1. PATTERN MATERIAL REQUIREMENTS OF A GOOD PATTERN MATERIAL: IT SHOULD BE
Easily worked, shaped and joined
Light in weight
Strong, hard and durable
Resistant to wear and abrasion
Resistant to corrosion
Dimensionally stable and unaffected by variations in temperature and humidity
Available at low cost

2. PATTERN ALLOWANCES Shrinkage or contraction allowance
The selection of the correct allowances greatly helps to reduce costs and avoid rejections.
Shrinkage or contraction allowance
Draft or taper allowance
Machining or finish allowance
Distortion or camber allowance
Rapping allowance

3. 1. Shrinkage/Contraction Allowance contd…
Types of shrinkage of metals:
liquid shrinkage – reduction in volume when the metal changes from liquid state o solid state at the solidus temp.--Compensated by riser in the mold.
Solid shrinkage – reduction in volume caused when the metal loses temp. in solid state.—compensated by providing shrinkage allowance on the patterns

4. Table 1: Rate of Contraction of Various Metals
MATERIAL
DIMENSION
SHRINKAGE ALLOWANCE(inch/ft)
GREY CAST IRON
Up to 2 ft
2 ft to 4 ft over 4 ft
0.125
0.105
0.083
CAST STEEL
2 ft to 6 ft over 4 ft
0.251
0.191
0.155
ALUMINIUM
Up to 4 ft
4 ft to 6 ft over 6 ft
0.143
MAGNESIUM
Oer 4 ft
0.173

5. Excercise 1.
The casting shown is to be made of cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimensions of the pattern.( All dimensions are in inches ).

6. The pattern drawing with required dimension is shown below:

7. Pattern Allowance contd…
2. Draft or Taper Allowance:
Withdrawal of vertical wall may damage mould cavity
Generally inner details require higher draft
More draft needed for hand moulding

8. Draft Allowances of various Metals
PATTERN MATERIAL
HEIGHT OF THE GIVEN SURFACE (INCH)
DRAFT ANGLE
(EXTERNAL SURFACE)
(INTERNAL SURFACE)
WOOD 1
1 TO 2
2 TO 4
4 TO 8
8 TO 32
3.00
1.50
1.00
0.75
0.50
2.50
METAL AND PLASTIC
2.00

9. Pattern Allowance contd…
3. Machining Allowance:
Poor finish of sand casting
Subsequent machining required
Extra material required to compensate the material lost in machining or finishing
Generally varies from 3 mm to 18 mm

10. Machining Allowances of Various Metals
DIMENSION(inch)
ALLOWANCE (inch)
CAST IRON
Up to 12
To 20
20 To 40
0.12
0.20
0.25
CAST STEEL
Up to 6
6 to 20
20 to 40
0.30

11. Exercise2. The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension o the pattern. (All dimensions are in inches).
The machining allowance for cast iron for size, up to 12 inch is 0.12 inch and from 12 inch to 20 inch is 0.20 inch
For dimension 18 inch, allowance = inch
For dimension 14 inch, allowance = 0.20inch
For dimension 8 inch, allowance = 0.12inch
For dimension 6 inch, allowance = 0.12inch

12. The pattern drawing with required dimension is shown in figure below

13. Pattern Allowance contd…
4. Rapping / Shake Allowance:
Before withdrawing pattern is rapped
Size of the mould cavity increases
Is insignificant in large castings
Negative allowance (0.5 – 1mm)
5. Distortion Allowance:
Sometimes castings get distorted, during solidification, due to their typical shape.
Reasons for distortion:
Internal stresses
Non-uniform cooling of casting

14. INVERSE REFLECTION

15. Core and core prints
Needs for cores-to creat holes and cavities inside a casting
Core prints- additional projections on the pattern