Help on Casting Large Items

[pnolan48]

I need some help in casting ship hulls about 13 inches long by 3 inches wide by 1.5 inches deep. That's about 50 cubic inches or 27 ounces liquid.

I think I do not want to cast these hulls as a solid block.

Can anyone here point me to links or sites that might help me learn how to cast these objects? I'm pretty good at determining release angles and how to build a mold for small parts.

I've received some resin-cast shells that obviously used a plug to produce, well, a shell, rather than a solid block.

I've been searching the web without success. My search skills have probably dulled with age. I'd appreciate any ideas.

All I can promise is some N-Scale ships, and perhaps a Baldwin Centipede, all totally original and scratch-built.

Thanks,

[victor miranda]

I've only done n-scale cars sized or smaller.

there are a lot of ways to build such molds
the people at smooth-on have a number of tutorials and products.

If I were taking a shot at thet size...
I'd use silicon mold rubber because it is what I have done in the past.

I would build the mold using plywood as a former box ( you will need two, top and bottom
to keep the rubber in proper shape.
the ship hull I would fill with mold rubber
and I'd put a layer of rubber into the bottom of each of the plywood box halves
I'd think about a 1/2 inch layer would do.


then those three were cured

I'd place the hull into one side with the rubber facing rubber
and fill enough rubber to at least touch the hull rubber everywhere.
or up far enough to be close the top of the box half.
wait for it to cure.
this next part is the tricky part.
I would cut away the mold rubber enough to get to the parting line of the hull
so that I can seperate the two,
think of a straight line from where the parting edge to the box side.
put some kind of mold release on the rubber of the just finished side.

stand the two halves up facing each other and fill the open half with rubber.
more than likely this hole will be where you plan to fill the mold
when you are casting the boat hull.

the only difference with the diesel shell is that you may not need the plywood
mold support.

v

[TrainCat2]

Here would be the method I would use,

Instead of a plywood box for large items, I use either plexiglass or acetate sheet. Simple to cut and bond together. It also offers a smooth surface for demolding. I will assume this to be a flat bottom hull. If not, you will need to build a support base of modeling clay for the hull that can be removed later. Afix the hull to flat smooth base with modeling clay. I like to make a 1/16 clay rope and put it at the bottom edge of the master and press it down to the base. This seals the master and holds it in place.

Normally, you only need 1/2 inch of rubber around your master, but 13x3 should get 1 inch to keep the walls from deforming. Cut two side sheets 2 inches longer and 1 inch higher that your master. cut two end sheets the same height of your sides and 2 inches wider. glue two 1/2 inch wide strips to the end near the top and bottom so they overhang the sides of the ends. These will align the ends with the sides and provide points to use rubber bands from end to end to keep the fence together.

I like to make small clay cones or square stock on the base between the master and the fence. Later when the mold is flipped over and the clay cones are removed, these become registration marks. Add 1/16 clay rope to the entire bottom of the fence and push down onto the base. Pour your rubber to the top of the fence which will be the bottom of the mold. Let it fully cure, remove the rubber bands and flip the mold over to the new bottom. Remove all clay from the bottom of the hull and mold. Use three or four lengths of large straws and some clay to create fill tubes. Put your fence back on to the mold with the top edge 1 inch above the the previous rubber level and spray/brush mold release to keep the mold top from sticking and fill to the top of the fence with rubber. When cured, demold and cleanup.

When ready to pour, the registration cones will be a huge help in keeping everything aligned.

[pnolan48]

Thanks so much for the practical methods!

Does anyone have any experience with shrinkage/warping with this size casting? It just seems that this size is between fiberglass layup methods, castings in 1:350 scale, and solid resin castings for smaller scales. I have a bow casting from the person whose work is now being sold by Savon-Arts on eBay. It's actually a very nice casting, keyed for a mid-section, which I presume is then keyed for the stern.

The hull of the Savon-Arts freighter is very good, in my opinion.

[victor miranda]

my experience is with one to one silicon rubber from smooth-on.

it expands when it is warm enough to cast pewter.
given my expansion issues  I am thinking shrinking ain't so bad...
with epoxy castings the seem the same size as original.

most rubbers will state shrinkage factors.

I recall 3 percent for one product.

victor