The Metal Artist Forum

When casting metals is it generally correct to assume that the ideal casting temperature is just above melting temperature?

I'm working with pewter and have been doing a lot of tests. I'm working with a bottom pouring pot. At just above melting temp the metal is thick and only drips from the nozzle - the cast items look "layered" but come out a bright, smooth silver colour. As I go up in temp the colour of the cast becomes dull, then yellow, then purple/blue and there are more pits/cavities and a more dull surface but it is less viscous and pours continously.

My theory is that the lower temp would give best results poured through a larger hole so the mould fills more quickly - I'm ready to drill-out the nozzle of the pot to a larger diameter UNLESS I find out I should be casting at the higher temps.

First I have not had any experience with the bottom pouring melting device you are describing. What type of metals was this device originally designed for? You will have to figure out what enlarging the hole will do to the pot. Will you still be able to contain the material in the pot, will it cause leaks etc.

(09-20-2009 09:24 PM)noir Wrote: [ -> ]When casting metals is it generally correct to assume that the ideal casting temperature is just above melting temperature?

Depends on the metal, the type and style of the mould.

Quote:I'm working with pewter and have been doing a lot of tests. I'm working with a bottom pouring pot. At just above melting temp the metal is thick and only drips from the nozzle - the cast items look "layered" but come out a bright, smooth silver colour.

As in the answer to your first post, knowing the alloy and what it's requirements are as to temp ect. can be important. As the alloys are designed to do certain things. Babbitt metal, tire weights, muzzle loading bullet lead all are mixed up to do certain things and react differently. If you can ask the supplier if they sell this alloy for other uses.
The general temps for the alloy of Britannia metal
Melting point:

244'C (471.F) solidus temperature
The solidus is the temperature at which metals began melting during heating,and the point it solidifies

295'C (536'F) liquidus temperature
The liquidus is a metals flow point at the temperature which melting is complete during heating or the point at which solidification starts during cooling.

Quote: As I go up in temp the colour of the cast becomes dull, then yellow, then purple/blue and there are more pits/cavities and a more dull surface but it is less viscous and pours continously.

The skin on the surface and the colors of it. Are the impurities in the metal and should be cleaned out and put in a discard can for later melting and collecting of any metal in an ingot mold.

Quote:My theory is that the lower temp would give best results poured through a larger hole so the mould fills more quickly - I'm ready to drill-out the nozzle of the pot to a larger diameter UNLESS I find out I should be casting at the higher temps.

Not knowing what your molds are made of,metal,plaster,sand etc or the amount of metal you are melting and pouring at one time. Everything is general in nature.

Metal molds are heated so the metal flows freely to all areas and doesn't develop cold shunts in areas.

Pinhole porosity is due to the evolution of gas in the metal due to shrinkage.

Porosity can be simply said to be the condition of metals containing blowholes,cavities caused by shrinkage and trapped gases. Which occurs from improper melting,venting, and overheating.

The only bottom feeding pewter type caster that I have seen. Is the ones that are used for casting the sheet pewter for the making of organ pipes. And that one is electrically heated, so the real long pipes can made in the fewest number of sections.

My set up is:

Melting pot - http://www.midwayusa.com/viewProduct/?pr...ber=637732 It is sold as a lead melter.

My mould is MDF with candle soot as the mould release. The MDF heats up and holds the heat after one or two pours.

I've found out the alloy I'm using is 97% tin and the rest is antimony and copper but not sure of the ratios. The manufacturer lists liquidus as 236'C and solidus as 235'C. (They must have a different definition for liquidus :S )

The colours I mentioned on the pewter aren't the oxides and impurities that rise to the surface of the melt - its just the colour that the surface of the whole cast takes on. (Kind of like the colours you can get by heating a piece of copper with a torch). The colours on the cast themselves aren't a problem but seem to indicate the temp the pieces are cast at - wasn't sure if they are a clue to what I'm doing wrong.

After reading your post I think cavities is a better description for most of the porosity problems - they are few but large. Also, at the higher temps I get what looks like a large grain structure but the surface is fairly smooth.

I've been scouring the internet since I posted and think I was a bit hasty in wanting to drill out the nozzle of the pot. I've found ideal casting temps as listed for various lead free, high tin pewters: 315-329'C, 250-370'C, 226-343'C. Looks like I may need to up my temp as I thought around 270-280'C was a good temp but it wasn't flowing and I should be able to go higher.

From my other reading tonight I'm thinking I should trial graphite powder as the mould release, add a reservoir and/or increase the sprue/button size and possibly add some venting lines.

I'm guessing MDF mighn't be the best mould material but have seen a few examples online where people have got much better results than me.

MDF is held together by glues, along with all kinds of ground up woods and pressure. The biggest problem will be overcoming the release of the trapped:
1 Moisture in the wood and fibers material
2 The epoxy or glues that are used to bind the material together. Will be out gassing the volatiles of the components.

