How are valves made in the sand casting process?

Sand casting is a common method used for manufacturing valves. This process is suitable for producing both simple and complex valve shapes, and it is particularly cost-effective for small to medium-sized production runs. Here&#;s an overview of the steps involved in making valves through the sand casting process:

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1.  Pattern Creation: 

   &#; The process begins with the creation of a pattern, which is a replica of the final valve. The pattern can be made from wood, metal, or other materials and is designed to create a cavity in the sand mold.

2.  Mold Preparation: 

   &#; The pattern is placed in a box, and molding sand is packed around it to create the mold. This process is known as ramming. The mold consists of two halves &#; the cope (upper) and the drag (lower).

3.  Core Production (if needed): 

   &#; Cores, which are inserts made of sand or other materials, may be used to create internal features of the valve, such as cavities or passages. Cores are positioned in the mold before the casting process.

4.  Mold Assembly: 

   &#; The cope and drag are assembled, creating a complete mold with the pattern or patterns enclosed. The mold cavity replicates the shape of the final valve.

5.  Molten Metal Pouring: 

   &#; The molten metal, often an alloy such as bronze or brass, is poured into the mold cavity through a channel called a sprue. The metal fills the space previously occupied by the pattern.

6.  Solidification: 

   &#; After pouring, the metal begins to cool and solidify within the mold. The solidification process may take some time, depending on the size and complexity of the valve.

7.  Mold Breakout: 

   &#; Once the metal has solidified, the sand mold is broken apart to reveal the rough casting. This process involves removing the cope and drag, as well as any cores, from the solidified metal.

8.  Cutting and Finishing: 

   &#; The rough casting is then cut from the sprue and gates (channels that guide the molten metal). Any excess material, called flashing, is removed. The valve may undergo additional finishing processes, such as machining, grinding, or polishing, to achieve the required dimensions and surface quality.

9.  Inspection and Quality Control: 

   &#; The finished valve is inspected to ensure it meets the specified standards and requirements. This may involve checking dimensions, surface finish, and other quality parameters.

10.  Post-Casting Heat Treatment (if needed): 

    &#; Depending on the material used and the desired properties, the valve may undergo post-casting heat treatment processes to enhance its mechanical properties.

Sand casting is a versatile process, and it is well-suited for the production of a wide range of valve types. While it may not provide the same level of precision as some other casting methods, it is cost-effective and allows for the production of larger and more complex components.

The casting of valve body is an important part of its manufacturing process. The quality of castings determines the quality of the valves. Here are several casting process methods commonly used in the valve industry:

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Sand mold casting is one of the popular methods in the valve industry. According to the different binders used, it can also be divided into: wet sand casting, dry sand casting, sodium silicate sand casting and furan resin self-hardening sand casting, etc.

1. Wet sand casting is a molding process using bentonite as a binder.
2. Dry sand casting is a molding process using clay as a binder, and a little bentonite can improve its wet strength.
3. Sodium silicate sand casting is a molding process using sodium silicate as a binder.
4. Furan resin self-hardening sand molding is a casting process with furan resin as a binder. At room temperature, the binder is chemically reacted under the action of a curing agent to solidify the sand.

In recent years, valve manufacturers have paid more and more attention to the appearance quality and dimensional accuracy of the castings. Because a good appearance is the basic requirement of the market, and it is also the positioning benchmark for the first machining process.

The most commonly used precision casting method in the valve industry is investment casting, which is briefly introduced as follows:

Two Process Methods of Investment Casting

&#; Using low-temperature wax-based mold material (stearic acid + paraffin), low pressure wax injection, water glass shell, hot water dewaxing, atmospheric melting pouring process, mainly used for carbon steel and low alloy steel castings with general quality requirements. The dimensional accuracy of castings can reach the national standard CT 7~9.

&#; Using medium temperature resin base mold material, high pressure wax injection, silica sol mold shell, steam dewaxing, rapid atmosphere or vacuum melting casting process. The dimensional accuracy of the casting can reach CT 4~6.

Features of Investment Casting

The casting has high dimensional accuracy, smooth surface and good appearance quality.

It can cast parts with complex structures and shapes that are difficult to be processed by other methods.

The casting materials are not limited to various alloy materials, such as carbon steel, stainless steel, alloy steel, aluminum alloy, high temperature alloy, and precious metals, especially those alloy materials which are difficult to be forged, welded and cut.

Good production flexibility and strong adaptability. It can be used for mass production, and also suitable for single or small batch production.

Investment casting also has certain limitations, such as cumbersome process flow and long production cycle. Due to the limited casting technology available, the pressure-bearing capacity of thin-shell valve casting can not be very high.

Analysis of Casting Defects

Any castings could get defects inside and the existence of these defects will bring great hidden dangers to the inherent quality of the castings. We can use welding repairs to eliminate these defects while it will also bring a great burden to the production process. In particular, as a thin-shell casting that bears pressure and temperature, the internal structure of the valve is very important.

Therefore, the internal defects of castings become a decisive factor affecting the quality of castings.

The internal defects of valve castings mainly include porosity, slag inclusions, shrinkage porosity and cracks, etc.

1. Porosity: It&#;s caused by gas during casting, and generated inside or near the casting surface. The shape is mostly round or oblong with smooth surface.
2. Shrinkage cavity/porosity: it is a continuous or incoherent circular or irregular cavity that is generated inside the casting, especially in its hot joint. It get rough inner surface, dark color, large metal grains, mostly dendritic crystals, and gathered in one or more places. It&#;s easy to lead leakage during hydraulic test.
3. Sand inclusion: It is commonly known as sand holes, the incoherent round or irregular holes inside the casting. The holes are mixed with molding sand or steel slag with irregular size, and gathered in one or more places, often in the upper part.
4. Cracks: Most of the cracks in castings are hot cracks with irregular shapes, penetrating or non-penetrating, continuous or intermittent. And the metal at the cracks is dark color or has surface oxidation.

By analyzing the causes of the above defects, we can find the existing problems, and take corresponding improvement measures to solve the casting defects, which is conducive to the improvement of casting quality.

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