Building a small electric furnace / kiln

[ASFParticipant @asf]

Posted by JasonB on 15/12/2015 13:09:13:

John, you should be OK now as you have been approved – they are not fussy

funny thing is they approved me last night as well. Must have been a two for one offer

[JasonBModerator @jasonb]

The guy who runs the site had been away so there must have been a bit of a backlog.

[AjohnwParticipant @ajohnw51620]

I can browse the site now Jason but get the same problem if I click on an attachment. No problem though.

Interesting to see the other method mentioned, charcoal. The design can be very similar to the propane ones. Air is just blown into the hole with a hair drier rather than fitting a propane torch.

I had lost the link to the backyard casting site but like many it's sketchy in places and I feel if people want to the trouble of making one they may as well use the correct materials.

John

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Edited By John W1 on 17/12/2015 23:32:49

[AjohnwParticipant @ajohnw51620]

I came across an interesting pdf on making furnaces. Sorry no link but it was from the University of Queensland.

It mostly talks about making tube furnaces but points out that the element can be wound on a former and the refractory cast around it. The reason being to allow the inner ceramic in this case tube to be replaced without making a new element.

The former is plastic tube with a slot cut in it plus end caps, all thread and a piece of metal to fill the gap. When that is removed the tube can be collapsed – big clips are needed to hold it all together so these could also be used to collapse it. Plastic ducting tube is available in all sorts of sizes. It also doesn't take all that many turns to loose say 20m of element wire round suitable tube sizes and no need to wind elements the usual coiled way. I'm thinking in terms of 200mm tube – on end but for heat treatment end on might be of interest.

A catch though.There always is. The element is modestly coated with high llumina cement. Looks to be 96% alumina. I've not found a UK source so far. It needs firing at 1000C but points out that this can be done in the furnace. The usual castables are used outside of this.

My immediate thought concerning the the ceramic inner tube is just to simply not have one. An alternative might be rigidised fibre blanket if some one wants a muffle. Lots of cheaper kilns are being made like this now but details on producing items like this are none existent. The cheaper rigidisers seem to limit max temperature too – 1250C which probably means lower in practice.

John

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[Les Jones 1Participant @lesjones1]

This MAY be the link to the information mentioned in the pevious post.

Les.

Edited By Les Jones 1 on 07/02/2016 15:27:20

[KWILParticipant @kwil]

Try these **LINK**

Suppliers of castable materials for high tenperatures.

[AjohnwParticipant @ajohnw51620]

That is the link. For some reason my history didn't catch it and I had been all over the place before it popped up.

Some might wonder why like that. My view anyway.

One aspect is size and cost. The size I want is pretty awkward in the usual size of insulating fire bricks and I want to keep the weight down which would mean cutting them into 1/2 thickness wise. It also means a lot of additional cutting to get the size. Loads of joints. The much larger bricks can reduce the joints to a more acceptable level but are not so easy to find and still need cutting to reduce the thickness. I've been thinking in terms of a chamber size 8" sq / dia and maybe 10" deep.

The other reason is elements. .Thicker wire improves the emission area figures but it starts getting more difficult to fit in and cut the grooves as the length required goes up. Using super cheap wire that should work out just needs 32 turns around the former. Spacing circa 1/4" No worry about coils being too close wound the usual way and hopefully less risk of hot spots. If this wire is too thin not much of a problem making another core using thicker wire.

Doors and lids are a pain. The cheaper furnaces seem to simplify this by moulding rigidised blanket to fit. I'me wondering if the other approach where the furnace is lifted off is easier and better too as it doesn't involve reaching in to lift things out. On the other hand there shouldn't be much of a problem using stainless for aluminium in an electric furnace.

One thing I don't like about the arrangement is that it's round. Probably have to make some rolls – myfordboy style as they are so simple and obviously work.

On the other hand it could still be in a square case really.

One thing I have no idea about is just how thin castable refractory can be and still have a reasonable strength. It slows down heating up times but the high alumina cement will need some backing if it's used. The castable might allow higher furnace surface area emissions too as it's more conductive than the usual bricks used.

I should say that I started of thinking castable was the way to go but making moulds with grooves is a bit of a nightmare. It can be done but.

John

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[AjohnwParticipant @ajohnw51620]

Posted by KWIL on 07/02/2016 16:00:41:

Try these **LINK**

Suppliers of castable materials for high tenperatures.

I'd already found them Kwil but the £200 min order rules them out completely for me. I'm thinking more in terms of £50 and it looks like I will achieve that too.

Edit or something much more like it.

John

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Edited By Ajohnw on 07/02/2016 16:41:57

[Les Jones 1Participant @lesjones1]

This is another place that may have what you want.

Les.

[AjohnwParticipant @ajohnw51620]

Artesianfoundary do have an item that may be of use. I wondered about making a case with it due to what it's used for. I need to handle some though

**LINK**

There are fair few suppliers about but shipping costs sometimes make local purchase the best option – if you can find one.

These people sell on ebay as well. I obtained a castable from them. I asked about a particular type and lo a listing appeared.

**LINK**

A well known supplier that does all sorts and also has a retail shop with some of the items in it. One of the furnace books uses their materials. For electric Myfordboy suggested I should buy it but none of the books use the technique more modern cheap kilns use that are basically made from blanket.

I'd rather work it out for myself anyway.

**LINK**

And one that may offer a high alumina cement but they don't state how high

**LINK**

There are lots about searching refractory bricks brings them up especially if insulating is included as it usually excludes fire bricks which are very dense and heavy. I did have another that listed all sorts of things while looking for something that turned out to be very expensive so didn't link to it. I might try another 1/2 hr with google

John

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Edited By Ajohnw on 07/02/2016 17:56:35

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