Padding In The Casting Process
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| Padding In Casting Process |
Padding In Casting
Padding in the casting process is extra material added to the casting for proper directional solidification during the cooling of the casting in the mould.
Padding is a wedge-shaped exothermal material mixed with metal powder in such a way that the padding metal piece is thicker near the feeder and starts thinning away from the feeder.
Padding made from metal should have higher thermal conductivity than mould material. The metal piece is placed at the end of the casting, melts and becomes part of the casting which is machined out later on.
This extra material added to the final casting is taken out from the casting by the machining process.
Padding in the casting process reduces shrinkage defects and porosity defects and promotes sound casting as cooling is achieved uniformly in all directions.
Exothermal material is used to generate heat to achieve uniform solidification by mixing oxides, aluminium and cast powder.
Padding and chills reduce shrinkage defects, voids, and porosity defects in complex casting.
Padding also serves as a core in the moulding process.
Padding in casting improves the quality of casting by reducing casting defects.
Casting is prone to porosity and shrinkage defects without using padding in casting.
Padding is an effective technique for directional solidification to reduce the cost of the risers in the casting process.
The riser neck is the entry point of the riser and the exit point of the mould cavity when molten metal flows from the mould cavity to the riser. Molten metal flows from the mould cavity to the riser from this neck which is of a smaller diameter. This neck connects the mould cavity to the riser.
It is important that this neck does not solidify during the casting process as the mould cavity needs to be fed with molten metal continuously maintaining the required temperature gradient.
"Riser padding" is placed at the neck of the riser to reduce the premature solidification of the molten metal in this riser section to avoid casting defects such as misrun, poured shot defect, cold shunt and shrinkage in sand casting products.
Padding serves an important insulating material for producing 3-dimensional casting medical objects without hotspot defects.
These hotspots are identified by finite element analysis identifying critical points during heat transfer at a particular thermal gradient and location.
Next step is to mark these spots and determine the amount of padding thickness and location of the feeder (riser). The thickness of the padding depends upon the material, solidification rate of molten metal, temperature gradient, thickness of the casting section, shape of casting section, complexity and expansion & contraction of the cooling molten metal.
Advantages Of Padding In Casting Process
- Hotspots are eliminated from the final product.
- Porosity and shrinkage defects are reduced.
- Improves casting yield.
- Increases the productivity and quality of casting.
- Reduces rejection of casting products and time.
- Improves directional solidification to produce complex shape casting parts.
- Saves casting metal and provides support and cushioning.
Padding Application
- Turbine blade casting.
- Hitch housing for trucks.
- Compressor housing.
- Commercial vehicle heavy engines.
Difference between padding and chills in the casting process.
The main difference between padding and chills in casting is as follows:
| Padding | Chills |
|---|---|
| Padding is done at the end of the casting to promote directional solidification. |
Chills in the sand casting process are provided throughout the
casting to achieve progressive solidification. |