Lecture 2: Casting

Question 1

What is casting in manufacturing?

Answer 1

Casting is the process of turning shapeless raw materials into a usable shape. This process involves melting the material and pouring it into a mold where it solidifies into the desired form.

Question 2

What types of materials are typically used in casting processes?

Answer 2

Liquid: Such as molten metal, poured into molds.
Viscous: Such as plastics, requiring pressure to mold.
Powder: Used in powder metallurgy for materials with very high melting points, compacted and then sintered.

Question 3

What are the different types of molds used in casting?

Answer 3

Expendable Molds: Made from materials like sand or ceramics, destroyed during the casting process:
i.Sand casting
ii. shell molding
iii. investment casting (low-wax process)
Permanent Molds: Reusable molds typically made from metals, used for large production runs.
i. Permanent-mold casting
ii. Die casting
iii. Centrifugal casting
Composite Molds: Combine features of both expendable and permanent molds

Question 4

What are the key 5 steps involved in metal casting?

Answer 4

Melting: The raw material is melted to a liquid state.
Pouring: The liquid metal is poured into a mold.
Cooling: The metal cools and solidifies into the shape of the mold.
Removal: The solid metal is removed from the mold.
Finishing: Additional processes to achieve the desired surface finish or dimensional accuracy.

Question 5

What factors influence the complexity and cost of a casting process?

Answer 5

The complexity and cost depend on the shape and size of the product, the material used, and the type of mold. Expendable molds are cheaper for small batches, while permanent molds are cost-effective for large quantities due to their reusability.

Question 6

What are the key considerations during the solidification and cooling phase of casting?

Answer 6

Shrinkage: As metal cools, it contracts; molds must account for this to avoid defects.
Heat Transfer: Efficient heat transfer is critical to control the cooling rate and prevent internal stresses and deformations.

Question 7

What are 4 common defects found in cast products?

Answer 7

Blowholes: Spherical cavities caused by trapped air.
Shrinkage Cavities: Void spaces formed by improper solidification.
Cold Shut: Occurs when two streams of molten metal solidify before completely merging.
Inclusions: Non-metal particles trapped within the metal.

Question 8

What are three common expendable mold casting processes?

Answer 8

1. Sand Casting: Uses sand-based molds.
2. Shell Molding: Uses a hardened sand-resin mixture to create a thin shell.
3. Investment Casting: Uses a wax pattern coated in ceramic material, which is then burnt out to leave a hollow mold.

Question 9

What are the advantages and disadvantages of sand casting?

Answer 9

Advantages: Low tooling cost, no limit to size or shape, suitable for almost all metal alloys.
Disadvantages: Coarse surface finish, less dimensional accuracy(accuracy is not high).

Question 10

Why is sand casting not suitable for plastics, and what types of sand are commonly used?

Answer 10

Plastics:

Not suitable due to adhesion to sand.
Sand Types:

Used types include naturally/synthetic bonded sand, silica sand

Question 11

What considerations are important during the simulation and design phase of sand casting?

Answer 11

Design Evaluation:

Probability of simulating designs for feasibility.
Critical Components:

Importance of risers for air escape.
Need for floating pins to prevent sand floating on metal.

Question 12

What roles do risers, vents, and cores play in sand casting?

Answer 12

Risers: Prevent shrinkage.
Vents: Prevent air bubbles.
Cores:
Internal cores for hollow regions.
External cores for complex models.
Machining often needed for precision.

Question 13

Why do sand cast products exhibit a parting line, and what is the typical surface finish?

Answer 13

Parting Line:
Present in every sand cast product.
Surface Finish:
Typically rough due to sand texture. . The product will not be very shiny, that has
to do with the roughness of the sand.

Question 14

What are the 3 advantages and 3 disadvantages of shell molding?

Answer 14

Advantages: High accuracy (Positioning holes are possible, Smooth surfaces, acute angles, thin walls), good surface finish, high production rate.
Disadvantages: Limited part size (Complex process which takes a lot of
handling), complex handling, and higher equipment cost.

Question 15

What distinguishes investment casting from sand casting in terms of mold material and complexity?

Answer 15

Investment Casting:

Mold material: Ceramics.
Complex process requiring extensive handling.
Expensive materials and labor.

Question 16

What are the characteristics of project geometry in investment casting?

Answer 16

Project Geometry:
Smooth surfaces.
No draft angle needed, as the mold is destroyed.
Primarily suited for small products due to complexity.

Question 17

What is the primary material output of investment casting?

Answer 17

Material Output:

Mainly metals, though virtually any material can be used (cast iron/steel, titanium alloy, nonferrous metal)

Question 18

How does investment casting differ from sand casting regarding the loss of the mold and pattern?

Answer 18

Investment Casting:

Both mold and pattern are lost.
Mold material consists of ceramics, unlike sand casting.

Question 19

What are permanent molds in casting?

Answer 19

Permanent molds are reusable molds typically made from metal. These molds can be used repeatedly to produce many castings.

