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 parameter to consider to obtain succesful casting operations such as sand casting of metal alloys. In here, we explain the shrinkage phenomenon in casting operations, and effects of it.
What Is ‘Shrinkage’ In Casting Operations?
In most basic approach, shrinkage is the phenomenon that occurs because of the contraction of material with the decreasing temperature. It has very specific mechanism in terms of metal casting and it needed to be understood for succesful casting processes.
The solidification of metals in casting molds takes place by starting to solidify from mold walls. But before thati the liquid must give some heat energy to surroundings to lower the pouring temperature to the level of melting temperature. During this temperature decrement in liquid phase, the decrement of volume of the liquid metal takes place.
When the solidification starts, temperature is not decreased for pure metals and decreased for metal alloys. But the liquid phase, changes into solid phase. So, solidification shrinkage takes place in this time. There is an another contraction because of the phase change. In general, solid state of a material is denser than 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 solid state.
Because of this shrinkage steps in casting molds, ‘shrinkage cavities’ occur inside the cast metal. These shrinkage cavities are major problems in terms of mechanical properties of produced part from cast metal.
How To Compensate Shrinkage In Metal Casting Operations?
During the design process of all mold and casting system of a metal part, shrinkage is considered in different means. The first solution to volumetric change due shrinkage phenomenon in casting operations; designing the mold cavity patterns higher in volume compared with original part. This shrinkage ratio is calculated by engineers, and patterns are designed bigger than original parts, to obtian exact dimensions after solidification.
As we stated above, shrinkage cavities are also important problems resulted from shrinkage thing. Risers are added to mold cavities and different places of mold gates to compensate 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% in average.
- Tin: 2.1%
- Magnesium Alloys: 2.1% in average.
- Yellow Brass: 1.3-1.6%
The volumetric calculations for shrinkage are done according to these values, to design patterns.
These are 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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