Precision casting

  Precision casting

  Precision casting refers to the general term for the process of obtaining precision-sized castings. Compared with the traditional sand casting process, the castings are obtained by precision casting have more precise dimensions and better surface finish. Its products are precise, complex and close to the final shape of the part. Can be used directly without processing or processing. It is an advanced process of near net shape.

It includes: investment casting, ceramic casting, metal casting, pressure casting, lost foam casting.

  Commonly used is investment casting, also known as lost wax casting. The investment mold is made by using a suitable investment material such as paraffin. The refractory coating and the refractory sand process are repeated on the investment mold. Hardened shell and dry. The internal melting mold is then melted off to obtain a cavity. The baked shell is obtained to obtain sufficient strength. The residual investment material is burned off and the desired metal material is poured. Solidification, cooling, shelling, sand cleaning. Thereby obtaining a high-precision finished product. Heat treatment and cold working and surface treatment according to product requirements.

  Process flow

  Waxing (wax mold)—repair wax—-wax inspection—-group tree (wax module tree)—shell (first paste, sand, re-slurry, finally Mould air drying)—Dewaxing (steam dewaxing)——-Mould roasting–chemical analysis–casting (casting molten steel in the mold shell)—-vibration shelling— Cutting and pouring of casting and pouring rod—-grinding gate—initial inspection (hairy inspection)—shot blasting—–machining—–polishing—finish inspection— Storage

  The casting production process is generally divided into wax, shell, casting, post-treatment, inspection.

  Pressure wax includes (pressure wax, wax repair, group tree)

  Wax—Use wax machine to make wax mold

  Repair wax—correct the wax mold

  Group tree—group the wax mold

  The shell includes (hanging sand, hanging pulp, air drying)

  Casting includes (baking, chemical analysis, also called spectroscopy, casting, shock shell, gate, gate)

  Post-treatment includes (blasting, shot blasting, correction, pickling)

  Inspection includes (wax inspection, initial inspection, intermediate inspection, finished product inspection)

  Skills requirement

  Method adaptation

  For example, sand casting. Mass-produced factories should create conditions that use technologically advanced modeling and core making methods.

  The productivity of the old-fashioned shock or vibratory molding machine production line is not high enough. Workers are in labor intensive and have high noise.

  It is not suitable for mass production requirements.It should be gradually modified.For small castings, it is possible to use a horizontal or vertical type high-pressure molding machine production line.The real model has high production efficiency and a small footprint. For the middle piece, you can use a variety of high-pressure molding machine production lines and air-punching molding lines. To meet the requirements of fast, high-precision modeling production lines. The core making method can be used: high-efficiency core making methods such as cold core box, hot core box and shell core.Medium-sized large castings can be considered for the application of resin self-hardening sand molding and core making.

  Manual casting is still an important method for heavy-duty castings produced in small batches. Manual modeling can be flexible to adapt to a variety of complex requirements. Does not require a lot of process equipment. Water glass sand type, VRH water glass sand type, organic ester water glass self-hardening sand type, clay dry type, resin self-hardening sand type and cement sand type can be applied. For heavy-duty castings produced in one piece, the pit modeling method is used. Low cost and fast production. The styling products for mass production or long-term production adopt multi-box shape and box molding method. Although molds, sand boxes, etc. start to invest high, they can be compensated for in terms of saving molding time and improving product quality.

  Casting methods such as low pressure casting, die casting, and centrifugal casting. Due to the high price of equipment and molds, it is only suitable for mass production.

  Conditional method should be suitable

  For example, it is also the production of castings such as large machine bed, generally using the core forming method. Do not make a pattern and a sand box, and assemble the core in the pit. The other factory uses a sand box modeling method to make a look. Different enterprise production conditions (including equipment, site, staff quality, etc.), production habits, and accumulated experience are different. It should be considered according to these conditions what products are suitable and what products are not suitable (or not).

  Precision requirements and costs

  The castings obtained by various casting methods have different previsions. Initial investment and productivity are also inconsistent. The final economic benefits are also different. So, to be more, faster, better, and more economical, we should take into account all aspects. A preliminary cost estimate should be made for the casting method chosen to determine the casting method that is economically efficient and that guarantees the casting requirements.

  From a long-term perspective, China’s precision casting industry still has certain hopes. The market demand has slowly begun to pick up, and with the strong strength of China’s foundry industry development, I believe that China’s foundry industry will certainly achieve gratifying results.

To fundamentally improve the level of casting technology, the following four points must be achieved.

First, develop simulation techniques to improve the accuracy of predictions. Strengthen process control and increase yield. The problem of regularity is not well mastered, which affects the yield in mass production.

Second, the combination of production, study and research. In addition to the enhancement of innovation consciousness and the improvement of research and development capabilities, the independent innovation of enterprises also needs to attach importance to and strengthen the combination of “production, learning and research” with precision casting enterprises as the main body.

Third, attach importance to material research and development. Materials are the foundation of industry, and there is still a lot of work to do. In the aerospace field, research and smelting technologies for some new materials such as alloy materials, especially high-temperature alloys, have yet to be improved, and the gap with foreign countries is still large.

Fourth, pay attention to the improvement of equipment technology. The process is mainly a matter of equipment. Many key equipments, such as some directional solidification equipment, mainly rely on imports. Therefore, the development of precision casting equipment is still the focus.

  Influence accuracy factor

  Under normal circumstances, the dimensional accuracy of precision castings is affected by many factors such as the structure of the casting, the material of the casting, molding, shelling, roasting and casting. Any one of the links set up and unreasonable operation will change the shrinkage rate of the casting. The dimensional accuracy of the casting is deviated from the requirements. The following are factors that can cause defects in the precision of precision castings:

  (1) The influence of the structure of the casting. a. The casting has a thick wall and a large shrinkage. The casting has a thin wall and a small shrinkage. b. The free shrinkage rate is large, which hinders the shrinkage rate.

  (2) The influence of casting material. a. The higher the carbon content of the material, the smaller the line shrinkage. The lower the carbon content, the greater the line shrinkage. b. The casting shrinkage of common materials is as follows: casting shrinkage K = (LM-LJ) / LJ × 100%, LM is the cavity size, and LJ is the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, casting temperature K4.

  (3) The influence of mold making on the shrinkage of the casting line. a. The effect of the wax temperature, the wax pressure, and the dwell time on the size of the melt is the most obvious. Followed by the wax pressure. The holding time has little effect on the final size of the investment after the injection molding is ensured. b. The linear shrinkage of the wax (molding) material is about 0.9-1.1%. c. When the investment mold is stored, further shrinkage will occur, and the shrinkage value is about 10% of the total shrinkage. However, after 12 hours of storage, the size of the investment was substantially stable. d. The radial shrinkage of the wax mold is only 30-40% of the shrinkage in the longitudinal direction, and the effect of the wax temperature on the free shrinkage is far greater than the effect on the resistive shrinkage (the optimum wax temperature is 57-59 ° C, The higher the temperature, the greater the shrinkage).

  (4) The influence of the shell material. Zircon sand and zircon powder are used because of their small expansion coefficient, which is only 4.6×10-6/°C, so it can be ignored.

  (5) Effect of shell baking. Since the expansion coefficient of the shell is small, when the shell temperature is 1150 ° C, it is only 0.053%, so it can be neglected.

  (6) The influence of casting temperature. The higher the casting temperature, the greater the shrinkage. The pouring temperature is low and the shrinkage rate is smaller. Therefore, the pouring temperature should be appropriate.