Question

1--Defects of various castings and suggest suitable remedies??

2-- Comparisons of quality aspects of metal casting processes ??

Answer 1

1) General defects that occur in castings and its remedies:

a) Blow holes

• This can be avoided by controlling the moisture content in the mould and avoiding the inclusion of moisture content in the molten metal.
• This can also avoided by baking the cores properly, which removes the moisture content.
• By providing adequate venting in mold and cores, which helps the evaporated gases escapes easily without and development of pressure inside the mold which forming blow holes.

b) Shrinkage:

• This defect can be avoided by ensuring the proper directional solidifiation by modifying or re-designing the gating system, riser and chills according to the need of the customer.

c) Porosity:

• ​​This is controlled by reglating the pouring temperature.
• Ensuring the mold does not have any gas inclusions due to evaporation. This can be done by providing sound vent holes.

d) Mis-runs and Hot tears

• Mis runs can be avoided by adusting the pouring temperature, modify the design and if not possible in modifying the desing then modifying the gating system.
• Hot tears are avoided by Design of the casting.

e) Metal penetration

• This means penetrating the metal into the mould. This can be avoidedd by using finer grain size sand, ramming the mould as hard as possible, and suitable pouring temperature.

f) Cuts and washers

• This can be avoided by using more binders to the mould in between facing and core sand.
• And core strength should be improved and improving gating system.

g) Fusion and Shift

• Fusion means, some of the mould is inclusionn with the molten metal, which intern gives a defective product. This can be avoided by improving the refractiveness of the molding sand, modifying the gating system, low pouring temperature and improving the facing sand quality.
• Shift means, shifting of the core or drag and cope boxes. This is avoided by providing adequate support to the prime objects. Increasing the strength of both mold and core.

2) Comparison of quality aspects of different casting processes:

a) Sand casting

The casting is designed to reduce the potential for tearing, cracking, and other flaws.

This type of casting is generally gives low accuracy, futhermore this process has tendency to yield products with a comparitively rough surface finish.

b) Ivestment castings

Investment castings uses a disposable wax pattern for each cast part. Some of the quality aspects are:

• It gives high degree of accuracy and precise dimensiona; results.
• As it is said to be highly expensive process, but it can be compensated by reducing in machinhing and tooling costs and gives quality surface results.

c) Plaster castings

Plster casting is similar to the sand casting process, using a mixture of gypsum, water in place of a sand and some binders.

This process offers a good surface finish. This process casts complex shapes with higher degree of dimensional accuracy. When quality of surface finish is an important factor, then this process is most cost effective and efficient process.

d) Die casting

Die casting is a method of moldinf materials under high pressure and involves non-ferrous and alloys.

This process offers close siza and shpe tolerances. High component dimensional consistency and uniform design and reduces post casting machining. Despite os all quality aspects, the die and tool cost is very high. It is also difficult to ensure the mechanical properties of the die cast component.