Furnace (part IV)

(Green-sand casting and flask building)

This project has been divided into sections. Navigate through them using the links at the bottom of the page.

After a few more foam castings, I decided to try greensand as a moulding medium. Greensand is a mixture of sand (~90%), bentonite (~10%) and a little bit of water. Crushed coal is sometimes introduced into this mix to aid in venting the mould space.

It took me a while trying to find a supplier of fine bentonite clay before giving up and grinding up cat litter (made of bentonite clay) instead. The current method I'm using to "grind" the bentonite is to dissolve it in water overnight and apply the wet mixture directly to the sand. I have found this works very well and eliminates any need for any sophisticated pulverising device (ie. ball mill).

Bentonite clay in the form of kitty litter

An open face "tri-marble" mould set inside flowerpot

Here is an open face greensand mould made from different sized marbles just to test the surface finish quality of this sand. The sand (masonry cement sand) was passed through a screen to filter out the rocks and any large grains prior to mixing with clay. A ceramic pot like the left is more suited to such a casting, more so than a vase from an Avas Flowers floral arrangement for example simply because its made of sturdier material.

And here is the resulting casting freshly made. Certain sections of the ball surfaces are pitted and fairly rough, but I suspect this was due to a few loose grains of sand which found their way into the moulds prior to pouring. the sides of all the balls are pristine!

A casting flask needs to be constructed in order to make full sand moulds - next item on the agenda.. here we go!

A "tri-marble" casting

Roast bananas for lunch everyone!

In the meantime, it has almost become tradition to roast something on top of this nice hot furnace after a melt. After all, why not get take full advantage of the BBQ charcoal?

This time it is banana filled with chocolate chips! YUM....

On we go to make the flasks! Flasks are basically square or rectangular boxes without a lid nor a base. It might seem strange to the uninformed, but flasks are the fundamental tool used in sand casting.

Here, strips of wood (either Pine or Balsa) are cut to size ready for drilling. My flask measures 20cm x 20cm and permits me to make some decent sized castings. The capacity of my furnace probably wouldn't suit any castings larger than this anyway.

The building blocks of a flask

A drill press bashing its way through pine

A drill press is employed to drill out some holes. Oh I do love having a press... having to drill critical holes like this without one always results in me drilling at a ridiculous angle.
Here are both sides of the flask assembled and labelled for future reference. The top section is known as the cope, whilst the drag is the lower section. The two sections can be used in a variety of ways, with each method used for its own unique purpose. For example, the most basic method used for casting flat patterns involves placing the entire pattern in the drag, with the cope accommodating the sprue and other "accessory" items.

The assembled skeleton of a flask

Alignment pins are screwed on the sides

The two parts of a flask must have a way of perfectly lining up, so that the mould will not end up misaligned. This is typically achieved using alignment pins, which insert into sockets as the cope is lowered onto the drag.

Here I have attached some crude alignment pins and sockets. They are also installed on the opposite side.

Finally, to finish off the flask, a ring of thin wood strips are nailed into the interior surface of the cope. This helps to grip onto the sand as the cope is lifted off and lowered back on the drag. I didn't feel obliged to install these on the drag, since it is rarely moved or flipped (without other support).

A wooden strip ribbing grips the sand harder

Moulding boards provide a solid foundation

Next, two moulding boards are constructed. These are just crude devices used to lay the flask on during the moulding process. Mine simply consist of two wooden rails screwed onto some Laminex board offcuts.

But of course, how could anything be complete without a full (albeit rather crude) set of moulding tools? From left to right, these are:

  • Sprue pin - tapped into the sand to create the sprue in moulds so that metal can enter the flask. (Mine is a bit off an old broom handle)
  • Ram - used to physically ram the sand in order to pack it in around the pattern object evenly. (Mine is a bit off an old table leg)
  • Venting wire - poked into the sand to provide venting holes which allow gas to escape while pouring metal (Mine is a tent peg)
  • Gate cutter - used to cut through sand in order to create a gate where metal can enter the mould. (Mine is a bent piece of sheet metal)
  • Strike-off bar - to "strike off" any excess sand left on top of the cope or drag. (Mine is a straight piece of sheet metal :)

A range of homemade moulding tools

Next - Furnace (part V)

Previous - Furnace (part III)

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