Gravity Diecasting

[itsme]

Hi there,

I am planning on casting a few small items in aluminium using carbon/mild steel dies that I will be machining.

What I'd like to know is if any kind of releasing agent should be used in the dies in order to aid in the removal of the cast part? What is used in industry and what could be used at home (that is if anything is used at all)?

I'd hate to spend hours making the dies and then discover that I'd 'brazed' them all together on the first pour...

Thanks
Warren

[davidmb]

Hi Warren,

You need something I think is called 'Dycote' Trade name, my local foundry supplier sells it, try 'Engineering Foundry Supplies' in Colne, Lancs.
http://www.ef-supplies.co.uk/ they have always been good both with small quantities and advice. If lancs is too far for you, try a local foundry and ask where they buy supplies.

[ironDigit]

could you post some more info on machining your molds

materials,conicity,finishing how many casts can a mold take?? etc. etc.

thx in advance

[itsme]

Hi there,

Thanks for the information, I will look into Dycote.

As you have probably guessed, I haven't done this before. Making the dies is going to be an interesting exercise. I will hopefully start making the dies in the next few days, using carbon/mild steel. For anyone that is interested, I will post here once I have got something to show for my efforts.

As far as die life goes, your guess is as good as mine. I am only planning on casting a few parts using the dies, that is why I will be using a cheap, low-tech steel.

Regards
Warren

[diecutter]

When I cast aluminum the amount left after the molds are filled is poured into a muffin tin made of pressed steel sheet. I have used the same tin for years and it is still in good shape. Also, the aluminum "muffins" simply slide out when the tin is turned upside down. The tin has eight compartments. These muffin ingots are perfect for remelting for latter pours.

Solid steel will cool the aluminum very quickly, possibly resulting in voids. Two things to do to avoid this are to preheat the steel mold and/or pour a couple test pieces until one comes out without voids. You can always remelt the test pieces. alternately, you can machine your mold from graphite which will not draw off any of the heat. I too would like to see pictures of your efforts.

[Geof]

Use hot rolled steel for your moulds or rough machine them to within .5mm or so of the final dimensions and then anneal or normalize them before finishing. Cast iron is really the best material for the moulds but steel works fine and you can get hundreds of parts out. Make sure all internal corners have a radius of around 1 to 2 mm and use a draft of about 3 or 4 degrees. Before using for the first time heat the moulds so you get a thoroughly oxidized surface past blue and into grey/black. You want a 'dirty' surface that will not easily braze. A powdered graphite slurry in water brushed on before preheating works as a release agent although once the moulds have a nice oxidized surface the aluminum will not stick very well. Put a bit of thought into the location of pouring gates and feeders before making the moulds. The mould should fill from the bottom up through the pouring gate but adjacent to heavy sections it may be necessary to have additional gates filling a reservoir volume that fills with molten metal and then feeds back into the mould as the metal cools and shrinks.

[itsme]

Hi there,

It seems that the steel may work fine without any expensive/hard-to-get releasing agents. Geof, now that you mention a 'dirty' layer on the mould, I think I should be good at getting that right, because whenever I try to braze something, I can never get it to stick...

What exactly would a graphite slurry involve? I have visions of going out and buying lots of cheap pencils to grind up and mix with water . Is the purpose of normalising the steel before finishing to prevent distortion?

I am planning on preheating the mould before use too, as I don't think it would work too well if it were cold. I also still need to make a furnace, which shouldn't take too long, as i only need something small. I will be needing less than 100g of aluminium at a time, which gives you an indication of the size of this mould. It is small.

Machining should start soon, provided the postman is quick with the tools that I haven't ordered yet...

Regards
Warren

[Geof]

Originally Posted by itsme ...What exactly would a graphite slurry involve? I have visions of going out and buying lots of cheap pencils to grind up and mix with water .

Or you could buy powdered graphite of the type that is available for blowing into locks to lubricate them. Just mix to a runny paste with water and a little soap.

Is the purpose of normalising the steel before finishing to prevent distortion?

Yes, with cold rolled steel you get a lot of distortion when you machine parts away and you would get a lot more when preheating and using the mould. The aluminum pouring temperature is close to the normalizing temperature.

[itsme]

Hi Geof,

When using a graphite/water/soap mix, should you let the water evapourate off before pouring the aluminium? Surely if it were still wet, you would have trouble with the steam (even with adequate venting)?

Regards
Warren

[Geof]

Notice I said before preheating. Yes the water evaporates off and just leaves a coating of the powder.

[Dougotio]

I think that if you lit your acetylene torch with out any oxygen and coated the mould with that sooty, pure carbon smoke it would work as a release.
Good luck,
Doug

[aggie_67]

I had heard that you could use a burning candle, held under the mold it will leave a carbon coating.