Casting made simple

[popper]

This is not how to cast but what goes on inside the cavity when you pour alloy into it. Understand the why and you can fix your problems. Ok, off to the races.
1) Our melt (M) is a mixture - not a fixed atomic crystal structure that doesn't move, whether solid (S) or liquid (L).
2) Casting is controlled by heat flow. I will refer to it as temperature (T). Our melt (M) is heavier and will displace air (A) IF it can. Nothing else counts!
3) Liquid (L) doesn't change to solid (S) at a specific heat (T) instantaneously. It is 'super' cooled and 'under' cooled, referring to the L concentration of components of M. This forms thin layers of different S. Solidification takes time.
4) Heat flows from high temperature to low temperature, the rate is determined by the difference in temperature. Therefore I will refer to heat as temperature (T).
5) Air (A) and liquid (L) contract when cooled - this is very important to understand.
6) Heat flows from the liquid (L) to the cavity (C), from the cavity (C) to air (A) AND from higher temperature (T) liquid (L) to lower temperature (T) liquid (L) plus lower temperature (L) to lower temperature (S).
OK, basic thermodynamics out of the way, what happens in the cavity when you pour?
Your L goes into the bottom of C. There isn't much T so L goes to S quickly. As C fills, T goes up, The volume of L goes up. Why? The S at the C wall shrinks. Air expands and must go somewhere. It can't go out the top as that is where new L enters. Trapped air causes wrinkles/voids - so we have vent 'lines' in the mould. Now we are at the groove. If 'new' L here is providing T to the S too quickly, new L turns to S and doesn't flow properly - poor fillout.
Now we are at the base. Really tricky here. Shrinking S will either pull A or L into C. If the sprue plate is too cool and the L under it is changing to S, the puddle will plug and only A can get sucked into (or remains in) C. Reject boolits.
When to cut the sprue? When the L under the plate has turned to S. How you know - at minimum you should see the dimple from L getting pulled into C.
Lets talk alloy Sb, Pb, Sn; in the order of solidifying temperature. Ok, Pb is heavy and our base - no need to discuss. Sb makes it harder/tougher - add % you want. Sn "it will make fillout better". Ok, how? Simple - it cools last and flows when the other stuff is mushy - refer to #3. Add if you want. Make your alloy do what you want at the terminal end. Use this info to cast it properly.
I post this to get casters up to speed quickly and solving their problems; not how you or I do it. PM if you have comments, we can discuss and modify where needed. As stated, this is K.I.S.S.

[runfiverun]

holy cow Popper.
pour some lead in a mold and let the air have a way out so the lead fills the cavity's all the way.
if the mold is too cold it freezes the lead too soon and you get poor fill out.

[Shiloh]

Ever watch "Good Eats" hosted by Alton Brown??

He gives the same scientific explanation only with food.

Shiloh

[Bruntson]

Tin also lower the surface tension of the alloy allowing the material to flow into the molds crevices easier, thus making a more detailed fill out of the mold.

[Yodogsandman]

This (T) confused(C) me(M).

[bangerjim]

Swag?

[dverna]

There is no science at all. It is art sometimes. It is magic sometimes. It is luck sometimes. Lead and its alloys do not follow the rules of thermodynamics. As a mechanical engineer I can attest to this as "fact".

[olafhardt]

There is no science at all. It is art sometimes. It is magic sometimes. It is luck sometimes. Lead and its alloys do not follow the rules of thermodynamics. As a mechanical engineer I can attest to this as "fact".

as a chemical engineer I say lead and lead alloys do follow the laws of thermodynamics but it's complicated. Generally the casting metals are true homogeneous solutions but may separate when solidify. To prepare a true and accurate model of boolit casting is possible but more complicated than this and I am not going to do it. I am much happier treating casting as an art than a science.

[kmw1954]

A question I was constantly asked while working with industrial refrigeration, Ammonia.

Does head draw cold or does cold draw heat?

[Cap'n Morgan]

Find a piece of bubble wrap and start popping the bubbles - it will do three things: 1) Annoy anyone around you. 2) Demonstrate that air is a gas and has mass. 3) Hammer home the point why venting is an important part of casting - especially the final venting of the base.

[runfiverun]

I think cold pulls away heat.
step out of a lake around here during the summer and you'll find out just how it works.

[dragon813gt]

A question I was constantly asked while working with industrial refrigeration, Ammonia.

Does head draw cold or does cold draw heat?

Second law of thermodynamics. Transfer of heat goes from a hot object to a cold object.

I think cold pulls away heat.
step out of a lake around here during the summer and you'll find out just how it works.

This is caused by evaporation. The change of state causes your skin to cool.

[popper]

Bruntson - totally bogus. Try using your soldering iron to solder high tin solder to aluminum, steel, brass. All you get is tiny balls of solder - that is surface tension. Think that is going to make your grooves sharper? Capillary action? Now heat copper (non-oxidized) and solder to it, wicks itself all over. Myth BUSTED.

[bangerjim]

Sn does lower the surface tension of Pb alloys. That allows it to flow into the grooves and crannies easier. Has nothing to do with "tinning" when soldering. That is a function of a flux and the correct metals and the right amount of heat. To "tin" and solder brass, copper, and even steel, you must have sufficient heat to get all the parts up to the liquidus temp of the solder you are using and a flux that will prevent oxidation of those surfaces so the solder will flow in, around, and on the surfaces.

In casting we are NOT wanting to "tin" the molds cavities, but Sn allows the hot Pb alloy to flow faster and easier into the inner reaches of the cavities for perfect fills. I use 2% Sn all the time and get pefect fills with a VERY hot mold, correct temp alloy, and correct casting procedures.

banger

[popper]

Wrong banger, look it up. From latest science measurements, Pb=467 dyne/cm, Sn=540 dyne/cm. Proof enough? Increased surface tension makes it pull away to spherical easier. and a flux Yup, it prevents/reduces oxidation and changes surface tension into capillary forces. As I originally stated, tin just makes the freeze temp lower so it can flow into the nooks and cranies. Same as "you can't shoot cast from a Glock". I jist get tired of haring stuff tht ain't right.
Edit: After my BP went down, well actually it didn't go up, My purpose here is to attempt to have a thread with real scientific reasons (in simple language) for what we do. As Run stated, the objective is to get the mould completely filled and cut the sprue without damaging the base.
We could add subjects like ladle, pressure pouring, venting, etc. so newbies, people with 'stubborn' moulds, etc can think out a solution without chasing all over to find answers. A common complaint here is people don't read to find answers. Can we clean up the snide comments and give purpose? PM to get agreeable info to place here? It's not about how you or I do it but what to do and why.

[wbrco]

A question I was constantly asked while working with industrial refrigeration, Ammonia.

Does head draw cold or does cold draw heat?

There is no such thing as "cold" or "dark", only lack of heat or lack of light. 😁