TYPES OF DIE
There
are four types of dies:
- Single cavity to produce one component
- Multiple cavities to produce a number of
identical parts
- Unit die to produce different parts
at one time
- Combination die to produce several different
parts for an assembly
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
a high startup cost.
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.
These tooling are made of alloy tool steels in at least two sections, the fixed die half, or cover half, and the ejector die half, to permit removal of castings. Modern dies also may have moveable slides, cores or other sections to produce holes, threads and other desired shapes in the casting. Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity.
The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant.
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. Aluminum alloy die usually have a life of 100,000 cycles, if the die is properly maintained.
Molds for die casting zinc last approximately 10 times longer than aluminium die casting mold due to the lower temperature of the zinc. Dies for zinc are often made of H13 and only hardened to 29-34 RHC. 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. Molds for die casting brass are the shortest-lived of all.
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.
These tooling are made of alloy tool steels in at least two sections, the fixed die half, or cover half, and the ejector die half, to permit removal of castings. Modern dies also may have moveable slides, cores or other sections to produce holes, threads and other desired shapes in the casting. Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity.
The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant.
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. Aluminum alloy die usually have a life of 100,000 cycles, if the die is properly maintained.
Molds for die casting zinc last approximately 10 times longer than aluminium die casting mold due to the lower temperature of the zinc. Dies for zinc are often made of H13 and only hardened to 29-34 RHC. 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. Molds for die casting brass are the shortest-lived of all.
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