What is metal casting?

Metal casting is described as a manufacturing process in which a mold cavity is produced by sand, and molten metal is filled inside a mold cavity. The desirable shaped object is achieved after the solidification of molten metal. Large metal objects like machine beds, motor bodies, and others can easily be manufactured using metal casting. The place or shop where metal casting is performed is called foundry.

Metal casting can also produce complex-shaped objects because the liquid or molten metal can easily reach the small sections of the mold cavity.

Terms associated with metal casting

Flask

Flask is a metal or wooden frame used to hold the sand mold interact. The top molding frame and bottom molding frame are called cope and drag, respectively. In a three-piece frame, the central frame is termed the cheek.

Parting line

The separating line between the cope and drag is termed as parting line.

Pattern

The pattern is a replica of the casting to be produced. The mold cavity in the mold is created by using the pattern.

Pouring Basin

A pouring basin is a funnel-shaped sink created at the top of mold into which liquid metal is discharged.

Sprue

A sprue is a portion through which liquid metal moves from the pouring basin to the casting cavity.

Runner

The runner is a parting passageway through which molten metal is guided before reaching the casting cavity.

Gate

Gate is an original entry place though metal enters into the cavity.

Riser

A riser is a reservoir of liquid metal in the casting so that hot metal can balance the metal shrinkage.

Core

The core is used to produce hollow cavities in the casting. Cores are generally made up of molding material.

Chaplet

Chaplets support cores inside the mold or casting cavity and overcome the metal poring force effect over the core.

Chills

Chills are made up of metal placed in the mold to increase the cooling rate of casting.

The diagrammatical representation of sand mold ready for pouring is shown as:

Pattern Allowance

Following are the various types of pattern allowances:

• Shrinkage or contraction allowance
• Draft or taper allowance
• Machining allowance
• Shaking allowance
• Distortion allowance

Shrinkage or contraction allowance

Shrinkage allowance is the allowance made to avoid the contraction of casting after the solidification of liquid metal.

Liquid shrinkage is neutralized by producing a riser to the mold, and solid shrinkage is neutralized by producing shrinkage allowance on the pattern.

Draft or taper allowance

The tapers of about half to two degrees are provided over the pattern to remove it from the sand mold easily. During taper allowance, the internal surface required more taper than external surfaces because of mold strength.

Machining allowance

The final produced casting has a poor surface finish and tolerance, removed later using machining operations. So, machine allowance is provided to the pattern so casting's desired dimension will not reduce.

Shaking allowance

During the pattern removal from the mold, clearance is required between pattern and mold for easy removal, which is produced by shaking the pattern. Due to shaking, the size of the cavity is increased; to overcome this size of the pattern can be reduced.

Distortion allowance

The complex shape objects like V-shape, W-shape, and others with less thickness components can distort, so to keep it in their shape, an excess amount of material in the form of distortion allowance is provided.

Types of Pattern

Following are the various types of pattern:

• Single piece pattern
• Loose piece pattern
• Split pattern
• Match plate pattern
• Skeleton pattern
• Cope and drag pattern
• Sweep pattern
• Follow board pattern

Single piece pattern

A single pattern is a simple shape of a casting object with suitable allowances. The diagrammatical representation of single piece pattern is shown as:

Loose piece pattern

A loose piece pattern is a pattern having some additional components connected to it. The diagrammatical representation of loose piece pattern is shown as:

Split piece pattern

A split piece pattern is a pattern that is an asymmetric part of the casting, and final casting is made by joining two pieces.

The diagrammatical representation of split piece pattern is shown as:

Match plate pattern

The match plate pattern is produced in a couple of parts mounted on both sides of a match plate conforming to the contour of the parting surface. The match plate is mounted accurately with the help of locating pins.

The diagrammatical representation of match piece pattern is shown as:

Skeleton pattern

A Skeleton pattern is used to produce large-size sheets and cylinders. A Skeleton pattern reduces the material consumption of preparing the pattern.

The diagrammatical representation of skeleton pattern is shown as:

Cope and drag pattern

Cope and drag patterns are two different pieces of complete casting; these patterns are used to cast heavy and inconvenient handling objects. It is also called a multi-piece pattern.

The diagrammatical representation of cope and drag pattern is shown as:

Sweep pattern

The sweep pattern is made of wood and used to generate surfaces of revolution in large castings. For developing a complex three-dimensional cavity using a two-dimensional plane pattern, a sweep pattern is used.

The diagrammatical representation of the sweep pattern is shown as:

Follow board pattern

Follow board pattern is used to support the pattern having overhanging portion and lack of strength than due to ramming force, it may distort.

The diagrammatical representation of following board pattern is shown as:

Properties of molding sand

The molding sand contains various ingredients like base sand, blinder, moisture, additives, and others. Following are the various properties of molding sand:

Refractoriness

Refractoriness is the ability of the molding sand to resist a significant temperature of molten liquid metal without fusion. It should be greater than the pouring temperature of molten metal.

Green Strength

Green strength is defined as the moisture contained by the molding sand. The molding sand should have sufficient green strength to attain proper shape.

Hot strength

Hot strength is defined as the strength of the sand required to hold the shape of the mold cavity after the elimination of moisture.

Permeability

The gas evolving capacity of the molding sand is known as permeability. It overcomes various gas defects in the casting.

Grain fineness number (GFN)

The grain fineness number represents the average grain size distribution of the given molding sand. The greater value of grain fineness number shows finer the grain size.

Flowability

The flowability is the ability of sand to flow due to ramming force to fill the molding flask.

