What are the main steps involved in the process of precision casting?

Precision casting, as the core process of high-end manufacturing, is widely used in key fields such as aerospace, military, medical equipment, and high-end equipment due to its high forming accuracy, excellent surface quality, and ability to manufacture complex structural components. Unlike ordinary sand casting, precision casting has a more complex process and finer steps. The operational standardization of each link directly affects the dimensional accuracy, mechanical properties, and service life of the castings. With the transformation of the precision casting industry towards high precision and high reliability, clarifying its complete process flow and defining the core requirements of each step has become the key to standardizing production, improving product quality, and enhancing core competitiveness for enterprises. It also provides strong support for the high-quality development of the industry.

According to research data from the Precision Casting Branch of the China Foundry Association, standardized process flow can reduce the defect occurrence rate of precision castings by more than 40% and increase the yield rate to around 95%; And non-standard process operation is one of the main reasons for defects such as casting size deviation, slag inclusion, and porosity. Some small and medium-sized enterprises have a product scrap rate of over 8% due to inadequate process control. Industry experts point out that precision casting is a systematic project, and the core process can be divided into six core steps: investment casting, shell preparation, roasting, molten metal melting and pouring, cooling and forming, cleaning and testing. Each step is interrelated and affects each other, and strict adherence to operating standards is necessary to achieve precise casting.

Investment casting is the starting step of precision casting and the fundamental link that determines the dimensional accuracy of castings. The core is to produce a "prototype" investment casting that is consistent with the shape and size of the final casting. This step mainly includes three sub steps: mold preparation, investment material preparation, and molding and cooling. Firstly, according to the casting design drawings, high-precision machining equipment should be used to make the mold. The mold size should be combined with the shrinkage characteristics of the investment material, and appropriate shrinkage allowance should be reserved to ensure that the mold accuracy meets the standard; The commonly used investment molding materials are easily melted and molded materials such as paraffin and resin. When preparing, the material ratio should be strictly controlled to ensure the stability of material fluidity and shrinkage rate.

During the molding process, the molten investment material is injected into the mold cavity. After the material cools and solidifies, the mold is removed to obtain the investment mold that matches the shape of the casting. Industry technicians explain that in investment casting, it is necessary to control the molding temperature and cooling rate well to avoid uneven material shrinkage and deformation of the investment mold; Priority should be given to using gradient cooling during cooling to ensure uniform cooling of all parts of the investment mold and reduce dimensional deviations. In addition, after the completion of the investment mold production, preliminary dimensional inspection is required to remove deformed and oversized investment molds, ensuring the smooth progress of subsequent processes from the source.

Shell preparation is the core key step of precision casting, which involves producing high-strength and high-temperature resistant shells on the surface of the investment mold as the final mold for casting. This step mainly includes three core steps: investment mold pretreatment, coating and sanding, and drying and hardening. Pre treatment of the investment mold requires cleaning of surface impurities and oil stains, and if necessary, polishing to ensure a smooth surface of the investment mold and avoid affecting the adhesion effect of the mold shell; The coating process requires the use of high-quality refractory materials with fine particles and low shrinkage rate. The refractory coating should be evenly applied to the surface of the investment mold, followed by the sprinkling of refractory sand to enhance the strength of the shell.

Coating and sanding need to be repeated multiple times until the shell reaches the specified thickness. Usually, the number of coating layers is 4-6, and each layer needs to be dried and hardened after coating. The drying temperature and time should be controlled to avoid cracking and peeling of the shell. After drying and hardening, the shell needs to undergo a visual inspection to ensure that the surface is defect free and the thickness is uniform. The quality of the shell directly determines the surface quality and dimensional accuracy of the casting, so this step requires strict control of the quality of refractory materials and operating standards, laying the foundation for precise casting.

Baking is an important pretreatment step after the preparation of the mold shell, with the core purpose of removing residual moisture, organic matter, and incompletely melted investment materials inside the mold shell, while improving the strength and dimensional stability of the mold shell. During the roasting process, it is necessary to strictly control the heating rate and roasting temperature, and adopt a gradient heating method to avoid cracks in the shell caused by sudden temperature increases and decreases; The calcination temperature is usually controlled at 800-1000 ℃, and the calcination time is adjusted according to the thickness of the shell, usually 2-4 hours. After the roasting is completed, the mold shell needs to be cooled to a suitable temperature. At this time, the molten mold inside the mold shell has been completely discharged, forming a mold cavity that is consistent with the shape of the casting and waiting for the pouring of the molten metal.

