popper
04-02-2016, 12:25 PM
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.
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.