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About die casting - Design geometry and Equipment

Knowledge  About Die Casting (II)

Design geometry

There are a number of geometric features to be considered when creating a parametric model of a die casting:

  • Draft is the amount of slope or taper given to cores or other parts of the die cavity to allow for easy ejection of the casting from the die. All die cast surfaces that are parallel to the opening direction of the die require draft for the proper ejection of the casting from the die. Die castings that feature proper draft are easier to remove from the die and result in high-quality surfaces and more precise finished product.
  • Fillet is the curved juncture of two surfaces that would have otherwise met at a sharp corner or edge. Simply, fillets can be added to a die casting to remove undesirable edges and corners.
  • Parting line represents the point at which two different sides of a mold come together. The location of the parting line defines which side of the die is the cover and which is the ejector.
  • Bosses are added to die castings to serve as stand-offs and mounting points for parts that will need to be mounted. For maximum integrity and strength of the die casting, bosses must have universal wall thickness.
  • Ribs are added to a die casting to provide added support for designs that require maximum strength without increased wall thickness.
  • Holes and windows require special consideration when die casting because the perimeters of these features will grip to the die steel during solidification. To counteract this affect, generous draft should be added to hole and window features.

Equipment

There are two basic types of die casting machines: hot-chamber machines and cold-chamber machines. These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 st (2,500 and 25,400 kg).

Hot-chamber die casting




Schematic of a hot-chamber machine

Hot-chamber die casting, 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 it is limited to use with low-melting point metals and that aluminium cannot be used because it picks up some of the iron while in the molten pool. Therefore, hot-chamber machines are primarily used with zinc-, tin-, and lead-based alloys.

Cold-chamber die casting




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. The 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.

  • Open tooling and injection nozzle

  • Complete working cell

    To be continued...