Sand Casting

It is most likely that the first metal casting was the result of some rocks around a camp fire that contained some metal ore and a nearby rock would have likely been a flux, which would have allowed the metal to melt at a lower temperature than the pure metal normally melts at.  This molten metal would have trickled to a puddle on the ground where it cooled.  The next phase would have likely been to shape the dirt on the ground to control the shape of the metal.  In its most primitive form this is sand casting.  

While sand casting originated as a very primitive form of metal casting it is now a vast and complicated topic.  It is a process capable of making incredibly complex and accurate metal parts and is widely used throughout the world  in a variety of industries.  But at the same time, it can still be a simple process. 

The sand system I uses is one that is most often used in the foundry industry for cores, which are the hollow portions of a casting.   I use it because it doesn't require a flask around the perimeter of the mold which makes it more flexible for me in terms of the shape and size of each mold I make.  It also makes a mold that is much more robust in holding up to drawing the patterns from the finished mold as well as handling the finished mold.  Another advantage is the system uses sodium silicate for the binder which is non toxic.

Other sand systems require the use of mixing machines called mullers and in industry even the system I use employs sophisticated metering and point of dispensation mixing equipment.  In the spirit of simplifying things I mix my sand in small batches in 5 gallon buckets using measuring cups and a bathroom scale to measure out all the ingredients.  In the picture below you can see every thing that I use to make my molds.  I mix 25lbs. of sand at a time, the catalyst is mixed first and then the binder is added and mixed,  then its dumped into the form.  I use what ever scrap of wood is handy to tamp the sand and to strike off the top to make it flat.  Like I said, simple.

What looks like a flask in the picture above is actually just some boards that are screwed together to make a form for the sand. I could have used anything to restrain the sand and have used concrete cinder blocks in the past.   In this instance the pattern is mounted on a board and the board is screwed to the forms with a space on each side i.e. I can fill up one side and once the sand sets up I can turn it over on the bench and fill up the other side .



The sand is added to the form in layers.  The idea is that you don't want to add so much sand that you can't tamp it down and eliminate the space between the grains, yet you want to dump in enough sand that when you tamp it down you don't push it aside and strike your pattern with what ever you are using to tamp the sand with.  A general rule is to add an inch or two at a time with the last layer or two being a bit more. 

In this next picture you can see the finished mold. along with the pattern mounded on a board.  The portion of the mold on the left is called the drag and when both halves are assembled and ready to be poured it will sit on the ground, it is because it is on the bottom that it is called the drag.  The portion on the right is called the cope and it it called the cope because it sits on top of the drag.

On two of the corners of the board with the pattern there are cone shaped black knobs, these are a commercial product that attach to each side of a hole drilled through the pattern board and form interlocking keys in the sand of the cope and drag. The keys accurately locate the cope and drag to allow the two halves of a mold to align the way they are supposed to.  The pattern is sitting on top of the drag and you can see a key in the corner, it is the raised cone of sand.   The cope is a bit more confusing because there is so much going on.  The holes on the diagonal corners are the keys that interlock with the drag.  The other four holes are actually called risers and they perform two tasks, they allow for the air displaced by the bronze entering the mold to escape, and they will feed the casting as it cools.  Because they are so much larger in cross section than any other part of the casting they will be the last part to chill and solidify.  This is important because as metal cools it shrinks, in the case of bronze it shrinks 3/16" per lineal foot.  Because the risers will stay molten longer they will feed the casting with metal as it cools to prevent a type of casting defect called a heat tear.  The three square pads near the top edge of the cope (as it sits in the picture) are the gates and they will connect the main sprue to the casting. The main sprue is the long trench in the drag. while this may all sound confusing it is basically just a system of plumbing for the bronze to get in and the air to get out, not completely unlike the plumbing on a typical house.  My intent here is that the bronze will enter the main sprue through the  pour cup, which is the rectangular hole between two of the gates, it will then fill up the main sprue and feed all the gates at the same time.  This will fill up the casting and in turn, fill up  the risers.  There is a lot of info I'm leaving out here such as head pressure, total cross sectional feed area fillet radii....  And while there is a great deal of knowledge and experience required to consistently get good castings it really can be a simple process.

With the binder and catalyst combination I use it takes about 45 minutes for the sand to cure.  At the end of that time it is about the strength of a medium hard sand stone and that allows for the form boards to be removed and the pattern to be drawn out of the mold.  I then use a cordless drill and a carpenters spade bit to drill the holes for the risers and after carefully removing any loose sand the cope and drag are assembled and the two are placed in a sand pit with weights on the cope to prevent it from floating up and the bronze squirting out the seam between the cope and drag.

Bronze is a generic term for a plethora of copper based alloys, I use a trademarked silicon bronze alloy named Everdur.  Each alloy has properties that make it best suited for specific uses.  The reason I uses Everdur is because it pours extremely well and is easy to weld.  This alloy of bronze melts at about 1,850F, but  to make sure it stays fluid long enough to fill the mold for this pour It was heated to 2,150F.  At that temperature if a drop of bronze lands on the concrete floor it will cause the atmospheric moisture in the concrete to instantly turn to steam. When water goes from a liquid to a gas it expands 1000x in volume, so imagine the energy of one tiny drop of water instantly expanding 1000 time in size.  What happens is the bronze hits the concrete, and the water expands so rapidly it is like a tiny explosion that can send a drop of molten bronze six or more feet in the air.  This is why all molds are placed in a sand pit when pouring, the sand is porous enough that if a bit of bronze spills the resulting stem can escape under and to the sides and not send that spilled bronze through the air towards the foundryman.

Once a mold is poured full of bronze the metal actually cools and solidifies very rapidly and within a few minutes the mold can be opened up.  the intense head causes the sand to look charred and a thin layer of sand may look like it is stuck to the casting but this comes off easily with water and a wire brush. 

In this casting I didn't have enough head pressure to fill one of the risers.  Luckily this didn't cause any problems.

Another 10 minutes remove the gates and risers with a portable band saw and another 10 minutes with a wire brush to remove the fire scale and the result is the picture below.  I'll need to do a small amount of chasing (restoring the original surface) in some of the corners but over all an afternoon well spent.  The gates and risers will be recycled in the next pour.