Sand Casting

                     Overview of Sand Casting

1. Most widely used casting process, accounting for a significant majority of total tonnage cast 
2. Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium 
3.      Castings range in size from small to very large
4.     Production quantities from one to millions  

Making the Sand Mold    

§The cavity in the sand mold is formed by

packing sand around a pattern, then

 separating the mold into two halves and

 removing the pattern

§The mold must also contain gating and

 riser system

§If casting is to have internal surfaces, a 

core must be included in mold 

§A new sand mold must be made for each

 part produced  

Steps in Sand Casting

1.Pour the molten metal into sand mold

2.Allow time for metal to solidify

3.Break up the mold to remove casting 

4.Clean and inspect casting

§Separate gating and riser system

5.Heat treatment of casting is sometimes 

required to improve metallurgical properties 

. 

The Pattern

A full‑sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting

§Pattern materials:

§Wood - common material because it is easy to work, but it warps

§Metal - more expensive to make, but lasts much longer 

§Plastic - compromise between wood and metal  

Types of Patterns

Figure 11.3  Types of patterns used in sand casting:

(a) solid pattern

(b) split pattern

(c) match‑plate pattern

(d) cope and drag pattern

Core

Full‑scale model of interior surfaces of part 

§It is inserted into the mold cavity prior to 

pouring

§The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces 

§May require supports to hold it in position in 

the mold cavity during pouring, called chaplets

 Desirable Mold Properties

§Strength ‑ to maintain shape and resist erosion 

§Permeability ‑ to allow hot air and gases to pass through voids in sand 

§Thermal stability ‑ to resist cracking on contact with molten metal 

§Collapsibility ‑ ability to give way and allow casting to shrink without cracking the casting

§Reusability ‑ can sand from broken mold be reused to make other molds?

Foundry Sands 

Silica (SiO2) or silica mixed with other minerals 

§Good refractory properties ‑ capacity to endure high temperatures 

§Small grain size yields better surface finish on the cast part

§Large grain size is more permeable, allowing gases to escape during pouring 

§Irregular grain shapes strengthen molds due to interlocking, compared to round grains

§Disadvantage: interlocking tends to reduce permeability

Binders

§Sand is held together by a mixture of water and bonding clay 

§Typical mix: 90% sand, 3% water, and 7% clay 

§Other bonding agents also used in sand molds:

§Organic resins (e g , phenolic resins)

§Inorganic binders (e g , sodium silicate and phosphate) 

§Additives are sometimes combined with the mixture to increase strength and/or permeability  

Types of Sand Mold

§Green‑sand molds - mixture of sand, clay, and water;

§“Green" means mold contains moisture at time of pouring 

§Dry‑sand mold - organic binders rather than clay

§And mold is baked to improve strength

§Skin‑dried mold - drying mold cavity surface of a green‑sand mold to a depth of 10 to 25 mm, using torches or heating lamps  

Buoyancy in Sand Casting Operation

§During pouring, buoyancy of the molten metal tends to displace the core, which can cause casting to be defective

§Force tending to lift core = weight of displaced liquid less the weight of core itself 

Fb = Wm ‑ Wc    

  where Fb = buoyancy force; Wm = weight of molten metal displaced; and Wc = weight of core