Introduction to Basic Manufacturing Processes and ... - always yours

Mold and Core Making 223
Molding sand
Pattern
(i) Required casting
(a)
Core print
Riser pin
Sprue pin
Cope
Drag
Pattern
Core print
(ii) Pattern to be used
Riser
Runner
(b)
(iii) Casting after being knocked out
Core
Mould cavity
Gate
(c)
Fig. 12.7 Example of making a mold
(d)
12.9 VENTING OF MOLDS
Vents are very small pin types holes made in
the cope portion of the mold using pointed edge
of the vent wire all around the mold surface as
shown in Fig. 12.8. These holes should reach
just near the pattern and hence mold cavity on
withdrawal of pattern. The basic purpose of vent
holes is to permit the escape of gases generated
in the mold cavity when the molten metal is
poured. Mold gases generate because of
evaporation of free water or steam formation,
evolution of combined water (steam formation),
decomposition of organic materials such as
binders and additives (generation of
hydrocarbons, CO and CO 2
), expansion of air
Parting
line
present in the pore spaces of rammed sand. If mold gases are not permitted to escape, they
may get trapped in the metal and produce defective castings. They may raise back pressure
and resist the inflow of molten metal. They may burst the mold. It is better to make many
small vent holes rather than a few large ones to reduce the casting defects.
Riser
Mold
cavity
Pouring basin
Cope
Sprue
Vent
holes
Drag
Gate
Vents made in the mold
Fig. 12.8 Venting of holes in mold