Reasons for the formation of brake disc air holes

　　The gas generated by the sand core of the brake disc casting disc under high temperature should be discharged horizontally outward or inward through the gap between the sand cores under normal conditions. The disc sand core becomes thinner, the gas path becomes narrow, and the flow resistance increases. One scenario is when a large amount of gas suddenly occurs when the molten iron quickly submerges the disc sand core; Or high-temperature molten iron may come into contact with a high water content sand mass (uneven sand mixing) at a certain location, causing gas explosion, causing choking fire, and forming choking pores; Another scenario is that the high-pressure gas formed invades the molten iron and escapes upwards. In the event that the mold cannot expel it in a timely manner, the gas spreads into a gas layer between the molten iron and the lower surface of the upper mold, occupying a portion of the space on the upper surface of the disc. If the molten iron is solidifying at this time, or if the viscosity is high and the fluidity is lost, the space occupied by the gas cannot be refilled, leaving surface pores. In general, the gas generated by the core cannot escape through the liquid iron in a timely manner, and it remains on the surface of the disc. Sometimes it is exposed as a single pore, sometimes it is exposed after shot blasting to remove the oxide skin, and sometimes it can only be discovered after mechanical processing, which will cause waste of processing time. When the brake disc core is thick, the time required for the molten iron to rise and submerge the disc core is longer. Before submerging, the gas generated by the core has more time to freely flow out to the upper surface of the core through the sand gap, and the resistance to flow horizontally outward or inward is also small. Therefore, surface porosity defects are rarely formed, but individual isolated pores may also occur. That is to say, there is a critical size between the thickness and thickness of the sand core that forms choking pores or surface pores. Once the thickness of the sand core is less than this critical size, there will be a relatively serious tendency towards pores. This critical size increases with the radial size of the brake disc and with the thinning of the disc core. Temperature is an important factor affecting stomata. The molten iron enters the mold cavity from the inner runner, bypasses the middle core when filling the disc, and intersects towards the opposite side of the inner runner. Due to the relatively long process, the temperature decreases more, and the viscosity also increases accordingly. The effective time for the bubbles to float up and discharge at this location is short, and the molten iron will solidify before the gas is completely discharged, making it easy for pores to occur. To this end, the effective time for bubbles to float and discharge at the opposite disc of the inner runner can be extended by increasing the temperature of the molten iron.