Just got an email from Harbor Castings with lots of pretty pictures of my parts going through the investment casting process! There’s something extra cool about seeing process pictures of your parts going through the process instead of some generic Google search images.
The wax parts I sent them looked like this:
The first step in the process is to attach the parts to a ‘tree’. The function of the tree is to create a large passageway for molten metal to fill the parts and to provide a reservoir of material to assist in keeping the molds fully filled as the metal cools and shrinks in volume. Arranging several parts on the tree allows the foundry to make the most use of the available pour size. These guys didn’t skimp on the tree or runner sizes! It may seem wasteful but all of the scrap can easily be recycled with minimal effort as they are a foundry. The small red tubes are vent lines to allow gas to escape as the molds are being filled with molten metal. The white cone at the bottom is a ceramic part that the investment bonds to and which forms the opening into which the metal is poured. Looks like a textbook tree.
Once the parts are on the tree it is time to ‘invest’ them. In production this is an automated process that repeatedly dips the tree assembly into a fine ceramic slurry and then lets dry. The composition of this material is important as it needs to transmit fine detail yet be strong enough to survive handling and thermal shocks. After several coats the wax parts now have a thin ceramic shell surrounding them. The shell is still uncured ‘green’ ceramic so is very fragile.
As more layers as added the part becomes decidedly puffy looking. After several layers of a fine slurry that closely mirrors the parts surfaces and textures a couple of thicker, more coarse layers of ceramic are added to make the assembly more damage resistant. Any damage from here on in makes the part scrap so it pays to pay attention and take care.
The tree is now ready for the next step in the process which is melting the wax out and curing the ceramic to a hard durable shell.
A steam autoclave melts the wax out and ensures that the inside of the ceramic mold is completely empty. Note that the hollow cone that was the ceramic base of the tree now seems to be an integral part of the entire assembly. The insides seem extremely clean, sign of a complete burn out with high quality wax.
The hollow tree is now fired in a high temperature oven to harden the ceramic. Yes, that is really hot.
While the ceramic is still hot it is put into a sand box and molten metal is poured into the opening at the top of the tree. A filter is placed at the opening of the tree to filter impurities from the metal from getting into the mold. The sandbox is to slow the cooling of the mold and the metal which minimizes built in casting stresses.
Yes, its still really hot.
Its like my own mini sun! Now to slow cooling even more a drum is placed over the filled mold.
Once the parts cool it time to unwrap them. The easiest method is to just shatter the ceramic off of the metal parts resulting in this stage.
Most of the outer material just falls away but the material inside the pockets requires more effort, or a vibratory tumbler, to get out. The central sprue of the tree is now a solid block of 4130 steel that can be recycled after being cut from the parts, which is happening here.
The parts are then given a light bead blast to remove any remaining investment material and mold scale.