Advantages, Disadvantages, and Applications of Shell Molding and Expendable Mold Casting, Study notes of Design

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METAL CASTING PROCESSES

• Sand Casting• Other Expendable Mold Casting

Processes

• Permanent Mold Casting Processes• Foundry Practice• Casting Quality• Metals for Casting• Product Design Considerations

Two Categories of Metal Casting

Processes

1. Expendable mold processes

• mold is sacrificed

to remove part–

Advantage: more complex shapes possible

Disadvantage: production rates often limited by timeto make mold rather than casting itself

2. Permanent mold processes

• mold is made of

metal and can be used to make many castings–

Advantage: higher production rates

Disadvantage: geometries limited by need to openmold

Figure 11.1 - A large sand casting weighing over 680 kg

(1500 lb) \

for an air compressor frame

(courtesy Elkhart Foundry, photo by Paragon Inc , Elkhart, Indiana)

Steps in Sand Casting

1. Pour molten metal into sand mold2. Allow metal to solidify3. Break up the mold to remove casting4. Clean and inspect casting5. Heat treatment of casting is sometimes

required to improve metallurgicalproperties

Figure 11.2 - Steps in the production sequence

in sand casting The steps include not onlythe casting operation but also pattern-makingand mold-making

The Pattern

A full-sized model of the part, slightly

enlarged to account for shrinkage andmachining 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

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 toform the casting's external and internalsurfaces

• May require supports to hold it in position

in the mold cavity during pouring, called chaplets

Figure 11.4 - Core held in place in the

mold cavity by chaplets (b) possible chaplet design(c) casting with internal cavity

Foundry Sands

Silica (SiO

) or silica mixed with other minerals 2

-^

Good refractory properties - capacity to endure hightemperatures

-^

Small grain size yields better surface finish on thecast part

-^

Large grain size is more permeable, to allow escapeof gases during pouring

-^

Irregular grain shapes tend to strengthen molds dueto interlocking, compared to round grains– Disadvantage: interlocking tends to reduce

permeability

Binders Used with Foundry

Sands

•^

Sand is held together by a mixture of water andbonding 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 themixture to enhance strength and/or permeability

Buoyancy in Sand Casting

Operation

•^

During pouring, buoyancy of the molten metaltends to displace the core

-^

Core displacement can cause casting to bedefective Force tending to lift core = weight of displaced

liquid less the weight of core itself

F

b^

W

m

W

c

where

F

b^

= buoyancy force;

W

m

= weight of molten

metal displaced; and

W

c^

= weight of core

Other Expendable Mold

Casting Processes

• Shell Molding• Vacuum Molding• Expanded Polystyrene Process• Investment Casting• Plaster Mold and Ceramic Mold Casting

Figure 11.5 - Steps in shell-molding: (1) a match-plate or

cope-and-drag metal pattern is heated and placed over abox containing sand mixed with thermosetting resin

Figure 11.5 - Steps in shell-molding: (2) box is inverted so that sand

and resin fall onto the hot pattern, causing a layer of the mixtureto partially cure on the surface to form a hard shell