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View Full Version : Casting Tools Part 3 - Furnace Design and Construction



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

RayG
6th Nov 2011, 09:38 PM
Continuing on from previous post, which describes the current furnace construction, this is how I would approach furnace construction, if I was to make another.

Apologies for the poor quality drawing, the concept is the important thing.

http://www.backsaw.net/pics/Casting/FurnaceDesign.jpg

As previously, the zirconia blanket fibres are dangerous, and need to be sealed, with ITC-100 which is an IR reflective coating.


One thing, that's a possibility, is to replace the pyrocrete base and plinth, with lightweight insulating bricks that can be easily shaped, the LBK28 isolite bricks would be ideal I think.

Material Suppliers.

I've listed these before many times, but it doesn't hurt to re-list them here.

Ceramic Fibre Blanket

IsoWool & Isolite Bricks
Refractory & Ceramic - (03) 9560 4477 (http://refractoryandceramic.com.au)
50 Geddes St, Mulgrave Vic 3170, Australia
Ph: +61 3 9560 4477


Cerachem
Thermal Ceramics
Factory 3-4 111-113 William Angliss Drive
Laverton North 3026
03 9360 0355

Pyrocrete 165
Foseco
19-21 Nicole Way
Dandenong 3175
03 9792 4033

ITC-100 High Temperature reflective coating $80 per pint
ITC-200 High Temperature ceramic repair $80 per pint

Pinches Alloys
21 Malua Street
Reservoir 3073
03 9460 2466

If you find other suppliers, feel free to list them here..

Next Installment, cope and drag meet sprue and riser

Regards
Ray

Stustoys
6th Nov 2011, 09:39 PM
Stop it Ray my to do list is long enough already and you are making it look to easy ;) The local scrppy was selling S/S beer kegs but I couldnt think of a use for one :doh: I knew it would come back to bite me.


"Here is the inside of the furnace with the ceramic fibre blanket removed."
Are you sure? or is that the Pyrocrete 165 in there?

Stuart

RayG
6th Nov 2011, 09:45 PM
Stop it Ray my to do list is long enough already and you are making it look to easy ;) The local scrppy was selling S/S beer kegs but I couldnt think of a use for one :doh: I knew it would come back to bite me.


"Here is the inside of the furnace with the ceramic fibre blanket removed."
Are you sure? or is that the Pyrocrete 165 in there?

Stuart

Hi Stuart,

Beer keg would be pretty close to the right size, might be a tad small if anything.. I'll help you empty it... :D

That picture is just the pyrocrete, it was taken after the cast-iron pour burnt a hole through the cerachem... it's since been rebuilt, and I wasn't going to pull it apart just for a picture...:)

Regards
Ray

Stustoys
6th Nov 2011, 09:55 PM
I'll help you empty it... :D

I wish, they didn't have any full ones :(

So the pyrocrete is "like concrete"? The lines(I guess from the formwork) made it look like bats. You wouldnt bother with the pyrocrete on the wall like that if you started over?

Stuart

RayG
6th Nov 2011, 10:04 PM
Hi Stuart,

I think they expect to get the empty's back if you buy a new full keg... pity.

I'd put a thinner pyrocrete lining in, the formwork lines you can see are left over from cardboard zigzag expansion joints that I put in when it was originally cast, you leave the cardboard in and it mysteriously vanishes when the temperature gets up to 1000C or so..:)

The idea was to stop the pyrocrete from cracking when it was fired, seemed to work, no major cracks developed.

But, to answer the question, If I didn't already have a spare bag of pyrocrete, I would just go with ceramic fibre blanket, and Isolite bricks on the base. The blanket stuff is amazingly good insulation, the outer shell never gets above 100C or so... Maybe 200-300 at the top near the flue.

The black colour in the pictures is high temperature VHT spray paint, and that seems to be surviving.

Regards
Ray

PS.. Yes, it's exactly like concrete to work with, mix with water and stir..

wheelinround
7th Nov 2011, 08:19 AM
:2tsup: ty RG

.RC.
7th Nov 2011, 08:35 AM
I have bought a few bags of refractory material from this company

high temperature environment, installation, trm, refractory, gunning, casting ,refractory brickwork, ceramic fibre (http://www.totalrefractorymanagement.com/)

matthew_g
7th Nov 2011, 09:18 AM
I have a spare beer keg if you are interested Stuart,
I can drop it off next time I'm in Melb if you are keen......I kept 4 of them when I sold the Pub, 2 have been made into a spit, ! is for a furnace and the other for who ever wants it...
Matt

Stustoys
7th Nov 2011, 12:12 PM
Hi Matthew,
I'll gladly take one off your hands if it doesnt trouble you that it will likely be a long term project.

Stuart

p.s. not to put you off or anything but have you thought about a coolant system for your lathe/mill?(cant remember what machines you have).

matthew_g
7th Nov 2011, 01:27 PM
Hi Stuart,
All my machines have their own coolant systems..I did think once I should have just 1 central system for all of them, But they are way to spaced out through the shed for that...

I will let you know a week or so before my next trip to Melbourne so we can organise it....

Matt