Material Selection - Mec08100 2016-7 Tr 2

Front 1

State a suitable material and manufacturing process for thefollowing application and give reasons for your choice:

Electric Drill Casing

Back 1

Polyurethane & Compression Moulding

Reasons: Good toughness, high impact resistance, non-conductor under most conditions.

Compression moulding is cheap, little waste material and can produce moderately complex parts.

Front 2

State a suitable material and manufacturing process for the following application and give reasons for your choice:

Yogurt Pot

Back 2

Polystyrene & Thermal Forming

Reasons: Low cost. Chemical resistance, keeps out moisture, recyclable. Good toughness, scratch resistance. Thermal forming is cheap, allowing for bulk items to be made for low cost and quickly

Front 3

Notes on Thermoforming

Back 3

Thermoforming: A sheet of extruded plastic is heated, then shaped by pressure and/or a vaccum.

It is used, typically, for producing smooth shapes with some details. It is relatively low cost. The deeper the mould the thinner the walls. Design is limited to relatively simple designs. Trimming is usually required afterwards.

Front 4

Notes on Extrusion

Back 4

Extrusion: Molten material is pushed through a shaped hole before being cooled. The Cross Section is continuous. The shape of die determines whether an object is a solid rod or hollow pipe.

Generally low cost, requiring simple dies. Can only produce continuous cross-section shapes.

Front 5

Notes on Injection Moulding

Back 5

Injection Moulding: Molten material is injected into a mould via screw or ram. It is then cooled and the object is ejected.

It can produce complex 3D shapes, consistent quality can be produced in high volumes. The initial setup costs are complex and moulds are complex, with larger moulds requiring more material to fill.

Front 6

Compression Moulding

Back 6

Compression Moulding: Moulding powder is heated and compressed onto a shape.

Process causes significant changes to material's form and structure. Most commonly used for shaping thermoplastics. Can produce moderately complex parts over long production runs. Start up costs are relatively low, with little waste product created

Front 7

Difference between a Thermoset and Thermoplastic

Back 7

Thermoplastics: Reusable plastics, typically less hard than thermoset plastics. High stress resistance, though versatile to serve in low stress environments.

Thermoset: Non-reusable once worked into its solid shape. Temperature resistant and more resistant to deformation.

Front 8

Define the Following:

- Toughness

- Yield Strength

- Stiffness

- Ductility

Back 8

Toughness: Ability to absorb energy and deform without fracture, enduring an applied load.

Yield Strength: The point of stress at which any further stress will cause plastic deformation.

Stiffness: Is the rigidity of an object, the ability to resist deformation in response to an applied force.

Ductility: The ability of an object to be stretched and drawn out into a thin wire.

Front 9

Explain a method of Non-Destructive Testing.

Back 9

Hardness Testing: An indent is made on a material from a very small, and very hard, material tip. This is then measured using microscopes to zoom and measure the size of the indent left on the material.

The harder the material is the more resistant it will be to the indentation caused by the tool tip. This would mean that any indentation mark would be smaller than if on a softer material.

Front 10

Write brief notes on Metal Rolling

Back 10

Metal Rolling: A metal stock is passed through one, or more, pairs of rollers to reduce the thickness of a metal and make its thickness uniform.

Depending on temperature, if it is above or below a metal's crystallisation temperature, then it is known as either hot rolling or cold rolling. Cold rolling increases a metal's strength via strain hardening. Hot rolling, however, prevents the metal from work hardening.

Front 11

List Five advantages of Plastics over Metals

Back 11

Plastics tend to have the following advantages over metals:

- Plastics tend to be more malleable and easily machined

- Plastics tend to be more resistant to creep than metals.

- Plastics tend to be cheaper to manufacture than metals.

- Plastics tend to be lighter than metals, making them easier to ship

- Plastics tend to be electronically resistant.

Front 12

List Five disadvantages of Plastics over Metals

Back 12

Plastics tend to have the following disadvantages over metals:

-Most plastics tend to have low heat resistance.

-Most plastics tend to have less impact resistance

-Plastics tend to be unable to conduct electricity

-Plastics have a lower yield point than most metals

-Plastics

Front 13

Ceramic, General Properties

Back 13

High Strength

High Fracture Toughness

High Hardness

Wear Resistance

Anti-Static

Low Thermal Conductivity

Chemical Corrosion Resistance

Front 14

Ceramics Engineering Benefits

Back 14

High Thermal Resistance

High Hardness

Low Electrical Conductivity (Insulator)

Corrosion Resistance (Acids and Alkali)

Low Coefficient of Resistance

Inexpensive

Front 15

Ceramics Engineering Drawbacks

Back 15

Brittle

Difficult to Machine

Weak in Tension

Poor Shock Resistance

Front 16

Alloying Elements and their Purpose

Back 16

Chromium:

Increases Corrosion Resistance (increasing with %Cr)
Nickel:
Promotes Austenitic Microstructure, increasing ductility and toughness.

Molybdenum:

Increases resistances to corrosion and slightly increases ferritic microstructure

Front 17

Alloying Elements and their Purpose (Pt2)

Back 17

Copper:

Enhances corrosion resistance to certain acids and promotes austentic microstructure.

Manganese:

Improves hot ductility and, depending on temp, acts as austentite stabilizer (low) or ferrite (high)

Silicon:

Increases oxidation resistance

Front 18

Stress/Strain Equations

Back 18

Tensile Stress: σ = Fn / A
Strain: ε = dl / lo

= σ / E

Young's Modulus: E = stress / strain

= σ / ε

= (Fn / A) / (dl / lo)

Front 19

Precipitation Hardening

Back 19

Also known as Age Hardening.

One Metal must slightly dissolve in another (in solid state). The solid solubility decreases rapidly as temperature falls.

Heat treatment needed:

Solutionise - heat so that all of solute metal is dissolved in solvent

Quench - Quench to room temperature.

Precipitate - heat to intermediate temperature so fine precipitates form

Normally Aluminium or copper allloys, some steels too.

Front 20

Work Hardening (Cold)

Back 20

Also known as Cold Working

Involves plastically deforming a material below 0.3Tm

When deformation takes place more dislocations are produced in crystal structure. Dislocations tangle and strain fields interact and pin each other increasing hardness and strength.

Front 21

Work Hardening (Hot)

Back 21

Does not provide strengthening.

Dislocations contain enough thermal energy to diffuse to surface and untangle.

Front 22

Tempering

Back 22

Usually Steel, few other metals can use this:

Based on Iron which has more than one crystal structure.

Copper and Aluminium don't work because crystal structures don't change with temperature.