cast brass surface conditioning

[pstarckt]

Hello all, new guy here with a couple of questions. I do sand casting of brass and aluminum and have a few issues with the surface quality, especially the brass. The surface is 'pitted' (the interior is solid) after casting. I've tried about every trick I know of to remove all the variables like loose sand, parting dust, trapped gas etc. Is there a process I need to go through to remove or mask the surface pits such as shot peening or glass beading before buffing/polishing? I can get a mirror finish otherwise but the pits show. They are small like pin pricks shot gunned across the surface.

How would I get a satin finish instead of a high gloss.

And last, how do you remove the buffer residue from tight detail spaces? Is there a chemical or procedure?

Thanks!

[CarWiz]

Need a few details about your green sand and process. What are you using for pouring temperatures? What are you using for alloy stock? I always spray my molds with Slide. It's a high temp mold release. Not so much for a release, although it works great, but it shows areas of the mold that are not smooth or has grains of sand on the surface. Inclusions can also be be caused by your pouring temperature and alloy as well as the sand mixture.

Buffing residue can be removed with acetone or lacquer thinner. If you plan to do further surface treatment (plating, dyeing, etc.) you'll want to wash the item in HOT SP degreaser or equivalent.

Caswell Inc. - Cleaners and Degreasers

[pstarckt]

Well since it's a backyard affair, I go by the known melting point of the stock. Wait till it turns liquid and give it 5 more minutes old rule of thumb. Not too scientific I know...

My stock is pretty much whatever I can get my hands on however these pieces have been from 'factory goofs' of a brass pipe connection. Very clean stuff.

I have tried blowing the mold clean and even vacuuming it.

The sand is veteran use Petro-Bond freshly mulled.

Tried using corn starch baby powder and (seperately of course) graphite for parting agents. No significant difference.

I don't think it's a metal gassing problem since there are no trapped bubbles in a cross section of the part. Only surface pits a few thousandths deep. Too much really to buff.

[CarWiz]

If you're happy with the grain size, you're probably not getting it hot enough. How long does it take to get to the melting point? Pour aluminum above 1400 and Brass above 1900. It should pour like water. Without a pyrometer, you're just guessing though.

[Rasper]

A satin finish can be obtained with automotive rubbing compound. I use the Number 7 brand. It used to be made by DuPont but they sold it to someone else. I rub it by hand with a wet pad.

R