I. Introduction
Machine tool castings, as the crucial basic components of machine tools, have a direct impact on the precision, stability, and service life of the entire machine tool. In the field of mechanical manufacturing, the casting process of machine tool castings holds a unique and significant position. It not only needs to meet complex shape and size requirements but also possess excellent internal quality and performance characteristics. This article will comprehensively elaborate on the casting methods of machine tool castings, deeply explore its process characteristics, quality influencing factors, and unique advantages, providing a comprehensive and in-depth technical reference for the production and manufacturing of machine tool castings, helping related enterprises optimize production processes, improve product quality, and stand out in the fierce market competition.
II. Overview of Machine Tool Casting Process
Metal Melting Stage
The casting of machine tool castings is a multi-step and complex process. Firstly, in the metal melting stage, according to the material requirements of the desired casting, various metal raw materials are precisely added to the furnace in a certain proportion for melting. For example, for common cast iron machine tool castings, pig iron, scrap steel, return material, and other raw materials need to be mixed and melted. During the melting process, parameters such as temperature, time, and furnace atmosphere must be strictly controlled to ensure that the chemical composition, fluidity, and purity of the molten metal meet the requirements. Generally, the melting temperature of cast iron is between 1300°C and 1500°C. Within this temperature range, various metal components can be fully fused, and impurities can be effectively removed.
Casting Process
When the molten metal meets the standards, it enters the casting process. The molten metal is carefully and quickly poured into a pre-prepared mold. The design and production of the mold are of crucial importance as it directly determines the shape and size accuracy of the casting. The mold is usually made of molding materials such as molding sand and core sand. For some machine tool castings with complex shapes, multiple cores may be required to construct internal cavity structures. During casting, parameters such as casting speed, casting temperature, and casting pressure need to be well controlled. If the casting speed is too fast, the molten metal may cause turbulence, entraining gas and impurities, resulting in defects such as gas holes and slag inclusions; if the casting temperature is too high, the shrinkage of the molten metal will increase, leading to shrinkage cavities and porosity; improper casting pressure will also affect the filling ability and feeding effect of the molten metal in the mold.
Cooling and Solidification Stage
After casting, it enters the cooling and solidification stage. During this process, the molten metal gradually changes from liquid to solid, and its internal structure and properties are constantly formed and changed. The cooling rate has a significant impact on the grain size, microstructure, and mechanical properties of the casting. Generally, a faster cooling rate will refine the grains, improving the strength and hardness of the casting, but it may also increase internal stress, leading to casting deformation or even cracking. Therefore, it is necessary to reasonably control the cooling method and cooling rate according to factors such as the material, shape, and size of the casting. For example, for some large machine tool castings, cooling water pipes can be installed in the mold to control the cooling rate by adjusting the flow rate and temperature of the cooling water.
Cleaning and Finishing Process
Finally, there is the cleaning and finishing process. This process includes removing the molding sand, core sand, gates and risers, as well as oxide scale, burnt-on sand, and other impurities on the surface of the casting, and performing surface treatments such as grinding and polishing on the casting to improve its surface quality and appearance flatness. The cleaning and finishing process also includes quality inspection of the casting. Through visual inspection, non-destructive testing, mechanical property testing, and other means, it is detected whether there are defects such as gas holes, shrinkage cavities, cracks, and slag inclusions in the casting, and whether its mechanical properties meet the design requirements. For castings with defects, corresponding repair or scrapping measures are taken according to the severity and type of the defects.
III. Characteristics of Machine Tool Casting Process
(I) Wide Adaptability
The casting process of machine tool castings can produce castings with extremely complex shapes and a wide range of sizes. Whether it is a bed, column with a complex internal cavity structure, or various uniquely shaped worktables, boxes, and other components, they can all be realized through the casting process. Compared with other processing processes, such as machining which is mainly suitable for the production of parts with relatively simple and regular shapes, the casting process is not overly restricted by the complexity of the part shape. For example, some machine tool bed castings may have multiple cavities, ribs, and various irregular outer contours. These complex shapes can be formed in one step through casting without the need for a large amount of subsequent processing. At the same time, the casting process can produce machine tool castings of different materials according to different design requirements, such as cast iron, cast steel, non-ferrous metal castings, etc., which can meet the usage of machine tools under different working conditions and performance requirements.
