to the people at Reynolds Advanced Material, they have and supply all kinds of materials and tools, and really know their stuff when it comes to making molds and molding processes.

Depending upon how big your mold would be, you might look at using a mechanical "tumbler" and put your mold into a bucket or round drum. If you look at the tumblers sold at Harbor Freight Salvage you can get them pretty cheaply. This would allow you to get an even coat around the sides of the mold, and still be able to pick it (the bucket or drum) up and get the bottom/top coated and also allow you to pour the material in without too much labor. You could also look for an old clothes dryer to use that drum and motor to do the same thing for you.


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## JustJimAZ (Aug 19, 2010)

I'll look into those options, Fontgeek. Thanks!


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## scourge999 (May 4, 2008)

Molding depends on how many castings you want to get before you need to make a new mold (they wear out) size and shape of model being molded. And yes, rotational casting for hollow parts. Be careful rotating 2 part foams. Closing/sealing a mold with expansive material can be quite explosive and rotating foam continually breaks down the cell structure of the foam as it tries to do its thing.


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