The design of the pouring system of cast steel valves includes determining the type of pouring system, calculating the dimensions of each part, designing the shape and layout, etc. The following are the specific contents:
Determine the type of pouring system
Scope of application of the bottom pouring pouring system
Complex shape valves: For cast steel valves with complex structures, many thin parts or deep cavities, the bottom pouring pouring system can make the molten metal fill the mold smoothly, reduce the scouring of the cavity, avoid defects such as sand scouring and sand inclusion, and help to ensure the integrity and surface quality of the casting.
Valves made of easily oxidizable materials: Cast steel materials are easily oxidized at high temperatures. Bottom pouring can make the molten metal rise steadily from bottom to top in the cavity, with a small contact area with air, which can effectively reduce the oxidation of the molten metal. It is suitable for cast steel valves with high oxidation sensitivity.
Large valves: Large cast steel valves are heavy and large in size. The bottom pouring pouring system can avoid the splashing and impact caused by the molten metal falling from a high place, which is beneficial to improve the intrinsic quality of the casting.
Precautions
Reasonably set the ingates: The number, position and size of the ingates should be reasonably determined according to the structure of the casting and the distribution of the heat nodes to ensure that the molten metal flows into the cavity evenly and smoothly, avoiding local overheating and defects such as shrinkage cavities and shrinkage.
Control the pouring speed: The filling speed of the bottom pouring system is relatively slow. The pouring speed should be reasonably controlled according to the size and complexity of the casting, so as to ensure that the molten metal can fill the cavity smoothly and avoid premature solidification of the molten metal in the pouring channel due to too slow pouring speed.
Strengthen the exhaust measures: Although the bottom pouring is conducive to gas discharge, since the molten metal is filled from the bottom, the gas in the cavity still needs to be discharged through a reasonably set exhaust channel, otherwise it may cause defects such as pores and gas marks. Exhaust holes or exhaust grooves can be set at high places and gas storage areas of the casting.
Pay attention to the riser design: The bottom pouring system is not conducive to the sequential solidification of the casting and the shrinkage compensation of the riser, so the riser needs to be carefully designed to ensure that the riser can effectively compensate for the shrinkage of the casting. Measures such as increasing the size of the riser, optimizing the position of the riser or using subsidies can be adopted to ensure that there is enough molten metal to supplement the casting during the solidification process.
Scope of application of the top-injection pouring system
Simple structure valves: For cast steel valves with simple shapes and small heights, such as some straight-through ball valves and gate valves, the top-injection pouring system can quickly fill the mold and effectively utilize the gravity of the molten metal to quickly fill the mold cavity and improve production efficiency.
Thick and large castings: When the wall thickness of the cast steel valve is large or it belongs to thick and large castings, the top-injection type is conducive to the sequential solidification of the casting from bottom to top, so that the shrinkage compensation effect of the riser is better, which can effectively reduce defects such as shrinkage cavities and shrinkage, and ensure the density of the casting.
Castings with low surface quality requirements: If the surface quality requirements of the cast steel valve are relatively low, and the main focus is on internal quality and mechanical properties, the top-injection pouring system can reduce the oxidation time of the molten metal in the mold cavity to a certain extent due to its fast filling speed, which is conducive to obtaining better internal quality.
Precautions
Prevent sand blasting: Since the molten metal is poured in from the top, it has a greater impact on the upper surface and wall of the cavity, which can easily cause sand blasting defects. Therefore, during molding, the compactness of the upper surface and the impacted parts of the cavity should be strengthened. If necessary, special molding materials or reinforcement measures can be used, such as using high-strength surface sand and setting reinforcement ribs.
Control the pouring height: If the pouring height is too high, the impact force of the molten metal will be too large, aggravating the scouring and oxidation of the cavity; if the pouring height is too low, it may cause uneven filling. Generally, the pouring height should be reasonably controlled according to the size and shape of the casting, and it should not exceed 1m.
Optimize the design of the ingates: The shape, size and number of the ingates should be reasonably designed so that the molten metal can be evenly distributed in the cavity to avoid local overheating and excessive concentration of the molten metal. Distributed ingates can be used to increase the number of ingates and reduce the cross-sectional area of each ingate to reduce the flow rate and impact force of the molten metal.
Pay attention to exhaust: During top pouring, the molten metal fills the mold from top to bottom, which easily wraps the gas in the mold cavity inside the casting, forming defects such as pores. Therefore, it is necessary to set up the exhaust channel reasonably to ensure that the gas in the mold cavity can be discharged smoothly. Exhaust holes or exhaust grooves can be set on the top and side of the casting, and the diameter of the exhaust hole is generally 3-5mm.
