Question

Consider the following statements:

In shell moulding

1) a single parting plane should be provided for mould

2) detachable pattern parts and cores could be included

3) minimum rounding radii of 2.5 mm to 3 mm should be used

4) draft angles of not less than 1° should be used

Which of the above statements are correct?
1. 1, 3 and 4 only
2. 1, 2 and 3 only
3. 2, 3 and 4 only
4. 1, 2, 3 and 4

Answer 1

Correct Answer - Option 1 : 1, 3 and 4 only

Explanation:

• Shell moulding is a process in which sand mixed with thermosetting resin (phenol-formaldehyde)  is allowed to come into contact with a heated metallic pattern plate so that a thin strong shell of mould is formed around the pattern.
• Additives (coal dust, pulverized slag, Manganese dioxide, Calcium Carbonate, Ammonium Boroflouride, and Manganese Silicofluoride) are added into sand mixtures to improve surface finish and avoid thermal cracking during pouring. Lubricants such as calcium stearate and zinc stearate are added to resin sand mixture to improve the flowability of the sand and permit the easy release of the shell from the pattern.
• The sand resin mixture is cured at about 150°C temperature so only metal patterns (Grey Cast Iron) are used. The average shell thickness increases with pattern temperature and contact time.

Advantages:-

1.Close tolerance of 0.25 mm for steel casting and 0.35 mm for grey cast iron under normal working conditions is possible.

2.Smooth surface finish is obtained of average roughness 3 to 6 microns.

3.Lower draft angle saves the material costs and machining time.

4.Very thin section (0.25 mm) of the type of air cooled cylinder heads can be easily made because of high strength of sand.

5.Permeability of the shell is high so no gas inclusion.

Limitations:-

1.Patterns are very expensive therefore it is economical to use for large production only (above 15000 pieces).

2.Highly complicated shapes can’t be made.

Applications:-

1.Cylinders and cylinders heads of air cooled IC Engine.

2.Automobile transmission parts.

3.Cast tooth bevel gear.

4. Brake beam, radome hubs, track rollers for crawler tractors, transmission planet carrier, steel eyes, gear blanks, chain seat bracket, refrigerator valve plate, small crank shafts, 4. etc.