Lost foam casting technical solution for back-spray and sand collapse: lost foam casting back-spray is a phenomenon in which a large amount of gas flows upward and makes a “puff” sound as soon as the molten metal enters the pouring cup, and the liquid also splashes out. Another phenomenon is that this phenomenon also occurs after a certain period of pouring.
1. Since more adhesive is used in the foam pattern during bonding, the amount of gas generated also increases accordingly, and a large amount of gas overflows during pouring, causing back-spray.
2. The pouring speed is too fast, which causes a large amount of cracking gas to burst out, resulting in spraying.
3. Due to the poor dehumidification capacity of the drying room, the moisture is not discharged, and even the coating is not completely dry, and back-spray occurs when high temperature and moisture come into contact.
4. The water in the molded foam pattern is not completely drained or the placement time is relatively short, and the mold is directly assembled, resulting in back-spray after pouring for a certain period of time.
5. There are many alcohol-based paints and places for re-brushing before modeling, which produce a large amount of gas and cause instant back-spray.
6. More white latex is added in the paint preparation or moldy fermented paint is used to cause back-spray.
In view of the above reasons, attention should be paid to the management of details in the actual operation on site; mark the beginning and end of each batch of molded foam patterns, weigh and record them, and put them into the drying room at about 35 degrees. After 8 hours, weigh and record them again or dry them in the sun. After 72 hours, check and pass them before bonding. Secondly, attention should be paid to the control and selection of glue when bonding foam patterns. Try to use cold glue and hot glue specially used for lost foam to reduce and eliminate the occurrence of back spray, keep a certain amount of metal liquid in the pouring cup, and keep the sprue full.
In the actual production process of lost foam, from foam molding to drying, runner design and bonding, and aggregate selection of coatings, all require rigorous and detailed management. Once a certain link is neglected, the product will have various defects after pouring, or even be waste. Therefore, various process controls and management in the early stage do not mean that this work is completed, because we all know that lost foam casting is from foam molding to product molding, which is completed by the replacement of molten metal, so the operation of the later process is more important. If any operation of the series of processes from drying room to box molding vacuum arrangement and water pouring is not strengthened, there will be back spray, local collapse, cold shut, wrinkled skin, convection punching, incomplete sand blasting, tumor growth and even box collapse, etc. The phenomenon of direct cross-box in lost foam casting production is a headache for every foundry worker.
In lost foam casting, the following factors may cause collapse and partial collapse:
1. During molding, the amount of sand at the bottom is too much and too high, the product is far away from the bottom, and the vacuum suction force is weak, resulting in insufficient demand on the upper part during the filling stage, coupled with the slow pouring speed, especially when there is only one bottom surface of the sand box for air extraction, causing the box to collapse.
2. The plastic film on the top covers less surface sand, and the ladle and the pouring cup are not aligned correctly, causing the plastic film to burn and leak, causing the box to collapse.
3. The pouring speed is too slow, especially in the case of interrupted pouring, the molten steel cannot seal the sprue, and a large amount of gas is sucked from the sprue, causing the vacuum in the sand box to drop sharply, causing the box to collapse.
4. The location of the pouring system is unreasonable, and there are many channels that borrow from each other, causing disorder during filling. After the foam is gasified, the metal liquid has no time to replace, forming a cavity and causing the box to collapse.
5. The coating configuration is unreasonable and the coating is relatively thin. During the configuration process, many things with large gas emission are added. The sand box structure is unreasonable. The exhaust chamber is through the triangular iron inside the sand box. In addition, there is too much dust and the vacuum tank is not cleaned, which causes negative pressure distortion, quality defects and box collapse.
6. When the large casting system is designed, the distance from the product is relatively close, which causes the dry sand to produce an overheating zone. The molten metal has not been fully filled, and the local part has been scalded, causing local collapse.
7. When the large part is top-cast, the pouring speed is slow, causing instantaneous gas intrusion and box collapse.
8. During the molding, due to the upper and lower structures, the vacuum obtained in the upper part is relatively small, causing box collapse.
9. For large machine tools and box parts, the upper half does not get negative pressure when the large plane is facing down, and the dry sand with the concave part is not sucked by the vacuum, and the metal liquid enters the moment it causes the box to collapse.
10. When the first burning and then pouring method is adopted, the paint has no strength and is not resistant to high temperature and erosion, and there are local paint cracks, causing box collapse.
11. When vibrating casting is used, the particles of the molding dry sand are relatively coarse, the coating is relatively thin and not resistant to vibration, and there is a lot of dust, which hinders the ability to evacuate the vacuum and causes the box to collapse.
12. The coating of the pouring system is relatively thin, the connection between the sprue and the pouring cup is not tightly sealed, and the pouring cup is small in size, so it cannot seal the pouring cup and causes the box to collapse.
