The coated sand casting process has a long history of use in the foundry field. Many companies use the coated sand casting process to produce a large number of castings; but the use of coated sand casting to produce precision steel castings also has its characteristics and some defects, which are worth us Thoughtful.
A. The characteristics of the coated sand casting process: the coated sand process has good strength performance; good fluidity, the prepared sand molds, sand cores have clear outlines, dense structures, and can produce complex sand cores; sand molds (cores) The surface quality is good, the surface roughness can reach Ra=6.3～12.5μm, the dimensional accuracy can reach CT7～CT9 level; the collapsibility is good, the casting is easy to clean. Coated sand can be used to make molds and sand cores. The molds or cores of coated sand can be used in conjunction with each other or with other sand molds (cores); it can not only be used for metal gravity casting or low-pressure casting, but also It can be used for sand-coated iron casting and thermal centrifugal casting; it can be used not only for the production of cast iron and non-ferrous alloy castings, but also for the production of steel castings.
B. Problems in the application of coated sand and their solutions. There are many types of core-making methods, which can be divided into two categories: thermosetting methods and cold-setting methods. The core making of coated sand belongs to the category of thermosetting methods. Any core-making method has its own advantages and disadvantages, which mainly depend on the comprehensive factors such as product quality requirements, complexity, production batch, production cost, product price, etc. to determine which core-making method to use. It is very effective to use coated sand for sand cores with high requirements on the surface quality of the casting cavity, high dimensional accuracy, and complex shapes. For example: sand cores for intake and exhaust passages, water passage sand cores, and oil passage sand cores of car engine cylinder heads, water passage sand cores and oil passage sand cores for cylinder blocks, shell core sand cores for intake manifolds and exhaust manifolds, Runner sand core of hydraulic valve, air passage sand core of automobile turbocharger, etc. However, some problems are often encountered in the use of coated sand. Here I will only briefly talk about the experience in work.
1. Storage period of coated sand cores: any sand core will absorb moisture, especially in southern areas where the relative humidity is high. The storage period of sand cores must be specified in the process documents, and the lean production method of first-in first-out production is used to reduce sand. The storage capacity and storage period of the core. Each enterprise should determine the storage period of the sand core based on its own plant conditions and local climatic conditions.
2. The method for determining the strength and gas evolution of coated sand. Under the premise of certain quality of raw sand and resin, the key factors affecting the strength of coated sand are mainly based on the amount of phenolic resin added. If the phenolic resin is added in a large amount, the strength is improved, but the amount of gas evolution is also increased, and the collapsibility is reduced. Therefore, in production and application, the strength of the coated sand must be controlled to reduce the amount of gas and improve the collapsibility. A balance point must be found when the strength standards are formulated. This balance point is to ensure the surface quality of the sand core and its strength under the premise of no deformation and no core breakage during pouring. In this way, the surface quality and dimensional accuracy of the castings can be guaranteed, the gas generation can be reduced, the pore defects of the castings can be reduced, and the sand production performance of the sand core can be improved. For the storage of sand cores, work station tools and sand core trolleys can be used in the transportation process, and 10mm-15mm thick sponges can be laid on them, which can reduce the loss rate of sand cores.
3. Control the quality of supply of coated sand: The quality assurance materials of the supplier must be attached to the coated sand when it enters the factory, and the company shall inspect it according to the sampling standards, and only after the inspection is qualified, can it be put into the warehouse.
4. Qualified coated sand finds core fracture and deformation during core making. The fracture deformation of sand core during core making is usually considered to be caused by low strength of coated sand. In fact, the core fracture and deformation will involve many production processes. In the event of an abnormal situation, the real cause must be found to be completely resolved. The specific reasons are as follows:
1) The mold temperature and mold retention time during core making are related to whether the hardened thickness of the sand core crust meets the process requirements. The process parameters specified in the process need to have a range, and this range needs to be adjusted by the skill of the operator. When the mold temperature is the upper limit, the mold retention time can take the lower limit, and when the mold temperature is at the lower limit, the mold retention time can take the upper limit. Operators need to be continuously trained to improve their operating skills.
2) Phenolic resin and sand particles will stick to the mold during core making. It must be cleaned in time and sprayed with release agent, otherwise the sand core will be broken or deformed when the mold is opened.
3) The spring ejector rod on the static mold of the hot core box mold will break or deform due to elastic failure due to long-term work under high temperature conditions. The spring must be replaced in time.
4) The movable mold and the static mold are not parallel or on the same centerline. When the mold is closed, under the pressure of the oil cylinder or the air cylinder, the front end of the positioning pin has a certain slope, and the mold will still close tightly, but the movable mold and the static mold will be tightly closed when the mold is opened. The mold will still return to its original state and the sand core will be broken or deformed. In this case, sand will run out when sand is shot, and the size of the sand core will become larger. The solution is to adjust the parallelism and coaxiality of the mold in time.
5) When hollow sand cores are produced on the shell core machine, when the unhardened coated sand is poured from the core and needs to be reused, the unused coated sand must be sieved and mixed in a ratio of 3:7. Only in this way can the surface quality and the strength of the core sand core be ensured.