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

234 Introduction to Basic Manufacturing Processes and Workshop Technology
12.19.6 Carbon-Dioxide Gas Molding
This process was widely used in Europe for rapid hardening the molds and cores made up of
green sand. The mold making process is similar to conventional molding procedure accept the
mould material which comprises of pure dry silica sand free from clay, 3-5% sodium silicate
as binder and moisture content generally less than 3%. A small amount of starch may be
added to improve the green compression strength and a very small quantity of coal dust, sea
coal, dextrin, wood floor, pitch, graphite and sugar can also be added to improve the collapsibility
of the molding sand. Kaolin clay is added to promote mold stability. The prepared molding
sand is rammed around the pattern in the mould box and mould is prepared by any conventional
technique. After packing, carbon dioxide gas at about 1.3-1.5 kg/cm 2 pressure is then forced
all round the mold surface to about 20 to 30 seconds using CO 2
head or probe or curtain as
shown in Fig. 12.14. The special pattern can also be used to force the carbon dioxide gas all
round the mold surfaces. Cores can be baked this way. The sodium silicate presented in the
mold reacts with CO 2
and produce a very hard constituents or substance commonly called as
silica gel.
Na 2
SiO 3
+CO 2
—————→ Na 2
CO 3
+ SiO 2
.xH 2
O (Silica Gel)
Co 2
gas
Plyboard or hard
board cover
Rubber
strip
Co 2
gas
Head
Mould
Mould
(a) Using Muffle Board
Hardened face
Hardened face
(b) Using Shower Curtain
Co 2
gas
Probe
Co 2
gas
Pattern
Hollow
pattern
(c) Using a Simple tube to provide entry to gas
(d) Using a Hollow Pattern
Fig. 12.14 Carbon dioxide molding