In metal casting, there are lots of kinds of parameters to consider, in the phase of mold and casting system design. One of these parameters is shrinkage. Shrinkage is one of the most important parameters to consider to obtain successful casting operations such as the sand casting of metal alloys. Here, we explain the shrinkage phenomenon in casting operations and its effects.
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What Is ‘Shrinkage’ In Casting Operations?
In the most basic approach, shrinkage is the phenomenon that occurs because of the contraction of material with the decreasing temperature. It has a very specific mechanism in terms of metal casting and it needed to be understood for successful casting processes.
The solidification of metals in casting molds takes place by starting to solidify from mold walls. But before that, the liquid must give some heat energy to the surroundings to lower the pouring temperature to the level of melting temperature. During this temperature decrement in the liquid phase, the decrement of the volume of the liquid metal takes place.
When the solidification starts, the temperature is not decreased for pure metals and decreased for metal alloys. But the liquid phase, changes into the solid phase. So, solidification shrinkage takes place at this time. There is another contraction because of the phase change. In general, the solid-state of the material is denser than the liquid phase.
After the completion of solidification of metal, temperature decrement goes on lowering up to room temperature. Another decrement of volume takes place for cast metal. This is the shrinkage phase for the solid-state.
Because of these shrinkage steps in casting molds, ‘shrinkage cavities’ occur inside the cast metal. These shrinkage cavities are major problems in terms of the mechanical properties of produced parts from cast metal.
How To Compensate Shrinkage In Metal Casting Operations?
During the design process of all mold and casting systems of a metal part, shrinkage is considered in different means. The first solution to volumetric change due to the shrinkage phenomenon in casting operations; designing the mold cavity patterns higher in volume compared with the original part. This shrinkage ratio is calculated by engineers, and patterns are designed bigger than original parts, to obtain exact dimensions after solidification.
As we stated above, shrinkage cavities are also important problems resulting from shrinkage thing. Risers are added to mold cavities and different places of mold gates to compensate for the shrinkage cavities. Risers must solidify after the part itself to compensate to fill shrinkage cavities with metal fluids.
What Are The Shrinkage Ratios Of Different Metals?
Here is the list of some shrinkage values of metals and metal alloys;
- Carbon Steel: 1.6-2.1%,
- Nickel: 2.1%
- Gray Cast Iron: 0.8-1.3%
- White Cast Iron: 2.1%
- Zinc: 2.6%
- Chrome Steel: 2.1%
- Magnesium: 2.1%
- Aluminum Alloys: 1.3% on average.
- Tin: 2.1%
- Magnesium Alloys: 2.1% on average.
- Yellow Brass: 1.3-1.6%
The volumetric calculations for shrinkage are done according to these values, to design patterns.
This is the general information about shrinkage in metal casting operations. Do not forget to leave your comments and questions about shrinkage in metal casting below.
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