Lost PLA First Attempt. Calling all casting experts.

No scraping off the dross or degassing before pouring?

Andrew

I did scrape of the dross. But didn't do anything with regards to degassing.

David

Flat horizontal surfaces can trap bubbles. Pre-warming the mould might help with this.

Frank

I think it was the bottom of the mould that went wrong with the full pressure head so not a venting issue, but it was the thinnest parts.
Could it be rapid cooling of the first flow down the ingate getting solidified?
Could some spec of plastic or other inclusion, even damp spec of dust that outgassed in the corner (though two corners so unlikely).
Perhaps burnt residue of the plastic got swept down.

Mostly I'm wondering if it was hot enough.

The mould was about 30 minute out of the fire, so it was probably at about one to two hundred centigrade inside.

I could see the corners before casting, and they looked to be good. And as you say, it was unlikely it would happen at both corners if it was fluke rather than systematic.

The aluminium (according to the thermocouple) was about 750 centigrade when I switched off the gas.

When you say 'Mostly I'm wondering if it was hot enough.' are you thinking about the mould or the aluminium ?

David

Hi David,

I think you needed venting at a couple of points, also (possibility) a shrink bob.

If you want much more in depth answers try here.

Hope this helps.

Peter.

Hi David,
First of all well done on producing a good casting,i think
these sort of problems plague would be casters i've had
more that enough over time.
Looking at the video at 1 min 36 the rounded form
may well be gas or even steam from the plaster
pushing back the metal, shrinkage normally has the
appearance of being more collapsed ie being pulled
from the inside ,yours looks pushed from the outside.

If you look at the Myford boy videos he has some very good
castings in nearly all of them the pouring and venting sprues
are extended in height allowing more of a head for the metal.

If you have the aluminium a 750 deg c i think you would be at the
upper end and could try to pour at a cooler temperature as the
sections you have for the sprues and the and the casting are
quite large.

In the photo in album Furnace and casting some small
lost wax castings and injected wax patterns ,aluminium
cast in investment powder flasks with vacuum assistance
as far as i can remember there was a 300 deg differential
between the metal and the flask temperature.

Keep on going your nearly there.

John

Thanks everyone – lots of helpful suggestions (as usual!), encouragement, and a few things to think about over the next few days until I can try again!

www.alloyavenue.com looks really interesting, with lots of beautiful work!

David

Hi David,

I spent 20 years in a aerospace foundry developing many rapid prototype methods and I bet your mold had plastic ash inside it. I would recommend burn out allow to cool down wash out and blow out with an air line then leave 24 hrs pre fire again when your happy there is no moisture left cast it. The early plastic Rapids gave us so much trouble with castings not forming, this is what we had to do. Long winded but you got all the ash out. Good luck I'm jealous of your set up.

Regards Senior

Read the Dan Calkin book a few years ago

A huge amount of detail and yet almost nothing on how the castings were done, and they had to be good

If memory serves only Dan knew the casting process to start with, he only ever shared it with one other person, and neither ever divulged their processes

-----------------------------

Dan had some clever  ideas for making castings
and wanted to keep these as trade secrets, so the
foundry area was off limits to all visitors.  One
thing Dan Calkin  did was to experiment  with
adding extra copper to the casting alloy. He found
that using an aluminum alloy with 5-10%  copper
in the casting alloy greatly improved the machin-
ability of the castings, while enhancing their
appearance.  Dan would weigh the aluminum to be
melted  and then added  a number of copper pen-
nies to the molten aluminum to get the desired
percentage.  At the Everett Street shop Dan usually
made all the castings himself  although  he some-
times shared the job with Dan Totten. All castings
used in all ELF engines were sand cast.

Edited By Ady1 on 04/07/2016 00:40:01

> He found that using an aluminum alloy with 5-10% copper in the casting alloy greatly improved the machinability of the castings

So he discovered Duralumin

Neil

He might have used green or oil sand for a good finish. Especially with oil sand. 2nd hand info from some one who did lots of casting.

There is a DIY recipe for oil sand here

**LINK**

Not at all sure where the cancer warning comes from. Best stay away from bentonite and sand it seems.

The person who did a lot if it bought oil sand but only used a limited amount of it around the pattern.

John

-