Die casting process

Details Die casting process Die casting process

1999-07-09

2001-06-26

Lin, Kuang Y. (Department: 1722)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S312000, C164S342000, C164S359000

Reexamination Certificate

active

06250365

ABSTRACT:

FIELD OF THE INVENTION
The present application relates to the die casting of metals, and particularly to methods and apparatus for reducing waste material in die casting.
BACKGROUND OF THE INVENTION
Most metal alloys undergo a volume contraction when they solidify. This volume contraction can result in a shrinkage porosity in a cast part unless an appendage, often called a “riser,” of appropriate size is properly placed on the casting. The riser serves to feed the shrinkage of the metal in the mold, and is cut off after casting. Typical risers need to have volumes approaching the volume of the casting in order to produce a pore-free casting, leading to large recycling costs. This problem is particularly severe in high-purity castings, such as typical magnesium and magnesium alloy castings, because the riser may become contaminated during post-processing of the casting, and must be repurified before the metal can be used in another casting.
It is an object of the present invention to provide a method and apparatus for casting which allows riser volume to be substantially reduced.
SUMMARY OF THE INVENTION
The invention comprises apparatus and methods of die casting which reduce waste, by providing heated channels to feed solidification shrinkage. The channels are heated to a temperature which prevents solidification in the channels, thus allowing the channels to be substantially smaller than conventional risers. An insulating layer in the mold prevents excessive heat loss from the channels to the mold cavity area.
In one aspect, the invention provides a die casting apparatus, comprising a die having upper and lower sections, means for maintaining a temperature differential between the die sections, and means for introducing molten metal into the die. The two sections of the die are separated by an insulating layer, and define a die cavity which spans the two sections. The die cavity includes the mold cavity, which defines the shape of the finished casting, a riser, and a gate connecting the two. The riser is in the lower section of the die, and the mold cavity is in the upper section of the die.
The gate may be tapered from a narrower riser to a wider mold cavity. The metal introducing means may be, for example, a compressed gas, mechanical, or electromagnetic pump. The riser may be substantially vertical (so that metal travels upwardly through the gate and into the mold cavity). The insulating layer may be, for example, a ceramic or a refractory metal.
In another aspect, the invention provides methods of die casting. The methods include introducing molten (or semimolten) metal into a die cavity comprising a mold cavity, a riser, and a gate connecting the two. The mold cavity is positioned in the upper section of the die, and the riser is positioned in the lower portion of the die. The two die portions are separated by an insulating layer, that acts to limit heat transfer from the hotter lower section of the die to the cooler top section. The lower section is maintained at a temperature which keeps the metal liquid or semisolid, while the upper section is cool enough to freeze the metal to form at casting. The methods may further comprise removing the casting from the mold. The methods of the invention may be used to cast magnesium, in which case typical temperatures for the upper and lower portions of the mold may be 230° C. and 675° C., respectively.


REFERENCES:
patent: 3959433 (1976-05-01), Sauers

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