Casting Techniques and Pattern Materials

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Questions and Answers

What is the primary purpose of the sprue base in a metal casting process?

To control the temperature of the liquid metal

To serve as a reservoir for the metal, reducing momentum (correct)

To increase the fluidity of the molten metal

To direct the flow of metal into the gate

Which statement correctly defines the volume flow rate in relation to the sprue?

Volume flow rate is calculated using the formula Q = A + v.

Volume flow rate is dependent solely on the velocity of the liquid metal.

Volume flow rate does not affect the quality of the final casting.

Volume flow rate is determined by the cross-sectional area and velocity of the liquid. (correct)

In the context of sprue design, what is the difference between a straight sprue and a tapered sprue?

A tapered sprue causes more turbulence than a straight sprue.

A straight sprue allows for higher speed than a tapered sprue.

A straight sprue requires less cross-sectional area than a tapered sprue.

A tapered sprue provides better control of metal flow compared to a straight sprue. (correct)

What does low pressure in the corners of the sprue refer to?

Decreased flow rate of molten metal Signup and view all the answers

Which component is primarily responsible for controlling the flow of molten metal into the casting mold?

Gate Signup and view all the answers

What is the primary purpose of a shrinkage allowance in pattern making?

To compensate for material shrinkage during cooling Signup and view all the answers

How does the pattern differ from the final casting?

It is larger in every dimension Signup and view all the answers

Which type of allowance is given to facilitate the removal of the pattern from the mould?

Draft allowance Signup and view all the answers

What happens to the pattern material when molten metal is poured into the mould?

It vaporizes and leaves a cavity Signup and view all the answers

Which of the following types of shrinkage occurs due to temperature reduction from pouring temperature to liquid point temperature?

Liquid shrinkage Signup and view all the answers

Which allowance is related to the precision of the final dimensions after machining?

Machining allowance Signup and view all the answers

What is meant by 'pattern allowances'?

Deviations in pattern dimensions from the final product Signup and view all the answers

Which of the following is NOT one of the allowed pattern allowances mentioned?

Vaporization allowance Signup and view all the answers

Which characteristic makes plastic a preferred material in casting?

Good corrosion resistance Signup and view all the answers

Which factor influences the selection of pattern material based on the size of casting?

Size of casting Signup and view all the answers

What is the primary use of a split pattern?

When solid pattern withdrawal is difficult Signup and view all the answers

Which type of pattern is characterized by having no joints or loose pieces?

Single piece pattern Signup and view all the answers

What advantage do match-plate patterns provide in casting?

Improvement in uniformity and production speed Signup and view all the answers

Which type of pattern is typically used for making castings with projections or undercuts?

Loose piece pattern Signup and view all the answers

Which type of pattern is best suited for mass production involving multiple cavities in one mold?

Gated pattern Signup and view all the answers

What material are disposable patterns typically made from?

Polystyrene (thermocol) Signup and view all the answers

What is the primary function of a runner in the casting process?

To distribute molten metal to several gates. Signup and view all the answers

What role does a riser play in the casting process?

It provides compensation for metal shrinkage. Signup and view all the answers

Why is it important for a riser to be the last part to solidify?

To ensure it is filled with molten metal. Signup and view all the answers

What creates pressure within a riser during the filling of a mould?

The hydrostatic head from the poured metal. Signup and view all the answers

Which of the following statements about gates in a mould is accurate?

They allow the molten metal to flow into the mould cavity. Signup and view all the answers

What is a key requirement for the volume of a riser?

It must be large enough to compensate for shrinkage. Signup and view all the answers

What characteristic must be maintained in the metal inside the riser?

Its fluidity must be maintained. Signup and view all the answers

Which type of riser is described as a vertical channel that remains open to the atmosphere?

Open riser. Signup and view all the answers

What does the continuity law express regarding the flow of liquid during pouring?

The volume rate of flow remains constant throughout the liquid. Signup and view all the answers

How is the time to fill a mold cavity calculated?

TMF = V / Q Signup and view all the answers

What does Chvorinov's rule relate to in casting?

Solidification time in relation to volume and surface area. Signup and view all the answers

If the cross-sectional area at the base of a sprue increases, what happens to the velocity of the flowing metal?

Velocity decreases. Signup and view all the answers

Which of the following is true about casting defects?

They generate imperfections that exceed quality limits. Signup and view all the answers

What does the variable 'c' represent in Chvorinov's rule for solidification time?

