Introduction and Forming Method of Investment Casting Precision Casting Technology

Investment casting is a traditional casting method with high forming accuracy, and its pouring form is gravity casting. The dimensional accuracy and surface roughness of the formed product are both very high, with a dimensional accuracy of CT4-6 level. Compared with the precision of CT5-7 for die casting and CT10-13 for sand casting, it should be the highest. The surface roughness can reach Ra1.6-3.2 μ m, which is also higher than that of castings produced by general casting methods. However, its molding process is relatively complex.

Investment casting, also known as lost wax casting, is formed as follows:

The first step is mold design. Design a metal mold based on the part drawing, taking into account the shrinkage rate of the parts, including the shrinkage rate of wax injection molding and metal casting molding; The mold is the first mock examination with several cavities; Location of parting surface; Is it necessary to design a side core pulling mechanism, etc. Its design is similar to that of injection molds.

The second step is mold manufacturing. According to the requirements of the mold design drawings, suitable metal materials are selected, and the mold components are processed by machining machines to complete assembly and trial molding. The material of the mold is generally mold steel or aluminum alloy. The characteristics of mold steel are high hardness, long processing time, and long service life, while the advantages of aluminum alloy are low hardness, easy processing, and no need for rust prevention. The disadvantage is relatively short service life.

The third step is to manufacture the investment mold (wax injection). Use a wax press to cast the paste like wax material into the mold for cooling and molding, and then open the mold to remove the wax mold. Small parts can be directly blown out of the mold with the help of an air gun, and some wax molds with large contact areas with the lower mold require the design of an ejector mechanism.

Step four, wax mold tree assembly. Use an electric soldering iron to bond the wax mold and pouring system together. When assembling, be careful not to drop the melted wax from the soldering iron onto the wax mold, otherwise it will affect the quality of the wax mold. Then perform wax mold degreasing, soak the assembled module in a specialized cleaning agent for cleaning, and wash it in a clean water tank to increase the coating performance of the module.

Step five, manufacture the mold shell. The process of making a mold shell is relatively complex. Now, a layer of pre configured silica sol is glued to the surface of the module, followed by a layer of sand. After the sand is glued, it is hung and dried. After the sand is dried, the above operation is repeated, usually at least 4-5 layers are glued, which are called surface layer sand, transition layer sand, and back layer sand from the inside out. Finally, a mold shell with certain strength is formed on the outside of the module.

Step six, melt loss of mold (dewaxing). Remove the lid from the dried mold shell and place it upside down in a high-temperature steam kettle for dewaxing. The high-pressure hot steam liquefies with the cold mold shell, releasing a large amount of heat. In a short period of time, the wax inside the mold shell melts, and the liquefied water and melted wax flow out of the steam kettle respectively,

Complete the process of dewaxing.

Step seven, shell baking. Put the mold shell into a resistance furnace for heating and baking. During the baking process, residual wax, moisture, and organic matter in the mold shell can be removed, which can improve its strength and breathability. When roasting, the temperature should not be too high, as it can actually reduce the strength of the mold shell.

Step eight, pouring the molten metal. The molten metal liquid is poured into the roasted mold shell by picking and pouring, and the pouring speed should be fast, otherwise the temperature of the mold shell will decrease. Some carbon steel parts that need to be isolated from oxygen need to be covered with boxes and wood chips should be placed in the boxes to prevent oxidation-reduction reactions.

Step nine, vibration shell and cleaning. Place the poured shell parts on the vibrating shell machine to remove the mold shell, and then perform cutting, grinding, shot blasting and other treatments to obtain the blank parts. When vibrating the shell, it is necessary to prevent the parts from being crushed and to avoid deformation of the parts after being subjected to pressure.

Finally, an inspection must be conducted to confirm that there are no defects before it can be stored.