Jump to content

Lost wax...failure


Recommended Posts

Hello every one,

today I have made my first experiment of fusion in bronze to lost wax, but it has been a failure.

The surface is much spongy, which is the cause? The mould was in chalk, is perhaps it does not go well? moreover the formed oxide is hard, how to eliminate it? I have tried with hydrochloric acid but he gets worse the situation because it corrodes the bronze!

 

These are some images:

 

mdf-miniforge640x480.jpg

The mini forge

 

mdf-minicrucible640x480.jpg

The crucible in iron, diameter 22mm

 

mdf-mould640x480.jpg

The mould in chalk

 

and te results, diameter 5.5mm

mdf-bronzeoxide640x480.jpg

 

mdf-bronze640x480.jpg

 

mdf-bronze2640x480.jpg

 

Thanks

 

CIAO

 

Marco

Link to post
Share on other sites

something which might help, is to take a shallow wide tin can, and put a wooden handle on the top, and stuff a few damp paper towels in it, once you have finished pouring the metal, put the can on top of the sprue, the residual heat will make the wet paper towels steam and the pressure will push the metal into the mold more forcefully than gravity and hopefully the gas out.

picture here:

(edited for typos and clarity)

steam_caster.JPG

Edited by Archie Zietman
Link to post
Share on other sites

I'm interested in the need for fluxing in bronze casting.

 

I've done dozens of investment castings in bronze and aluminum in the educational institutions, and I've never seen a process that has used any kind of flux. Even when we are remelting old sprues and pour cups, we've never needed any. We simply skim the crucible well before the pour, removing the slag/dross from the top of the melt. What are the benefits of fluxing and degassing a bronze melt? Does this apply to sculpture as well as industrial castings?

 

Josh

Link to post
Share on other sites

I know it is commonly done with industrial castings - I am not sure or unfamilar with scuplture type castings. Perhaps the chalk was a bit damp....It really doesn't take much for the moisture to cause a problem. The benefits are reduced gas porosity, and reduced inclusions. Also most use a bottom pour for industrial sized castings to reduce gas porosity.

Link to post
Share on other sites

Josh, Aluminium is not as prone to gassing up as most copper based alloys, and in an educational setting, you were likely using silicon bronze ? It is nearly foolproof. Start fooling with making copper based alloys from scratch, however, and you better figure out some way to degas the metal, or likely you will never get any usable parts. Phos-copper just before pouring seems the best to me.

Link to post
Share on other sites

Howard,

 

Yes, Silicon bronze. I'll remember to flux and degas when I start working with different alloys.

 

Thanks for the info, folks.

 

I agree that the issue with the casting in the original post is probably moisture in the chalk.. If it were very much moisture, though, would it not have violently evacuated the mold from steam explosion? I've had improperly heated ingot molds spit a good amount of bronze, even though they can't have had much moisture in them at all.

 

Josh

 

 

 

Josh, Aluminium is not as prone to gassing up as most copper based alloys, and in an educational setting, you were likely using silicon bronze ? It is nearly foolproof. Start fooling with making copper based alloys from scratch, however, and you better figure out some way to degas the metal, or likely you will never get any usable parts. Phos-copper just before pouring seems the best to me.
Link to post
Share on other sites
Howard,

 

Yes, Silicon bronze. I'll remember to flux and degas when I start working with different alloys.

 

Thanks for the info, folks.

 

I agree that the issue with the casting in the original post is probably moisture in the chalk.. If it were very much moisture, though, would it not have violently evacuated the mold from steam explosion? I've had improperly heated ingot molds spit a good amount of bronze, even though they can't have had much moisture in them at all.

 

Josh

 

I Josh,

I used to be involved with large Bronze melts at several different foundry years ago. Silicon bronze is basically fool proof because the silicon content is a built in deoxidizer. You will find some precious casting alloys also contain silicon as a deoxidizer. It is bad if you want to roll your metal later. Not usually an issue with casting to shape :)

As I was told by an old time caster I work with a few times "Nobody likes scooping the flux and dross off a 500 pounds of F2100 metal". Flux is generally the second choice to deoxidizing in today's foundry. Phosphorus copper works for deox and there are many recipe for deox additives in the melt. They are generally thrown in a few minutes before pouring.

