Big Miller Bro
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- Joined
- Feb 8, 2007
- Messages
- 109
Well this is one of the few forums for tips so here are some of my newest ones concerning pour cavity and venting:
Vocabulary
Pour cavity- the large funnel shape void in the mold that the molten metal is poured into
Figure Cavity-the female shape of the figure/miniature
Venting-the small channels cut into the RTV to allow the metal to fully fill every space of the figure cavity; vents allow better flow of metal as well as escape of air pockets.
Size of Pour Cavity
To get good detail you need to have high pressure forcing the metal into the figures cavity. When a large volume is above a smaller volume, there is more pressure on the smaller volume. The greater the size of the larger volume the greater the multiple of force concentrated into the smaller figure.
Ideally each figure cavity should be fed by its own pour cavity as each additional figure cavity reduces the pressure applied to all figure cavities.*
For 54mm sized figures I use 3-4x the volume of pour cavity and after 2-3 casts the mold produces perfect casts. For 12mm sized figures I have been using 20x the volume to really cram that metal in and often the pressure is such that the molds dont even need to be prewarmed.
Venting
Venting should be thought of as the place for air to escape. Venting is a 'learned' technique and requires some understanding of fluid dynamics.
Think of the mold as casting a 'U' shape. The pour cavity and figure cavity make one 'arm' of the U. The venting is the curve of the U that leads all the way up to the other arm. The force of the metal running down one arm pushes all the air up and out the other arm of the U.
A beginner needs to make vents and so long as you follow the following rules you'll be good:
There should only be 1 main vent from the very bottom most of the mold to the very top- this vent will carry excess air and metal out the bottom of the casting. This is the 'J' part of your 'U'.
All other vents should be angled down toward the bottom of the mold to meet the main vent. You can have as many vents as needed so long as it aids the metals flow to fill the figures shape. warning- If you cut a vent right across then the metal will push the air back into the bottom of your U and you'll be left with an 'H'- the pour cavity, the venting and maybe a casualty figure. So remember always vent down
Vocabulary
Pour cavity- the large funnel shape void in the mold that the molten metal is poured into
Figure Cavity-the female shape of the figure/miniature
Venting-the small channels cut into the RTV to allow the metal to fully fill every space of the figure cavity; vents allow better flow of metal as well as escape of air pockets.
Size of Pour Cavity
To get good detail you need to have high pressure forcing the metal into the figures cavity. When a large volume is above a smaller volume, there is more pressure on the smaller volume. The greater the size of the larger volume the greater the multiple of force concentrated into the smaller figure.
Ideally each figure cavity should be fed by its own pour cavity as each additional figure cavity reduces the pressure applied to all figure cavities.*
For 54mm sized figures I use 3-4x the volume of pour cavity and after 2-3 casts the mold produces perfect casts. For 12mm sized figures I have been using 20x the volume to really cram that metal in and often the pressure is such that the molds dont even need to be prewarmed.
Venting
Venting should be thought of as the place for air to escape. Venting is a 'learned' technique and requires some understanding of fluid dynamics.
Think of the mold as casting a 'U' shape. The pour cavity and figure cavity make one 'arm' of the U. The venting is the curve of the U that leads all the way up to the other arm. The force of the metal running down one arm pushes all the air up and out the other arm of the U.
A beginner needs to make vents and so long as you follow the following rules you'll be good:
There should only be 1 main vent from the very bottom most of the mold to the very top- this vent will carry excess air and metal out the bottom of the casting. This is the 'J' part of your 'U'.
All other vents should be angled down toward the bottom of the mold to meet the main vent. You can have as many vents as needed so long as it aids the metals flow to fill the figures shape. warning- If you cut a vent right across then the metal will push the air back into the bottom of your U and you'll be left with an 'H'- the pour cavity, the venting and maybe a casualty figure. So remember always vent down
