1. Function ofexhaust system
(1) The exhaust system ofdischarges the medium-sized cavity of sand mold, sand core and various gases precipitated by liquid metal.
(2) Reduce the gas pressure in the mold cavity during mold filling and improve the mold filling ability of liquid metal.
(3) The exhaust system of sand casting discharges the low-temperature liquid metal and scum that first filled the mold cavity.
(4) It is convenient to observe the state and filling degree of metal filled cavity.
2. Classification of exhaust system for sand casting
There are two main types of exhaust systems for sand mold casting: mold cavity exhaust and sand core exhaust; In addition, it also relates to exhaust plug exhaust in the process of molding and core making and ignition and air introduction exhaust during pouring.
Air outlet is the general name of various air outlet risers, sand mold and sand core exhaust channels in the mold cavity. It is a sheet or needle cavity set in the mold or core. The exhaust of sand mold casting cavity mainly includes air outlet, air outlet needle, air outlet sheet, exhaust filter sheet and sand particle (mold) gap; Various top side light and dark risers, blank holder risers, overflow risers, thermal insulation and heating exhaust risers, exhaust plug exhaust, ignition and air introduction, etc. The sand core exhaust mainly includes air outlet, air outlet needle, exhaust sheet, exhaust groove, exhaust rope, exhaust channel, filling combustion medium, increasing gap material, sand (core) gap and exhaust plug.
3. Design principles of sand casting exhaust system
When designing the exhaust system of sand mold casting, the principle should be that the cavity gas and sand core gas should be discharged separately, and the direct exhaust of sand mold casting body should be avoided as far as possible.
(1) The setting position of the air outlet shall not damage the feeding conditions of the sand casting. Generally, it is not suitable to set it at the hot joint and thick wall of the sand casting, so as to avoid shrinkage defects of the sand casting due to rapid cooling of the air outlet.
(2) Before designing the exhaust system of sand casting, the molding and core making method and process formula shall be reasonably selected according to the structure and quality requirements of sand casting, and the sand core with low air generation and good air permeability shall be selected to prevent air hole defects of sand casting.
(3) The air outlet hole, air outlet needle, exhaust plate and riser are generally set at the highest point of the pouring position of the sand mold casting, the last part of the mold filling liquid metal, the part with more gas generation and storage in the sand core, the “dead corner” where the gas in the mold cavity is difficult to be discharged and above the cold iron.
(4) The air outlet, air outlet needle, exhaust plate and riser shall not be directly connected with the mold cavity as far as possible. The outlet passage can be used to connect with the mold cavity (outlet air outlet) to prevent loose sand from falling into the mold cavity due to sand falling and other reasons.
(5) For large and medium-sized complex sand mold castings and core modeling by multiple sand cores, exhaust channels shall be designed on each sand core, and the main exhaust port shall be set on the appearance. The gas in the sand core shall be discharged from the inside to the main exhaust port through the exhaust channel. In order to prevent the metal liquid from blocking the exhaust hole of the sand core, the sand core, the core head and the core seat shall be sealed in place, or the core head shall be filled with sealing strip, rock wool, etc.
(6) The air outlet needle, exhaust hole and exhaust sheet are only used for exhaust, and liquid metal is not allowed to enter; The air outlet plate, air outlet needle and overflow riser not only play the role of discharging scum, but also play the role of feeding, overflow cold metal liquid and chilling.
(7) It is generally believed that for sand castings without open risers, the minimum total cross-sectional area of the root of the air outlet hole should be equal to the total cross-sectional area of the choke, so as to ensure that the air outlet hole can discharge the gas in the mold cavity smoothly. For resin sand molding castings, high-pressure wet molding parts, cylinder, hydraulic, turbocharger turbine shell and intermediate shell complex thin-walled sand castings, the total exhaust area is often 1.5 ~ 2.5 times larger than the sprue and flow blocking area, so as to make the cavity gas discharge smoothly and ensure the quality of sand castings.
(8) The direct air outlet should not be too small. If necessary, an overflow cup can be set at the root of the air outlet to discharge dirty liquid metal and prevent air holes at the root of the air outlet.
(9) The diameter (or thickness) of the root of the air outlet shall not be greater than 1 / 2 of the thickness of the casting at the setting, and the diameter of the indirect air outlet can be larger.
(10) During pouring, ignition and air introduction shall be carried out in time to discharge the gas generated in the mold cavity, reduce the air pressure in the mold cavity, and prevent defects such as air hole, fire choking and explosive sand sticking of sand castings.