RayG
6th Nov 2011, 07:11 PM
Hi All,
Before starting on the furnace construction, it might be worthwhile to re-visit some temperatures.
Pure elements melt at higher temperatures than they do when alloyed with other elements, so pure copper melts at a higher temperature than brass or bronze.. Pure Iron melts at a higher temperature than cast iron, and so on..
Here are a few Melting Points for common alloys.
http://www.backsaw.net/pics/Casting/Temperatures.jpg
Just melting the raw material isn't quite what we want to do, we need to go above the melting point to a temperature that pours and cast nicely, the above pouring temperatures are just a guide, and sometimes trial and error is needed to get the flow through the pattern to work.. this is the "black magic" part of pattern making.
The way you choose to construct the furnace is going to be determined by the maximum temperatures. Let's say we are going to do cast iron. That means we will be running at 1300-1400 degrees C and probably sometimes even a bit higher.
Ok, let's start.. forget any of those designs you've seen on the internet that use heavy castable refractory, this is a design that uses light weight zircon(ceramic) fibre blanket, the result is a lightweight highly efficient foundry that heats up quicker and to higher temperatures. For home foundry use it's more than durable enough to last for a few years.
Begin with the biggest crucible you are going to use.. and add 2-3 inches to the radius, that gives a rough internal diameter, now we have layers of zircon fibre blanket insulation, Let's use 1400C rated ISOWOOL CB25H or CB50H at least 3 layers (the standard rolls are 25x600x7200 or 50x600x3600)
That gives a rough guide to the overall diameter of the furnace, and the outer shell can be sheet metal. If you can find a suitable steel shell, a drum or whatever, in my case I used a scrap water pressure tank.
Next we need a base and a lid.
The lid can be a lift off style, or a lift and rotate style. There are lots' of options, but there are a few things to remember, the inside of the lid is going to be at casting temperatures when you lift it, so make sure that it opens away from gas lines and anything flammable.
Here's my (over-engineered as usual) lift and rotate mechanism.
http://www.backsaw.net/pics/Casting/DSCN0950.JPG
http://www.backsaw.net/pics/Casting/DSCN0951.JPG
http://www.backsaw.net/pics/Casting/DSCN0952.JPG
http://www.backsaw.net/pics/Casting/DSCN0953.JPG
It has an overcenter locking action, so that once you raise the lid, it stays there.
And you just rotate clockwise (away from the burner side).
The internal construction, I used was one I wouldn't recommend, I used a lining of Pyrocrete 165, which I don't think is needed if you use a good layer of ceramic blanket.
http://www.backsaw.net/pics/Casting/DSCN0947.JPG
It's good stuff, rated to 1649 C, but I really don't think it's needed.
Here is the inside of the furnace with the ceramic fibre blanket removed.
http://www.backsaw.net/pics/Casting/DSCN0416.JPG
Note the drain hole in the bottom, this is to cater for a broken crucible, rather than end up with a solid block of molten metal in the
bottom of the furnace, it can run out on the bottom.
In use the hole is covered by a plinth, cast from Pyrocrete 165, that the crucible sits on. It's a good idea to put a piece of cardboard under
the crucible to stop it from sticking to the plinth. I have had crucibles stick to the plinth and usually, you have to break the pyrocrete to get
the crucible loose.
The inside of the lid is lined with pyrocrete and ceramic fibre blanket, the ceramic fibre blanket is retained by titanium wire, I used to use
stainless steel tig welding wire, but that eventually melted during a cast iron pour... this picture shows the melted stainless steel wires.
http://www.backsaw.net/pics/Casting/DSCN0417.JPG
I didn't have a picture of the current setup with the titanium wires, so this is the best I could get
http://www.backsaw.net/pics/Casting/DSCN0946.JPG
The ceramic fibre blanket sits up a little, so that it forms a seal of sorts when it is closed
The ceramic fibre blanket is dangerous stuff, you need to wear a face mask to stop breathing the fibres, and it need to be sealed to stop any loose fibres floating around, the
sealer I used was ITC-100, which is a brush-on IR reflective furnace lining compound, it's water based, and it's a bit like brushing on a layer of mud...