(II) Requirement of Multiple Materials and Equipment
Casting machine tool castings requires a variety of types of materials. Firstly, there are metal raw materials. As mentioned above, according to different casting material requirements, various pig irons, scrap steels, alloy elements, etc. need to be prepared. Secondly, there are molding materials, including molding sand, core sand, binders, coatings, etc. The properties of molding sand and core sand directly affect key performance indicators such as the strength, permeability, and refractoriness of the mold. For example, quartz sand is a commonly used molding sand material. It has a relatively high refractoriness and thermal stability, but its permeability is relatively poor. Therefore, some binders such as bentonite need to be added to improve its performance, and some additives such as sawdust may also need to be added to improve permeability. The coating is used to coat the surface of the mold to improve the surface finish and anti-burn-on sand ability of the mold.
In terms of equipment, it is necessary to equip furnaces, casting equipment, molding equipment, sand treatment equipment, cleaning equipment, and various testing equipment. The furnace is used for metal melting. Common ones include cupola furnaces, electric furnaces, etc. Different furnaces have different characteristics and application ranges. Casting equipment includes ladles, gating systems, etc., which are used to accurately and stably pour the molten metal into the mold. Molding equipment such such as molding machines and core-making machines can improve the efficiency and accuracy of molding and core-making. Sand treatment equipment is used for the preparation, recovery, and regeneration of molding sand and core sand to ensure the stable performance of molding materials. Cleaning equipment such as shot blasting machines and sandblasting machines is used to remove impurities and oxide scale on the surface of the casting. Testing equipment includes X-ray flaw detectors, ultrasonic flaw detectors, hardness testers, metallurgical microscopes, etc., which are used for comprehensive testing and analysis of the quality of the casting.
(III) Environmental Pollution
The casting production process inevitably causes environmental pollution. During the metal melting process, a large amount of dust and harmful gases are generated. For example, when a cupola furnace melts cast iron, it will produce dust-laden flue gas, which contains dust particles such as silicon dioxide and aluminum oxide, as well as harmful gases such as sulfur dioxide and carbon monoxide. If these dust and gases are directly discharged into the atmosphere, they will cause serious pollution to air quality, endangering human health and the ecological environment. In the molding process, the handling and use of molding sand will also cause dust pollution. During the casting cleaning process, such as when using shot blasting and sandblasting methods to remove surface impurities, a large amount of dust will also be generated. In addition, various mechanical equipment in the casting workshop will generate noise pollution during operation, such as the noise emitted by molding machines, furnace fans, cleaning equipment, etc. Long-term exposure to a high-noise environment will damage the hearing of workers.
To reduce the environmental pollution caused by casting production, enterprises need to adopt a series of effective environmental protection measures. For example, in the furnace flue gas emission system, install high-efficiency dust removal equipment such as bag filters and electrostatic precipitators to purify the dust-laden flue gas, so that the emission concentration of dust and harmful gases meets the national environmental protection standards. For the dust generated in the molding and cleaning processes, closed production equipment and ventilation and dust removal systems can be used to collect and treat the dust. In terms of noise control, sound insulation and vibration reduction treatments can be carried out on noise source equipment, such as installing sound insulation covers around molding machines, installing vibration reduction pads between the fan and the foundation, and at the same time, providing workers with personal protective equipment such as earplugs and earmuffs.
