What is casting?
Casting may be defined as a manufacturing process in which a liquified material (such as aluminium, cast iron, steel) is poured into a mold of any shape and is left to solidify. The solidified part is also referred to as casting and is removed from the mold to be given various finishing touches before it is ready to be used as a final product. The whole process of casting is most widely used in terms of creating intricate or complex solid or hollow shapes. The casting products are used in applications such as automotive parts, etc.
Due to the addition of new manufacturing practices, casting process have also acquired various forms. These are:
a) Sand Casting
b) Investment Casting
c) Die Casting
d) Low Pressure Casting
e) Centrifugal Casting
f) Gravity Die Casting
g) Vacuum Die Casting
h) Squeezing Die Casting
i) Lost Foam Casting
j) Continual Casting
The control of the entire casting operation plays a very important role in ensuring quality casting. Inspection of casting must be done regularly or in compliance with any regulations. As such, inspection and testing of casting is classified into five categories. These are as follows:
a) Casting Finish
b) Dimensional Accuracy
c) Mechanical Testing
d) Chemical Composition
e) Casting Soundness
Each and every step of the casting process should be done carefully, from the pattern-making to heat treatment. This is to avoid the problems related with the soundness, casting finish, mechanical factors and final dimensions of the final casting.
But even with such considerations for efficiency, there is still the requirements of the castings for undergoing inspection. This is to ensure quality control. Inspections also let’s one enjoy the assurance of having a quality casting.
Following are some of the defects and discontinuities of casting
1) Surface Defects
Surface Defects include roughness or unevenness of the surface caused due to the mold cracking at very high temperatures, fast cooling, stripping defects (which is caused due to gas being trapped inside mold) and burned sand.
2) Inclusion Defects
a) Sand inclusion is detected when the non-metallic materials create pockets in the casting.
b) Slag inclusion is required in small quantities but too much of sand inclusion can ruin a casting. This is detected when the sand in the mold gets trapped in the metal.
3) Molding and Pouring Defects
a) Underpour defects may happen when not enough molten material introduced to fill up the mold.
b) A raising or floating core may happen if the mold has not been properly clamped down.
c) Flashing is caused when a thin layer is created due to liquid which seeps in between the joints on the mold.
d) Mold mismatch or mold shift will happen if the top and bottom of the mold is not alighned properly.
e) Sand instability is the result when the there is loss of fine details on a casting or the wavering edges.
f) Cold laps occur when the temperature of either the mold or the molten material is too low.
g) Gas porosity occurs when gas in the mold seeps out through the molten material, leaving in its wake voids or bubbles once it cools down.
4) Cooling Defects
a) Hot cracks are caused due to quick cooling which are identified by ribbon like cracks on the metal.
b) Cooling deformation takes place when the metal casting warps away from the designated shape while cooling.
c) Chilling defects may either take place when the mold is too cold or the casting is removed very quickly.
d) Shrinkage cavities takes place when the metal is not enough to fill in the space when the casting cools down and shrinks. This can weaken the casting.