Lost foam and sand cores stage manufacturing technology

Details Lost foam and sand cores stage manufacturing technology Lost foam and sand cores stage manufacturing technology

2000-03-15

2001-10-23

Dunn, Tom (Department: 1725)

Metal founding

Process

Shaping a forming surface

C164S028000

Reexamination Certificate

active

06305458

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a foundry tooling process and method using destructible material in casting operations for producing complex internal shapes requiring a high degree of surface finish. More particularly, the invention relates to a process for producing impellers and diffusers for centrifugal submersible pumps.
BACKGROUND OF THE INVENTION
Many cast parts require the formation of voids therein. An example of such castings includes impellers of pump stages in centrifugal submersible pumps.
One method for forming such castings is the lost foam method. In the lost foam method, consumable patterns, such as polystyrene foam plastic, are coated with a permeable refractory wash and then embedded in foundry sand to be replaced by molten metal. The molten metal vaporizes the foam-type pattern and a metal duplicate of the pattern is the result. The lost foam method is advantageous because it allows for net shape capacity, i.e., the ability to produce a finished or near-finished casting that requires little or no machining. However, the lost foam method is unsuitable for the geometric particularities of the hydraulic passages required in complex castings, such as impellers and diffusers. The lost foam process is not suitable to form some complex castings because of process shortcomings including limitations in thickness of the walls as well as major costs associated with tooling, low consistency in the molding stage, low surface finish and low compression strength. Additionally, particular types of geometries result in warping of the castings. Therefore, such castings do not achieve a near net shape.
Another method for forming castings is sand molding. Sand molding uses high strength chemically bonded sand. Sand molding is not a preferred choice to form net shape parts because of mold wall movement. Additionally, some specific requirements of sand molding may only be met by high quality foundries.
Therefore, it is desirable to develop a method that results in near net shape, that is low in cost, and that avoids the shortcomings of the lost foam process. One solution is proposed in U.S. Pat. No. 4,691,754, which teaches molding a destructible layer around a first core by placing the first core into a molding machine. Partially pre-expanded polystyrene pellets are applied to the core and fully expanded, via a steam expansion step. However, a drawback with this method is that the core is exposed to moisture during the steam expansion step. It is desirable to avoid exposing the core, which is typically hygroscopic, to moisture. If the core is exposed to moisture, molten metal will come in contact with the moisture absorbed by the core when the molten metal is poured into the core. The exposure of molten metal to moisture is undesirable.
SUMMARY OF THE INVENTION
A casting method that combines a lost foam method and a sand molding method provides desired benefits without undesirable effects. The present invention is a lost foam and sand core casting method that includes the step of forming a core of bonded granular material, preferably sand or ceramic, containing voids. The core has voids formed in it that have at least two exposed sides. A first layer of lost foam destructible material is then preformed in a separate mold, preferably by an injection process. Similarly, a second layer of lost foam destructible material is preformed.
The first foam layer is positioned on one side of the core, and the second foam layer is positioned on another side of the core. The layers are glued to the core, closing off the exposed sides of the voids formed in the core.
The assembly is attached to a down sprue and lowered in a mold cavity, which is then packed with a mold media that is preferably the same as the media of the core. The assembly is vibration compacted and/or vacuum compacted within the ceramic and/or sand media. The foam layers prevent entry of mold media into the voids. Molten metal is then delivered to the mold cavity to form the casting. The lost foam layers are gasified and replaced by the molten metal as it enters the mold. The voids in the core as well as the spaces previously occupied by the foam layers are filled with the advancing molten metal. After cooling, the cast component is removed from the mold cavity, and the material of the core, which is no longer bonded, is removed from the component.


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