Measures to Eliminate Cast Iron Skin Defects
Selecting suitable foam plastic for casting: There are two main ways to reduce the liquid products of polystyrene: first, improve the vaporisation conditions of the moulding material (such as increasing the pouring speed and temperature, improving the permeability of the mould, etc.) to promote its full vaporisation; Second, select low-density foam plastic as the moulding material based on the type of casting alloy, the shape of the casting, and the characteristics of the sand, to ensure minimal residue, minimal smoke, and rapid vaporisation of the foam plastic. This also helps minimise the formation of tar-like liquid residue and solid-phase decomposition products when the foam plastic comes into contact with the molten metal, thereby improving the quality of the casting.
Increasing pouring temperature and pouring speed: Reasonably selecting the pouring temperature plays a significant role in controlling casting defects. Pouring at a lower speed under high-temperature conditions facilitates top-down solidification of the casting, increasing the temperature difference between horizontal interfaces and adhering to the principle of sequential solidification. When using a stepped or slit pouring system, to achieve sequential solidification, the internal pouring channel should be introduced from the thin-walled area of the casting, while increasing the number of internal pouring channels. Adopting a low-temperature rapid pouring process reduces liquid metal shrinkage and prevents the formation of shrinkage cavities and porosity. Increasing the pouring temperature of cast iron by 20°C to 80°C and accelerating the pouring speed while ensuring smooth metal flow can compensate for heat loss caused by plastic combustion, vaporisation, and liquid flow within the mould, providing sufficient heat to ensure foam plastic vaporisation. This improves foam plastic vaporisation conditions, facilitates rapid metal filling of the mould, and promotes the escape of residues and gases.
Select an appropriate pouring position: Based on the characteristics of foam plastic mould combustion and gasification within the mould, the pouring position for solid castings should preferably use the bottom pouring method regardless of the alloy type. However, for certain cast iron parts, under suitable conditions such as appropriate shape and pouring system layout, as well as ideal mould permeability, other pouring positions may also be selected. However, it is essential to ensure smooth and rapid filling of the mould with molten metal. When setting the pouring position, the shape of the casting must also be fully considered. For large-area or tall castings, it is advisable to adopt a dispersed, multi-stage, layered pouring method to avoid excessive concentration of the pouring channels. This facilitates the smooth and rapid filling of the mould with molten metal, thereby improving casting quality.
Reasonable design of risers: If the number, position, and size of risers are not designed reasonably, they will fail to promote sequential solidification of the casting, thereby failing to eliminate shrinkage cavities and porosity. When the top of a hidden riser is not equipped with a vent riser, gases may cause shrinkage cavities and porosity in the casting. Cold irons are often used in casting design, and their positioning is also critical. Improper positioning can disrupt solidification sequence and cause shrinkage cavities and porosity in localized areas.
Improving the permeability of the mould: A mould with good permeability is a necessary condition for ensuring the production of high-quality solid castings. Practice has shown that there are three main ways to improve the permeability of the mould: improving the permeability of the moulding sand, increasing the number of vent holes or vent risers, and setting up several external ventilation channels inside the mould according to the shape of the pattern. Due to the mould's good permeability, sufficient gaps are maintained between the molten metal and the foam plastic during pouring, thereby improving the gasification conditions and rate of the foam plastic mould, facilitating the escape or penetration of its high-temperature decomposition products into the mould sand.
Improving the surface layer of the foam plastic: Spraying a layer of nitrocellulose or other coatings that vaporise more rapidly than plastic onto the surface of the foam plastic mould not only improves the surface quality of the pattern but also aids in the vaporisation of the foam plastic and prevents the formation of high-temperature decomposition products. Improving the flowability of the molten metal is beneficial for enhancing the quality of the casting and eliminating defects.
Selecting reasonable process layout and pattern structure: For large, thick-walled patterns, hollow structures and internal/external ventilation channels within the pattern or mould can be used to improve the gasification conditions of the foam plastic mould. A serial moulding method can also be adopted to concentrate defects at the top of a single casting, ensuring the quality of the remaining castings.
In summary, the factors contributing to wrinkling defects in cast iron parts are multifaceted and cannot be resolved by simply altering a single material or process. Instead, it is essential to comprehensively consider the influence of all the aforementioned factors, develop the optimal process, and ensure the production of cast iron parts free of wrinkling defects.
