Lecture 1-2 Processes Flashcards by Tyler Oxley

Casting is a process whereby

a part is produced by solidification to take the shape of a mould

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Casting processes were first seen in the —– ages.

bronze

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5 main steps in casting process

Pattern/mould making
Melt preparation
Pour molten metal into the mould
Allow to solifify
Remove part from the mould

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Net shape means that

the product is ready for direct use

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Near net shape means that

the product requires some additional processing/finishing required before it can be used

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Why is using metal in casting versatile?

As any metal can be heated to be in the molten state

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Casting can be used to form how big of a piece?

Wide range from mm to m in one piece

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—– shapes are able to be produced through casting. The shapes can also have —– —–.

Complex, internal cavities

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Economically, casting is

a relatively cheap process

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Through casting a product, the outcome parts are

net shape or near net shape.

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Foundries are places

where casting takes place

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Expendable mould examples

Sand casting
Investment casting

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Permanent mould example

Die casting

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3 considerations regarding the molten material chosen for casting

Reactivity with air
Gas solubility
Safety

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Cores are used in casting to produce

internal features

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Cope and drag are

the two halves of the sand mould

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The entrance and the initial downward “tube” where the molten metal are poured down are called what?

Pouring cup at the top, which leads to the downsprue

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The parting line is

the line between the two moulds

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A riser is

a reservoir of metal to ensure that the cavity is filled with metal

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A riser is used because

A riser reduces the chance of —- occurring in the mould where there isn’t enough metal to go around

when metal is solidifying it shrinks.

voids

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The metal after entering the mould, flows down the —- to get to the cavity

Runner

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The finished product from sand casting usually requires —– ——. Why?

Post processing, low surface detail

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4 Cons of sand casting

High tolerances = low precision in the finished product
Flow and shrinkage must be monitored as it is gravity driven
Safety precautions
Labour intensive with long cycle time

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Pros for sand casting

Low production volumes
Cheap moulds
No size and shape limits to the cast
Moulds can be changed easily

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Die casting uses what sort of pressures? Why?

High pressure, to be able to inject the metals into the die

Die casting produces excess material where the two moulds meet. Name term used to describe this

Flash

Average cycle time for die casting

~ 10 secs

Die casting could be expensive due to (2 points)

- the tool grade steel that is required - needing the machines capable of closing dies and injecting molten metals

2 pros for die casting

- Process is semi or fully automated - Very high production rates

Two different options for die casting

- Hot chamber - Cold chamber

Basic outline of steps for die casting:

1. Lubrication of the dies 2. Closing and locking of dies 3. Molten metal is forced into the cavity 4. Dies are held together under pressure til the metal solidifies 5. Die opens 6. Casting is ejected

In Hot chamber die casting, common metals used are:

Zinc, lead, tin, and magnesium

Disadvantage of cold chamber die casting compared to hot chamber die casting:

extra step of pouring molten metal from a ladle into the piston

Cold chamber compared to hot chamber die casting has a

slightly lower production rate

In Cold chamber die casting metals commonly used are:

Aluminium, brass, magnesium alloys

5 Pros of die casting

- Cheap for large production - Good accuracy - Good surface finish - Able to do thin sections - Rapid cooling results in small grain size and good strength

Why would you choose cold chamber over a hot chamber die casting method? How does this apply to the difference in process?

The metal you are casting is corrosive - it cannot sit in the hot chamber for long without reacting with the container

Why is the runner an important section of the casting process?

It helps to direct the molten metal and allow it flow at a sensible rate (faster causes turbulence)

The downsprue helps to decrease the

- momentum in the molten metal - regulates the air going into it

In the Bernoulli equation, what do the letters represent? p v h g F ρ

p - pressure v - velocity h - height g - gravity F - energy losses due to friction ρ - density

Defects that can come about due to turbulent flow:

- entrap air from the atmosphere - air will react with the metal - voids and air bubbles will form in the metal

A lower Reynolds number means that

the flow is more laminar

A higher Reynolds number means that

the flow is more turbulent

How do we test a metal's fluidity index

spiral mould test

A higher metal's fluidity index means that the metal has

a higher fluidity index

In the Chvorinov's rule, the letters represent C V A n

C - constant for the alloy being cast V - volume of the casting A - the surface area of the casting n - constant (between 1.5-2.5)

Where does the solidification front occur?

It occurs at the melting point temp

If you are not given an n number for a calculation involving the Chvorinov's rule, then we assume

that n = 2

Relationship between grain size and cooling rate

Grain size is inversely proportional to cooling rate

In the columnar zone what sort of grain is present? What strength is this?

Longer grain - weaker

In the chill zone what sort of grain is present? What strength is this?

Finer grain - stronger

Where are the two zones found, columnar and chill?

Columnar are more central and chill is on the outer edge

In a cooled down alloy, what feature is seen in the centre of the columnar zone?

Equiaxed zone

What is the main property of the equiaxed zone?

There are no directions to the grains - it is isotropic

Why is a fully equiaxed structure preferred over mixed structures?

Isotropic properties and consistent throughout

Ways to counteract shrinkages

- Use of the riser design - Remoulding the mould to be slightly larger

A misrun is

solidification occurs before filling the cavity

Cold shuts are

lack of fusion around a core due to cooling

Cold shots are

metal splatter which leads to air entrapment

Shrinkage cavity is

solidification shrinkage which leads to a lack of metal in the mould

Hot tearing or cracking is

contraction during the final stages of cooling due to shape of the mould

8 Main steps in investment casting

1. Injection of wax into a cast 2. Combining of the wax models onto a larger frame 3. Coat in ceramic 4. Apply stucco 5. Heat to remove and melt internal wax models 6. Ceramic shell is fired to strengthen 7. Ceramic shell is ready to be used for casting 8. Separate the ceramic metal filled casts from the frame

Lecture 1-2 Processes Flashcards

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