A Sandcasting Primer

 
  Sandcasting is a process for casting metal in sand molds. The mold is made by packing the sand around a pattern and is used only once. The sand is recycled after each use (with only small loss), so the process is efficient. There are several different types of casting sand, and the process can vary greatly in scale from casting multi-ton parts for industrial machines to casting items as small as a ring. This article will cover a small scale implementation of the process which can be done in the artist's studio. I'm still working on this page, so please forgive the incompleteness. I have plans to illustrate some of the more tricky techniques, which I haven't seen elsewhere on the web.  
  The Sand: Here, I am using Petrobond brand sand. It is a mixture of fine white silica sand, 30 weight motor oil (without detergents), the Petrobond powder (a specially modified clay), and a splash of liquid catalyst. The mix is prepared in a machine called a muller, which stirs and mashes the sand.

I Keep my sand in a rolling box under the molding bench. It holds up to 500 pounds of sand, which is plenty for my current needs. Petrobond always works better when it's warm, so in cool weather I put a small electric heater in the box and put a piece of foam insulation over the top. Another way to do it is to have some heat lamps over your sand pile.


If you can't find a muller to use, you can buy Petrobond premixed, which would be perfectly fine for a small amount, but which might get a bit expensive if you want a lot of sand. There are other types of sand - some use water instead of oil, but I don't have much experience with them. One nice thing about Petrobond is that you don't have to re-mull it, once it has been made. One not so nice thing about Petrobond - it kind of stinks.

The Tools: The process can be done with a bare minimum of equipment, most of which can be made in the studio.

Flasks: A flask is a pair of interlocking frames that are used to contain the mold. One side is traditionally called the cope and the other the drag. In a proper foundry, flasks are generally made of metal, but for the small scale operation, perfectly satisfactory flasks can be made of wood. Yes, Wood! They get a little burnt here and there, but they can actually last pretty long.

The Furnace: A furnace is what makes the heat for melting metal. I constructed this one from lightweight firebrick, cut to fit and banded into an octagon with fiberfrax (a refractory fiber) gaskets to seal between the bricks (not absolutely necessary but nice). The "flamethrower" part of the furnace began with a salvaged electric leaf blower. Then I made a mixer of steel pipe, with a replaceable pipe nipple for the business end.


#4 crucible in the furnace with aluminum ready to pour


#10 crucible in furnace with a recycled leaf blower

 
 

Crucible Handling Tools: When the crucible is glowing hot, with a load of molten metal in it, one has to be able to pick it up out of the furnace and pour the metal into the molds. A crucible gets a little soft at these temperatures, so you can't just grab the edge of it with some big channel locks - you might be left holding a piece of the crucible's rim with hot metal all over the floor!

Much better to have some special tongs that go around the crucible and hold it firmly from several contact points. Again, I made my own, but you can buy them, too. The configuration varies a lot with the size of the crucible. A small crucible like a #4 or #10 can easily be handled by one person with some big plierlike tongs. But say a #40 crucible requires large 2 person tongs to lift the crucible out of the furnace, and a seperate pouring ring for filling molds.

The Crucible: The crucible is what the metal is melted in. They are made of graphite clay ,silicon carbide and some other materials. I haven't tried making my own yet (I know some people who make them out of steel), so I buy them. If one is careful with them, they'll last a long time. I blew one up! .

Parting Dust. Is used to keep the sand from sticking to the pattern and to keep the two sides of the mold from sticking together.


Rams, Slicks, Sandworking Tools: You can buy these. Or make them all yourself, as I have done in this example.


Patterns:
We can't really have a discussion of sandcasting without talking about patterns. A pattern is what the "original" is called. It can be made from anything rigid enough to withstand being rammed in sand. Patterns can be made from any combination of wood, cardboard, plastic, metal, masonite, bondo, plaster... Since a sand mold is not flexible, the pattern has to be able to slip out without hanging in the mold. This means that undercuts need to be eliminated, and the features of the pattern need to be made with good draught angles. For art casting, we have a little more lattitude, and it may be acceptible for some sand to stick in the pattern (for example in crevices in a natural object).

Making a mold: I'm going to start with the most simple situation for making a mold. That would be a pattern with a flat back that can be laid on the surface of the bench.

We can simply dust the pattern with parting dust, set one half of the flask over the pattern, and proceed to ram the first side of the mold.

It should be noted that a pattern that will slip out easily should be placed on a board so that the first side of the mold can be flipped while keeping the pattern in.
The Next Level of Complexity would be a pattern which has a hollow back, but which still lays flat against the table. A packed sand fill helps keep the pattern from deforming during ramming.


An even more complicated pattern has an irregular back which must be placed in a sand bed. In this method, one side of the flask is packed with sand, and the pattern is pressed in. Then the parting line is defined by packing and smoothing the surface with a sand slick.

A sandcast project

 
 
Making Sand Molds
 
  Here I am making a louver for one of the shiny aluminum cases

Click on images to see larger


The Mold ready to put together
 
  Here the louver pattern is a metal casting, which was cast in a mold made from a bondo original. It is sitting on a plaster follow board.Which supports the pattern while the bottom side of the mold (called the drag) is rammed.

The drag has been rammed and flipped, and the follow board has been removed, leaving the pattern in the drag. Note the plywood runner pattern placed alongside the louver and the smaller pattern on the uppet right. It's all dusted and ready put the top side of the mold (called the cope) on and ram the second side of the mold

Left: A louver casting. Note this one has a hand cut runner.

Right: Castings from the cored mold illustrated below.

Here you can see the two sides of the mold finished and ready to put together. Note the three gates cut in the cope to let the metal get from the runner to the piece. Note also the small hole that feeds the metal from the pouring cup (on top of the inverted cope) to the runner.
 
 
Steps in mold making
 
   
  Riddling. This is a situation with the pattern flat against the table. The first bit of sand is sifted through a piece of mesh onto the dusted pattern. Riddling is only necessary on the layer of sand near the pattern and parting surface. Tucking. After riddling in about an inch of sand, the sand is pushed into the corners and crevices with the fingers. be careful not to slide the sand around on the pattern. Pay special attention to push the sand into deep crevices in the pattern. Light Ramming. More sand is added and the sand is rammed lightly, being careful not to disturb the pattern or distort the sand. My ram has a wedged end which I use to get into the corners.  
   
  More Ramming. Put in more sand. Now you can put some force into the ramming. Pack the mold to overflowing with sand. Striking. Here I am using a steel bar to level off the outer surface of the mold. This is a mold ready to pour.
...Except for clamping!!!
 
 
Castings with simple cores
 
 

 
  Top: A hinged core mold. Middle: A plaster cylinder made from the core mold (I waxed the mold liberally). Bottom: Bondo patterns made on the plaster cylinders (after they dried a couple days). Bedded Patterns. The procedure is to pack the drag mostly full of sand. Then riddle on a thin layer on sand. Dust the back of the pattern lightly and press it in. Then pack and smooth the parting surface with the sand slicks. Packing the core mold. After lightly dusting, I pack riddled sand so that it fills each side just a little more than full.  
   
  Close the mold and squeeze it together (use a clamp, if necessary), strike off the top. Open the mold, remove the core. Place the cores Gently in the mold. Note the knocked off corners of the core to prevent a hole in the casting  
 
Always clamp, molten metal can exert enough pressure to lift a mold up and shoot out the side.

Coming up:

getting a surface shell from an object using a pair of matched flasks.

The gooey Bondo patternmaking technique.


Pouring metal Definitely should be wearing safety glasses here, but then I have been known to cast barefoot!

 
    a sandcast computer case    
 

 

a sandcast sculpture    

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