Design of shrinkage feeding system for wheel-type steel castings in lost foam ne

In numerous literature, the design of the shrinkage system for steel casting in lost foam negative pressure casting is derived from cavity casting, with few detailed descriptions and mostly just a passing mention. Although there are similarities between the two, most previous theories are not applicable to the former. How to correctly design the shrinkage system for steel castings in lost foam negative pressure casting?
1. Design of unified shrinkage compensation system for vertical casting and pouring products: The structure of the wheel consists of three parts: the rim, the spokes, and the hub.
Generally, sand casting is done in a horizontal position, with risers and chill blocks placed at the rim and hub. However, the processability of lost foam negative molds is different from that of cast iron and non-ferrous metals. The principle of the shrinkage system for wheel-type castings is: uniform pouring and risering. Place the risers on the circular arc surface of the rim, and use round lightweight insulation risers. During molding, place them upright and pour from the only riser on top. After pouring, cover the risers with at least 15% of their weight of heating agents, adding them in two stages, and spot-pour the risers 1-3 times. Fill the axial hole at the hub with resin-coated chromite sand or iron sand. If the hot spot on the rim is relatively large, an external chill block can be placed. If the hot spot on the hub exceeds 200mm, forced cooling is implemented to achieve simultaneous solidification with the spokes. After pouring, maintain a vacuum degree for more than 10-20 minutes depending on the size of the hot spot.
II. The unified shrinkage system design for vertical pouring and pouring risers has the following advantages: 1. The dry sand cavity of the lost foam mold has low thermal conductivity, which is not conducive to the formation of a dense crystalline structure in steel castings. By placing an external chill iron on the lower half of the rim and reducing the risers, the process yield can be increased to 80-85%, refining the initial grain size, and optimizing the quality of the castings.
2. The open-type quick stopper should be activated as quickly as possible without causing backflow. The combustion residue floats on the surface of the molten steel and is pushed downwards through the sprue, leaving no dead zones where slag can accumulate. This reduces slag inclusion defects at the rim and hub of the wheel.
3. The air inhaled during pouring causes most of the EPS to burn, generating CO, CO2, and a small amount of free carbon, which is then discharged from the mold cavity along with the high-temperature gas, reducing the probability of carbon buildup in the steel castings. At the same time, the reducing gas in the mold cavity prevents secondary oxidation of the molten steel due to pouring splashes.
4. EPS combustion releases a large amount of heat, which slows down the temperature drop of the molten steel. The actual pouring temperature of this process is almost the same as that of sand molds, avoiding the defects that arise from increasing the pouring temperature in general lost foam negative pressure casting.
III. Precautions in the Implementation of the Shrinkage Compensation System: 1. Do not add any flame retardant additives to the EPS used for molds.
This process is also applicable to double-width wheels.
2. To prevent secondary oxidation of molten steel more effectively, measures such as refining, purification, and combined deoxidation are implemented during the melting process. The addition amount of deoxidizer is increased by 0.05%, which should contain elements such as RE, Al, Ca, Ba, etc.
3. The combustion of EPS produces 10 times more gas than pyrolysis. To prevent backflow, the permeability of the coating should be between 40% and 60%, allowing 60% to 40% of the gas to escape through the vent.
How can we achieve such performance in the coating? Wheel sizes vary, and there is a significant difference in heat spots. The coating must also resist impact and long-term high-temperature erosion. From practical experience, we have found that: when the heat spot of the wheel rim is below 100mm, the coating should use aggregates of around 200 mesh, with a thickness of around 2mm. When the heat spot is between 100-200mm, use a combination of 180-200 mesh and 120-160 mesh aggregates, with a coating thickness of 2-6mm. When the heat spot is above 200mm, or the weight exceeds 1500kg, use a combination of 180-200 mesh, 120-160 mesh, and 60-100 mesh aggregates, with a coating thickness of over 6mm. For dry sand molding, use chromite sand or pearl sand.
4. For wheel-type castings with a diameter exceeding 1000mm, a stepped gating system shall be established separately. The ingate must be perpendicular to the workpiece and shall not be located at the hub. The uppermost ingate shall be positioned at the root of the riser, 50mm away from the workpiece. The gating system must be rigidly integrated with the casting.
5. This process involves a thicker coating. When adjusting the permeability of the coating material, alumina powder and quartz powder are used in combination with aggregates of various particle sizes. Once the coating achieves the desired permeability, the presence of as few organic compounds as possible is preferred. Most theories suggest that organic compounds in the coating can enhance high-temperature permeability. However, this only applies to coatings thinner than 2 millimeters. When the coating is thicker, under rapid pouring conditions, the organic compounds in the coating may not have time to decompose before the molten steel fills the mold cavity. If there are too many organic compounds, they can form a large number of micropores under high temperature, high pressure, and vacuum conditions. The molten steel will be drawn out of the mold cavity along with these micropores, resulting in hard-to-remove iron-coated sand.
4. For other process measures such as mass production, the following two methods can also achieve satisfactory castings.
1. Increase the amount of organic matter in the coating to enhance the dry strength of the coating. First, bake, then pour, ensuring the coating thickness is over 4mm.
2. Apply silicone coating 3-4 times, with each application followed by a layer of dry sand. The coating thickness should be greater than 20mm. After baking at 950 degrees Celsius, cast the hollow shell under negative pressure.