IV. Influencing Factors of Machine Tool Casting Quality
(I) Selection of Process Parameters
Process parameters play a crucial role in the quality control of machine tool castings. For example, the casting temperature of the molten metal directly affects its fluidity and solidification characteristics. If the casting temperature is too high, the shrinkage of the molten metal will increase, easily leading to defects such as shrinkage cavities and porosity in the casting; if the casting temperature is too low, the fluidity of the molten metal will become poor, and problems such as misrun and cold shut may occur. For example, for gray cast iron machine tool castings, the casting temperature is generally controlled between 1350°C and 1450°C. Casting speed is also an important parameter. A suitable casting speed can ensure that the molten metal smoothly fills the mold cavity and avoid entraining gas and impurities. If the casting speed is too fast, the molten metal will cause turbulence in the mold cavity, entraining gas and impurities, forming gas holes and slag inclusions; if the casting speed is too slow, the molten metal may cool down too quickly during the filling process, resulting in misrun or cold shut. In addition, process parameters such as the preheating temperature of the mold, the cooling rate, and the design of the gating system also have a significant impact on the casting quality. In actual production, it is necessary to carefully optimize and select each process parameter through multiple process tests and experience accumulation to ensure the quality of machine tool castings.
(II) Quality of the Mold
The quality of the mold is one of the important factors affecting the quality of machine tool castings. The strength of the mold must be high enough to withstand the impact and pressure of the molten metal and prevent accidents such as mold wall rupture and mold collapse during the casting process. If the mold strength is insufficient, it may lead to the leakage of the molten metal and the scrapping of the casting. The permeability of the mold should be good so that the gas in the mold cavity can be smoothly discharged after the molten metal is poured into the mold. If the permeability is poor, the gas will accumulate in the mold cavity, hindering the filling of the molten metal and forming gas hole defects. The refractoriness of the mold is also crucial. It must be able to withstand the high-temperature action of the molten metal and prevent chemical reactions between the mold material and the molten metal during the casting process, resulting in defects such as burnt-on sand. For example, when casting high-temperature alloy machine tool castings, higher requirements are placed on the refractoriness of the mold. High-refractoriness molding materials need to be selected, and a high-quality refractory coating needs to be applied on the surface of the mold. In addition, the dimensional accuracy and surface finish of the mold will also directly affect the shape and surface quality of the casting. A high-precision mold can ensure that the size of the casting meets the design requirements, and a surface-finished mold can reduce the surface roughness of the casting, reducing the cleaning workload and the probability of surface defects.
(III) Quality of the Molten Metal
The quality of the molten metal is directly related to the internal quality and performance of machine tool castings. The chemical composition of the molten metal must meet the design requirements. Any deviation in the content of alloy elements may affect the mechanical, physical, and chemical properties of the casting. For example, in cast iron, the content of elements such as carbon, silicon, manganese, phosphorus, and sulfur has an important impact on the structure and performance of cast iron. If the carbon content is too high, the toughness of cast iron will decrease, and cracks are prone to occur; if the silicon content is too low, it will affect the casting performance and graphitization degree of cast iron. The purity of the molten metal is also very critical. The content of inclusions and gases should be as low as possible. Inclusions will disrupt the continuity of the metal, become stress concentration points, and reduce the strength and toughness of the casting; gases will form gas holes during the solidification of the molten metal, reducing the density of the casting. To improve the quality of the molten metal, high-quality metal raw materials should be used in the melting process, and the melting process should be strictly controlled. For example, refining technology can be used to remove inclusions and gases in the molten metal, and modifiers can be added to improve the crystal structure of the metal.
V. Advantages of Machine Tool Castings
(I) Good Vibration Damping
Machine tools will generate various vibrations during operation, and the good vibration damping property of machine tool castings can effectively absorb and attenuate these vibrations. This is mainly due to the presence of graphite in cast iron. Graphite has a flake structure. It is equivalent to a series of tiny "cracks" in the cast iron matrix. When the vibration wave propagates to the graphite flakes, due to the weak interface bonding force between graphite and the matrix, the vibration energy will be reflected, refracted, and scattered at the interface, thus consuming a large amount of vibration energy and playing a good vibration damping role. For example, in some high-precision machine tools, the bed is made of cast iron castings, which can provide a stable working environment for machine tool processing and reduce the impact of vibration on machining accuracy. Compared with other materials, such as cast steel or aluminum alloy, the vibration damping property of cast iron is more prominent. The structure of cast steel is relatively dense and lacks a vibration damping structure like graphite; although aluminum alloy has a relatively low elastic modulus and can absorb vibrations to a certain extent, its vibration damping effect is not as good as that of cast iron.
