Will concrete (cement) blast if i heated around 630 degrees sentigrade?

[Willen]

I designed round heatsink myself. Design is ready on concrete (cement). Now I going to melt an aluminium and going to pour this liquid aluminium on concrete (where hole for heatsink is ready) to give its size. Can I do like this way? Won't concrete damage at 630 deg centigrade? The concrete (designed shape) will hold such hot liquid just a few second, then I will use water to cool it down.

 

[ericgibbs]

hi,
A VERY important point, the concrete mould MUST be perfectly dry else the mould could explode in your face when the molten metal is poured.
You should really use the correct type of moulding sand, go to your local casting foundry for a sample.
E

 

[Nigel Goodwin]

I agree with Eric, use proper casting sand - NOT concrete.

It might be worth mentioning as well that heatsinks are normally extruded, not cast - it's also cold extruded, no heating required.

 

[ClydeCrashKop]

I have occasionally used a cutting torch near concrete. When the slag hits the concrete, it pops a crater in it, as deep as 1/4 inch. Those flying chips of concrete would be no fun with molten aluminum on them.

 

[Willen]

There are no experience person or molding manufacturer around my area for the molding process. Once I've seen such molding process live but I forgot totally. I have no required hardware too. So trying for simple and alternative way. i thought to use general soil but soil might change its exact designed shape while heating with liquid metal. Are there any simpler way?

 

[dr pepper]

I've lit a fire in a pallet 6" away from a concrete floor.
I wont be doing that again.

 

[Nigel Goodwin]

Perhaps the OP might like to read this:

https://en.wikipedia.org/wiki/Sand_casting

 

[jpanhalt]

You (Willen) will not get what you expect to get simply by pouring molten aluminum into a concrete mold.

People have mentioned being sure the mold is dry. Dry at room temperature is not "dry" at 630°. Aside from moisture, you have to consider air. Air in the pores of the mold will also expand greatly. That can cause splashing, but it will also cause defects in the cast ingot. Also, as concrete cures by reaction with CO2 (e.g, Ca(OH)2 + CO2 ==> CaCO2 +H2O ;although, the chemistry of concrete is much more complex), you will need to be aware that other gases may also be produced. That is one reason some fine castings are done in a vacuum.

With sand casting, you need vents and such and will get a rough casting, unless you are expert. With more rigid molds, such as concrete and plaster, you can get finer detail, but you will also need to design the mold differently. You need vents. Dentists and others use a centrifuge to force the molten metal into the mold. I suggest that you look up and read a description of "lost wax" casting, as based on what you have said, that sounds like what you are trying to do.

Finally, have you considered machining rather than casting? However you cast, unless it is a very well done lost wax casting or a boat anchor, will require machining before use.

John

 

[alec_t]

I have no required hardware too.

How are you proposing to get the ali to melting point? What will you hold it in while melting and pouring? This sounds a very dangerous project

 

[dr pepper]

I have a crucible and casting sets, never used them, I was going to make a furnace out of a fire extinguisher, home casting supplies are tricky to get, casting black sand especially.
The 'proper' home shop casting method would be to make a mould in the shape you want from wood in 2 halfs or just one if simple enough, use that to create a positiv mould in sand then cast it, followed by machining to size.
Another way is to make the part in polstyrene and pack into the sand, then burn it out with molten ally, 'lost foam' is the correct term I think.

 

[Nigel Goodwin]

Another way is to make the part in polstyrene and pack into the sand, then burn it out with molten ally, 'lost foam' is the correct term I think.

Out chief instructor used to make his own medals to hand out to students, for various different awards - he used a lost wax technique, where (I think?) the mould is centrifuged and the molten metal if forced out in to the mould, displacing the wax. The medals themselves where formed in a tree shape, and cut off afterwards.

He did this because he actually used to work at a factory where they made casting powders (amongst many other things), and the batches of powder had to be tested. So the company were quite happy for him to test their product for them, using their equipment (in his own time), and he just had to pay for the metal he used (bronze or something similar?).

The actual casting powder was for the jewellery industry, and used for casting rings etc. (on a similar tree structure to the medals).

Edit:

This is the company he used to work for:

https://www.srs-ltd.co.uk/

and I think this was the product used:

 

[dr pepper]

Yes thats a specialized industry pretty much exclusive to jeweellery.
Must have been a talented chappy.

 

[Willen]

Yes! Fristly I designed positive mould (real shape) with wax. Then my problem is to make negative mould. Can I find this casting sand in nature? Or is it manufactured?

 

[Nigel Goodwin]

Yes thats a specialized industry pretty much exclusive to jeweellery.

Only a small part of their business, they also provided the lining material for the Channel Tunnel

Must have been a talented chappy.

Not particularly, the existing testers in the lab showed him how to do it - as it saved them doing it

 

[Nigel Goodwin]

Yes! Fristly I designed positive mould (real shape) with wax. Then my problem is to make negative mould. Can I find this casting sand in nature? Or is it manufactured?

I imagine it's mainly manufactured now, but certainly in the past it would have just been quarried - and is probably the reason some foundries are where they are.

 

[jpanhalt]

Here's a description: https://en.wikipedia.org/wiki/Molding_sand

Since your pattern is wax, have you abandoned doing the lost wax method? It is easier than doing a split sand casting.

John

 

[ClydeCrashKop]

Maybe this would work.
https://en.wikipedia.org/wiki/Plaster_mold_casting
Plaster mold casting is a metalworkingcasting process similar to sand casting except the molding material is plaster of paris instead of sand. Like sand casting, plaster mold casting is an expendable mold processes, however it can only be used with non-ferrous materials. Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments

 

[Willen]

I read all and found that it is not easy without necessary materials. Plaster method is also hard for me because it needs few extra special chemicals. Wax lost method also needs special molding sand or such materials to face around 630 degrees sentigrade.

Actually I am trying to make a simple round (like a flower) heatsink for TO-39 case transistor. It has been unusual component around my area so that I cannot find in market! Oops!

 

[ericgibbs]

I read all and found that it is not easy without necessary materials. Plaster method is also hard for me because it needs few extra special chemicals. Wax lost method also needs special molding sand or such materials to face around 630 degrees sentigrade.

Actually I am trying to make a simple round (like a flower) heatsink for TO-39 case transistor. It has been unusual component around my area so that I cannot find in market! Oops!

hi,
If your dimensions are critical, be aware that aluminium shrinks as it cools, by a few percent.
E
**broken link removed**
**broken link removed**

 

[jpanhalt]

Is this something like what you are intending to make with sand casting:

If so, that will be very difficult by any casting method. I suggest making it out of a soft alloy or only half-hard alloy of aluminum.

Can you post a sketch of what you want to make?

John