Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-05-08
2003-10-28
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S133000, C425S261000, C264S297700, C426S516000, C141S113000
Reexamination Certificate
active
06637497
ABSTRACT:
TECHNICAL FIELD OF INVENTION
This invention relates to the apparatus and processes of molding and casting, particularly to molten metal casting, for such as aluminum alloy wheels, but also to other thermally or chemically hardening liquids such as solidified foodstuff, plastics, rubber or other polymers formed in molds into solid articles for such as the automotive, aerospace industries, and food technology.
BACKGROUND AND PRIOR ART
For centuries, molds have been filled with molten metal to solidify into desired shapes. The processes have been adapted to other natural and synthetic hardening fluids ranging from chocolate to plastics, for example. The fluid or liquid may be poured under gravity from a vessel into an opening in the top of the mold. It may be pumped or flow under pressure into any area of the mold. The mold generally consists of a top and bottom section joined along a somewhat horizontal parting line. A cavity within the assembled mold corresponds to the desired solid shape. The opening for down pouring, or the “sprue”, is usually cut or molded through the upper section of the mold, with some difficulties.
As most molten metals are highly reactive at the elevated temperatures required, defects often form in proportion to the speed of the filling or the height of the falling stream, both of which increase the turbulence of fluid flow and the reactive exposure; to oxygen, for instance. The mold itself may suffer erosion or liquid penetration of the mold media. The mold may even rupture under the forces applied.
Liquids usually undergo shrinkage of volume when cooled and changed into solids. This may cause defects or less than ideal mechanical properties. Additional feed liquid must remain in contact long enough to compensate for the shrinkage.
As casting has developed from an art into a science, skilled artisan foundrymen have continued to gently fill the mold cavity. They may allow time for a protective shell to solidify against the mold wall, and then fill an additional column or riser to compensate for the on-going shrinkage. The second operation is called “topping off”. In this way, the mold is not exposed to high fluidity hot metal at the higher pressure. It is a time/temperature/pressure dependent physics issue. The additional pressure (as well as volume) of the higher riser(s) is necessarily added to penetrate the lattice-like dendritic structure of the solidifying metal, filling the micro voids of shrinkage with feed metal.
The artisan would also carefully cut or mold filling channels, being careful to streamline the flow as much as possible. These filling access channels, called “gating”, distribute the liquid throughout the mold. Bottom filling gates have long been known as being most effective for quiescent flow. Special gating techniques within the mold, such as “horn gates” (the biological name implying structure) could achieve true bottom-filling after the initial downpour through the sprue.
To control defects today, the mold is designed at great expense to accommodate fluid flow principles and to provide the risers in one filling operation. This speeds the production process but requires higher quality molds, at higher cost, capable of withstanding the early pressure of the riser height for a longer time against hot liquid (time/temperature/pressure). The molds may be of precise aggregate media formulation, or of semi-permanent material, or permanent material (i.e. metal molds). A form of “horn gate” is often incorporated. A disadvantage of the gating and risers is reduced cast yield.
Demand for low cost, high speed production has led to highly automated molding machines. Today, the speed of the molding operation can be very rapid, perhaps a mold every eight seconds (i.e., advanced, vertically parted, green sand molding at 500 molds per hour). Unfortunately, fluid dynamic calculations may recommend the mold be filled at a considerably slower rate, perhaps thirty seconds. Production speed or quality is often compromised, necessarily.
Mechanical devices to gently and consistently pour the metal have largely replaced the manual pouring of molds. Production speeds and quality are often improved. Radiant energy losses are exceedingly high whether manually or automatically poured. Spillage, spatter and runout also pollute the plant environment and create hazards. The heat, smoke, fumes and hazards have long made the foundry an icon of harsh industrial conditions. Filling devices that contact the mold have been proposed with the potential of substantial energy savings and quality benefits. However, these necessitate stopping or slowing the automated mold movement with loss of production speed.
Demand for increased quality in castings has led to advanced molding techniques such as low pressure permanent molding, lost foam molding and ceramic investment molding. These and other high quality systems are notoriously slow processes.
Perhaps the best combination of quality and productivity (certainly the most commercially successful) was described by the specification of U.K. Patent No. 848604 also known as the DISA ®process. This is a metal casting apparatus in which green sand mold halves are arranged one behind the other, providing a succession of molds with primarily vertical parting lines. This was revolutionary. The molds are conveyed or pushed in a tightly booked line through a gravity pouring zone and sequentially filled. The sprue is molded without difficulty along the vertical parting line.
In rare instances, movable ladles or launders have been indexed to the vertical mold's movements. This enables more suitable pouring speeds that are longer than mold cycle times, further enhancing quality and productivity. These techniques were short-lived, however, as vertical molding cycle times continued to decrease with innovations in programmable controllers and the hydraulic and pneumatic valves and cylinders of the sand compaction equipment.
In a modification of the DISA process, described in the specification of U.K. Patent No. 1,357,410, the molds are bottom filled. The velocity and pressure of the liquid metal cannot be controlled, however, to the extent required for casting of light metal alloys, such as aluminum. Vertical molding has not been widely applied to light alloys for other reasons also, discussed below. This is unfortunate for the automotive and aerospace industries. High integrity aluminum castings are critically needed. Much of the huge demand (for instance: automotive alloy wheels) has been met by low pressure permanent molding (LPPM) at slow speed and high operational cost.
The foam molding casting method has high capability for aluminum casting. It comprises embedding a pattern of foam plastics material (i.e. expanded polystyrene) or other replaceable material in loose sand. The process is quite slow compared to vertical green sand molding.
The foam molding technique also suffers from the disadvantage of sporadic filling defects. Further attempts to provide a method of casting whereby this problem is reduced as disclosed in the specification of U.S. Pat. No. 4,693,292, which comprises the step of feeding molten metal generally upwardly against the force of gravity. This is again a form of the ancient artisan's prior art “horn gate” which was called a “riser tube” (not to be confused with the traditional elevated shrinkage “riser”). These and other so-called “counter gravity processes” are exceedingly slow by waiting for solidification before the next operation. The mold must stay connected to a metal source for a time sufficient for the casting(s) within to at least become self-supporting. For high rates of productivity, multiple casting stations and sets of expensive molds are necessary.
The desired direction of solidification is always toward a source of liquid feed. In bottom filling, this is initially from the coldest liquid metal at the top of the mold towards the hot metal at the bottom. Natural convection within the mold, however, attempts to move the hot metal to the top of the mold over a period of time. This changes the
Cook, Esq. Dennis L.
Fowler White Boggs Banker P.A.
Lin Kuang Y.
LandOfFree
Automotive and aerospace materials in a continuous,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Automotive and aerospace materials in a continuous,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Automotive and aerospace materials in a continuous,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3159151