Vacuum Casting help please.

[BlueLeaf]

Hi everyone.
I have been having terrible problems vacuum casting silver. Melting with an electric melting furnace and graphite crucibles
It seems to be impossible to vacuum cast silver without some form of tiny gas porosity. This porosity only shows itself after polishing and ultrasonic cleaning the rings as dozens and dozens of tiny pits.

I have some very basic casting questions which hopefully experienced people my know.

1. Does it matter how you pour the melted metal ? For instance a slow steady pour or tip it in as fast as possible.

2. Should I leave the casting machine vacuum pressure on after the pour whilst the can and molten metal cool slightly, or turn of the vacuum immediately after pouring the metal into the can.

3. Do graphite crucibles oxidize after only a few uses ( 3 to 5 ) and therefore the crucible may cause contamination of the melting silver.

4. Should I use borax in the graphite crucible.

5. I melt and then cast at between 1020 and 1040 celsius. Anything outside this range causes worse porosity.

6. I remove the can from the kiln after 8 hours and the temperature I remove the can is 550 celsius although with a laser thermoter pointed inside the can the temperature would then be 400 approx.

7. I allow 15mins to cool before I quench the can.

8. I have been using almost always brand new grain so old scrap is not a factor in my process.

9. I am using standard injection wax patterns, 3d printed wax is not a factor.

If anyone can see any problems here which im missing or can answer some of the basic questions I would be delighted with any help.

Best Wishes
David

[josef1]

Hi David,

Whats your burn out procedure is it correct for the investment. porosity can be from over heating the metal .

1. Not really I use a system that just drops the metal in with gravity.

2. I do for a minute or so

3.you should brush them out with a stiff brush ceramic crucibles are better if you can get them for your melter

4. I don't, this can cause porosity and breakdown the crucibles but others may tell you different

5.depnds what metal but sounds ok for sterling silver

6. you need to check you are following a proper burnout cycle that's recommended for your investment powder and your kiln is somewhere near the temps you are using

7. sounds ok, red gold I dunk quicker it brings the copper out.

8.this is the way when trouble shooting

9. Also good, resins can add another level of complication.

cheers Joe

[BlueLeaf]

Thank you for the reply Joe most appreciated.

Im using protocast investment and they recommend a 12 hour burnout. But I had to halve it. Its just not feasible to run a kiln for 12 hours for small 2.5 inch cans.

I used a graphite lid for the graphite crucible today in an attempt to further protect the silver from absorbing oxygen in the electric melter.

Today I lowered the casting temperature of the sterling silver to 1015.

The inside temp of the can was 450 C the outside was 510C determined by laser thermometer.

I released the vacuum straight away after it filled.

Let the can sit and cool 5 mins before quench as its only a small 2.5 inch can. let it cool in a closed pot avoid too much oxygen.

The result is as bad as ever. Hundreds of tiny dentric pores just under the surface of the silver.

In the past ive tried

borax / no borax.

Pouring trough a flame to protect from oxygen.

Thicker sprues.

Ive been up and down the temperature scales for cans and melting temps.

Ive tried thicker investment ratios and so thin that they have burst.

Ive tried wax webbing to increase the pull of the vacuum.

Ive tried different investment powders and different burnout cycles.

Ive put vents onto the far end of the sprue.

Nothing has worked so far.

Ill try making the investment a bit thicker and adjust the burnout again but im running out of ideas after that.

Ill try 1h ramp to 230C, hold at 230 C for 1.5h. 2h ramp to 730C. Hold at 730C for 2h. 30m climb down to casting temp 600C hold for 1h.

This I hope gives with working time an actual internal can temp of 450.

[josef1]

Its frustrating I know. I run 12 hours for 3"x3" flasks for burnout. If you want to send me some of your metal and a wax I will cast it with my setup and send it all back to you back to compare. I can give you all the specs and temps I use then you will have a base line to work from if that helps ? PM me.

[BlueLeaf]

Thanks Joe,
Thats a very nice offer Im going to try a few last things here. One thing I have noticed is the date printed on the bottom of the investment powder is 1/3/22. So if that is the expiry date then the tools company I have been buying from has been shipping expired investment to me.
Could expired investment be the cause of all these tiny porosity holes?

[josef1]

Investment is hygroscopic it absorbs water this can effect it if its been left open for ages. But if its been sealed up I dont think it will effect things so much but its not going to help been out of date I guess. I have had bad bags of investment though. the things I found effected porosity for me was using borax and metal/flask to hot but the only way to do it is keep ruling things out one by one .once you get the formula stick with it It can be frustrating.