Collapsibility and Toughness

The ability of the molding sand to not offer any resistance against the contraction of casting material is called collapsibility. The toughness of sand represents its ability to resist impact loads.

Adhesiveness and Cohesiveness

Adhesiveness represents the bond between dissimilar materials, and cohesiveness represents the bond between similar materials. The molding sand should have sufficient adhesiveness and cohesiveness.

Types of casting

Following are the basic types of casting:

• Sand casting
• Die casting
• Shell mold casting
• Investment casting
• Centrifugal casting

Sand Casting

Sand casting is defined as the easiest method to produce aluminum casting. The mold cavity is created with the help of a wooden pattern by placing it into a flask and molding sand. Sand casting is generally used for the small production of castings. The advantages of sand casting are low setup cost, versatile casting process, low gas porosity, and others.

Die Casting

Die casting is described as a process in which liquid metal is poured into a steel mold or dies to achieve casting after solidification. The significant advantage of die casting is single mold can be used again and again, which increases the production rate. The dies are made up of aluminum, steel, and other alloy materials for better durability.

The size of casting is limited up to a specific size, and the initial investment is high.

The die casting is classified as:

• Hot chamber die casting
• Cold chamber die casting
• Gravity die casting
• Vacuum die casting

Shell mold casting

Shell mold casting is a process in which the sand mixed with a thermosetting resin, phenol, and urea-formaldehyde is allowed to contact a heated metallic pattern plate, creating a thin and robust sheet of mold around the pattern. The casting produced has a better surface finish and close tolerance.

The advantages of shell mold casting are dimensionally accurate, smooth surface finish, high permeability, no gas inclusion, and others.

Investment casting

The investment casting is also termed the lost wax casting process. The pattern used in investment casting is made up of wax or polystyrene by rapid prototyping techniques. The pattern is made by injecting molten wax or polystyrene into a metal die in the shape of a pattern. The pattern is dipped into a slurry of refractory material such as fine silica and binders. After the initial dried coating, the pattern is coated again to increase its thickness.

Investment casting is used to manufacture typical mechanical components like gears, valves, cams, and others.

The diagrammatical representation of investment casting is shown as:

Centrifugal Casting

Centrifugal casting is when the mold is rotated about its central axis as the metal pours into it. Due to centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag oxides and other lighter inclusions are separated from the metal and accumulate at the center.

Following are various types of centrifugal casting:

• True centrifugal casting
• Semi centrifugal casting
• Centrifuge casting

The diagrammatical representation of the centrifugal casting is shown as:

Casting Defects

Following are the various casting defects in metal casting:

Shrinkage

The unavailability of feed metal as the metal solidifies in the casting is termed a shrinkage defect. It is eliminated by producing a riser to the mold.

Blowholes

The blowholes are developed over the casting surface by the gases captured by solidifying metal. It is eliminated by generating vents.

Cold shut

Two metal streams do not fuse in a cold shut due to a lack of fluidity in liquid metal. It can be eliminated by using proper pouring temperature.

Misrun

In misrun, liquid metal is unable to fill the mold cavity completely. It can also be eliminated by using proper pouring temperature.

Mismatch

In mismatch, shifting of molding flashes generated irregularity in casting. Appropriate molding boxes and closing pins can eliminate it.

Inclusions

In inclusions, a foreign substance is introduced to the casting, changing dimension and developing a poor surface finish. It can be eliminated by using pure liquid metal and clean casting pieces of equipment.

Common Mistakes

Following are the common mistakes performed by students:

• Sometimes, students get confused between the various terms of casting components like caplet, chills, riser, and others.
• Sometimes, students get confused between the different types of patterns and their different allowances.
• Sometimes, students get confused with pattern material like aluminum pattern, alloy pattern, steel patter and others used in various casting processes.

Context and Applications

The topic metal casting is significant in various professional exams and courses in undergraduate, graduate, post graduate, doctorate level and others. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Production Engineering
• Master of Technology in Production Engineering
• Doctorate of Philosophy in Foundry

Related Concepts

• Manufacturing processes
• Production methods
• Foundry
• Alloy Manufacturing
• Liquid metal production

Practice Problems

Q1. Which of the following is a negative allowance?

1. Machining allowance

2. Shaking allowance

3.  Draft allowance

4.  None of these

Correct option: (b)

Explanation: In shaking allowance, the size of the shaft is more than the hole size, hence the allowance is considered as negative allowance.

Q2. The passage from the pouring basin to the mold cavity is known as ______?

1. Sprue
2. Runner
3. Riser
4. Core

Correct option: (a)

Explanation: In the casting process, the sprue is described as the passage that allows molten metal to move from the pouring basin to the mold cavity.

Q3. Grain fineness number represents _____?

1. Grain size
2. Number of grains
3. Grain boundary
4. Grain type

Correct option: (a)

Explanation: In order to evaluate the average size of the grain for any particular sample, the grain fineness number is calculated.

Q4. The casting process in which mold is rotated about its central axis is?

1. Sand casting
2. Investment casting
3. Centrifugal casting
4. Die casting

Correct option: (c)

Explanation: Centrifugal casting is utilized for making cylindrical objects like pipes, tubes, and others. Also, in the centrifugal casting process, the mold is rotated around its central axis for producing the molds.

Q5. Which of the following defect is eliminated by providing veins?

1. Shrinkage defect
2. Blowhole defect
3. Misrun
4. Inclusion

Correct option: (b)

Explanation: In metal casting, the presence of excessive amount of gas in the molten metal causes blowhole defect. In order to eliminate the blowholes level of ventilation is increased.