I was wondering about at what point one might switch from lost-wax to sand casting. The two primary factors I see are 1.) amount of detail required and 2.) size of cast. It seems that lost-wax lends itself well to smaller more detailed casts where sand might be better for larger less detailed. Would you agree with that statement?
At what point do you generally pick one over the other? I have heard that sand casting can be done at the ton level, what is the largest lost-wax cast you know of and how was it achieved?
Don't forget die casting if you're going to crank out a ton of parts that need good precision.
I know that you can sand cast model airplane engine crank cases on a fairly routine level -- there is a famous Fox .36 engine from the 50's or 60's that was sand cast, and there are hobbyists who are sand casting .09 inch displacement engines -- think 1.5 inches long by maybe 1.25 inches across the motor mounts, with details down to 20 mils or so.
I know that some companies are lost foam casting engine blocks for cars. It's not lost wax, but at the top level its similar.
I would imagine that as your piece got very large you'd have issues with the wax form holding up long enough to make the investment. Never mind the cost -- at some point a wax form is going to collapse.
Ultimately I would expect it to boil down to cost and precision issues
-- lost wax casting is fussy and expensive. Sand casting may have more steps up front to get the forms built, but the process itself goes a lot quicker. No matter what you do you're going to have to post-machine any mating or working surface, so sand cast is going to be good enough for a lot of applications.
Generally speaking the main considerations is, does the casting have undercuts?
Lost wax, also called investment casting can have extreme undercuts whereas sand casting must have draft to allow the pattern to be removed from the sand.
It's not a question of size, since statues are investment cast. Although some may have welded or brazed additions.
Another person mentioned lost foam casting, which is another way to achieve undercuts and cores.
"chips-'n-swarf" wrote: (clip) It's not a question of size, since statues are investment cast. Although some may have welded or brazed additions. (clip) ^^^^^^^^^^^^^^^^^^ The bronze foundry in Berkeley that I like to visit assembles all their larger pieces by welding. There are a couple of reasons for this:
1.) A very large wax pattern would be hard to make.
2.) If the piece is too large, the metal tends to solidify during the pour.
3.) There is a practical limit to how large a mold they want to work with. They have to lift and tip a red hot crucible full of molten metal, big enough to fill the mold. The workers are exposed to the heat radiating from this, and they wear reflective suits. As you go bigger, this becomes more intense.
When they do a REALLY big sculpture, they exceed the capacity of their furnace to melt that much metal at once.
The foundry referred to below uses the ceramic shell method exclusively. There's nothing wrong with that, but it does limit the size of a wax part to that which can be dipped in the tank of slurry. When people have wanted to make very large lost-wax sculptures without welding them up from smaller parts, they've found ways around the difficulties outlined below.
A wax pattern can be made by first creating the sculpture in core material, then applying a layer of wax. This gets away from the need to create a large self-supporting wax form.
There's no problem with the metal solidifying in the mold as long as this happens in an orderly manner, filling the mold from the bottom up. The trick is to keep adding fresh metal so that it flows into liquid metal in the mold, avoiding a juncture between solid and liquid.
The size of the mold only matters if you have to pick it up. Very large molds have a kiln built around them for burning out, and the wax is conveyed away with tubes let into the bottom. These are plugged before filling the mold with metal.
In order to cast very large sculptures, it's not necessary to melt all the metal in a single crucible. In casting their monumental Buddha sculptures, for instance, the Japanese of centuries ago constructed a series of furnaces on the hillside above the mold, and allowed the metal to flow into it through a network of refractory channels, like tributaries flowing into a river.
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