Pattern:
A replica of the the final casting, used to form the mold cavity.
A pattern is an element used
for making cavities in the mould, into which molten metal is poured to produce
a casting. It is not an exact replica of the casting desired.
There are certain essential
differences. It is slightly larger than the desired casting, due to the various
allowances (shrinkage allowance, machining allowance etc.).
Pattern materials:
1. Wood:
Advantages:
1. Light
in weight
2. Comparatively
inexpensive
3. Good
workability
4. Lends
itself to gluing and joining
5. Holds
well varnishes and paints
6. Can
be repaired easily
Disadvantages:
1. Inherently
non uniform in structure
2. Posses
poor wear and abrasion resistance
3. Can
not withstand rough handling
4. Absorbs
and gives off moisture, so that it varies in volume, wraps and thus changes its
mechanical properties
2.
Metals:
Advantages:
1. More
durable and accurate in size than wooden patterns
2. Have
a smooth surface
3. Do
not deform in storage
4. Are
resistant to wear, abrasion, corrosion and swelling
5. Can
withstand rough handling
Disadvantages:
1. Expensive
as compared to wood
2. Not
easily repaired
3. Heavier
than wooden patterns
4. Ferrous
patterns can get rusted
The common metals used for
pattern making are:
1. Cast
iron
2. Brass
3. Aluminum
4. White
metal
3. Plaster gypsum cement
4. Plastic compound
Advantages:
1. Facilitates
the production process.
2. Makes
it more economical in cost and labor.
3. Plastic
patterns are highly resistant to corrosion, lighter and stronger than wood
patterns.
4. Molding
sand sticks less to plastics than to wood.
5. No
moisture absorption.
6. Smooth
surface of patterns.
7. Plastic patterns are Strong. And so they are dimensionally stable.
5. Wax
Wax is the best material for surface finish.
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