In casting production, casting porosity and casting shrinkage cavity are sometimes associated, and sometimes exist independently. When there are air holes and shrinkage holes, we can quickly determine whether the defect is air holes or shrinkage holes to solve the problem is very necessary. In order to quickly identify these two defects, we must first have a good understanding of their characteristics. Today, we will teach you to quickly identify stomatal and shrinkage defects by understanding their characteristics.

Stomatal defects and prevention methods

In the casting production, the hole defects are common defects, but also one of the defects that cause great losses to the foundry. Cavitation defects are divided into stoma and shrinkage. Porosity is due to the metal liquid in the invasion, wrapped in, involved in the gas. The causes of the porosity in castings vary with the location of the porosity in castings. This requires our casting technicians to master the principle of various kinds of pores and what kind of special birth when judging the cause of stomatal defects. Only in this way can the remedy be applied, and the stomatal defects will be solved. Next, we take a look at the characteristics of the pores generated by different reasons: (1) Involved pores: the metal liquid in the filling process due to the involvement of gas and the formation of pores in the casting, mostly isolated circular or oval large pores, the position is not fixed, generally partial to the upper part of the casting. (2) Intrusion pores: the pores produced by the mold, core, paint, core support and cold iron invade the surface of the casting and form pores. They are pear shaped or oval, large in size, smooth in the hole wall, and oxidized in the surface. (3) Reaction pores: pores with group distribution formed by chemical reactions between some components in the metal liquid or between the metal liquid and the type and core on the interface. The needle-shaped or waist-round reaction pores located on the surface of the casting are called surface needle pores and subcutaneous pores, which are caused by the interface reaction between the metal liquid and the coating. Needle reaction pores scattered or clustered in the whole section of the casting or in a local area. Formation reasons (1) Due to the wet, rust, oil, wet climate, melting tools and pouring bags are not dried, improper composition of metal liquid, alloy liquid refining and refining is insufficient, so that metal liquid contains a lot of gas or gas substances, resulting in the precipitation of pores or reaction pores in the casting. (2) Type, core is not fully dried, poor air permeability, poor ventilation, excessive moisture and gas material, paint is not dried or too much gas content, cold iron, core support rust spots, oil or not dried, metal type exhaust is poor, the formation of intrusion pores in the casting. (3) The unreasonable casting system, casting and filling speed is too fast, the metal type exhaust is bad, so that the metal liquid in the casting and filling process produces turbulence, eddy current or flow break and involved in the gas, the formation of involvement in the casting air hole. (4) The alloy liquid is easy to inhale, and effective refining, protection and purification measures are not taken in the process of melting and pouring, so that the metal liquid contains a lot of gas, slag and gas components, forming precipitation pores and reaction pores in the filling and solidification process. (5) Improper preparation of molding sand, core and coating, and metal liquid interface reaction, the formation of surface pinholes and subcutaneous pores. (6) The casting temperature is too low, the metal type temperature is too low, the metal liquid slag is poor, the viscosity is too high, so that the gas involved in the casting and filling process and the gas precipitated from the metal liquid have no time to discharge the casting or float to the riser or the air outlet. (7) In the humid season, when the alloy that is easy to inhale is smelted, a large amount of alloy liquid is inhaled, resulting in batch scrap of castings. (8) The resin and curing agent of resin sand is added too much, the fluoride content of resin is too high, the angular coefficient of raw sand and recycled sand is too high, the particle size is too fine, the burning reduction and micro powder content is too high, so that the gas production of molding sand is too high, and the permeability is too low. Prevent method (1) when non-ferroalloy smelting, the charge, solvent, tools and pouring bag should be fully preheated and dried, rust and oil. The addition of multiple refurnaces should be appropriately limited. (2) To prevent excessive oxidation and suction of the metal liquid in the melting process, to deoxidize, degassing and slag, to cover the surface of the metal molten pool in the pouring package with solvent, to prevent the secondary oxidation of the metal, suction and harmful impurities back to the molten pool. When using aluminum to deoxidize cast steel and cast iron, the amount of residual aluminum should be strictly controlled. For liquid steel with serious aspiration tendency, aluminum deoxidization should be avoided as far as possible. AVD, VOD, porous plug blowing inert gas and powder spraying method can be used for refining the liquid steel outside the furnace to remove the gas and harmful impurities in the liquid steel; For nodular cast iron, desulfurization should be strengthened, the flow rate of the original soup should be reduced, under the premise of ensuring spheroidization, the addition of spheroidizing agent should be reduced as far as possible, the amount of residual magnesium in cast iron should be reduced, and the inoculation treatment should be strengthened. (3) The flow of metal liquid shall not be interrupted during casting, and the filling speed shall not be too high. The setting of casting position and pouring system shall ensure that the metal liquid can fill the cavity smoothly, and facilitate the smooth discharge of gas in the opening cavity. (4) During casting, the exhaust flow of the casting and core should be ensured, the exhaust channel should be opened in the sand core, and the gap between the core head should be filled when the mold is closed, so as not to plug the exhaust channel by drilling into the metal liquid. (5) Increase the height of the direct runner to improve the liquid static pressure of the filling metal. (6) Reduce the amount of resin and curing agent added to the resin sand, use low nitrogen or no nitrogen resin and raw sand and reclaimed sand with rounded shape material, moderate particle size, burning reduction and low micro-powder content, in order to reduce the gas output of the resin and improve the permeability of the resin sand

Shrinkage cavity defects and prevention methods

And the casting shrinkage cavity, shrinkage loose, there are many reasons, there are casting and shape design reasons, there are inappropriate sand box design reasons, there are gating risers design reasons, there are sand type to avoid displacement reasons, in addition to the system reasons, there are improper metal chemical composition deployment reasons, there are improper smelting link operation reasons, there are pouring reasons. Because there are many reasons for shrinkage holes in castings, it often takes time to find the root cause. How to quickly determine the internal causes of casting shrinkage cavity requires us to compare casting shrinkage cavity defect cases, memorize theoretical characteristics and strengthen learning, so as to improve the understanding and solving ability of such defects.