Scope of application of side injection pouring system
Medium complexity valve: For cast steel valves whose structure is neither particularly simple nor too complex, such as some stop valves and check valves with certain bosses and ribs, the side injection pouring system can better take into account the requirements of filling stability and shrinkage compensation, and can make the molten metal flow into the mold cavity more evenly, reducing turbulence and impact during the filling process.
Valves with multiple wall thicknesses: When the cast steel valve has multiple wall thicknesses and the wall thickness difference is not particularly large, the side injection type is conducive to controlling the flow direction and temperature distribution of the molten metal, so that different wall thickness parts can solidify more evenly, which helps to prevent defects such as shrinkage cavities and shrinkage from occurring at the wall thickness transition.
Valves with certain requirements for surface quality: This pouring system has relatively little scouring on the cavity, which can guarantee the surface quality of the casting to a certain extent. It is suitable for cast steel valves with certain requirements for surface roughness and flatness, such as some valves that require high-precision processing.
Matters needing attention
Selection of the position of the ingode: The position of the ingode on the side should be accurately selected according to the structural characteristics and heat node distribution of the casting. Generally, the ingode should be avoided from facing the core or thin-walled part to prevent the direct impact of the molten metal from causing defects such as sand holes and erosion. At the same time, the ingode should be as close as possible to the thick and large part of the casting to facilitate shrinkage compensation.
Preventing air entrainment: Although the side injection filling is relatively stable, gas may still be entrained in the process of the molten metal flowing into the cavity. Therefore, it is necessary to control the angle of the ingode and the flow rate of the molten metal so that the molten metal flows into the cavity at a suitable angle and speed to reduce the possibility of gas entrainment. If necessary, a slag retaining wall or a flow stabilizing device can be set in the cavity to stabilize the molten metal flow and avoid air entrainment.
Consider the influence of the parting surface: Since the side-injection pouring system is closely related to the parting surface, the position and shape of the parting surface should be fully considered during design. The ingates should be set on the parting surface as much as possible to facilitate shaping and cleaning, and at the same time, the ingates should be prevented from being cut by the parting surface and affecting the flow of the molten metal. If the parting surface is a curved surface or an irregular shape, the shape and layout of the ingates should be reasonably adjusted to ensure that the molten metal can flow smoothly into the cavity.
Balance filling and shrinkage: It is necessary to balance the needs of filling and shrinkage by reasonably designing the size, number and distribution of the ingates, and matching appropriate risers. The shrinkage effect should not be ignored because of the excessive pursuit of smooth filling, nor should the shrinkage effect be emphasized, resulting in unsmooth filling. It is usually necessary to optimize the design of the pouring system through simulation analysis or process testing to achieve the best filling and shrinkage effects.
Stepped gating system: Multiple layers of ingates are opened at different heights of the casting to allow the molten metal to fill the mold in layers and smoothly, which can effectively avoid excessive oxidation of the molten metal and facilitate sequential solidification and shrinkage compensation. It is often used for cast steel valves with large heights and complex structures.
Calculate the size of the gating system
Determine based on empirical data: For some common types and sizes of cast steel valves, the size of each part of the gating system can be determined by referring to the empirical data in the relevant manual. For example, the sprue diameter of a small cast steel valve may be 15-20mm, and the ingate width is 5-10mm; the sprue diameter of a medium-sized cast steel valve is 20-30mm, and the ingate width is 10-15mm, etc.
Design the shape and layout of the gating system
Shape design: Each part of the gating system should be designed as smooth and streamlined as possible, avoiding sharp turns and cross-sectional changes to reduce the flow resistance and turbulence of the molten metal. The sprue is usually designed as a cone with a large top and a small bottom to facilitate the smooth flow of molten metal; the runner and ingates are mostly trapezoidal or semicircular in cross section to facilitate the flow of molten metal and slag blocking.
Layout design: The layout of the pouring system should be determined according to the structural shape, size and parting surface of the casting. The opening position of the ingates should avoid the molten metal directly impacting the core and the cavity wall, and try to make the molten metal flow into the cavity evenly. For complex cast steel valves, multiple ingates may be required to ensure uniform filling. At the same time, the position and direction of the sprue and ingates should be reasonably set to make the layout of the entire pouring system compact and reasonable, which is convenient for molding and pouring operations.
