Low & High Pressure Diecasting | Choosing a Pressure Casting

				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		2° 2°	1° 1°	1° 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

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.