What is the sand casting
In the production preparation process of sand casting, it is necessary to develop a reasonable casting process plan and draw a casting process diagram.
The casting process diagram is a graphical representation of the casting process plan with various process symbols in the part diagram, including the casting pouring position, mold parting surface, number of cores, shape, fixing method, and order of cores, as well as the remaining processing quantity, draft angle, shrinkage rate, gate system, riser, size and layout of cooling iron, etc. The casting process drawing is the basic process document to guide the design of the model (core box) ), production preparation, mold manufacturing, and casting inspection.
Based on the casting process diagram, combined with the chosen modeling method, pattern and box diagrams can be drawn. The enlarged dimensions of the casting used for cutting are called machining allowances. The value of the machining margin depends on the production lot of the casting, the type of alloy, the dimensions of the casting, the distance between the machined surface and the reference surface, and the position of the machined surface during casting.
The margin can be reduced by using machine modeling with high casting accuracy; the margin should be increased for larger errors in manual modeling. Due to the rough surface of cast steel parts, the margin should be increased; non-ferrous alloy castings are expensive and have smooth surfaces, and the margin should be smaller than that of cast iron. The larger the size of the casting, or the distance between the machined surface and the reference surface, the larger the dimensional error, so the margin should be increased accordingly. In the casting process, the casting towards the top surface has a higher possibility of defects, and the margin should be greater than the bottom and sides.
Advantages of Sand Casting
The molding methods of sand casting are divided into the manual molding and mechanical molding.
The characteristics of manual molding: are convenient and flexible operation, strong adaptability, and short preparation time for mold making, but low productivity, high labor intensity, and difficulty in ensuring the quality of castings, only for single-piece small batch production.
Mechanical molding characteristics: the main method of mass production of sand mold can significantly increase labor productivity, improve working conditions, and improve the dimensional accuracy and surface quality of the castings, and reduce the machining allowance.
What is the investment casting
Investment casting, also known as a lost wax casting, includes processes such as pressing wax, repairing wax, assembling trees, dipping, melting wax, casting molten metal, and post-treatment.
Lost wax casting involves using wax to make a wax mold of the part to be cast, and then applying clay, or clay molds, to that wax mold. After the clay mold is dried, it is placed in hot water to melt the internal wax mold. The clay mold is removed from the melted wax mold and fired into an earthenware mold. Once baked. Usually, the clay mold is made leaving a pouring spout from which the molten metal is then poured. After cooling, the desired part is made.
Advantages of Precision Casting
The dimensional accuracy of fine casting is high, usually up to CT4-6 (CT10~13 for sand casting and CT5~7 for die casting). Of course, due to the complexity of the investment casting process, there are many factors that can affect the dimensional accuracy of the casting, such as the casting mold. The shrinkage of the material, the deformation of the molten die, the linear change of the shell during heating and cooling, the shrinkage of the alloy, and the deformation of the casting during solidification, so the dimensional accuracy of the ordinary molten die casting high temperature, but still, need to improve its consistency (the dimensional consistency of the casting using medium and high-temperature wax should be greatly improved).
When the molten die is pressed, the surface finish of the molten die is also relatively high because a pressure type with a high surface finish of the cavity is used. In addition, the shell is made of high temperature resistant coating and refractory material which is made of special high temperature resistant binder and refractory material and coated on the molten die, and the inner surface of the cavity which is in direct contact with the molten metal has high smoothness. As a result, the surface finish of fine castings is higher than that of ordinary castings, usually reaching Ra.1.6 ~ 3.2μm.
The biggest advantage of fine castings is that they have high dimensional accuracy and surface finish, so they can reduce machining work, but leave a small amount of machining margin on parts that require higher parts, and even some castings retain only grinding and polishing margin. You can use it without machining. As you can see, using the investment casting method can result in significant savings in machine tool equipment and machining hours, as well as significant savings in raw metal.
Another advantage of the investment casting method is that it can cast complex castings in various alloys, especially high-temperature alloys. For example, jet engine blades, streamlined profiles, and cooling cavities are difficult to form by machining techniques. Investment casting production not only enables mass production ensuring the consistency of the casting but also avoids the stress concentration of residual tool marks after machining.