Equipment of Die Casting

There are two basic types of die casting machines: hot-chamber machines (a.k.a. gooseneck machines) and cold-chamber machines. These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 short tons.

Hot-chamber 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 gas or oil 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 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. This machine works by melting the material, first, 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.[7]

The dies used in die casting are usually made out of hardened tool steels because cast iron cannot withstand the high pressures involved. Due to this the dies are very expensive, resulting in high start-up costs. Dies may contain only one mold cavity or multiple cavities of the same or different parts. There must be at least two dies to allow for separation and ejection of the finished workpiece, however its not uncommon for there to be more sections that open and close in different directions. Dies also often contain water-cooling passages, retractable cores, ejector pins, and vents along the parting lines. These vents are usually wide and thin (approximately 0.13 mm or 0.005 in) so that when the molten metal starts filling them the metal quickly solidifies and minimizes scrap. No risers are used because the high pressure ensures a continuous feed of metal from the gate. Recently, there's been a trend to incorporate larger gates in the die and to use lower injection pressures to fill the mold, and then increase the pressure after its filled. This system helps reduce porosity and inclusions.

In addition to the dies there may be cores involved to cast features such as undercuts. Sand cores cannot be used because they disintegrate from the high pressures involved with die casting, therefore metal cores are used. If a retractable core is used then provisions must be made for it to be removed either in a straight line or circular arc. Moreover, these cores must have very little clearance between the die and the core to prevent the molten metal from escaping. Loose cores may also be used to cast more intricate features (such as threaded holes). These loose cores are inserted into the die by hand before each cycle and then ejected with the part at the end of the cycle. The core then must be removed by hand. Loose cores are more expensive due to the extra labor and time involved.

A die's life is most prominently limited by wear or erosion, which is strongly dependent on the temperature of the molten metal. Dies for zinc are often made of H13 and only hardened to 29-34 HRC.[11] Cores are either made of H13 or 440B, so that the wearing parts can be selectively nitrided for hardness, leaving the exposed part soft to resist heat checking.

Typical die temperatures and life for various cast materials

Zinc Aluminum Magnesium Brass (leaded yellow)
Maximum die life [number of cycles] 1,000,000 100,000 100,000 10,000
Die temperature [C° (F°)] 218 (425) 288 (550) 260 (500) 500 (950)
Casting temperature [C° (F°)] 400 (760) 660 (1220) 760 (1400) 1090 (2000)

Other failure modes for dies are:

  • Heat checking: surface cracks occur on the die due to a large temperature change on every cycle
  • Thermal fatigue: surface cracks occur on the die due to a large number of cycles
sumber : wikipedia

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