Sand Casting and Precision Casting (Investment Casting)

What is the Sand Casting Process?

During the production preparation for sand casting, it is necessary to formulate a reasonable casting process plan and draw a casting process diagram.A casting process diagram is a graph that shows the casting process plan with various process symbols on the part drawing. It includes the pouring position of the casting, the parting surface of the mold, the number, shape, fixing method and sequence of cores, as well as the amount of remaining machining allowance, draft angle, shrinkage rate, gating system, riser, and the size and layout of chillers. The casting process diagram is a basic technical document that guides pattern design (core box), production preparation, mold manufacturing and casting inspection.

Based on the casting process diagram and combined with the selected molding method, the pattern drawing and flask drawing can be drawn. The enlarged dimension of the casting for machining is called the machining allowance. The value of the machining allowance for castings depends on the production batch of the castings, the type of alloy, the size of the castings, the distance between the machined surface and the reference surface, and the position of the machined surface during casting.

When machine molding is adopted, the casting precision is high, and the allowance can be reduced; manual molding has large errors, so the allowance should be increased. Due to the rough surface of steel castings, the allowance should be increased; non-ferrous alloy castings are expensive and have smooth surfaces, so their allowance should be smaller than that of cast iron. The larger the size of the casting, or the greater the distance between the machined surface and the reference surface, the larger the dimensional error, so the allowance should be increased accordingly. During the casting process, the upward-facing surface of the casting has a higher possibility of defects, so the remaining allowance should be larger than that of the bottom and side surfaces.

Advantages of Sand Casting

The molding methods of sand casting are divided into manual molding and mechanical molding.

Characteristics of Manual Molding:

• Convenient and flexible operation, strong adaptability, and short preparation time for mold making.

• Low productivity, high labor intensity, and difficulty in ensuring casting quality.

• Only suitable for single-piece and small-batch production.

Characteristics of Mechanical Molding:

• The main method for mass production of sand molds, which can significantly improve labor productivity, improve working conditions, and enhance the dimensional accuracy and surface quality of castings, while reducing machining allowance.

What is the Investment Casting Process?

Investment casting, also known as lost-wax casting, includes processes such as wax pressing, wax repairing, tree assembly, slurry dipping, wax melting, pouring of molten metal, and post-processing.

Lost-wax casting involves using wax to make a wax pattern of the part to be cast, and then coating the wax pattern with clay to form a clay mold. After the clay mold dries, it is put into hot water to melt the internal wax pattern. The clay mold is taken out from the melted wax pattern and fired into a ceramic mold. Usually, a sprue is left when making the clay mold, and then molten metal is poured from the sprue. After cooling, the required part is made.

Advantages of Precision Casting (Investment Casting)

High Dimensional Accuracy

The dimensional accuracy of precision castings is relatively high, usually reaching CT4-6 (CT10-13 for sand castings, CT5-7 for die castings). Of course, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of castings, such as the shrinkage of the mold material, the deformation of the wax pattern, the linear change of the shell during heating and cooling, the shrinkage of the alloy, and the deformation of the casting during solidification. Therefore, the dimensional accuracy of ordinary high-temperature investment castings is high, but their consistency still needs to be improved (the dimensional consistency of castings using medium and high-temperature wax should be significantly improved).

Excellent Surface Finish

When the wax pattern is pressed, since a pressure mold with a high surface finish of the cavity is used, the surface finish of the wax pattern is also relatively high. In addition, the shell is made of high-temperature resistant coating and refractory material. The high-temperature resistant coating is made of special high-temperature resistant binder and refractory material, which is coated on the wax pattern. The inner surface of the cavity that is in direct contact with the molten metal has high smoothness. Therefore, the surface finish of precision castings is higher than that of ordinary castings, usually reaching Ra 1.6-3.2 μm.

Reduced Machining Work

The biggest advantage of precision castings is that due to their high dimensional accuracy and surface finish, they can reduce machining work. Only a small amount of machining allowance is left on parts that require higher precision, and even some castings only retain grinding and polishing allowances and can be used without machining. It can be seen that the investment casting method can greatly save machine tools and processing man-hours, and significantly save metal raw materials.

Ability to Cast Complex Castings

Another advantage of the investment casting method is that it can cast complex castings of various alloys, especially high-temperature alloy castings. For example, the blades of jet engines, with their streamlined profiles and cooling cavities, are difficult to form by machining technology. Investment casting production can not only realize mass production and ensure the consistency of castings, but also avoid stress concentration caused by residual tool marks after machining.

What are the Main Differences Between Sand Casting and Investment Casting?