question

============== That the bubbles are inside the casting indicates that these may not be bubbles but rather voids left when the metal solidified and shrank. Water expands when it freezes but metal shrinks by quite a large amount in volume when it changes from liquid to solid. In casting it is common to provide a large volume of liquid metal in the mold above the casting [the riser] to feed metal into the casting as it cools and solidfies. The sprues and runners that feed the metal into the mold cavity must be large enough so these will not solidify first so that additional liquid metal [from the riser] can fill any voids in the mold cavity. Design and creation of molds to allow ouring of sound castings is an art. Lindsay books has a good selection. see

Mmmm, I think it would be not the problem, because the mold is an open mould, so there is a lot of metal over it. Think it's a cylinder, so the mold only have an entry. I will try to draw with characters:

******** ********* ******** ********* ******** ********* ******** ********* Think in the figure as a box with a cylinder hole in the middle. *********************** ***********************

The * are the mold, and the [spaces] the cavity of the mold. I fill it all with melt bronze. The result piece I get is like that:

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

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

-------------------[]

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

-------------------- Think in the figure as a Cylinder

--[]-----------------

---[]-----[]---------

--[][]----[]------[]--

Being the [] holes in the bronze resulted piece. Of course the "-" are solid bronze. It seems there were captured some bubbles in the piece.

I will try to make some pictures right now and post here.

Thanks JRL

"F. George McDuffee" escribió en el mensaje news: snipped-for-privacy@4ax.com...

Consulting my US Navy Foundry Manual pages 221-222 [see #20072 in

] indicates two possible problems. [I highly recommend this book -- it is low cost and filled with practical information.]

(1) Your ladle and/or mold may not be completely dry allowing the melt to pick up hydrogen from the water vapor.

(2) Your melting procedure is allowing the metal to pick up excessive hydrogen from somewhere. This can be caused by too slow/long a melt or by a reducing [hydrogen rich] atmosphere. Manual suggests an oxidizing atmosphere. In addition, your melt might be at too high a temperature when you are making the pour.

You may need to degas your melt before pouring, especially if you are using scrap/salvage metal. If the top of your casting is domed or convex when it has cooled this is an indication of excessively gassy metal. There should be a pronounced dent or dip [called a pipe] on the top when the casting cools with good metal.

Which bronze alloy are you using? There are several cheap things you can try to degas the melt depending on the composition.

You may also need to add more vents to the mold to allow the air to escape. As my foundry teacher hammered home to me "if the air can't get out, the metal can't get >Mmmm, I think it would be not the problem, because the mold is an open

Could be you are not degassing?

"jr" wrote:

jk

Yes, this could be the answer, maybe one or both.

I will try tomorrow to make another new mold and to heat to 800 ºC before fill with melt bronze, to be sure it's completely dry. Besides, I will melt the bronze at 1000 ºC instead the 1100 ºC I used last times.

I will tell you the result.....

Thanks JRL

"F. George McDuffee" escribió en el mensaje news: snipped-for-privacy@4ax.com...

Yes, could be, but I don't see how to put a new degassing tube in the mold, this is a very easy and simple mold. Mmmmm, As I have just said in my last post, I will make another test tomorrow,now it's 23:00 here...:-).

I will try to add some vents to let the gass go out. I have read the plaster is not so porous, so I will try it.

Thanks. JRL

"jk" escribió en el mensaje news: snipped-for-privacy@4ax.com...

I don't know which bronze alloy I'm using, this come from a garden piece I have melted....

The top of the casting is horizontal, so no domed or convex.....

hu? How to know the alloy I'm using?

JRL

"F. George McDuffee" escribió en el mensaje news: snipped-for-privacy@4ax.com...

it have still moisture....

The news servers will not forward pictures for this newsgroup! Use the dropbox at:-

There are instructions at the "Using the Dropbox" link.

Good luck with the casting.

Mark Rand RTFM

Thanks Mark,

Images already sent. The links sould be:

And the images correspond to the base and the border of the cilynder. Note that I have still not try the George's ideas. My new mold it's still not dry.

JRL

The images has been renamed, follow this link instead the old:

JRL

jr wrote on 26 Aug 2005:

and on 28 Aug 2005:

Is that 2nd image a side view, a different try, or what?

Anyhow, from the 'base' picture I imagine the mold was wet. Plaster molds used for lost-wax casting are heated to about red heat (to cook out all organics) over a period of four to eight hours, and are poured into almost immediately upon coming out of the furnace. Also, such molds are made with "investment plaster", which is not the same as "plaster of paris".

Where did you get your "plaster + kaolin (25% + 75%)" recipe? I don't see how that would work, because AIUI kaolin is a clay, so your mold would not have adequate porosity to allow gases to escape when you pour. For the simple cylinder you are making, perhaps you should try a "green sand" mold. Green sand is fine sand mixed with perhaps 10% clay and 5% moisture, although some recipes use half or twice as much. The main idea is to use just enough clay and water so the mold holds its shape. Too much clay will decrease porosity and cause bubbles; too much water gives a wet mold and bubbles.

-jiw