DIE CASTING - Metrics

DIE CASTINGS

DIE CASTING Process

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mould during the process.

There are two basic types of die casting machines: hot-chamber machines and cold-chamber machines

Hot-chamber die casting
Hot-chamber die casting, also known gooseneck machines, rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the “gooseneck”. The pneumatic– or hydraulic-powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times (approximately 15 cycles a minute) and the convenience of melting the metal in the casting machine.
hot-chamber machines are primarily used with zinc
Cold-chamber die casting
These are used when the casting alloy cannot be used in hot-chamber machines; these include aluminium, zinc alloys with a large composition of aluminium, magnesium and copper. The process for these machines start with melting the metal in a separate furnace.[14] Then a precise amount of molten metal is transported to the cold-chamber machine where it is fed into an unheated shot chamber (or injection cylinder). This shot is then driven into the die by a hydraulic or mechanical piston.
Advantages of die casting:[10]
- Excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm (0.004 inch for the first inch) and 0.02 mm for each additional centimeter (0.002 inch for each additional inch).
- Smooth cast surfaces (Ra 1–2.5 micrometres or 0.04–0.10 thou rms).
- Thinner walls can be cast as compared to sand and permanent mould casting (approximately 0.75 mm or 0.030 in).
- Inserts can be cast-in (such as threaded inserts, heating elements, and high strength bearing surfaces).
- Reduces or eliminates secondary machining operations.
- Rapid production rates.
- Casting tensile strength as high as 415 megapascals (60 ksi).
- Casting of low fluidity metals.

The main die casting alloys are: zinc, aluminium, magnesium, copper,.

- Zinc: the easiest metal to cast; high ductility; high impact strength; easily plated; economical for small parts; promotes long die life.
- Aluminium: lightweight; high dimensional stability for very complex shapes and thin walls; good corrosion resistance; good mechanical properties; high thermal and electrical conductivity; retains strength at high temperatures.
- Magnesium: the easiest metal to machine; excellent strength-to-weight ratio; lightest alloy commonly die cast.
- Copper: high hardness; high corrosion resistance; highest mechanical properties of alloys die cast; excellent wear resistance; excellent dimensional stability; strength approaching that of steel parts.

Manufacture of parts using die casting is relatively simple

Most die castings are made from non-ferrous metals, zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low.

Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.

Die casting can produce metal parts with complex shapes and do so with closer tolerances than many other mass production processes.
Die casting yields especially high production rates, with parts that require little or no machining.
Die casting results in parts that are durable, dimensionally stable, and project the feel and appearance of quality.
Parts that have been die cast are stronger than plastic injection moldings, which offer similar dimensional precision. Wall castings are stronger and lighter than those possible with other casting processes.
Die castings can be produced with thinner walls than those obtainable by other casting methods. Die castings are generally much stronger than plastic injection moldings with the same dimensions.
The dies used in die casting can produce thousands of identical castings within specified tolerances before additional tooling may be required.
Zinc castings can be easily plated or finished with a minimum of surface preparation.
Die castings can be produced with surfaces simulating a wide variety of textures.
Die cast surfaces, as cast, are smoother than most other forms of casting such as sand, permanent mold, and investment casting processes.

Holes in die castings can be cored and made to tap drill sizes.
External threads on parts can be readily die cast.
Die castings provide integral fastening elements, such as bosses and studs, which can result in assembly economies.
Inserts of other metals and some non-metals can be die cast in place.
The corrosion resistance of die casting alloys rates from good to high.
Die castings are monolithic. They combine many functions in one, complex shaped part. Because die castings do not consist of separate parts that have been welded or fastened together, the strength of the material determines the strength of the part, rather than a strength that depends on threads or welds.
Die casting features high accuracy and repeatable reproduction of designs of varying complexity and level of detail.
The use of a single mold design (die set) allows multiple features to be imparted in one process.
Generally, die casting results in reduced cost from one process versus a process that requires several distinct production steps. It can also save money by reducing waste material and scrap.
Die casting usually results in faster production rates or speeds.