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

226 Introduction to Basic Manufacturing Processes and Workshop Technology
8. Chills
In some casting, it is required to produce a hard surface at a particular place in the
casting. At that particular position, the special mould surface for fast extraction of heat is to
be made. The fast heat extracting metallic materials known as chills will be incorporated
separately along with sand mould surface during molding. After pouring of molten metal and
during solidification, the molten metal solidifies quickly on the metallic mould surface in
comparison to other mold sand surfaces. This imparts hardness to that particular surface
because of this special hardening treatment through fast extracting heat from that particular
portion. Thus, the main function of chill is to provide a hard surface at a localized place in
the casting by way of special and fast solidification. Various types of chills used in some
casting processes are shown in Fig. 12.12. The use of a chill in the mold is depicted in
Fig. 12.13.
Fig. 12.12 Types of chills
Chill
Chill
Core
Rammed
Molding sand
Fig. 12.13 Use of a chill
12.11 FACTORS CONTROLING GATING DESIGN
The following factors must be considered while designing gating system.
(i) Sharp corners and abrupt changes in at any section or portion in gating system
should be avoided for suppressing turbulence and gas entrapment. Suitable
relationship must exist between different cross-sectional areas of gating systems.
(ii) The most important characteristics of gating system besides sprue are the shape,
location and dimensions of runners and type of flow. It is also important to determine
the position at which the molten metal enters the mould cavity.
(iii) Gating ratio should reveal that the total cross-section of sprue, runner and gate
decreases towards the mold cavity which provides a choke effect.
(iv) Bending of runner if any should be kept away from mold cavity.
(v) Developing the various cross sections of gating system to nullify the effect of
turbulence or momentum of molten metal.
(vi) Streamlining or removing sharp corners at any junctions by providing generous
radius, tapering the sprue, providing radius at sprue entrance and exit and providing
a basin instead pouring cup etc.