MOULDING SAND cont...
COMPOSITION:
•BASE SAND (upto 85%)
–Mainly SILICA GRAINS (SiO2)
–Main source is river sand
–Zircon, Chromite, Olivine Sand
SILICA GRAINS
ZIRCON SAND CHROMITE SAND OLIVINE SAND
MOULDING SAND COMPOSITION
•BINDER (upto 11%)
–Provides REQUISITE STRENGTH
–CLAY is generally used
•Kaolinite
•Bentonite : absorbs more water
•MOISTURE (upto 4%)
–Activates the CLAY
helps in bonding
TYPES OF SAND MOULD CASTING
(i) CONVENTIONAL MOLDING PROCESS
(a)Green sand mold.
(b)Dry sand mold,
(ii) CHEMICAL SAND MOLDING PROCESS
1. Shell molding
2. Sodium Silicate Molding
3. No-Bake molding
(iii) PERMANENT MOLD CASTING
1. gravity die casting
2.pressure Die casting
(iv) SPECIAL CASTING PROCESSES
1.Centrifugal casting
2.Investment casting
3.Vacuum sealed casting
4.Slush casting
5.Squeeze castingetc.
Green sand moulding
•The term “green” denotes the presence of moisture inthe molding sand.
•Composition
+ + MOISTURE
•Advantages
a.Most metals can be cast by this method.
b.Pattern costs and material costs are relatively low.
c.No limitation with respect to size of casting & type ofmetal or alloy used.
DISADVANTAGES
•POOR SURFACE FINISH OF THE CASTING.Machining is required to achieve the finishedproduct.
•Improper properties of moisture and cclayresult in casting defects.
•Unclean environment.
DRY SAND MOLDING
Need for dry Sand Molding
- to avoid pinholes
- to increase the strength of the mold
Types of dry Sand Molding
1. Skin drying
(using torches,infrared lamps,hot air etc.)
2. Complete mold drying
( by using large ovens)
SHELL MOLDING
•FEATURES:
Metallic pattern is used
Molding material: Fine sand + Thermosettingresin (Phenol-formaldehyde)(5kg of phenol-formaldehyde in 100 kg of sand)
Heating arrangement for the pattern
TYPES OF SAND MOULD CASTING contd…
Shell Moulding
POINTS TO REMEMBER:
A cope-drag metalpattern heated at 200– 2300C
Dump boxcontaining sandmixed withthermosetting resininverted for 30 sec
Shell skin about 4-10 mm
Oven for heatingthe shell for 60 sec at3150C temp
Gap betweenclamped shell andMoulding Flask,backing sand gravel,coarse sand or metalshot
Shell Moulding
Applications:
Valves bodies, Bearing caps,Gears, Very suitable for likepetrol engine cylinder
Advantages:
Excellent surface finish
Good dimensional accuracy oforder of 0.002 to 0.003 mm
Molds can be stored
Suitable for Mass production
Sand metal ratio relatively low
Less Skill-ness required
Limitations:
High initial cost
Specialized equipment
Expensive Resin binder
Limited for small size
Cost not justified for smallscale productions
Need for Sodium Silicate molding
•poor strength of green sand mold (for cores)
•Poor surface finish of green sand mold
•Inability of green sand mold to retain smalldetails
SODIUM SILICATE Molding cont…
MOLDING MATERIALS:
Fresh silica sand
Sodium silicate (binder)
Hardener (CO2 gas, portland cement, blast furnace slags , etc)
Types:
1. Self hardened
Hardener: portland cement, blast furnace slags, ferro-silicon, etc
Setting time: 2 -24 hours
2.CO2 hardened
Hardener: CO2 gas
Setting time: few minutes
TYPES OF SAND MOULD CASTING contd…
Carbon-dioxide moulding
Points to Remember:
Mould material: Dry silica
sand (no clay) + (3-5%) sodium
silicate + moisture (<3%)
CO2 at 1.3-1.5 kg/cm2 forced
for about 20 to 30s
CO2 head or probe or curtain
is used
CO2 reacts with the sodium
silicate to cure, to form hard
silica gel
Na2SiO3 +CO2 —————→ Na2CO3 + SiO2.xH2O (Silica Gel)
Carbon-dioxide moulding
Applications:
Aluminum cylinder; Pump,Compressor Impeller , casing;Parts of Diesel engine
Advantages:
Quick & simple operation
Requires semi-skilled worker
accommodates a wide range ofcore and mold sizes
When used for making cores,the process can be automatedfor long durations &speedyproductions runs
Good dimensional tolerance
Good casting surface
Limitations:
Recovery of used sand difficult
Binder is hygroscopic andabsorbs water
collapsibility slows downproduction
