Green Sand Casting
Green Sand Casting
During my project work in 2001 we had to make a specialist fitting to be used in mounting our equipment on Breda Light Rail Vehicles (trams) at San Francisco Municipal Railway (SF MUNI). I chose Bentech (The Philadelphia Pipe Bending Company) as the primary supplier, and they in turn used their long term partner Dent Casting / Forging as the principal manufacturer. I visited Dent to oversee the first production run to ensure the item measured up to our needs before we made thousands of them. I won't document the design stage in this page except to say that we used an existing similar unit to build a CAD model which was used in a CNC machine to make the reversible mould. To reduce casting costs we (and Bentech) elected to fund the extra cost of a multi unit mould which turns out twenty half units with each casting. We made the parts out of AluMag35 which is ideal for this sort of work in that it is readilly machinable but tough enough for public transportation use.
The mould is a plate of steel machined in a CNC milling machine. The plate is placed between the two halves of a casting box and the box is filled with green sand. Green sand is a refined sand which includes particular trace elements that lend themselves to makeing a good mould which fills and parts easilly. The sand is kept in a tower which keeps it at a particular temperature and is completely dry to the touch. The sand is carried by sealed conveyor to each work station where it is held in a temporary hopper and dispensed as needed into moulds.
After the mould is full it is pressed. After pressing the mould box is removed leaving only the sand itself to hold the shape. I cannot begin to explain how the dry sand holds together.
After pressing the edges of the sand around the mould are clean and sharp. In this image, the casting box has been removed and the mould has been removed from the sand. You can see the detail left behind in the sand for the metal to flow into. Note that the deep lines are the sprue lines which carry the metal to each part to be cast. Two of these halves are put together then sent to the casting floor to make a complete mould.
On the pouring floor a ceramic crucible is dipped into the furnace where it fills with molten metal. The crucible is brought to the slag table and handed to a pourer. The slag is removed from the top of the metal and the pourer begins pouring into the moulds.
The pourer quickly gets into position and pours each mould quickly and evenly until metal appears in the vent holes. Each mould is poured in about 4 to 5 seconds. The pourer moves on until his crucible does not hold enough metal for another mould. He pours around 12 to 14 moulds each round.
Out of each mould comes the raw castings, the green sand provides a clean smooth finish only requiring a light polishing before heading off to the machining room to have the bores cut and the bolt holes drilled.
After machining the article is complete and ready for use.