Melting and pouring of molten metal is the core process of injecting molten metal into the shell and cavity to achieve casting formation, which directly affects the density and mechanical properties of the casting. Firstly, according to the requirements of the casting material, select metal raw materials with qualified purity and put them into the furnace for melting. During the melting process, it is necessary to strictly control the melting temperature and insulation time to ensure that the composition of the metal liquid is uniform and the flowability meets the standards; At the same time, deoxidation and degassing treatment should be carried out to reduce the gas and impurity content in the molten metal, and to avoid defects such as porosity and slag inclusion in the castings.

The pouring process needs to control the pouring temperature and pouring speed. The pouring temperature should be adjusted according to the characteristics of the metal material and the size of the casting, usually 50-100 ℃ higher than the melting point of the metal, to ensure good fluidity of the metal liquid; The pouring speed should be smooth and uniform to avoid deformation caused by the metal liquid impacting the mold shell due to too fast a speed, or to prevent the metal liquid from cooling too quickly or filling incompletely due to too slow a speed. During pouring, it is necessary to ensure that the metal liquid completely fills the shell cavity to avoid defects such as material shortage and cold sealing. After pouring is completed, pouring records should be kept in a timely manner for subsequent quality traceability.

Cooling molding is a key step in casting molding, with the core being to control the cooling rate and uniformity of the metal liquid inside the mold shell, reduce uneven shrinkage and deformation of the casting, and ensure dimensional accuracy. During the cooling process, a gradient cooling method should be used. Based on the material, size, and structure of the casting, the cooling temperature and time should be set reasonably to avoid cracking of the casting caused by too fast a cooling rate, or prolonging the production cycle and affecting the uniformity of the casting structure due to too slow a cooling rate. After cooling to room temperature, the casting and the shell are preliminarily bonded, and the next step of cleaning can be carried out.

Cleaning and testing are the final steps in precision casting, with the core being the removal of impurities such as mold residue and burrs on the surface of the casting, and a comprehensive inspection of the casting quality to ensure that the product meets the standards. The cleaning process mainly includes shell removal, burr polishing, surface polishing, etc. Shell removal can be done by mechanical cleaning or chemical cleaning to avoid damaging the surface of the casting; Burr polishing requires the use of appropriate tools to ensure that the surface of the casting is smooth and meets the surface quality requirements.

The inspection process is an important guarantee for ensuring the quality of castings, and high-precision inspection equipment is required to comprehensively inspect the dimensional accuracy, surface quality, and internal defects of castings. Dimensional accuracy testing requires comparing with the design drawings and using precision measuring instruments to ensure that the dimensional deviation of the castings is within the tolerance range; Surface quality inspection requires checking for defects such as cracks, sand sticking, and missing materials on the surface of castings; Internal defect detection can be carried out using ultrasonic testing, radiographic testing, and other methods to check for the presence of pores, slag inclusions, cracks, and other issues inside. Only qualified castings can be stored, while unqualified castings need to be repaired or scrapped according to the defect situation, and the cause should be traced and the process optimized.

Industry experts remind that the precision casting process is complex and requires strict requirements. Enterprises need to combine their own production needs and product positioning to develop comprehensive process operation specifications, clarify the operating standards, parameter requirements, and quality control points for each step; Strengthen the training of operators, enhance the professionalism of practical operations, standardize the operation of each link, and reduce human errors; Establish a full process quality traceability system, promptly identify problems in the process, continuously optimize the process flow, and improve production efficiency and product quality.

In practice, many precision casting enterprises have achieved a dual improvement in product quality and production efficiency by standardizing process flows and optimizing operational details. For example, a certain aerospace precision casting enterprise has standardized the processes of investment casting, shell preparation, and baking, accurately controlled the pouring and cooling parameters, and combined with full process testing to stabilize the dimensional tolerance level of castings between CT5-CT7, reducing the defect occurrence rate by more than 45%; A certain medical device enterprise has optimized the metal liquid melting and cleaning process to ensure smooth surfaces and precise dimensions of precision castings, fully meeting the stringent requirements of the medical field.

With the continuous upgrading of precision casting technology, the process flow is also continuously optimized. Related enterprises are continuously developing intelligent preparation equipment and automated detection systems to assist them in achieving automated and precise operations in areas such as investment casting, shell preparation, and pouring cooling; Precision casting enterprises optimize mold design and process parameters, shorten production cycles, and improve process control by introducing technologies such as 3D printing and big data. In the future, with the continuous advancement of technology, the precision casting process will become more standardized, efficient, and accurate, helping the industry develop towards higher quality and higher end, and providing more reliable core component support for high-end manufacturing.