yvonne@nydcasting.com 
Wrinkle defects often appear at the last place where the molten metal flows or the “cold end” of the liquid flow, generally appearing at the top of the casting, vertical sidewalls or dead corners. The surface of this type of defect is often covered with light and shiny carbon flakes, and the depression of the defect is filled with soot carbon. There are four types of wrinkle defects according to appearance: corrugated, dendritic, dripping and slag inclusion.
1. Formation mechanism
Wrinkle defects are mainly caused by intense combustion of pattern material at high temperatures. Oxygen presence causes large amounts of free carbon to form during combustion. Free carbon has a much lower density than molten metal. It rises to the surface or accumulates in corners of the mold. If not discharged before solidification, it causes surface wrinkle defects.
2. Influencing factors
2.1 Foam plastic pattern
FPS forms wrinkles more easily than EPMMA and STMMA. This is because FPS has higher carbon content. FPS contains 92% carbon, STMMA has 69.6%, and EPMMA has 60.0%. Higher carbon means more residue after combustion. Denser patterns produce more liquid during decomposition. More liquid increases the chance of wrinkle defects.
2.2 Influence of chemical composition of castings
Low-carbon cast iron can dissolve some pattern carbon. Wrinkle defects are less likely to form. High-carbon cast iron, like ductile iron, forms wrinkles more easily. At pouring temperature, foam patterns produce carbon-rich gas. About 50% of pattern weight becomes cracked carbon. Ductile iron has high carbon and carbon equivalent. Extra carbon gathers on the surface or front of molten iron. These deposits lead to wrinkle defects.
2.3 Influence of the pouring system
The pouring and riser system has a significant impact on the flow field and temperature field of the iron liquid filling, and directly determines the pyrolysis products of the mold material and its flow direction. After increasing the cross-sectional area of the vertical, horizontal and inner runners, wrinkles are easy to occur. The probability of wrinkles in top pouring is smaller than that in bottom pouring, so setting a top riser is conducive to reducing wrinkles.
2.4 Influence of casting structure
The smaller the ratio of the volume to the surface area of the casting, that is, the modulus, the more conducive it is to the discharge of the pattern pyrolysis products, and the less prone to wrinkle defects.
2.5 Influence of pouring temperature
Pouring temperature affects pyrolysis product types and metal filling speed. Below a critical temperature, liquid products dominate. As temperature rises, filling speed increases. Above the critical temperature, gas products dominate. Then, higher temperature causes filling speed to decrease.
2.6 Influence of coating layer and permeability of molding sand
The higher the permeability of molding sand, the more conducive it is to the discharge of pyrolysis products of the mold, and the tendency to form wrinkles is reduced. The thinner the coating, and the coarser the particle size of the coating aggregate and molding sand, the more conducive it is to exhaust and reduce wrinkle defects.
2.7 Influence of negative pressure
Increasing the pouring vacuum of cast iron is conducive to smoke and exhaust, thereby improving the gasification conditions of foam plastics, and is conducive to the rapid filling of the mold with molten metal, prompting the escape of residues and gases. However, too high a vacuum is prone to sand sticking defects.
2.8 Influence of process coordination
Depending on the size of the casting, the pouring speed should be properly controlled, and the vacuum degree should be coordinated at the same time. If the coordination is improper, when the pouring speed increases, the stream becomes thicker, and the vacuum degree is not increased accordingly, wrinkle defects often occur.
3. Preventive measures
During the entire pouring process, we strive to avoid the melting and burning of foam plastics to prevent the formation of high-temperature decomposition products of foam plastics. We hope that it will shrink in volume when it contacts the high-temperature metal, and immediately transform into gas and overflow the mold like sublimation. This is an effective way to eliminate the wrinkles of cast iron parts.
3.1 Select suitable foam plastics for casting
In view of the high carbon content and easy wrinkles of ductile iron parts, low-density foam plastic mold materials for casting, such as EPMMA copolymer materials, are selected.
3.2 Increase the pouring temperature and pouring speed
Increasing the pouring temperature can reduce the wrinkle defect. Since the temperature needs to be reduced by 50-100℃ during the spheroidizing process, in order to ensure the pouring temperature, the molten iron must have a much higher tapping temperature than ordinary gray cast iron.
Speeding up the pouring speed (based on the principle of ensuring the smooth flow of molten metal) can make up for the heat loss of foam plastic combustion and gasification in the casting mold, and have sufficient heat energy to ensure the gasification of foam plastic, thereby improving the gasification conditions of foam plastic, which is conducive to the rapid filling of the casting mold with molten metal and causing the residue and gas to overflow.
3.3 Increase the amount of air pumped and the vacuum degree of the casting mold
Practice has proved that with the increase of the negative pressure of the casting mold, it is conducive to reducing or eliminating the wrinkle defect. Because the higher the negative pressure, the faster the filling speed, the shorter the pouring time, the low-viscosity liquid phase product has no time to transform into a high-viscosity liquid phase decomposition product, and the bright carbon appears less; the higher the negative pressure, the more the pyrolysis product of the mold is sucked and discharged into the molding sand through the coating layer, which is conducive to reducing the wrinkle defect. The recommended vacuum control is -0.06~-0.09Mpa.
3.4 Choose a suitable pouring system
According to the characteristics of the foam plastic mold gasification in the mold, the pouring method of the lost foam casting can be selected from bottom pouring, step pouring, top pouring and rain pouring. However, it must be noted that the metal liquid flow should be ensured to fill the mold smoothly and quickly.
3.5 Improving the permeability of the mold
Good permeability of the mold is an important condition to ensure the acquisition of high-quality vacuum lost foam castings. Practice has shown that the main way to improve the permeability of the mold is to use coarse sand, increase the amount of air pumped and the vacuum degree during pouring, and select reasonable coatings and coating thicknesses.
3.6 Others
Use easily vaporized EPS pattern binder. A small amount of cryolite (Na3AlF6) is added to the coating to form highly active NaF, AlF3, etc. at high temperatures. These highly active substances can adsorb the carbon produced by thermal decomposition of the pattern, so that it does not precipitate on the surface of the casting, thereby eliminating the wrinkle defect. In addition, adding a certain amount of potassium permanganate to the coating can reduce the wrinkle defect of ductile iron castings and ensure the appearance quality of the casting.