Quote:Medium-density fiberboard (MDF) is an engineered wood product formed by breaking down hardwood or softwood residuals into wood fibres, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature and pressure.

Again the important info is WAX and RESINS.
Another problem will be they manufacture of the board you don't know what types or the temperature the wax and resins will react at and the moisture content of the MDF.

Generally only metal molds need to be heated so it doesn't chill the pour before it gets to the bottom of the mold. when using 2 or multiple parts molds.

Yes that is a bullet molding melter.

You might want to try a propane torch and a large dia pipe cap with a handle attached or a plumbers pot. I have a feeling that will cure most of your problems. As the bullet melting furnace is made to deliver a controlled small stream of lead to the bullet molds which are either a single or multiple cavity mold. Not larger surface area.

[attachment=11089]
In the picture here is my buckle casting rig for classes and workshops.
A typical plumbing propane torch on a fat boy cylinder, a ladle used for melting plumbers lead for leading joints in iron sewer(waste) sitting on a coupler from the auto parts store Exhaust system that fits the ladle bowl.
I add a screw the length with a couple washers and a wing nut to hold the ladle level. This way you can pretty much heat the metal to pour, one handed.

The style pot in the picture if it cast iron can be also used, this one is a heavy wall aluminum that was going to become a holder of pitch.

I think if you try this you might solve most if not all your problems.
If required you can always have more than one ladle if you are doing multiple casting at once.

The coloring on the metal is still a very thin layer of oxidation as tin and the other white metal alloys are known for the rapid oxidation. Basically as fast as you scrape it off it reforms! That is just a fact of the material.

[attachment=11090] [attachment=11091]

[attachment=11092] [attachment=11093]

Any of these torch set ups will work, One of the larger pewter smiths here uses an open large melting pot over a gas ring and uses a ladle to dip and pour his molds or for his sheet.
Don't get yourself in a bundle by over thinking or engineering some thing, as the simplest way some times is still the best way.

Glen has given you a most excellent response. Casting into MDF is a new one to me. It sounds like an open invitation to a gassing problem. I'd choose a system like Glen is using over your bottom feeder. because there are fewer limitations ie. sprue placement and size of the object to be cast. Sounds like your thoughts are correct, up the temp, increase sprue size, vent. use the graphite as a mold release. Even then you might not be delivering enough metal at a fast enough rate. It would help if we knew the general shape and size of what you are casting. Some shapes need ventng and careful sprue placement, It sounds like you've got that under control. Brad

great topic......I'm right in the middle of a lost wax casting class. This is one of the "hand outs" that I have related to this topic. Thought I'd toss it out there......I'll be back at the end of the day to see if there is anything else I can add.

Bill

[attachment=11095]

Photos of your set up would really be helpful in enabling us to give you more specific information.

Quote:I've found out the alloy I'm using is 97% tin and the rest is antimony and copper but not sure of the ratios. The manufacturer lists liquidus as 236'C and solidus as 235'C. (They must have a different definition for liquidus :S )

Well don't know where they are getting the numbers, but they are much to close together. And I have corrected the definitions of both liquidus and solidus. I have went back to the authority, Oppi's opus "Jewelry concept and technology" for clarification. Sort of which type of mud is clearer
but the corrected info under the terms in my first post.

I have a great video by this pewter foundry als Pewter foundry
I can't seem to find it on their site, but it covers all the classes they offer and shows him soldering with a smith little torch!

Quote:After reading your post I think cavities is a better description for most of the porosity problems - they are few but large. Also, at the higher temps I get what looks like a large grain structure but the surface is fairly smooth.

Problem is the moisture and binder inside the MDF. Which you have to remember steam expands from 1,600 times the volume of water. When you pour the hot metal even at the relative cool temps that it melts at. It is still flashing any liquids in the molds into a vapor which is why it drives people crazy when they say well the mold was dry! When it really isn't.

Quote:From my other reading tonight I'm thinking I should trial graphite powder as the mould release,

Candle soot is what in the old days was lamp black a pure form of carbon, which was collected and used as a coloring pigment. In paints and rubber etc. You can use graphite and any pure powered carbon.

If when using graphite as heat barrier/ solder stop it doesn't brush evenly into all areas. You can try mixing a small amount of alcohol with the graphite. Then paint it on so you have an even covering over the inside and play a torch over it to remove the alcohol content. with a nice smooth solid coating you should get numerous casting without a re coat.

Quote:add a reservoir and/or increase the sprue/button size and possibly add some venting lines.

This is referring to a two piece mold not an open face mold where you are pouring into negative cavity to form a positive cast piece.

You only need any of the above when it is a closed mold and you have to make sure that the point of the pour is in to and spreads out to the smaller thinner areas.
If it isn't it will cool to rapidly.


As to the casting temperature of pewter or the other tin alloys. The use of a pine stick to tell temp, I use paint stirring sticks here in the states the ones with a pronounced grain are generally P-S-F Pine-Spruce-Fir wood or just slit a 2x4 up the wood chars around the proper temperature for pouring. You can use it to move the scum off the top just before pouring to.

