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Casting Process

SAND CASTING, GRAVITY OR HIGH PRESSURE DIE CASTING?

Once the most appropriate process has been selected, designers should decide upon the alloy which has the mechanical and physical properties most appropriate to the particular application, whilst also considering whether the casting characteristics are appropriate to the chosen process.

The table below will assist you to choose the process most appropriate to your requirements, however it is often necessary to obtain quotations for more than one process in order to decide which will be the most economic.

 

 

Sand Casting

Gravity Die Casting

High Pressure Die Casting

Tolerances on

Critical Dimensions

Up to 25mm

Each Additional cm

Across Parting Line Ad.

± 1mm
± 0.39mm
± 1mm

± 0.4mm
± 0.02mm
± 0.25mm

± 0.1mm
± 0.016mm
± 0.12mm

Minimum Taper

On Inside Walls

On Outside Walls



0.5°

Minimum Wall Thickness

On Small Castings

On Large Castings

3mm

5mm

3mm

4mm

1mm

2mm

Minimum Size of Cored Hole

15mm

3mm

2mm

Ability to use Sand Cores

Yes

Yes

No

Surface Finish

Poor

6.5-12.5 m

Fair

4-10 m

Good

1.5 m

Alloys commonly used in this form

Aluminium

Zinc

Copper

LM5

LM6

LM9

LM16

LM24

LM25

LM27

LM31

ZA3

ZA8

ZA12

ZA27

LG2

PB2

 

*

*

*

 

*

*

 

 

 

*

*

*

*

 

*

*

*

 

*

*

*

 

*

*

*

*

*

 

 

*

 

 

 

*

Typical Production Quantities per annum

1-500

250-50,000

2,000-500,000

Typical Tool Life

5,000

100,000

500,000

Types of Casting

Sand

Sand Casting

Sand casting offers the simplest medium for production of non-ferrous castings, by pouring molten metal into a sand mould. This process is chosen for the production of small numbers of castings, for complex shape castings requiring intricate cores, or for large castings. Advantages include low tooling costs, largest casting sizes achievable, very low gas porosity is possible, and general versatility of the process is good. Limitations include low casting rate, 3-5mm minimum wall thickness, poor linear tolerance (e.g. 4mm/m), rough surface finish (6.5-12.5mm), and a coarser grain size than gravity die castings.

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Gravity

Gravity Die Casting

Gravity die casting is produced by pouring molten metal into permanent cast-iron moulds. This process produces chill castings. The process has a higher casting rate than sand casting, but the metal moulds are higher cost than sand patterns. Advantages include the possibility of low gas porosity, and fine grain sizes can be obtained. Less finishing and polishing will be required than for sand castings. Gravity die casting tends to produce the highest quality castings, however the accuracy achievable is limited. Minimum wall thickness possible is c. 3-5mm.

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Low Pressure

Low Pressure Die Casting

Low pressure casting is a development of the permanent mould process, in which the metal is introduced into the chill mould from below. Gas pressure holds the metal in the die until it solidifies. As with high pressure diecasting the process requires complex machinery. It is repetitive, and may be automated. Production rates are fair, but not as good as high pressure. Minimum wall thickness' are as little as 2-3mm. It has high yields of over 90%, as runners and risers are excluded, also reducing fettling and trimming costs. Pore-free castings are obtainable. Machine size will limit the size of castings. Surface finishing and minimum wall thickness' are both better than gravity die casting, but poorer than high pressure die casting.

High Pressure

High Pressure Die Casting

High pressure die casting, often shortened to Pressure Die Casting, is a repetitive process where identical parts are cast at high production rates by injecting molten metal under pressure into a metal die. Again, this process requires complex machinery. High pressure die casting is ideally suited to high production rates, and wall thickness' can be as little as 1-2.5mm. It provides the best surface finish (1.5m), and a very fine grain surface can be obtained. Pressure die casting has high strength. Castings may be quenched from the die. Machine size will limit casting size. Sound thick sections are difficult to cast, and core configurations must be complex to enable disassembly. High pressure die castings may suffer from porosity, although evacuated chamber and other techniques may reduce this. High start up costs are only reduced by long casting runs, thus enabling low unit cost with a high volume production. Pressure die castings cannot be fully heat treated.

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Investment

Investment Casting 

This casting method involves producing a "wax pattern" by injecting wax or plastic into a pattern die. The pattern is attached to gating and runner systems and this assembly is dipped in a hard setting refractory slurry which is then cured. The pattern is melted out of the mould to leave an exact cavity. The mould is heated to cure the refractory and to volatilize the remaining pattern material. The moulds are baked and molten metal is poured into the mould cavity. On solidification of the casting, the mould material is broken away from the casting.

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