Casting has become an important method for producing rough castings in mechanical manufacturing and holds a significant position in industrial production. With the development of society, competition in the casting industry has become increasingly intense. How to gain a foothold in the market under such fierce competition has placed high demands on the casting industry itself, including the updating of equipment, technological advancements, and improvements in new processes. In casting production, wrinkling is a characteristic surface defect of solid-moulded cast iron parts, and it is one of the primary factors affecting casting quality and hindering cast iron production.
Wrinkling Defects and Their Formation Mechanism
Generally, wrinkling defects are commonly found on the upper part of the casting, in dead corners, or on the vertical surfaces of thin-walled castings with wall thicknesses less than 15 mm. Based on appearance, there are four main types of wrinkling defects: branched, cold-cut, droplet-like, and slag-included. Among these, branched wrinkling is shallower, while cold-cut, droplet-like, and slag-included wrinkling are deeper. The surfaces of such defects are often covered with lightweight, shiny carbon flakes, and the recesses of the defects are filled with sooty carbon. The presence of such defects and solid polystyrene residues severely affects the surface quality of cast iron parts. In solid mould casting, if the foam plastic fails to fully vaporise, its decomposition products can thicken the originally thin honeycomb-like membranes of the foam structure, disrupting the foam structure and forming a thick, hard crust. During the solidification of molten iron, the surface tension of residual liquid polystyrene differs from that of molten iron, causing contraction. After the metal liquid cools and solidifies, this forms discontinuous wavy wrinkles. Cold-separated and droplet-like wrinkled skin defects primarily occur at the convergence points of two or more streams of undercooled molten iron or on the upper surface where unvaporised liquid or solid polystyrene residues remain. After the casting cools and solidifies, these carbon residues become trapped on the casting's surface, forming irregular inclusions in the form of wrinkled skin defects.
Factors influencing wrinkled skin defects
Influence of pattern material: When molten metal flows over heated patterns, the gasification and decomposition of foam plastic are incomplete, leaving some material in a liquid state. Even under sufficiently high temperatures, the time required for complete gasification of the pattern material always exceeds the metal filling time. These residual liquid pattern materials may accumulate on the surface of the molten metal or adhere to the mould walls, potentially forming various casting defects under unfavourable process conditions. As such, moulding material is the primary factor causing or influencing wrinkling defects in cast iron. The fewer high-temperature decomposition products of the foam plastic in liquid (or solid) form, the lower the likelihood of defects occurring.
Alloy Influence: Production experience shows that cast steel and cast aluminium have better surface quality and no wrinkling defects; malleable cast iron has fewer defects than grey cast iron; high-grade cast iron has fewer defects than low-grade cast iron, which may be related to the carbon content of the alloy. Practice also shows that the higher the carbon content in the alloy, the more severe the defects; conversely, the lower the carbon content, the fewer the defects.
Influence of pouring temperature and speed: Practice shows that cast iron wrinkling defects decrease with an increase in metal liquid pouring temperature, while sand adhesion defects become more severe; conversely, the lower the pouring temperature, the more severe the wrinkling defects. Cast steel, cast iron, cast aluminium, and other alloys poured at different temperatures will result in castings with significantly different surface qualities. Foam plastic undergoes a series of reactions when exposed to high-temperature molten metal in the mould, the most significant being the endothermic reaction of vaporisation. This inevitably reduces the temperature and fluidity of the molten metal, affecting its ability to fill the mould. When pouring different alloys, it was observed that foam plastic undergoes different transformations between the molten metal and the mould. If the pouring temperature is close to the vaporisation point of the foam plastic, only white smoke is produced during pouring, without the formation of black decomposition products; when the temperature rises to the pouring temperature of cast iron, oily substances seep out from the sand box joints and a large amount of black smoke is produced; and when the temperature continues to rise to the pouring temperature of steel, the plastic may undergo another transformation that helps improve defects.
Increasing the pouring speed can also allow the foam plastic to absorb more heat from the molten metal during the short period of metal filling, compensating for the rapid cooling rate of solid-moulded castings, while also accelerating the vaporisation rate of the foam plastic mould.
Influence of the pouring system and pouring position: An improperly designed pouring system or an inappropriate pouring position may cause shrinkage cavities and porosity. Inappropriate selection of the dimensional parameters of the pouring channels in the pouring system may also lead to such defects. When the internal pouring channel is located at the thickest part of the casting wall, the dimensions of the internal pouring channel are selected to be thicker. After the casting is poured, the liquid flow in the internal pouring channel remains in a liquid state for an extended period. When using conventional cast iron processes (such as the rain-style pouring system) to pour solid cast iron parts, defects such as inclusions, scaling, gas holes, and wrinkling are severe, and the quality is consistently unsatisfactory.
The effect of moulding sand permeability: The vaporisation of foam plastic primarily occurs in areas directly adjacent to molten metal. Once a layer of high-temperature decomposition gas forms between the foam plastic and molten metal, the rate of foam plastic vaporisation primarily depends on the speed at which this high-temperature decomposition gas permeates the moulding sand and dissipates. Improving the permeability of the moulding sand helps increase the distance of the gas phase, enabling the gas to dissipate through the moulding sand over a larger area. Experiments also indicate that improving the permeability of moulding sand is an important factor in ensuring the quality of castings.
Influence of casting shape: Castings of different shapes and sizes have varying effects on surface quality. While this influence follows certain patterns, it does not mean that more complex shapes result in poorer surface quality or that simpler shapes guarantee better quality. Rather, the larger the ratio of surface area to volume, or the larger the upper surface area, the more likely the upper surface is to develop widespread concave defects. For cast iron components, wrinkling defects are particularly common. Even when compared to cylindrical castings of the same volume, flat plate components with simple shapes often exhibit poorer surface quality (especially on the upper surface) due to their larger surface area or higher surface area-to-volume ratio. Under identical manufacturing conditions, the surface quality of flat plate components is generally inferior to that of cylindrical castings.
