Causes and prevention methods of orange peel defects on the surface of coated sand-ash cast iron
1. Characteristics of orange peel defects on coated sand-ash cast iron parts:
Appearance: The surface of the casting presents an uneven appearance similar to orange peel, with many small protrusions and depressions, which are densely distributed and visible to the naked eye. In severe cases, it will affect the overall appearance quality and dimensional accuracy of the casting.
Roughness: The surface roughness of the casting with orange peel defects is significantly increased, and it can be felt to be obviously rough when touched by hand, which is quite different from the smooth texture of the normal casting surface.
Color change: The color of the defective part may be slightly different from the normal surface of the casting, usually appearing darker or more uneven, which may be due to the change of the surface microstructure causing different light reflection.
Distribution pattern: Orange peel defects are generally distributed more evenly on the surface of the casting, but may be more serious in some parts with complex structures, large wall thickness changes, or turbulent metal liquid flow during filling.
2. Causes of orange peel defects on the surface of gray iron castings produced by coated sand
Mold aspect
Surface roughness: The mold surface roughness value is too large and not smooth, so that the casting surface cannot replicate the mold surface well during the molding process, resulting in orange peel-like unevenness.
Demolding slope: Insufficient demolding slope increases the friction between the casting and the mold surface when demolding, which may damage the casting surface and form orange peel defects.
Coated sand aspect
Particle size distribution: Uneven particle size of coated sand and excessive fine sand will lead to poor air permeability of molding sand. When pouring molten iron, gas is not discharged smoothly, resulting in orange peel-like pores on the surface of the casting.
Resin content: Too high resin content will increase the toughness of the molding sand and make the strength too high. When the casting solidifies and shrinks, the molding sand cannot give way in time, thereby generating stress on the casting surface, resulting in orange peel defects.
In terms of pouring process
Pouring temperature: If the pouring temperature is too high and the fluidity of the molten iron is too good, the scouring force of the molten iron on the molding sand will increase, resulting in a rough surface of the casting and the formation of orange peel defects.
Pouring speed: If the pouring speed is too fast, the molten iron will have serious turbulence in the mold cavity, which will involve a large amount of gas, and will also increase the scouring of the molding sand, which is easy to cause orange peel defects on the surface of the casting.
In terms of environmental factors
Humidity: If the ambient humidity is too high, the coated sand will easily absorb moisture, resulting in changes in the properties of the molding sand, such as reduced strength and poor air permeability, which will cause orange peel defects on the surface of the casting.
For the above reasons, corresponding solutions can be taken, such as optimizing the mold surface quality and demolding slope, adjusting the particle size distribution and resin content of the coated sand, controlling the appropriate pouring temperature and speed, and improving the humidity of the production environment, so as to reduce or eliminate the orange peel defects on the surface of gray iron castings.
3. How to avoid orange peel defects in coated sand gray iron castings?
Mold design and manufacturing
Optimize surface quality: Finely process and polish the mold surface to make its surface roughness reach a low level. Generally, the Ra value is required to be between 0.8μm - 1.6μm to ensure that the casting surface can well replicate the mold surface.
Reasonably design the demoulding slope: According to the structure and size of the casting, design a suitable demoulding slope, generally between 1° - 3°, to reduce the friction between the casting and the mold during demoulding and avoid scratching the casting surface.
Coated sand selection and processing
Control particle size distribution: Select coated sand with uniform particle size, generally requiring its mesh number to be between 40 - 70 mesh, and the fine sand content not exceeding 10% to ensure the air permeability and fluidity of the molding sand.
Adjust the resin content: According to the requirements of the casting and the production process, reasonably adjust the resin content in the coated sand, generally between 2.0% - 3.0%, to ensure that the molding sand has sufficient strength and good yield when the casting solidifies and shrinks.
Pouring process control
Strictly control the pouring temperature: Determine the appropriate pouring temperature according to the material and structure of the casting. For gray iron castings, the pouring temperature is generally controlled between 1350℃ and 1450℃ to avoid excessive temperature causing excessive scouring force of molten iron on the molding sand.
Precisely control the pouring speed: Use a reasonable pouring speed, generally between 0.5m/s and 1.5m/s, so that the molten iron can fill the mold smoothly, reduce turbulence and gas entrapment, and reduce the scouring of the molding sand.
Production environment management
Adjust the humidity of the environment: Control the humidity of the production environment between 40% and 60% to avoid moisture absorption by the coated sand and ensure the stability of the molding sand performance. Humidity can be controlled by installing dehumidification equipment or placing desiccant in the workshop.
Post-processing of castings
Timely cleaning and polishing: After the casting is demolded, the surface molding sand and burrs are cleaned in time, and polished, which can effectively improve the surface quality of the casting and reduce the impact of orange peel defects on the appearance of the casting.
