Valve Casting Manufacturer

This section introduces targeted process measures taken in the design of the casting and feeding system for valve castings based on their structural and usage characteristics, with the aim of reducing defects such as looseness and shrinkage holes and improving the internal quality of the castings. Different casting system design schemes and empirical formulas for determining the dimensions of each part of the casting system are proposed based on the structural and dimensional characteristics of butterfly valve bodies, ball valve bodies, and gate valve bodies.

Valve castings must undergo pressure testing, and the requirements for their working and usage states determine that they cannot have looseness or shrinkage holes and must have good compactness. Therefore, the rationality of the casting and feeding system design is crucial.

1.1 Feeding and Shrinkage Form of Butterfly Valve Body

Butterfly valves account for a large share of valve castings and are difficult to cast. Figure 6-11 shows a tree diagram of a butterfly valve assembly, and Figure 6-12 shows a tree diagram of an irregular butterfly valve assembly.

There are two long holes with a diameter of 25mm at both ends of the valve body (which need to be processed). With the user’s consent, the long hole with the necked end is not cast, and a manifold feeding and shrinkage system is adopted. The top injection vertical pouring is used, and the inner pouring channel is designed as a square with a size of 65mm × 65mm. The diameter of the riser is D_R = 1.3D, so the top diameter is 110mm. The height dimension is calculated according to the Lushi Yang formula, H_R = D_R（1+0.2h)/D= 135mm, and the cone angle is 58°0.

Since the casting and feeding system has sufficient reserve metal liquid to supply the casting for feeding and shrinkage, there is no looseness or shrinkage holes in the neck, and the process yield is high.

Figure 6-12 shows a tree diagram of an irregular butterfly valve assembly, which is basically similar to Figure 6-11. The only difference is that a cube and a rectangular parallelepiped are connected to the outer circle of the valve body at both ends, with not too large dimensions. Obviously, the hot spots are located at the wall thickness of the valve body and the centroid of the geometry. The equivalent thermal node diameter is about 31mm. The simplest top riser is used, and the riser is replaced by the inner pouring channel. The top of the riser is directly connected to the transverse pouring channel, and the diameter of the riser is calculated as DR = 1.3b. The height dimension of the riser is as follows:

 

• h_R= DR[（1+0.1h)∕D-d)]

Figure 6-13 is a tree diagram of a bottom-pouring butterfly valve assembly. It is based on Figures 6-11 and 6-12 and is designed to meet the requirements of casting two long holes with a diameter of 25mm, as well as the presence of a natural shrinkage section in the casting. The structural design of a local feeding and shrinkage system using a bottom-pouring vertical pouring channel (also known as a split binary riser feeding and shrinkage system) is adopted. The molten metal flows through the straight pouring channel to the transverse pouring channel and then to the inner pouring port from the bottom of the mold cavity, gradually flowing upward with smooth flow, thus avoiding the formation of pores and slag inclusions, ensuring smooth exhaust, and providing sufficient molten metal for feeding and shrinkage. Moreover, the amount of molten metal used in the straight and transverse pouring channels is small (only as a flow channel for molten metal pouring and feeding and shrinkage to ensure smooth feeding and shrinkage during the solidification process of the casting), resulting in high yield and process yield of the castings.

Figure-6-11：Valve wax mold diagram

The process parameters are as follows:

Inner gating thickness： h2=1.2D1=l.2X25=30mm

Inner gating width： b=h2

Transverse gating length： 115mm

Transverse gating thickness： h3=2/3h2

Riser height： h4=15mm

Sprue diameter： D=2.5D1=62.5mm

Sprue cone angle： 15°

Sprue neck height： h1=D1

Sprue height： h=3D1=75mm

The shape of the sprue neck is shown in Figure 6-13 A-A section.

1.2 Gating and feeding system for large-scale ball valve body casting

The valve body is generally sand cast, while some customers require precision casting. Figure 6-14 shows the tree diagram of the ball valve body. There are two different shapes in Figure 6-14. One has a protruding step inside the diameter, and the other does not have a step. Regardless of whether there is a step inside the diameter, in terms of casting method, this type of casting should avoid horizontal pouring and should be poured vertically. As for the feeding method, it should avoid central direct gating and vertical gating, but transverse gating and runner feeding are appropriate. The above-mentioned casting and feeding design principles will be explained through the analysis of the following casting structures.

Firstly, from the sectional view in Figure 6-14, it can be observed that the component has a structure dominated by two flange bodies connected by a cylindrical section. For flange components, the most common and effective casting method is used.

The feeding and shrinking system is a top-gated transverse gating system that utilizes the gravity of the cylindrical cavity to generate a large static pressure head, providing sufficient molten metal for feeding and reducing the self-consumption of the riser metal. In Figure 6-14, the dimensions of the inner gating connecting the flange bodies are 100mm in diameter and 190mm in height. It is evident that the inner gating has a large cross-section and a high cooling modulus, which is necessary to accommodate the solidification shrinkage of the large casting.

Secondly, the necks at both ends of the valve body are connected to two small flanges. To address the feeding and shrinking issues of the neck and small flanges, two additional risers are added to form multiple sets of feeding and shrinking patterns. Views A and B represent different riser sizes. The molten metal from the two risers is introduced through the transverse gating system, forming a “person”-shaped runner, which also supports smooth investment removal, wax drainage, and venting during casting.

