Casting (Upload)

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Casting

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Introduction

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 _{Casting is a manufacturing process in which a molten metal is}

usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process.

 _{First casting: 5000-3000 BC}  _Versatility

 _{Many types of metals}  _{Rapid production}

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Categories of Casting

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1. Expandable mold processes - mold is sacrificed to remove part

 _{Advantage: more complex shapes possible}

 _{Disadvantage: production rates often limited by time to 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}

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Sand Casting

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Overview of Sand Casting

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 _{This is an expandable mold process.}

 _{Most widely used casting process, accounting for a significant}

majority of total cast .

 _{Nearly all alloys can be sand casted, including metals with}

high melting temperatures, such as steel, nickel, and titanium.

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Terms Used in Sand Casting

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 _{Pattern- approximate duplicate of the part to be cast.}

 _{Molding material- material that is packed around the pattern}

to provide the mold cavity.

 _{Flask- rigid frame that holds the molding aggregate.}  _{Cope- top half of the flask.}

 _{Drag- bottom half of the flask.}

 _{Core- sand or metal shape that is inserted into the mold to}

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Terms Used in Sand Casting

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 _{Mold cavity- combination of the mold material and cores.}

 _{Riser-additional void in the mold that provides additional}

metal to compensate for shrinkage.

 _{Gating system- network of channels that delivers the molten}

metal to the mold.

 _{Pouring cup- portion of the gating system that controls the}

delivery of the metal.

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Terms Used in Sand Casting

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 _{Gates- controlled entrances.}

 _{Parting line- separates the cope and drag.}

 _{Draft- angle or taper on a pattern that allows for easy removal}

of the casting from the mold.

 _{Casting- describes both the process and the product when}

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Terms Explained

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Steps Involved in the Process

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1. Mold Cavity is produced having the desired shape and

size of the part

 _{Takes shrinkage into account}  _{Single-use or permanent mold}

2. Melting

 _{Provides molten material at the proper temperature}

3. Pouring

 _{Molten metal is poured into the mold at a proper rate to ensure}

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Steps Involved in the Process

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4. Solidification process

 _{Controlled solidification allows the product to have desired properties.}  _{Mold should be designed so that shrinkage is controlled.}

5. Removal from the mold

 _{The casting is removed from the mold}

 _{Single-use molds are broken away from the casting.}

 _{Permanent molds must be designed so that removal does not damage}

the part.

6. Cleaning, finishing, and inspection operations

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Making the Sand Mold

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 _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.

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Flow Diagram of the Process

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The Pattern

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 _{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 (a}

deviation from flatness due to uneven drying of wood).

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Core

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 _{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}

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Core in Mold

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(a) Core held in place by chaplets (b) Possible chaplet design

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Desirable Mold Properties

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 _{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}_‑

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Foundry Sands

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Silica (SiO₂) or silica mixed with other minerals. Properties of a good sand:



_{Good refractory (A refractory material is one that}

retains its strength at high temperatures

.)

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.

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Binders

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 _{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}

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Types of Sand Mold

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 _{Green sand molds}_‑

 _{Green means mold contains moisture at the time of pouring}

 _{Binder used with Green Sand is molasses}

 _{Dry sand mold - organic binders rather than clay.}

 _{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 ‑

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Advantages and Limitations

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Process Advantages:

•

_{Product is ~finished right out of mold.}

•

_{High complexity with few steps (usually)}

•

_{No machining waste}

Process Disadvantages:

•

_{Expensive and time-consuming patterns/molds/dies}

•

_{Solidification issues: shrinkage, porosity, ~low}

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Die Casting

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Overview of Die Casting

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 _{This is a permanent mold process.}

 _{In this process molten metal is injected into mold cavity under}

high pressure.

 _{Pressure is maintained during solidification, then mold is}

opened and part is removed.

 _{Molds in this casting operation are called dies .}

 _{Use of high pressure to force metal into die cavity is what}

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Die Casting Machines

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 _{Designed to hold and accurately close two mold halves and}

keep them closed while liquid metal is forced into cavity.

 _{There are two types of die casting:} 1. Hot chamber machine

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Hot-Chamber Die Casting

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Metal is melted in a container, and a piston injects liquid metal under high pressure into the die.



