Everything You Need to Know About Die Casting:
Hot-Chamber and Cold-Chamber Processing
There are two basic die casting processes: hot-chamber and cold- chamber. The hot-chamber process is used for zinc alloy ZA-8 and magnesium Alloys AZ91B and AZ9ID. The cold-chamber process is used for aluminum alloys ZA-12 and ZA-27, and magnesium alloys AZ91B and AZ91D, particularly for large castings.
In the hot-chamber die casting process, the plunger and cylinder are submerged in the molten metal in the holding furnace. The energy to pump the metal into the die cavity is provided by a hydraulic pump and stored in an accumulator. Oil is supplied at a rate that will bring the pressure to the desired operating level each time a slot is made.
The operating sequence for the hot-chamber cycle: when a shot is made, the plunger is driven downward, forcing molten metal through the gooseneck and nozzle into the die cavity. After the cavity is filled, the metal is allowed to solidify, the casting is ejected, and the cycle repeated. A die lubricant, or mold release compound, is periodically sprayed onto the die to facilitate release of the casting. The system refills automatically when the plunger is withdrawn because the gooseneck and plunger are submerged in the molten metal.
The operating sequence for the cold-chamber process: the operating sequence is essentially the same as for hot chamber, except for the manner in which the molten metal is fed into the shot system prior to injection into the die. In the cold-cha ber process molten metal is ladled, either by hand or machine, or pumped into a horizontal shot cylinder. A hydraulically actuated plunger then advances to force the metal into the die.
The differences in metal feed give the hot-chamber process several advantages:
There are five significant characteristics common to both cycles which should be noted:
- Precise control of metal temperature improves fluidity, so that injection pressures can be lower, placing less strain on the equipment. Also, better fluidity promotes sounder castings and permits thinner walls.
- The submerged shot cylinder, which fills automatically, eliminates a human factor, reduces the cycle time, and makes it easier to control metal temperature.
- There is no cooling of the charge, as there may be when molten metal is manually transferred to the shot cylinder of the cold-chamber machine.
- The molten metal is less subject to oxidation from atmospheric exposure.
- The casting, as normally ejected from the die, is attached to extraneous metal. The entire shot is transferred to a trim die, which separates the casting from the extraneous metal. The metal that is removed is subsequently recycled.
- The metal injection portion of the cycle normally terminates with a spike, or sharp intensification in metal pressure. Intensification is developed by increasing the hydraulic pressure on the plunger to pack metal in the die cavity and feed shrinkage during solidification. It is beneficial to the extent that it promotes complete filling of the die cavity and uniformly sound metal density. Excessive intensification is undesirable, because it forces molten metal between die menbers, forming flash.
- The peak pressure multiplied by the projected area of the entire shot generates the force imposed on the machine structure. Die casting machines must be sized according to this force, and they are rated in terms of number of tons of locking force they are capable of developing in long-term, continuous operation.
- Peak pressure often dictates the size of the machine required to make a casting, which can attest the cost of the product.
- Intensification is being controlled to advantage by many diecasters who employ high-technology die casting.
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