metal castin g.docx

8/14/2019 Metal Castin g.docx

1/3

Design and manufacture of consumer products.

20/02/2012

Metal working

Metal casting:Casting involves pouring molten metal into a mould. When it solidifies, it forms

the shape of the mould. It can be used to produce a complex product in one piece,

such as an engine block.

First used around 4000B.C to make copper arrowheads and ornaments.

Major categories of moulds include:

Expendable moulds; made of sand/plaster/ceramic with additional binders.

Can be single use or multiple use, but most likely single use in this category,

expecially sand casting.

Permanent moulds; made of metals that retain strength at high temperatures

and do not posess much thermal expansion properties. Can be used repeatedly.

Cast items are easily removed.

Composite moulds; made of 2 or more different materials, e,g sand, graphite

and metal. Can have a permanent and an expendable section to improve

strenght, control cooling rates and make less work as the whole mould does not

have to be used again.

Casting can be used to manufacture many useful products. Processes such as diecasting are used for a huge number of products.

Defects in the mould can lead to defects in the finished product, therefore the

cast must be made without imperfections to make sure the finished product is of

a good quality.

Casting is advantageous in situations where:

Metal rods/blocks (Stock bars/billets) need to be transported

Certain alloys cannot be hot or cold worked.

Mechanical working would require heavy & expensive machinery

The product has a complex internal shape

A large item needs to be made in one piece

A different material is needed to be inserted (a composite product)

Solidification properties of metals:

When a metal cools, it undergoes a solidification process. How quickly it cools

determines the crystal structure of the metal internally.

(see handout)

Different rates of cooling produce different microstructures. A slow cooling willproduce equiaxed crystals all over. Medium cooling rates produce an internal


2/3

equiaxed zone with a chill zone on outside and a comumnar boundry around the

equiaxed zone. Fast cooling just produces a chill zone on outside and a coumnar

zone running through the inside with no equiaxed zone. The microstructure

affects the end properties of the metal.

Complications

The molten metal must be viscious enough to fill the mould fully and flow into

the mould easily.

The rate of solidification must be tailiored to the usage and type of material.

7 basic factors:

The mould

The energy to melt metals

The mould filling

The cooling rate

Shrinkage allowances

The removal of product from the mould

Finishing operations

1: The mould

must consider size, shrinkage allowance as it solidifies. Any complex parts must

be present in the mould features.

Whether it is a permanent or expendable mould. The mould must also withstand

the heat of the molten metal.

2: The energy to melt

Need to have a suitable way of melting metal. Must also have an adequate

temperature, but also needs to be as environmentally friendly as possible.

3. Mould filling

must design how the mould is to be filled.

4. Cooling rate

Must optimise cooling process to make the best microstructure for the task

needed.

5. Shrinkage

Metals shrink when they solidify. Therefore, the metal will shrink in the mould

and reduce its volume. Mould must accommodate this and not restrain the

shrinkage. Cast may split or crack. Must design so mould will not split or crack.

6. Removal from mould

must be able to remove, and design for this. Not a problem with expendable but

problem with permanent

7. Finishing operationsmust be able to finish product e.g removing mould overflow.


3/3

Factors affecting casting

Type of metals:

Alloys can have different melt temps. This can cause problems.

Mechanical properties:

Different types of mould provide different cooling methods which affects

microstructure.

Casting alloy and proces may be dictated by required product strength.

Production and economic factors.

Sand/mineral grain moulds are cheap but need to be made at equal volumes to

the product, metal moulds can be reused again and again.

Casting defects

ABCDEFG

A- Metalic projections fins and burrs sticking outB- Cavity -rounded or rough internal or exposed cavitiesC- Discontinuties- cracks, cold or hot tearing and cold shutsD- Defective surface folds, laps scars, sand layers and oxide scaleE- Incomplete casting- misruns due to premature solidificationF- Incorrect dimensions or shape due to improper shrinkage allowanceG- Inclusions ( voids/non metal areas formed during

melting/solidification/moulding)

metal castin g.docx

Documents