_{High production rates - 500 parts per hour not}

uncommon.



_{Applications limited to low melting point metals that do}

not chemically attack plunger and other mechanical

components.

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Hot Chamber Die Casting

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Hot Chamber Die Casting

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Cold Chamber Die Casting Machine

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Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity.



_{High production but not usually as fast as hot chamber}

machines because of pouring step.



_{Casting metals: aluminum, brass, and magnesium}

alloys.



_{Advantages of hot chamber process favor its use on}

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Cold Chamber Die Casting Machine

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Cold Chamber Die Casting Machine

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Molds for Die Casting

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 _{Usually made of tool steel, mold steel.}

 _{Tungsten and molybdenum (good refractory qualities) used}

to die cast steel and cast iron.

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Advantages and Limitations

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 _{Advantages of die casting:}

 _{Economical for large production quantities}  _{Good accuracy and surface finish}

 _{Thin sections are possible}

 _{Rapid cooling provides small grain size and good strength to casting}

 _{Disadvantages:}

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Casting

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Investment

Casting

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The mold is made by making a pattern using wax or some other material that can be melted away.

This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin.

This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved.

After this, the entire pattern is placed in an oven and the wax is melted away.

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Applications

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Metals that are hard to machine or fabricate are good candidates for this process.

This process can be used to make parts that cannot be produced by normal manufacturing techniques.

Typical examples of parts made by this process are:

•_{Turbine blades (complex shapes)}

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Investment

Casting Process

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1. Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece.

2. Mold creation - The pattern is dipped into a slurry of a ceramic material and then dried to form a ceramic shell around the patterns and gating system.

3. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity.

4. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. 5. Casting removal - After the molten metal has cooled, the mold

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Steps Involved in the Process

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Advantages and Limitations

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 _{Advantages of investment casting:}

 _{Parts of great complexity and intricacy can be cast.}  _{Close dimensional control and good surface finish.}  _{Wax can usually be recovered for reuse.}

 _{Additional machining is not normally required this is a net shape}_‑

process.

 _{Disadvantages}

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Comparison

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Additional Steps After Casting

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 _Trimming

 _{Removing the core}  _{Surface cleaning}  _Inspection

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Casting Quality

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 _{There are numerous opportunities for things to go wrong}

in a casting operation, resulting in quality defects in the product.

 _{The defects can be classified as follows:}

 _{General defects}

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General Defects

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General Defects

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General Defects

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General Defects

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Metals for Casting

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 _{Most commercial castings are made of alloys rather than}

pure metals

 _{Alloys are generally easier to cast, and properties of product are}

better

 _{Casting alloys can be classified as:}

 _Ferrous

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Product Design Considerations

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 _{Geometric simplicity:}

 _{Although casting can be used to produce complex part geometries,}

simplifying the part design usually improves cast ability.

 _{Avoiding unnecessary complexities:}

 _{Simplifies mold making}  _{Reduces the need for cores}

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Product Design Considerations

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 _{Corners on the casting:}

 _{Sharp corners and angles should be avoided, since they are sources}

of stress concentrations and may cause hot tearing and cracks.

 _{Generous fillets should be designed on inside corners and sharp}

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Product Design Considerations

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 _{Draft Guidelines:}

 _{In expendable mold casting, draft facilitates removal of pattern from}

mold

 _{Draft = 1}_{for sand casting}

 _{In permanent mold casting, purpose is to aid in removal of the part}

from the mold

 _{Draft = 2}_{to 3}_{for permanent mold processes}

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Product Design Considerations

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Product Design Considerations

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 _{Dimensional Tolerances and Surface Finish:}

 _{Significant differences in dimensional accuracies and finishes can be}

achieved in castings, depending on process:

 _{Poor dimensional accuracies and finish for sand casting}  _{Good dimensional accuracies and finish for die casting and}

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Product Design Considerations

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 _{Machining Allowances:}

 _{Almost all sand castings must be machined to achieve the required}

dimensions and part features.

 _{Additional material, called the machining allowance, is left on the}

casting in those surfaces where machining is necessary.

 _{Typical machining allowances for sand castings are around 1.5 and 3}

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Typical Shrinkage Allowances

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Typical Pattern Machining Allowance

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