The casting of the valve body is a crucial step in the valve manufacturing process, and the quality of the valve casting directly determines the quality of the valve itself. Below are some commonly used casting processes in the valve industry:
Sand casting commonly used in the valve industry can be further divided according to the binder: wet sand, dry sand, water glass sand, and furan resin self-hardening sand, etc.
(1) Wet sand casting is a molding process using bentonite as a binder.
Its characteristics are: the finished sand mold does not require drying or hardening treatment; the sand mold has a certain wet strength; the sand core and shell have good collapsibility, facilitating the cleaning and removal of sand from the casting. It boasts high molding efficiency, a short production cycle, low material costs, and is easy to organize into assembly line production.
(2) Dry sand casting is a molding process using clay as a binder; adding a small amount of bentonite can improve its wet strength.
Its characteristics are: the sand mold needs to be dried; it has good permeability; it is less prone to defects such as sand erosion, sand adhesion, and porosity; and the internal quality of the casting is relatively good.
(3) Water glass sand is a molding process using water glass as a binder. Its characteristics include: water glass has an automatic hardening function upon contact with CO2, offering advantages similar to gas-hardening molding and core making. However, it suffers from poor shell collapse, difficulty in cleaning castings, and low recycling rate of old sand.
Furan resin self-hardening sand mixing process: Resin self-hardening sand is best prepared using a continuous sand mixer. Raw sand, resin, and hardener are added sequentially and mixed rapidly, allowing for immediate preparation and use.
The order of adding various raw materials during resin sand mixing is as follows:
Raw sand + hardener (p-toluenesulfonic acid aqueous solution) - (120～180S) - Resin + silane- (60～90S) - Sand discharge
In recent years, valve manufacturers have increasingly focused on the appearance quality and dimensional accuracy of castings. A good appearance is a basic market requirement and serves as a positioning benchmark for the first machining process.
Investment casting is a commonly used precision casting method in the valve industry. A brief introduction is as follows:
(1) Two process methods of investment casting:
① Using low-temperature wax-based molding compound (stearic acid + paraffin), low-pressure wax injection, water glass mold shell, hot water dewaxing, and atmospheric melting and casting, this method is mainly used for carbon steel and low alloy steel castings with general quality requirements. The dimensional accuracy of the castings can reach the national standard CT7-9 grade.
② Using medium-temperature resin-based molding compound, high-pressure wax injection, silica sol mold shell, steam dewaxing, and rapid atmospheric or vacuum melting and casting, the dimensional accuracy of the castings can reach CT4-6 grade.
(2) Characteristics of investment casting:
① High dimensional accuracy, smooth surface, and good appearance quality.
② It can cast parts with complex structures and shapes that are difficult to machine using other processes.
③ There are no restrictions on casting materials; various alloy materials such as carbon steel, stainless steel, alloy steel, aluminum alloys, high-temperature alloys, and precious metals are acceptable, especially alloy materials that are difficult to forge, weld, and machine.
④ It offers good production flexibility and adaptability. It can be used for mass production as well as single-piece or small-batch production.
⑤ Investment casting also has certain limitations, such as a complex process and long production cycle. Due to the limited casting techniques available, its pressure-bearing capacity cannot be very high when used for casting pressure-bearing thin-shell valve castings.
