Nowadays, the precision casting industry is developing very rapidly in today’s society. Of course, it is inseparable from the advantages of precision castings, such as high dimensional accuracy, small surface roughness, suitable for castings with complex structure and shape, etc. it is also inseparable from our designers. Precision casting has design principles:
1、 Reasonable wall thickness. Under the condition of meeting the small allowable wall thickness of the casting, try to be thinner as much as possible, which can not only ensure the good mobility of the molten metal, but also avoid the casting defects (such as shrinkage cavity) caused by excessive shrinkage.
2、 Uniform wall thickness. It means that the wall thickness and cooling rate of each part are uniform. The inner wall and partition wall shall be thin and the surrounding walls shall be thick. Objective: to reduce stress, deformation and cracking; Prevent shrinkage of hot joints.
3、 Transition connection. Structural fillet: avoid the formation of hot joints; Improve stress distribution; Avoid sand mold damage and sand holes. Uniform joint: when the walls or ribs in different directions on the casting are connected, metal accumulation (hot joint) and shrinkage cavity shall be avoided. Fillet, inclined plane and cone are used for gradual transition. Purpose: to prevent cracking due to stress concentration.
4、 The large plane is inclined. Purpose: use filling and exhaust to check.
5、 Reduce deformation. (same as heat treatment) symmetrical structure and additional reinforcing ribs.
6、 Free contraction. Objective: to reduce the stress cracking and deformation caused by shrinkage stress.
The great advantage of silica sol casting is that due to the high dimensional accuracy and surface finish of investment castings, machining work can be reduced, only a little machining allowance can be left in the parts with high requirements, and even some castings can be used without machining. It can be seen that the investment casting method can greatly save machine tool equipment and processing hours, and greatly save metal raw materials.
The manufacturing process of Silica Sol precision casting is actually very complex. Firstly, the processing materials of Silica Sol precision casting should be clean. Wood chips, grass chips, sand, iron chips and other sundries must not be mixed in during use. The furnace lining must be tamped and compacted evenly. The newly built furnace lining must be sintered according to the process requirements and can be used after sintering.
In the process of using silica sol precision casting, a common problem is that there are pores in the product. For some parts with high requirements, pores are not allowed. Therefore, Silica Sol precision casting needs to prevent pores during processing.
In the face of complex thin-wall castings, in order to effectively improve the air permeability of precision casting shell, Silica Sol precision casting can effectively set its exhaust hole and reasonably set its gating system directly above the surface in the process of processing.
Silica Sol precision casting can effectively prevent its pouring air entrainment, which can be conducive to the discharge of gas in the mold cavity. In the process of pouring, the pouring temperature can be appropriately increased, the distance between the ladle nozzle and the pouring speed also needs to be reduced.
Nowadays, precision casting is a very important processing technology, and its processing is inseparable from precision casting. However, when pouring, we need to determine the pouring surface, so how to determine this position?
1. The important processing surface or main processing surface of precision casting faces down or is located on the side: during pouring, the gas, slag in the liquid metal and sand particles in the mold will float up, which may cause defects such as pores, slag inclusions and sand holes in the upper part of the casting, while the structure is dense under the possibility of defects in the lower part of the casting.
a. At the pouring position of the machine bed, the guide rail shall face down to ensure the quality of this important working face. The quality requirements of its circumferential surface are high. The vertical pouring scheme can make the circumferential surface on the side and ensure uniform quality.
2. The large plane of precision casting is poured downward or obliquely: due to the strong heat radiation of the hot liquid metal on the upper part of the mold during pouring, the sand mold on the top surface expands, arches or even cracks, resulting in sand inclusion, sand holes and other defects on the large plane. The production casting defects of large plane can be avoided by using the method of large plane downward or inclined pouring.
3. The thin wall of precision casting is downward, sideways or inclined: in order to prevent cold insulation and pouring defects at the thin wall of the casting, the thin wall with large area shall be placed at the lower part of the casting or in the side wall or inclined position.
4. Most of the thick parts of precision castings should be placed on the top or on the side of the parting surface. The main purpose is to facilitate the placement of risers at the thick parts for feeding.
Stainless steel precision casting has good fluidity to some extent. Due to its small size of transition zone and good mechanical properties, stainless steel precision casting can meet its fluidity and no bright crystal to a certain extent. The general temperature is 715 ~ 740 ℃. The hot cracking tendency of stainless steel precision casting flat ingot is high, and the temperature of stainless steel precision casting is relatively low, generally at 680 ~ 735 ℃. In addition to the temperature, the following operating procedures should be noted:
First make the electrode of the required blank, and then use the electrode corrosion mold to form a cavity. Then cast wax by casting method to obtain the original wax mold. Brush the wax mold with high temperature resistant liquid sand layer by layer. After the stainless steel precision casting obtains sufficient thickness, it is dried, and then heated to melt the internal wax mold and obtain the cavity consistent with the required blank. Then cast molten iron in the mold cavity and peel off the shell after curing, so as to obtain the finished product of precision manufacturing.
Precision steel casting technology comes from silica sol casting, with a complete set of advanced equipment and material composition detector. The casting has high dimensional accuracy and good surface finish, which can avoid reducing machining and save materials and processing costs. Cooling is a very important process in processing:
1. When alloy steel castings are air cooled, they should be scattered in a dry place;
2. The workpiece shall be tempered in time after quenching, and the time interval shall not exceed 4H. The carbon content (mass fraction of carbon) of the alloy steel casting used for the workpiece is low, and the shape of the workpiece is simple, which shall not exceed 16h;
3. Cleaning, cryogenic treatment or tempering can be carried out after cooling to room temperature;
4. For welded assemblies composed of alloy steel castings, the time interval between welding and subsequent heat treatment shall not exceed 4H.
Post time: Apr-15-2022