Thought I'd start by sharing links to other people's projects casting into MDF moulds:

http://www.denfordata.com/bb/viewtopic.php?t=1116

http://i567.photobucket.com/albums/ss116...endant.jpg
(from this forum thread: http://cr4.globalspec.com/thread/38928/S...r-Castings )

http://www.tep.org.uk/PDF/NV10/NV10_Pewter.pdf

http://www.technologystudent.com/equip_flsh/pewtt1.html - this is a really good description of the type of mould I'm using - kind of a 2-part mould with 3 layers.

Sorry I left out one of the most important bits of information in my last post - I'm casting a small circular pendant 2.6mm thick and 25mm diameter.

I was initially casting using a small hand held milk pot over a camp fire stove flame but was getting dross and oxides in the cast - they could be seen as small lumpy dark coloured bits in the casts. I got the bottom pour pot to overcome this problem - it seemed like a worthwhile investment as I hope to do a lot of small pendants. I have overcome the dross and oxides in the cast problem - the pot only pours clean metal as everything else stays at the top of the pot. I agree it wouldn't pour fast enough (even at the higher temps) for larger items but feel my pendants are of comparable volume to the sinkers and bullets I've seen (on youtube) cast with the same type of pot. I would like to persevere with this set up.

Thank you for the replies - its all food for thought. About the resins and moisture in the MDF - at first I was looking through my garage for hardwood to use for moulds, realised all I had was a stack of MDF and jumped to the same conclusion about the resin. When I got online and saw other people were successfully using MDF moulds I thought I'd give it a try. I thought as it worked for others it could work for me... I wanted to perfect my process with the MDF mould and then, once I'd nailed that, also try adding silicon and aluminium moulds to the fold. I'm starting to think now it might be time to just give up on MDF and move on to a different mould material. I will try some modifications to my MDF mould today though.

Well the first thing That I see is the material they are calling MDF is a slightly different composition and particle size than in the states. We have some stuff that looks like that but they are making it out of the brown cardboard and the resins and pressure. It is used in the real cheap press board furniture. Some Use the chip board/particle board larger pieces of the woody material including the bark and the knot material.

Looking at the pictures of the one forums and the castings a lot look like they have a lot of surface porosity. None of them look to be using a mould separator (graphite) which with pewter you shouldn't need. It is used with the steel molds for the bullets and sinkers to keep the pewter or lead from sticking to the molds as most people overheat the pour and it sticks to the steel.
Some of the other photo's also look to have some issues. with the surface.

I do the pewter with sand casting, the soft fire brick, home style investment casting but only with a thin shell over the wax master.
An the occasional high temp silicone caulking from the auto parts store.

Quote:I'm casting a small circular pendant 2.6mm thick and 25mm diameter.

For the metrically challenged that is about 1"dia and the thickness of a #37 drill just a little thicker than a dollar coin.

This is a good source for mold making info great videos
This is for the hobbyist and there aren't for pewter casting but resins other then the working temp of the silicone. it is the same

This is the industral division again great videos

This is another great site he is into lost foam but does cover some other materials

Here is a world wide source but they are the grandaddy for the video's on line or to purchase
I can highly recommend them and their vid's I have a number of them and they are great. They are so have distributors around the world.

Here is the place for some fast and easy new product for low temp casting Haven't used it yet but this may be what you need.

You can make a divider for your pouring pot that has the out line of the bowl with two or more half circles cut out along the bottom so the melt flows and the dross stays behind the baffle. Will have to see if I can find one to show you.

You can keep the dross etc. off the pour with the pine stick.
The oxide's will form as soon as any of the melt comes into contact with the oxygen in the air and its starts cooling.
The oxides should be nothing more the a skin on the pour. any chucks etc are dross or impurities.

PTSideshow, firstly, thanks for the size conversion - couldn't do that myself because anything that isn't base 10 does my head in - I really can't deal with fractions of inches...

Thanks for the links - that last link for the castaldo quick-sil is a product that's had me interested for some time. Not sure how many moulds it makes but looks like it could get expensive to do a bunch of moulds.

So... today I played some more with temp ranges and found that the best way to get a good flow from my pot is to fill the pot right up.... still, temp seems important as I got best results in the 270-280'C range - and this time the metal actually flowed out nicely.

I cut the button down deep as there wasn't really enough length on the sprue to add a separate reservoir. It seems to have improved casting results to a limited extent. So far I've got the best surface on the cast yet - no longer porous, no cavities but quite frosty in the centre on front and back.

I tried cutting some lines with a scalpel from the pendant cavity out to the edges of the mould but it really made no difference. Think I'll make a better effort to vent the mould before my next cast...

All in all though... think its about as good as its going to get now so I'm ready to try some other mould materials. If I get time tomorrow think I'll try to get some cuttlefish bones to cast with... I'm also planning to see what I can do with aluminium scraps...

I'll come back to update my progress as I try different things....