That's the current setup, I'll draw some sketches of what changes I would make if I was to build another. (that can be a new post)
Regards
Ray
Before starting on the furnace construction, it might be worthwhile to re-visit some temperatures.
Pure elements melt at higher temperatures than they do when alloyed with other elements, so pure copper melts at a higher temperature than brass or bronze.. Pure Iron melts at a higher temperature than cast iron, and so on..
Here are a few Melting Points for common alloys.
http://www.backsaw.net/pics/Casting/Temperatures.jpg
Just melting the raw material isn't quite what we want to do, we need to go above the melting point to a temperature that pours and cast nicely, the above pouring temperatures are just a guide, and sometimes trial and error is needed to get the flow through the pattern to work.. this is the "black magic" part of pattern making.
The way you choose to construct the furnace is going to be determined by the maximum temperatures. Let's say we are going to do cast iron. That means we will be running at 1300-1400 degrees C and probably sometimes even a bit higher.
Ok, let's start.. forget any of those designs you've seen on the internet that use heavy castable refractory, this is a design that uses light weight zircon(ceramic) fibre blanket, the result is a lightweight highly efficient foundry that heats up quicker and to higher temperatures. For home foundry use it's more than durable enough to last for a few years.
Begin with the biggest crucible you are going to use.. and add 2-3 inches to the radius, that gives a rough internal diameter, now we have layers of zircon fibre blanket insulation, Let's use 1400C rated ISOWOOL CB25H or CB50H at least 3 layers (the standard rolls are 25x600x7200 or 50x600x3600)
That gives a rough guide to the overall diameter of the furnace, and the outer shell can be sheet metal. If you can find a suitable steel shell, a drum or whatever, in my case I used a scrap water pressure tank.
Next we need a base and a lid.
The lid can be a lift off style, or a lift and rotate style. There are lots' of options, but there are a few things to remember, the inside of the lid is going to be at casting temperatures when you lift it, so make sure that it opens away from gas lines and anything flammable.
Here's my (over-engineered as usual) lift and rotate mechanism.
http://www.backsaw.net/pics/Casting/DSCN0950.JPG
http://www.backsaw.net/pics/Casting/DSCN0951.JPG
http://www.backsaw.net/pics/Casting/DSCN0952.JPG
http://www.backsaw.net/pics/Casting/DSCN0953.JPG
It has an overcenter locking action, so that once you raise the lid, it stays there.
And you just rotate clockwise (away from the burner side).
The internal construction, I used was one I wouldn't recommend, I used a lining of Pyrocrete 165, which I don't think is needed if you use a good layer of ceramic blanket.
http://www.backsaw.net/pics/Casting/DSCN0947.JPG
It's good stuff, rated to 1649 C, but I really don't think it's needed.
Here is the inside of the furnace with the ceramic fibre blanket removed.
http://www.backsaw.net/pics/Casting/DSCN0416.JPG
Note the drain hole in the bottom, this is to cater for a broken crucible, rather than end up with a solid block of molten metal in the
bottom of the furnace, it can run out on the bottom.
In use the hole is covered by a plinth, cast from Pyrocrete 165, that the crucible sits on. It's a good idea to put a piece of cardboard under
the crucible to stop it from sticking to the plinth. I have had crucibles stick to the plinth and usually, you have to break the pyrocrete to get
the crucible loose.
The inside of the lid is lined with pyrocrete and ceramic fibre blanket, the ceramic fibre blanket is retained by titanium wire, I used to use
stainless steel tig welding wire, but that eventually melted during a cast iron pour... this picture shows the melted stainless steel wires.
http://www.backsaw.net/pics/Casting/DSCN0417.JPG
I didn't have a picture of the current setup with the titanium wires, so this is the best I could get
http://www.backsaw.net/pics/Casting/DSCN0946.JPG
The ceramic fibre blanket sits up a little, so that it forms a seal of sorts when it is closed
The ceramic fibre blanket is dangerous stuff, you need to wear a face mask to stop breathing the fibres, and it need to be sealed to stop any loose fibres floating around, the
sealer I used was ITC-100, which is a brush-on IR reflective furnace lining compound, it's water based, and it's a bit like brushing on a layer of mud...
That's the current setup, I'll draw some sketches of what changes I would make if I was to build another. (that can be a new post)
Regards
Ray