In many references, the design of the feeding system for lost foam vacuum cast steel is derived from cavity casting, but detailed descriptions are scarce, and most are only briefly mentioned. Although there are similarities between the two, most previous theories are not applicable to the former. So, how should the feeding system for lost foam vacuum cast steel be properly designed?

I. Design of a unified feeding system for vertical pouring with combined gating and riser:
The structure of wheel-type castings consists of three parts: the rim, the spokes, and the hub.

In general sand casting, the casting is placed horizontally, with risers and chills placed at the rim and hub. However, the process characteristics of lost foam vacuum casting differ from those of cast iron and non-ferrous metals. The process principle for the feeding system of wheel-type castings is: unified gating and riser. The riser is placed on the arc surface of the rim, using a lightweight insulating cylindrical riser. During molding, the casting is placed vertically, and pouring is done through the single riser on top. After pouring, the riser is covered with an exothermic compound weighing no less than 15% of the riser weight, added in two batches, and the riser is fed 1–3 times. The shaft hole at the hub is filled with resin-bonded chromite sand or iron sand. If the rim's thermal junction is relatively large, external chills can be placed. If the hub's thermal junction exceeds 200mm, forced cooling should be applied to achieve simultaneous solidification with the spokes. After pouring, the vacuum should be maintained for 10–20 minutes or more, depending on the size of the thermal junction.

II. The unified feeding system for vertical pouring with combined gating and riser offers the following advantages:
1. The dry sand cavity of lost foam has low thermal conductivity, which is not conducive to forming a dense crystalline structure in steel castings. By placing external chills on the lower half of the rim while also reducing the riser size, the process yield can be increased to 80–85%, refining the primary grain size and improving casting quality.
2. Open rapid filling—when back-spraying does not occur, faster filling is better. Combustion residues float on the molten steel surface and are pushed into the riser along the arc, eliminating dead zones for entrapment and reducing slag inclusion defects at the rim and hub.
3. Air drawn in during pouring causes most of the EPS to burn, generating CO, CO2, and a small amount of free carbon, which are then expelled from the cavity with the hot gases. This reduces the risk of carburization in the steel casting. Meanwhile, the reducing atmosphere in the cavity prevents secondary oxidation of the molten steel caused by splashing during pouring.
4. The combustion of EPS releases a large amount of heat, slowing the cooling of the molten steel. This process results in actual pouring temperatures similar to those in sand casting, avoiding defects commonly associated with elevated pouring temperatures in conventional lost foam vacuum casting.

III. Precautions for implementing the feeding system:
1. The pattern should be made of EPS without any flame-retardant additives. This process is also applicable to dual-wheel type castings.
2. To more effectively prevent secondary oxidation of the molten steel, measures such as refining, purifying, and combined deoxidation should be carried out during melting. The amount of deoxidizer added should be increased by 0.05%, and it should contain elements such as RE, Al, Ca, and Ba.
3. The gas volume from EPS combustion is 10 times greater than that from pyrolysis. To prevent back-spraying, the coating permeability should be controlled at 40%–60%, meaning 60%–40% of the gases escape through the riser. How can this coating performance be achieved? Since wheel types vary in size and thermal junction, the coating must also withstand impact and prolonged high-temperature erosion. Based on experience: For rim thermal junctions below 100mm, use aggregate around 200 mesh with a coating thickness of about 2mm. For thermal junctions between 100–200mm, use a combination of 180–200 mesh and 120–160 mesh aggregates, with a coating thickness of 2–6mm. For thermal junctions above 200mm or castings over 1500kg, use a combination of 180–200 mesh, 120–160 mesh, and 60–100 mesh aggregates, with a coating thickness over 6mm. The dry sand for molding should be chromite sand or ceramsite sand.
4. For wheels with diameters exceeding 1000mm, a stepped gating system should be provided, with the ingate perpendicular to the workpiece and not located at the hub. The uppermost ingate should be positioned at the base of the riser, 50mm from the workpiece. The gating system must be rigidly connected to the casting.
5. This process requires a thicker coating. To adjust coating permeability, alumina powder and quartz powder are used with each aggregate size. Once the coating achieves the required permeability, organic content should be minimized. Most theories favor organic additives to increase high-temperature permeability, but this only applies to coatings under 2mm. With thicker coatings and rapid filling, the organics may not decompose in time before the mold is filled. Excessive organics, under high temperature, pressure, and vacuum, can create numerous micropores, allowing molten steel to be drawn through the coating and causing difficult-to-remove burn-on sand.

IV. Other process measures:
For batch production, the following two methods can also produce satisfactory castings:
1. Increase the organic content in the coating to improve dry strength, pre-burn the coating before pouring, with a coating thickness over 4mm.
2. Use silica sol coating applied in 3–4 layers, each layer sprinkled with dry sand, resulting in a coating thickness over 20mm. After baking at 950°C, the shell is buried and poured under vacuum.