Why do shrinkage occur in castings?

Replies

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  saptm

  See in casting we uses molten metals in the mould or pattern.As we all know in liquid molecules are loosely packed.So as the casting solidifies,the molecules packed themselves tightly as a result the shrinkage happens. That's the reason we are using drifts or a bigger pattern or mould for casting to avoid these types of problems.
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  CE Designer

  saptm

  That's the reason we are using drifts or a bigger pattern or mould for casting to avoid these types of problems.

  Would you mind explaining this a little more?
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  saptm

  ya sure dude [😀]
  we already knows that after solidification,the cast contracted.And since,we usually use sand dust for a mold or pattern due to which there come some roughness in the cast. So that's why we make the pattern bigger than the actual size of the cast so that we just get rid of all these allowance/defects.
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  ISHAN TOPRE

  saptm

  ya sure dude [😀]
  we already knows that after solidification,the cast contracted.And since,we usually use sand dust for a mold or pattern due to which there come some roughness in the cast. So that's why we make the pattern bigger than the actual size of the cast so that we just get rid of all these allowance/defects.

  Yes that is correct. But can we deal with atomic structures a bit. I am curious too 😀
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  saptm

  Issue

  Yes that is correct. But can we deal with atomic structures a bit. I am curious too 😀

  What types of atomic structure you are taking about,i mean FCC,BCC HCP or SCC?
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  ISHAN TOPRE

  saptm

  What types of atomic structure you are taking about,i mean FCC,BCC HCP or SCC?

  I am talking about the grain structures. You see, as a casting is cooled, it shrinks. It is more of a function of its grain structure. Can you explain about it more? 😀
• 

  CE Designer

  Issue

  I am talking about the grain structures. You see, as a casting is cooled, it shrinks. It is more of a function of its grain structure. Can you explain about it more? 😀

  I am not certain of what you are seeking but basic cooling of iron goes like this:
  Delta ferrite - Molten iron (BCC Structure)
  Austenite - (FCC Structure)
  Delta ferrite - Stable form of iron (BCC Structure)
  
  At high temperatures the grains will have higher Internal energy so they will vibrate eratically and move further away from eachother (expand). Upon solidification this internal energy is released by giving up the heat energy it took to become a molten liquid. As they lose their heat energy the amplitude of the particles' vibrations is reduced significantly and they revert back to the solid form.