A mould constant dependent on material and temperature. Signup and view all the answers

What will happen to the solidification time of an insulated cube compared to a non-insulated cube of the same size and material?

It will be longer. Signup and view all the answers

What does the term 'blow' refer to in casting defects?

Small round voids opened to the casting surface. Signup and view all the answers

Study Notes

Plastics in Casting

Low weight, easily formed, moisture resistant, and corrosion resistant.

Pattern Material Selection

Casting size influences material choice (large castings favor wood (W) over plastic (P) over metal (M)).
Casting shape impacts material choice (metal more suitable for complex shapes).
Dimensional accuracy favors metal patterns.
Machinability is best with wood patterns.
Production quantity influences material choice (plastic preferred for high volume).
Economy favors wood patterns.

Types of Patterns

Single piece (solid, flat-back): Simple shapes.
Split pattern: For complex shapes where removal from mold is difficult; joined at parting line.
Loose piece pattern: Used for undercuts and projections; removed after main pattern.
Gated pattern: Used for multiple small cavities in mass production.
Match-plate pattern: Split patterns attached to a plate for increased production and uniformity.
Disposable pattern: Made from polystyrene; left in mold during casting.

Single Piece Pattern (Solid Pattern)

Simplest pattern type; used for castings with simple shapes.
Made in one piece without joints or loose pieces.

Split Patterns

Used for shapes too complex for a single-piece pattern.
Split into two parts joined by dowel pins.
Splitting facilitates pattern removal.

Loose Piece Patterns

Used when pattern removal is difficult due to undercuts or projections.
Main pattern removed first, then loose pieces.

Gated Patterns

Used for mass production of small castings in multi-cavity molds.
Patterns, gates, and runners are joined with a common runner for molten metal.

Match-Plate Patterns

Split patterns attached to a plate on both sides.
Improves production and casting uniformity; gates and runners are also attached.

Disposable Patterns

Made of polystyrene (thermocol).
Left in the mold; vaporizes when molten metal is poured (cavity-less method).

Pattern vs. Casting Dimensions

Patterns are larger than castings to account for: shrinkage allowance, machining allowance, draft allowance, distortion allowance, and rapping/shake allowance.

Pattern Allowances

Intentional dimensional differences between pattern and final casting.
Includes shrinkage, machining, draft, distortion, and rapping allowances.

Shrinkage Allowance

Compensates for material shrinkage during cooling.
Pattern is made larger than the final casting size.
Allowance depends on material type and composition.

Types of Shrinkage

Liquid shrinkage: Contraction as liquid metal cools from pouring temperature to liquidus temperature. Prevents turbulence.

Sprue

Metal pouring cup to channel molten metal into the mold.
Straight or tapered sprue designs.
Sprue base acts as a reservoir to reduce molten metal momentum and prevent mold erosion.

Runner

Distributes molten metal from the sprue base to multiple gates.

Gates

Passages for molten metal to fill the mold cavity.
Top, bottom, and parting gates are examples.

Riser

Reservoir to feed molten metal to the casting, compensating for shrinkage.
Allows air escape, creates pressure, ensures complete cavity filling, and allows for visual confirmation of cavity filling.
Blind and open riser types.

Riser Requirements

Solidifies last, sufficient volume for shrinkage compensation, covers the casting section needing feeding.
Maintains metal fluidity.

Pouring Process Analysis – Velocity and Flow Rate

Flow velocity is calculated using sprue height and velocity at the base.
Continuity during pouring ensures constant flow rate (Q = vA).

Mold Filling Time

Calculated using mold cavity volume and volumetric flow rate: TMF = V/Q.

Numerical Problem (Mold Filling)

Example problem calculating velocity, flow rate and mold filling time given sprue dimensions and mold cavity volume.

Solidification Time (Chvorinov's Rule)

Solidification time (t) is proportional to (Volume/Surface Area)^2.
Riser solidification time should exceed casting solidification time.

Numerical Problem (Solidification)

Example problem comparing solidification times of two cubes with different insulation conditions.

Casting Defects

Imperfections exceeding acceptable quality limits.

Blow Defects

Small, round voids open to the casting surface.

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Description

Explore the essential concepts of pattern material selection and types used in casting. This quiz covers the advantages of various materials, including plastics and metals, and discusses their suitability based on casting size, shape, and production quantity. Test your knowledge on different pattern types and their applications in the casting process.