Valve bodies with protrusions inside the diameter often develop shrinkage cavities on the flat surfaces or intersections with a width of 14mm. This is due to the presence of thermal nodes, as indicated by the circle “De” in the sectional view of Figure 6-14. The diameter (CDc) of the internal tangent circle within this thermal node is approximately 22mm. Referring to the formulas for calculating the sprue diameter (D) and sprue height (h) based on the diameter of the internal tangent circle.

Figure 6-14 Ball valve body wax mold

D=(2.2~ 2. 5)Dc

h=(3~ 3.S )Dc 

Design a hidden riser with dimensions of 80mm X 60mm X 60mm. It is welded to the lower part of the thermal node and connected to the bottom plane of the transverse gating system. This form is adopted to eliminate shrinkage cavities and shrinkage defects in the casting.

The dimensions of the inner gating connecting the flange bodies for such castings are determined based on production experience. If the flange diameter is greater than 500mm, the width of the inner gating is 1/6 to 1/5 of the flange diameter, and the height of the inner gating is 1/3 to 1/4 of the flange diameter. If the flange diameter is less than 200mm, the width of the inner gating is 1.5/5 to 2/5 of the flange diameter, and the height of the inner gating is 1/4 to 1/5 of the flange diameter.

1.3 Pouring and shrinking system of gate valve body

The pouring and shrinking system of the gate valve body is basically the same as that of the ball valve and butterfly valve, but two issues are emphasized here.

First, regarding the production of the transverse gating system, many manufacturers make the transverse gating system in a rectangular flat-bottomed mode. In Figure 6-15, the shape of the transverse gating system has been improved, with the middle part of the transverse gating system made into an arc shape and the ends being square, allowing the molten metal to flow smoothly and concentrate during pouring.

Secondly, there is a rectangular protrusion at the top of the gate valve body [see the half-sectional view in Figure 6-15(a)], which significantly increases the local wall thickness. This is used to hide the riser for feeding and shrinkage under the arc of the transverse gating system, and when the shell is formed, the coating connects them to form a hidden riser, which is also convenient for cutting the riser.

Figure 6-16 shows the assembly tree diagram of the valve cover casting, and the transverse gating system also adopts an arc-shaped design. This not only provides good feeding and solidification effects but also improves the production efficiency and yield of the process.

1.4 Comparison of two valve plate feeding and solidification methods.

The gate plate assembly schemes can be seen in Figure 6-17 and Figure 6-18, with the same shape and dimensions. Only the feeding and solidification methods differ between the two. The feeding and solidification effect in Figure 6-17 is better than that in Figure 6-18.

 1.5 Design and calculation of risers

Correct understanding of the design and calculation methods of risers is key to solving shrinkage defects. With the development of the machinery industry, some parts with complex shapes, uneven wall thickness, and pressure requirements need to be obtained by precision casting. This has brought higher requirements for the design of the precision casting process. If the gating and riser design is not improved in time with the changes in the structure of the casting, shrinkage defects may easily occur, which will increase the scrap rate and welding repair rate of the casting. In response to this issue, the author started from the principles of metal crystallization and solidification, collected relevant technical information for analysis and research, combined the design and calculation methods of gating and risers, and achieved ideal results in actual production for reference by peers.

 Figure 6-15: Sluice valve body

Figure 6-16: Sluice valve body casting cover

Formation mechanism and solutions for shrinkage and porosity. The formation process of shrinkage and porosity is mainly affected by the heat absorption of the mold shell and the temperature difference with the outside world when the liquid metal fills the mold cavity. A hard shell solidifies on the surface of the casting and tightly encloses the liquid metal inside. With further cooling, the hard shell becomes thicker and the liquid level follows to generate liquid and solidification.

As a result, a cone-shaped shrinkage and an axial porosity are formed at the thermal section, central area, and upper part. A complete gating and riser design system must be equipped around this area. To set up a reasonable gating and riser system, we should first start from the end area of the casting, determine the direction of sequential solidification, analyze the filling and non-filling areas, and then decide the position of the gating and riser. At the same time, the size of the gating and riser should be calculated according to the modulus method, and necessary allowances should be added if needed.

The compensation distance of the riser is the sum of the dense riser area and the dense end. The determination methods for the compensation distance include flat shape, square shape, step-shaped cocoon shape, circular shape, upright shape, etc., as shown in Figure 6-19 to Figure 6-21.

The modulus calculation method establishes a proportional relationship between the length of solidification and the volume V and surface area S of the casting. That is, Ma = VIS (mm), and this ratio Ma is called the solidification modulus of the casting. In process design, the modulus of the riser must be greater than the modulus of the casting at the compensating shrinkage section to ensure smooth supply of liquid metal from the riser to the casting. Considering fluid mechanics and physical chemistry theories, the ratio K between the riser modulus M_riser and the casting modulus M_casting should be greater than 1.3 to achieve better results.

The formulas for calculating the casting modulus can be found in Table 6-2. The constant proportion relationship among the casting modulus M_casting, the inner gate (riser neck) modulus M_inner, and the riser modulus M_riser is: M_casting : M_inner : M_riser = 1:1.2:1.3. In the case of high mechanical performance requirements, the proportion relationship is: M_casting : M_inner : M_riser = 1:1.3:1.5. By using the formulas, suitable dimensions for the riser and riser neck can be obtained from Table 6-3 and Table 6-4.