Regards,

Patrick

Link to post
Share on other sites
Many many thanks for the information!

I suspicion however that the chalk was humid (the air was much humid one).

 

CIAO

 

Marco

 

The mould material could be at fault. The mould needs to be composed of at least some refractory materials and have a certain level of porosity once cured to let super heated gases escape ahead of the metal. It looks to me like perhaps the mould did not have the proper porosity and probably broke down too rapidly from the intense heat. A good ready made Investment for sculptures casting might help.

 

Some good references here, but let me point out that Fluxing and Degassing are two different approaches.

Fluxing in nonferrous melts is meant to create a barrier keep oxygen from being absorbed by the melt. You have to have metal that is clean of the oxygen to begin with for this to be very effective. This works well if your air to fuel ratio is not very controllable as the barrier is generally effective against badly adjusted oxidizing flame environments. A draw back to flux is you have to keep it out of your casting. There are ways to deal with that, but it is nice when you can skip it. Some fluxes are corrosive and will eat away as your crucible at temp.

Degassing/deoxidizing can be used without flux or with it if need be. This is where you assume that the melt has absorbed oxygen or will have by the time your ready to pour. Towards the end of the melt you drop in some elements or compounds that more aggressively bind to the oxygen and steel it away from the molten alloy. If done a few minutes prior to pouring the oxygen is removed and the metal is "degassed".

To make life simple you can use "Silicon bronze" which has a built in deoxidizer.

 

In my work I don't do any casting to shape anymore. I cast ingots to break them down into sheet stock. I don't want to deal with flux. Deoxidizers tend to make the metals less rollable. So I have a dedicated set up with an electric melter using an Argon gas shield. I always start with clean metals. No flux and no deox required. That is not the set up for the occasional caster. I just mention it for reference.

Regards,

Patrick

Link to post
Share on other sites
  • 1 month later...

Thanks to all, finally I have caught up some result. the problem was the chalk. I have made three tried with various pastes and is much best, nothing gas problems! the best one is sand and 10% of concrete (Thor's hammer).

M.D.F.___Bronze.jpg

This evening I make new tried using my new forge for the fusions. soon I insert a pictorial on its realization

 

I hope to learn this technique soon for my guards

 

CIAO

 

Marco

Link to post
Share on other sites

Hi Marco, just noticed this thread.

a couple things I would sudjest. If you can get some silicon bronze to work with it is a great way to get good at casting, it is an almost fool proof metal and very attractive for finishing or patinating; and needs no flux!

I would also sudjest that you buy some profesional investment solution from a jewellery or sculpture suply, I found a huge difference in the quality of my castings when I started using premixed investment powder, but for larger stuff I still use a three part solution of one part 00 silica powder or "flower", one part plaster, and one part fine silica sand, wich works quite well for gravity casting bigger sculptural stuff.

a couple more things that I would sudjest are, if you are gravity casting, make a long sprew so that you build up a bit of head pressure when you pour.

Put several small gass vents comming off of small parts of the objects as well as the bottom and have them come out next to the pouring cap so that hot gases can escape and draw the bronze into the hard to get at areas.

And finally make sure your mold is pre-heated, around 800 degrees ferenhite is ideal, this way the bronze dosen't cool to quickly and you can be sure that there is no moisture in your mold, this also helps get greater detail retention in the final product.

I hope none of that info was redundant, I just had time to skim the rest of the thread.

good luck to you

Jake

Link to post
Share on other sites

I have very limited casting experience, and none with copper alloys - yet. But in the process of reading up to try homebrewing some tin bronze, some folks with more experience told me that zinc is a good deoxidizer for copper alloys. It's pretty readily available, too. (Think pennies.) Might be worth a try. Just a thought.

Link to post
Share on other sites

many thanks Jake,

these informations are very important for me.

 

Yesterday I have done mine third attempt of fusion with the new constructed forge, exceptional!

Gas problems do not have more, not flux use but the fusions are compact, without blowings.the bronze I make it from me with copper and tin.

 

The problem is in the moulds, yesterday has added also 10% of clay, but I have gotten worse, the edges in fact are all rounds,I will try with your mixture.