Question 20

3 Advantages of Permanent Molds

Answer 20

They provide good surface finish, high dimensional accuracy, and are suitable for large production volumes

Question 21

Why are draft angles essential in Permanent Molds, and what is their role in preventing mold wear?

Answer 21

Draft Angles:

Essential to prevent mold wear.
Facilitate easy removal of products from the mold.
Ensure smooth production process.

Question 22

What are the advantages and limitations of Permanent Molds in terms of surface finish, accuracy, and part complexity?

Answer 22

Advantages:

Good surface finish and high accuracy due to metal molds.
Suitable for simple products with smooth surfaces.
Limitations:

Limited part complexity due to contact between product and mold.
Difficulty in removing complex parts from the mold.

Question 23

What materials are typically used for Permanent Molds, and what are the characteristics of product geometry?

Answer 23

Mold Material:

Always metal, with occasional use of sand cores.
Product Geometry:

Smooth surface.
Simple products.
Draft angle essential for mold release.
Nonferrous metals or cast iron used for product material.

Question 24

Why are ejectors frequently used in Permanent Molds, and how do ejector pins serve as an indication of the casting method?

Answer 24

Ejectors:

Prevent cooled molten metal from sticking to the mold.
Necessary if the product is hard enough.
Ejector Pins:

Serve as a good indication that a product was made with permanent mold casting

Question 25

What are 3 types of permanent mold casting?

Answer 25

1.Gravity Die Casting: Uses gravity to fill the mold.
2.Low-Pressure Die Casting: Uses low pressure to push the molten metal into the mold.
3.High-Pressure Die Casting: Uses high pressure for rapid filling of the mold, suitable for high-volume production.

Question 25

What are the 4 characteristics and 4 advantages of Permanent Mold Casting done with pressure?

Answer 25

Characteristics:
1.High dimensional accuracy.
2.Very good surface finish.
3.High production rate.
4.Long lead time and high die costs.

Advantages:
1.Metal mold material and core.
2.Limited product size.
3.Thin walls.
4.Nonferrous metals.

Question 25

Describe the process of Vacuum Casting.

Answer 25

Process:

Mold pushed into molten metal.
Vacuum created to fill entire mold.
Useful for eliminating air bubbles.

Question 26

Differentiate between Hot Chamber and Cold Chamber High Pressure Die Casting processes

Answer 26

Hot Chamber Die Casting:

Oven part of the machine.
3-30 MPa pressure.
Cooling by water or oil.
Cold Chamber Die Casting:

Oven outside the machine.
70-200 MPa pressure.
More than 70% of machine dedicated to mold closure mechanism

Question 27

Explain the process and characteristics of Rotational Molding. (7)

Answer 27

Process:

1. Machine rotates, melting granulated pellets.
2. Molten material sticks to mold walls and forms product.
3. Moderate production rate.
4. Relatively cheap molds.
5. Suitable for hollow and complex products.
6. Primary material: plastics.
7. Mold material: thin metal.

Question 28

What are the distinguishing features and applications of (Semi-)Centrifugal Casting?

Answer 28

Features:

Faster than rotational casting.
Limited part shape due to high rotational speed.
Mold material: metal or graphite.
Applications:

Hollow, cylindrical parts.
Non-ferrous and ferrous metals and alloys.
Limited melting temperature.

Question 29

What are some challenges associated with permanent molds?

Answer 29

Designing molds that can effectively release the cast without damage is crucial. This often requires draft angles and proper cooling channels to manage the thermal dynamics during casting.

Question 30

In what scenarios are permanent molds preferred?

Answer 30

Permanent molds are ideal for producing non-ferrous metal parts like aluminum and magnesium alloys, commonly used in automotive and aerospace industries for their ability to produce high-integrity structural components.

Question 31

What is a key design rule for simplifying casting processes?

Answer 31

Adjust the design to the simplest casting process available to minimize complexity and cost, ensuring that the part design is compatible with casting capabilities.

Question 32

Why should metal accumulation be avoided in casting designs?

Answer 32

Accumulation of metal in certain areas (like thick sections or sharp corners) can lead to hot spots during cooling, causing defects such as shrinkage or residual stresses.

Question 33

What is the role of ribs in casting designs?

Answer 33

Ribs are used to strengthen and support designs without significantly increasing the weight or thickness of the part, improving structural integrity while maintaining material efficiency.

Question 34

What factors should be considered when choosing the right casting process?

Answer 34

Factors include:

Material type: Suitability of the material for the chosen process.
Product size and complexity: Larger and more complex shapes may require specific casting methods.
Dimensional accuracy and surface finish: Desired precision and aesthetic requirements.
Cost considerations: Equipment, labor, and production rate influence the choice of process.

Question 35

How should casting designs account for shrinkage and stress?

Answer 35

Designs should limit areas of stress concentration by using fillets and radii to smooth transitions between different sections. Also, account for material shrinkage during cooling when defining the final dimensions.