Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-06-14
2003-11-25
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S305000, C164S312000
Reexamination Certificate
active
06651727
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods and apparatus for die-casting, and more particularly, to systems for reducing the inclusion of gaseous voids in die castings.
BACKGROUND OF THE INVENTION
The use of die-casting and plastic molding has been extended to the manufacture of larger and larger articles. Such large automotive parts as internal combustion engine blocks and the housings for automatic transmissions are now commonly manufactured with die-casting as the first step in formation of the part. Such parts have extensive and complex surfaces with close tolerances; and die-casting permits their formation, eliminating costly machining operations and saving metal. Die-casting requires extreme pressures exerted on the liquid metal and large amounts of heat are released from the molten metals as they change state. Massive dies are required to maintain dimensional tolerances within the limits making such operations economically attractive and to provide the strength to withstand the stresses resulting from high pressures and forces. The die-casting molds for such large automotive parts as automatic transmission housings are, for example, frequently seven to eight feet (2.1-2.5 meters) tall, seven to eight feet (2.1-2.5 meters) wide, and six to seven feet (1.8-2.1 meters) thick when closed, and must be manufactured from high-grade, high-tensile strength steel. (The words “mold” and “die” are used interchangeably herein.)
Such molds frequently include one stationary element, one movable element operated by the die-casting machine to close the mold, and several slideable elements referred to as “slides,” that move transversely of the direction of movement of the die-casting machine to provide a mold cavity with intricate and re-entrant surface configurations. The mold slides, which slide transversely of the direction of movement of the die-casting machine, are generally moved by hydraulic cylinders to their proper positions.
Die-casting has become desirable as a manufacturing method for parts such as automobile engine blocks and transmission housings because it can produce intricately shaped parts to close tolerances. Thus, die-casting can provide such parts with strength and intricately shaped surfaces without extensive and expensive machining operations. Such parts have wall thicknesses designed to take advantage of the economy of die-casting operations. Misalignment of the mold parts due, for example, to warping of the mold, misalignment of the mold on the molding machine, or non-parallelism in the molding machine platen surfaces or their direction of the movement, can vary wall thicknesses and distort part surface dimensions to unacceptable limits and result in a substantial waste of die-cast parts. In addition, the inclusion of voids within the walls of a casting can create stress concentration sites and can provide undesirably thin areas of the casting's walls. The detection of voids in casting walls is difficult, and failure to detect poorly cast parts before machining can result in further waste.
In die-casting operations, high pressure is needed to fill quickly the intricate cavities of die-casting molds and to avoid premature solidification of the molten metal as the die cavity is being filled. Die-casting machines typically include a “shot tube” connected to a stationary die element so its central cylindrical cavity is in communication with the mold cavity. Molten metal is introduced into the central cavity of the shot tube through a pour hole and is forced into the mold cavity by a piston-driven tip or plunger, (referred to as a “shot tip”) that is reciprocally moved in the shot tube cavity. In filling the mold cavity pressures of up to 5500 to 20,000 psi (386-1400 kg/cm
2
) are exerted by the piston on the molten metal in the shot tube and the mold cavity in each “shot.”
In a typical die-casting operation, the shot tube is only partially filled with the volume of metal corresponding to the volume of the die cavity. A shot with a partially filled shot tube is called an “open shot” During an open shot, a wave forms in front of the shot tip as it advances. This wave can entrap air bubbles with the molten metal, ultimately resulting in the formation of voids within the casting. Accordingly the shot portion of the die-casting operation, shot tubes and their operations have been the subject of extensive study and development. Examples of such efforts are disclosed in U.S. Pat. No. 5,601,136 and Japanese Patent Publication Nos. 58-148066, 59-921157, 62-101360 and 63-188465, and Chapter 5, Plunger Velocity & Force, of Die Casting Process Engineering and Control, published by The North American Die Casting Association of Rosemont, Ill., 1991. Japanese Patent Publication 63-188465 discloses one attempt to reduce the inclusion of air from the shot tube by adding, to the shot tube, a slot extending from the pour hole in the direction of the die to act as an air vent and reduce the air forced into the die cavity during the shot, but such slots weaken the ability of the shot tube to withstand the high internal pressures exerted on the molten metal and can lead to structural failure of the shot tube and provide an extended avenue for the escape of molten metal as the shot tube tip is advancing, both of which can provide unsafe operating conditions.
To produce higher quality castings, “closed shot” assemblies have been developed. A closed shot tube has a volume corresponding to the volume of the die cavity. Consequently, the sleeve is completely filled with molten metal and the pour hole is closed before the plunger advances. Such closed shot assemblies require complex moving part assemblies that are exposed to the molten metal and extreme pressures and are not preferred in the die-casting industry.
It is believed that none of the prior developments of the shot portion of a die-casting operation and shot tubes and their operation have addressed the combined effects of heat transfer, wave formations, and air within a shot tube during the injection of molten metal into the die cavity. A need continues to exist for a die-casting method and apparatus, which can be operated reliably to substantially reduce or substantially eliminate voids within die cast parts.
BRIEF STATEMENT OF THE INVENTION
The invention provides an improved apparatus and method for injecting molten metal into the die cavity and rests on the belief that air voids within a die casting may be minimized by controlling, during the shot process, the venting of gas from the shot tube and the movement of metal within the shot tube to avoid entrapped gas and prematurely solidified aluminum particles in the molten metal being injected into the die cavity during the shot process.
In the invention a shot tube for use with open shots in injecting molten metal into a die cavity is provided with a vent opening having a diameter D
V
, preferably from about 18% to about 27% of the diameter of the shot tube, located downstream of the pour opening, and upstream of the distal end of the shot tube a distance L
V
, which is substantially equal to V
M
and divided by V
T
times L
T
, where V
M
is the volume of the metal poured into the shot tube, which is somewhat greater than the total volume of the die cavity, and V
T
and L
T
are, respectively, the total volume of the shot tube and the length of the shot tube between the shot tip and the distal end of the shot tube (the shot stroke length), and the shot tip is accelerated through the shot tube at a rate maintaining a non-turbulent rising wave of metal in the shot tube until the shot tip is adjacent the vent opening, and is thereafter accelerated to fill the cavity as quickly as possible. Upon filling the cavity, very high pressures are applied to the shot tip by the intensifier to compress the molten metal in the die cavity. In addition, a vacuum can be applied to the die cavity after the shot tip is adjacent the vent opening.
A die casting apparatus of the invention can comprise a die having a part-forming die cavity with a volume V
M
, a shot tube having a central bor
Prince Greg A.
Richey Harry
Smith Richard L.
Brinks Hofer Gilson & Lione
Delaware Machinery and Tool Co., Inc.
Lin Kuang Y.
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