 

 

However it is only my third attempt, creed that sooner or later I will succeed to obtain turns out and then I can dedicate to guards and pommels.

 

Thanks still for these profits info, I add some photos of yesterday. moreover I make a new one post with a pictorial of the constructed forge, can be useful to someone.

 

IMG_0026.jpg

The postation

 

IMG_0023.jpg

modified tongs

 

IMG_0022.jpg

 

IMG_0016.jpg

copper used

IMG_0029.jpg

the temperature is very hot

IMG_0033.jpg

pouring

IMG_0034.jpg

 

IMG_0040.jpg

 

IMG_0044.jpg

 

IMG_0049.jpg

 

IMG_0054.jpg

 

the statue seems very much ancient one, now tries to oxidize it, seems one old roman votive statue :lol:

 

CIAO

 

Marco

Edited by MDF
Link to post
Share on other sites

OK - since it seems rounded at the ends - it is cooling off too fast. You need to get the melt much hottor. It apparently has inadequate fluidity to reach the rends of the casting. With enough super heat - it will work well. You are doing well - much better than before!. I like the statue!

 

Scott

Link to post
Share on other sites

It would make a huge difference for your molds to be hot..... 900 deg F is typical for investment casting.

 

I'm assuming your pouring with room temperature molds?

 

If your metal is fluid and the surface is glassy, its hot enough. Too much heat will cause serious shinkage in your parts.

Link to post
Share on other sites
It would make a huge difference for your molds to be hot..... 900 deg F is typical for investment casting.

yes, creed that the molds were too much cold, I have heated them, but too much little.

 

 

Also, if you configure your wax models in the mold with the larger portions at the bottom it will help with heat retention as the large sections will cool more slowly and allow the smaller parts above to fill.

 

Optimal trick

 

I will try these ulterior trick and inform you on my new results

 

CIAO

 

Marco

Link to post
Share on other sites
  • 2 months later...

The molds should be three-four hundred degrees F colder than the melting point of the metal being cast. The round edges on your castings could be caused by: 1) cold molds. 2) Not enough gravity pressure to force the molten metal into place... this is why small castings benefit from centrifugal force, steam pressure or vacuum assistance. 3) Not having a means for air to escape. You can add small extensions to the mold meant to be cut off later which provide a place for air to go. Modern investment compounds / gravity or pressure assistance usually allow enough air flow that this is not needed. 4) It is unlikely that the round edges are caused by not heating the molten metal enough, and overheating the molten metal is a problem that can result in boiling and pits much like the ones you have. In fact, that was my first thought when I saw your initial pictures. Don't go more than one or two hundred over the melting point... you want a swirling surface on the molten metal but not a bumpy, jittery surface. Flux it with some borax and skim the dross with a carbon stirring rod.

 

Do not overheat the investment mold... at 1450 F and up it degrades; however, to properly vaporize the wax residue you need to get to at least 1250 F. In addition to overheating your metal, wax residue in the mold AND overheating the mold can both cause surface pitting like some of the pitting I see.

 

Also... remember that when you start casting it is pretty normal to screw up frequently. New jewelry students HATE casting because of the failure rate.

 

I can't remember the last time I screwed up a casting.

 

You should also try sand-casting, as it is very repeatable with one original model, much easier than lost-wax and would lend itself well to your designs.

Link to post
Share on other sites

Thanks,

today...another failure!

the result is always the same: rounded edges.

The molds are hot (I built a new forge), but no difference, the bronze when poured into the molds boil.

The gravity is not a problem because if the molds are sand are good, but the finish is bad, too coarse.

The air sprew go I tried to change positions and even more than one, but it is the same.

Maybe it is overheating to be a problem, but in the crucible bronze is calm, but when I start to mold into boiling.

I tried with borax and clean the waste but the problem remains the same, even changing all these things ... always rounded edges!

As I said the only well have come with molds of sand 90% and cement 10%, net edges, but finishing too coarse.

Now I do not know what to do

 

 

Again excuse for my bad English, and thanks for your patience :unsure:

 

 

The forged for heating the molds

IMG_0013.jpg

 

the results

IMG_0035.jpg

IMG_0040.jpg

 

result in sand mould, the better, no round edges

IMG_0041.jpg

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...