Introduction to Die casting

Investment casting uses a pattern that may be made from wax, plastic, or other material.After the mold is made, the pattern is melted out. Thus a mechanized method of casting a great many patterns is necessary.

The mold material is dependent upon the melting point of the cast metal. Thus a plaster mold can be used for some materials while others would require a ceramic mold.

After the pattern is melted out, the mold is baked or fired; when firing is completed, the molten metal may be poured into the hot mold and allowed to cool. If a number of castings are to be made, then metal or permanent molds may be suitable.

Metal-mold castings are also known as die castings and centrifugal castings.there are two basic types of die casting machines: hot-chamber machines and cold-chamber machines.

Hot-chamber machines

Schematic of a hot-chamber machine

 

Hot-chamber machines, also known as 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. The disadvantages of this system are that high-melting point metals cannot be utilized and aluminium cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.

Cold-chamber machines

A schematic of a cold-chamber die casting machine.

 

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. 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. This biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold-chamber machine.

Advantages and disadvantages

Advantages:

  • Excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm (0.005 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 mold 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).

The main disadvantage to die casting is the very high capital cost. Both the casting equipment required and the dies and related components are very costly, as compared to most other casting processes. Therefore to make die casting an economic process a large production volume is needed. Other disadvantages include: the process is limited to high-fluidity metals and casting weights must be between 30 grams (1 oz) and 10 kg (20 lb).In the standard die casting process the final casting will have a small amount of porosity. This prevents any heat treating or welding, because the heat causes the gas in the pores to expand, which causes micro-cracks inside the part and exfoliation of the surface.

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