Design of feeding system for wheel steel castings in lost mold negative pressure

In many literatures, the design of EPC negative pressure cast steel feeding system evolved from cavity casting. There is little detailed description, and most of them have been mentioned in one stroke. Although the two have similarities, most of the previous theories can not be applied to the former. How to correctly design the feeding system of EPC negative die casting steel parts?
1、 Design of unified feeding system for vertical pouring and pouring products:
The structure of wheel has three parts: rim, spoke and hub.
Generally, sand casting is laid flat, with risers and chills placed at the rim and hub. However, the processability of EPC is different from that of cast iron and non-ferrous metals. The process principle of wheel feeding system is: unified pouring and pouring. Place the riser on the circular arc surface of the rim, and use a circular lightweight thermal insulation riser. Place it vertically during modeling. Pour from the only riser above. After pouring, cover the riser with a heating agent not less than 15% of the weight of the riser, add it twice, and dot the riser for 1-3 times. The shaft hole at the hub shall be filled with resin chrome ore or iron sand. If the hot joint of the rim is relatively large, external cold iron can be placed. If the hot joint of the hub exceeds 200mm, forced cooling shall be implemented to achieve the purpose of solidification at the same time with the spoke. After pouring, the vacuum degree shall be maintained for more than 10-20 minutes according to the size of hot joint.
2、 The design advantages of the unified feeding system for vertical pouring and pouring products are as follows:
1. The dry sand cavity of EPC has low thermal conductivity, which is not conducive to the formation of dense crystalline structure of steel castings. At the same time, the grain size of the outer riser can be refined to 85%, and the quality of the casting can be improved at the same time.
2. Open type quick hold, the faster the better without reverse injection. The combustion residue floats on the surface of liquid steel and is pushed down into the riser along with the arc. There is no dead corner to stop, which reduces the slag inclusion defects at the rim and hub.
3. The air inhaled during pouring makes most of EPS burn to generate co.co2 and a small amount of free carbon, which is then discharged from the cavity with high-temperature gas, reducing the carbon increase probability of steel castings. At the same time, the reducing gas in the cavity prevents the secondary oxidation of liquid steel due to pouring splash.
4. EPS combustion releases a lot of heat, which slows down the temperature drop of liquid steel. The actual pouring temperature of this process is almost the same as that of sand mold, which avoids the defects caused by increasing the pouring temperature in general EPC negative pressure casting.
3、 Precautions during the implementation of feeding system:
1. EPS is used for appearance without any flame retardant additives.
This process is also applicable to double width wheels.
2. In order to more effectively prevent the secondary oxidation of molten steel, refining, purification, combined deoxidation and other measures shall be taken for molten steel during melting. The addition amount of deoxidizer is increased by 0.05%, which should contain elements such as re, Al, CA, Ba, etc.
3. EPS combustion is 10 times larger than pyrolysis gas production. In order to prevent backflow, the permeability of the coating should be 40% - 60%, that is, 60% - 40% of the gas escapes from the riser.
How can the coating achieve this performance? The size of wheels is different, and the heat section is very different. At the same time, the coating has to resist impact and long-term high-temperature erosion. It is concluded from practice that the hot spot of the rim is less than 100mm, the aggregate of about 200 mesh is used for the coating, and the coating is about 2mm. The hot spot is between 100-200m, with 180-200 mesh and 120-160 mesh aggregate mixed, and the coating is between 2-6mm. If the hot spot is more than 200mm or the weight is more than 1500kg, the coating shall be more than 6mm with three kinds of aggregates of 180-200 mesh, 120-160 mesh and 60-100 mesh. Chrome ore or pearl sand is used for molding dry sand.
4. For wheels with a diameter of more than 1000mm, a stepped gating system shall be set. The inner sprue shall be perpendicular to the workpiece and shall not be opened at the wheel hub. The riser root at the place where the last ingate is 50mm away from the workpiece. The gating system must be hard bonded to the casting.
5. The coating of this process is thick. When preparing the permeability of the coating, aluminum oxide powder and quartz powder shall be used to mix the aggregate of each particle size. After the coating reaches the air permeability, the less organic matter, the better. Most theories tend to: the organic matter in the coating can increase the permeability at high temperature. However, it can only be applied to coatings below 2mm. The coating is thickened. Under rapid pouring, the organic matter in the coating is not decomposed and the molten steel has filled the cavity. If there is too much organic matter, a large number of micropores will be formed under the action of high temperature, high pressure and vacuum, and the liquid steel will be drawn out of the mold cavity with the micropores to form iron clad sand that is difficult to remove.
4、 Other process measures
In case of mass production, satisfactory castings can also be obtained by the following two methods.
1. Increase the amount of organic matter in the coating, improve the dry strength of the coating, burn first and then pour, and the coating is more than 4mm.
2. Silica gel coating shall be applied for 3-4 times, and a layer of dry sand shall be sprayed once. The coating shall be greater than 20mm. After roasting at 950 ℃, the empty shell shall be buried for negative pressure pouring.