Building expressly for casting

[Spacephrawg]

How do I build something so that it will cast well? I've only ever built things for the purpose of creating a model. The only times i've ever built things for casting they've been made of clay, i.e. sculpture, and have been a totally different ball game. I've seen resin kits before and like everyone I'm a verteran of all injection kits.

I am planning to resin cast this thing (Its not presentable yet so i dont have a picture). With any luck I'll be able to degass/pressure cast it but even if I can't, I have to build it so that it will make an easy kit with *as few parts as possible*.

Does anyone have any tips/tutorials/ideas for how to go about this?

Thanks very much.

[TER-OR]

avoid big undercuts. If you have to have something like this, think about parts breakdown.

Think about how molds are made. A flat piece is best, so if you have fins or whatever, they must be separate.

Think about assembly, key parts when you can, so you can assure good alignment.

[tonyG2]

And read the various articles and links dotted around SSM both at the main site and in the construction forum. Great hints and tips. Helped me a lot.

For example if you can set up a side pouring channel so the resin fills the mold from the bottom up, it helps discplace any trapped bubbles and so you get a better result if you don't have access to pressure casting systems.

[Spacephrawg]

The thing that really puzles me is how to design things to have keys. I dont know why this confuses me but it does. Like how to make things that arnt attached such that when they are put together, they line up. I have no CAD system to help me. I'm sure theres a way to do it analog. Any thoughts? Again thanks so much.

[Treadhead]

The thing that really puzles me is how to design things to have keys. I dont know why this confuses me but it does. Like how to make things that arnt attached such that when they are put together, they line up. I have no CAD system to help me. I'm sure theres a way to do it analog. Any thoughts? Again thanks so much.

Here's what I have done:

I have a ship that is basically an upper and lower hull. As I build each piece, I continually fit them together as I go along. Then, I use white glue and tack both parts together and apply filler and or sand so that both fit as seamlessly as possible.

I don't think that I'll ever get a perfect fit -- all I can do is minimize the amount of puttying that will be needed when the thing goes togther.

Fortunately I'm building for myself, so when I do cast the darn thing a little bit of filling on the final product isn't going to be that big of a deal...

[jack wendt]

*as few parts as possible*.

here in lies the problem. break the kit down into smaller pieces will ease your efforts in casting, or build it but plan on having someone like david or erin cast it for you. they can tell you where to seperate the parts for their ease in casting.

just an IMHO.

[tonyG2]

And you can build in "keys" as well.

Like Treadhead says for mastering the part. When its done, pop the two halfs open. Get some short stubs of sprue and glue at appropriate points on the mating surface of one part. Then paint the exposed faces of the stubs and bring the two parts together (as accurately as you can).

Then drill out a hole of the right size at the points on the other part now marked by paint spots.

Its NOT 100% accurate but it can help.

To be honest though, for most parts, you just follow Treadheads suggestion. Gluing the cast parts isn't a problem normally unless the mating surface is very small and in that case, resin lugs and holes don't help much and you are better off drilling a hole in both pieces and using a wire insert.

Some resin kits do have embeded wire to strengthen fragile parts (the VA Klingon Ktinga for example) but I've no idea how they do that!.

[Chacal]

The thing that really puzles me is how to design things to have keys. I dont know why this confuses me but it does. Like how to make things that arnt attached such that when they are put together, they line up. I have no CAD system to help me. I'm sure theres a way to do it analog. Any thoughts? Again thanks so much.

Like a nacelle pylon on a trek ship or a wing on an airplane? If I had to do it, I would build the tab on the pylon/wing with a slab of sheet styrene slightly pared down so it is a tad (0.25mm) narrower and thinner on the tip than on the root. Then I'd open up a corresponding hole on the receiving part (secondary hull/fuselage) and fill that hole with epoxy putty (if the part is thin, I'd back the hole up with a styrene sheet little "box", if the part is solid, I'd make the hole a bit bigger and opening out toward its bottom – by tilting a dremel cutting tool a bit while I cut). Then I'd insert the tab (coated with a release agent) into the putty, adjust it to its correct position and brace it on a jig until fullt cured. After that, I'd pull it out and hopefully have a nice hole for the key tab to fit in. After that I'd file away a half milimeter off the end of the tab and I'd slightly round it off too, so there's some space left when the tab is inserted on its socket. Of course, the tab can't be too long, because the RTV for the mold would droop if the tab on the mold (negative of the socket) gets too long and thin. And if it droops, the part won't align correctly.

[Sparky]

Blappy has a tutorial from wonderfest:
http://www.spacemodelmania.com/models/w ... ut2005.pdf

Here's some tips from looking at a small master.
http://www.kc6sye.com/techmages_2_8_04_b.html