(II) High Temperature Resistance
Machine tools may face high-temperature environments during certain processing operations, and machine tool castings need to have a certain high-temperature resistance. The graphite in cast iron has good stability at high temperatures. It can prevent the rapid transfer of heat to a certain extent and play a role in heat insulation. At the same time, cast iron can maintain a certain strength and hardness at high temperatures and will not soften or deform rapidly due to temperature rise. For example, in some hot working machine tools or machine tool components close to heat sources, cast iron castings can withstand the test of high temperatures and ensure the normal operation of the machine tool. Different types of cast iron also have differences in high-temperature resistance. For example, ductile iron has relatively better performance at high temperatures due to its spherical graphite. The retention rate of its strength and toughness at high temperatures is relatively high, so it is widely used in some machine tool castings with high requirements for high-temperature resistance.
(III) Good Processability
Machine tool castings have excellent processability, which makes them have high feasibility and economy in the casting production process. Firstly, the melting point of cast iron is relatively low. Compared with other metal materials such as cast steel, it requires less energy for melting and has relatively lower equipment requirements. For example, the melting point of gray cast iron is generally between 1150°C and 1250°C, while the melting point of cast steel is usually above 1450°C - 1550°C. The lower melting point reduces the difficulty and cost of casting production. Secondly, cast iron has good fluidity. It can more easily fill the mold cavity during the casting process, and even some castings with complex shapes can be successfully formed. This is because the carbon in cast iron exists in the form of graphite, and graphite can play a role in lubricating and improving fluidity in the liquid metal. In addition, the shrinkage rate of cast iron during solidification is relatively small, reducing the possibility of defects such as shrinkage cavities and porosity in the casting and improving the yield of the casting. Moreover, cast iron has good cutting performance. Due to the presence of graphite, it can play a role in chip breaking and lubricating the cutting tool during the cutting process, reducing the cutting force and tool wear, facilitating subsequent machining, and enabling machine tool castings to better meet the shape and size accuracy requirements of the design.
VI. EATHU's Outstanding Contribution in the Field of Machine Tool Casting
(I) Application of Advanced Environmental Protection Technology and Equipment
EATHU attaches great importance to environmental protection issues in the casting process and actively introduces advanced environmental protection technology and equipment. In terms of waste gas treatment, a highly efficient multi-stage dust removal and purification system is adopted. For the dust-laden flue gas generated during the melting process, large particles of dust are first removed by an inertial dust collector, then finely filtered by a bag filter, and finally harmful gases are removed by an activated carbon adsorption device to ensure that the waste gas emission is far below the national environmental protection standards. In terms of noise control, professional sound insulation and noise reduction facilities are equipped for various types of equipment in the casting workshop. For example, a customized silencer cover for the furnace fan, high-performance vibration reduction pads installed at the bottom of the molding machine and cleaning equipment, etc., effectively reduce the noise level in the workshop and create a relatively comfortable working environment for employees.
(II) Precise Optimization and Control of Process Parameters
EATHU, relying on its rich experience and advanced technical means, has carried out precise optimization and strict control of the process parameters of machine tool casting. Through the establishment of a digital casting process simulation platform, parameters such as casting temperature, speed, mold preheating temperature, cooling rate, etc. are simulated and analyzed before actual production to predict possible defects and make optimization adjustments. For example, when producing a high-precision machine tool bed casting, the simulation platform determines that the optimal casting temperature range is 1360°C - 1420°C, and the casting speed is controlled at 0.5 - 0.8 meters per second, ensuring the smooth filling of the molten metal in the mold and effectively reducing the occurrence of defects such as gas holes and slag inclusions, greatly improving the quality stability and yield of the casting.
(III) Research and Development and Supply of High-Quality Materials
EATHU is committed to the research and development and supply of high-quality materials for machine tool castings. In terms of metal raw materials, it has established long-term cooperative relationships with multiple high-quality suppliers to ensure the stable and reliable quality of purchased pig iron, scrap steel, and other raw materials. At the same time, a series of alloy additives for different machine tool casting performance requirements have been independently developed. By precisely controlling the composition and proportion of these additives, the quality of the molten metal can be effectively