The difference between the gas porosity and shrinkage cavity in casting process （1）
Click here casting production, casting gas porosity, and casting shrinkage cavity are sometimes associated, sometimes in separate groups. When gas porosity and shrinkage cavity occur, we quickly determine whether the defect is a gas porosity or a shrinkage cavity is necessary to solve the problem. To quickly judge these two defects, you know their characteristics well at first. Today, we teach you how to quickly judge gas porosity and shrinkage cavity from the understanding of their characteristics.
The defects and the method of prevention and control of the gas porosity
In foundry production, hole defects are common defects, but also to the foundry caused a relatively large loss of defects. Hole defects are divided into gas porosity and shrinkage holes. Most of the gas porosity is caused by the intrusion, infusion, and involvement of the gas in the metal liquid.
Casting gas porosity appears in different positions on castings for different reasons. This requires our casting technicians in the judgment of the cause of gas porosity defects, to master the principle of various types of pores occurred, with what kind of special life. Only in this way can the disease be drugged, the gas porosity defects will appear to solve.
Below, let’s look at the gas porosity characteristics generated for different reasons.
1) Involved in gas porosity
The metal liquid in the filling process because of the injection of gas in the formation of gas holes in the casting, much-isolated existences of circular or oval atmospheric holes, the position is not fixed, generally partial casting in the upper part.
2) Incursion to the gas porosity
By the type, core, paint, core support, cold iron generated by the gas holes into the surface of the casting and form a hole, mostly pear-shaped or oval, larger size, smooth hole wall, the surface is more oxidizing color.
3) Reaction porosity
A porosity that is distributed by a chemical reaction between certain components of the metal liquid or between the metal liquid and the type and core in the interface. The needle-shaped or lumbar circular reaction porosity located on the surface of the casting are called surface needle-empty and subcutaneous porosity, which are formed by the interface reaction between the metal liquid and the type and core coating;
(1) Because of the damp burden, rust, oil pollution, and damp climate, the non-drying of the melting tools and pouring ladle, the improper composition of the liquid metal, and the inadequate refining and refining of the alloy liquid, the liquid metal contains a lot of gases or gaseous substances; Resulting in precipitation or reaction blowholes in the casting.
(2) insufficient drying of type and core, poor air permeability, poor ventilation, excessive moisture, and gas-generating substances, paint not dried or containing too many gas-generating ingredients, rust spots, grease stains or not dried on cold iron and core brace, poor venting of metal type, an invasive blowhole is formed in the casting.
(3) the unreasonable pouring system, too fast pouring and filling speed, bad venting of metal mould, which makes metal liquid entrapped with gas in the process of pouring and filling, and form entrapped gas hole in the casting.
(4) the alloy liquid is easy to be aspirated, and no effective refining, protection, and purification measures have been taken in the melting and pouring process, so that the metal liquid contains a lot of gas, slag, and gas entrapment components, in the process of filling and solidifying, there are two kinds of pores, namely, precipitation pores and reaction pores.
(5) improper preparation of molding sand, core and paint, and metal liquid interface reaction, forming surface pinholes and subcutaneous pores.
(6) the pouring temperature is too low, the mold temperature is too low, the slag removal from the liquid metal is not good, and the viscosity is too high, the gas involved in the pouring and filling process and the gas released from the liquid metal can not be discharged from the mold or floated to the riser or outlet in time.
(7) when melting the alloy which is easy to be aspirated in the humid season, the alloy liquid aspirates a great deal, causing the casting to be scrapped in batches.
(8) resin sand contains too much resin and curing agent, too much fluorine in Resin, too high angle coefficient of raw sand and reclaimed sand, too fine grain size, too high ignition loss and too high content of micro powder so that the gas emission of molding sand is too high and the air permeability is too low.