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Detailed explanation of 3D printing casting process

The steps of 3D printing casting and the key technologies and optimization methods in the process are described

Basic concepts of 3D printing casting

3D printing casting process is a manufacturing method that combines traditional casting process and 3D printing technology. Traditional casting process has a long history and is a metal hot working process. For example, liquid metals (such as copper, iron, aluminum, tin, lead, etc.) are cast into cavities (casting molds, materials can be sand, metal, or even ceramics) that match the shape of the parts, and then cooled and solidified to obtain parts or blanks. 3D printing technology appeared in the late 1980s to early 1990s, also known as rapid prototyping technology. Its principle is to use 3D digital model files as input, and use adhesive materials such as powdered metals or plastics to construct objects by printing layer by layer. In terms of image, ordinary printers output 2D images or graphic digital files to paper through ink; 3D printers output real raw materials into a thin layer, and then repeatedly layer by layer, finally becoming a spatial object.
The 3D printing casting process combines the two, with the core idea of using 3D printing technology to construct molds, and then manufacturing parts through traditional molten metal or alloy casting. The biggest feature of this process is that it can flexibly and quickly print complex and personalized molds according to design requirements. For example, in the manufacturing process, customized castings can be made. Traditional casting processes are difficult and expensive to make molds with complex shapes, while 3D printing casting can customize castings with various shapes and structures through 3D printing technology to meet personalized needs. Metal parts with complex microstructures are used in fields such as microscopic devices, bionics, and medical apparatus, or the production of unique artworks and decorations is a way to break through the limitations of traditional casting and showcase creative and personalized designs.
In addition, this process has some advantages, such as rapid sample verification. Traditional casting takes a long time to make samples and is not easy to modify. 3D printing casting can quickly manufacture sample molds to quickly cast samples for verification. In the manufacture of complex structural castings, traditional casting is difficult to achieve. 3D printing casting can accurately print molds with complex structures, achieve filling of complex shapes and internal channels during the casting process, and make the manufacture of complex structural castings feasible. In addition, in terms of mixed material castings, traditional casting usually relies on a single metal or alloy material, while 3D printing can mix different material powders for printing, achieve mixed material casting manufacturing, and open up more possibilities for material combination and Performance optimization.

The main steps of 3D printing casting

Taking 3D printing resin investment casting as an example, the steps are as follows:
  • CAD modeling, 3D printing lost foam
      Firstly, it is necessary to use CAD software to design a digital file of the melt casting model. CAD (Computer – Aided Design) software is a tool used by engineers to design 3D models. There are many free and professional CAD programs compatible with 3D printing on the market. Reverse engineering can also be used to generate digital models through 3D scanning. Then export the file in .STL format. The .STL (Stereolithography) file uses triangles (polygons) to describe the stereoscopic parameter information of the object, which is a key file format conversion in the 3D printing process and can be recognized by the 3D printer. Afterwards, use a 3D printer (SLA technology 3D printer is recommended) to print it out. This printing process generally takes only a few hours. SLA (Stereolithography Apparatus) technology, which uses liquid photosensitive resin as raw material and solidifies layer by layer under ultraviolet light, has high resolution and detail expression, and is suitable for making complex and precise models.
  • Check the melt casting model for cavities
      Surface polishing and other post-processing work are carried out on the 3D printed model to remove surface layer patterns and improve the surface quality of the model. Polishing work requires the use of suitable tools and materials, such as sandpaper. Afterwards, the model should be carefully checked for holes or cracks, because the holes and gaps on the model will affect the subsequent casting work, such as the metal liquid may leak during the casting process, leading to casting failure.
  • Surface coating
      After sending the model to the foundry, first coat the surface of the model with ceramic slurry. This layer of slurry should closely adhere to the investment casting model, and the quality of the first layer of slurry will directly affect the surface quality of the final casting part. Because this layer of slurry will form the surface layer of the casting part, if there is unevenness or poor adhesion to the model, defects will appear on the surface of the finished product.
  • Shell making
      After coating the ceramic slurry, sand is stuck on its outer layer. After it dries, repeat the steps of coating the slurry and sticking sand until the shell reaches the ideal thickness. This shell will play an important role in the subsequent firing process. It needs to have sufficient strength to withstand high temperature and the impact of internal model combustion, ensuring that the overall shape will not collapse.
  • Roasting, cleaning
      After the shell is dried, it is put into the furnace for incineration until all the molten casting models inside are burned clean. At this time, the shell will become pottery as a whole due to heating. During the baking process, pay attention to controlling parameters such as temperature and time. After being taken out of the furnace, the inner surface should be thoroughly cleaned by washing with water and other methods, and then dried and preheated to prepare for the next casting step.
  • Casting
      By pouring, pressure, vacuum suction, centrifugal force and other methods, the molten liquid metal is filled into the empty shell. During the casting process, parameters such as temperature and flow rate of the liquid metal will affect the quality and molding effect of the casting. Different metal materials may require different casting temperatures and speeds. For example, the casting temperature and speed of aluminum alloy are different from those of steel. If the temperature is too high, defects such as shrinkage and looseness may occur; if the flow rate is inappropriate, it may cause pores or uneven surfaces inside the casting. After casting, wait for the liquid metal to cool down.
  • Demoulding
      After the liquid metal is completely cooled and formed, the ceramic shell outside the metal is completely cleaned by mechanical vibration, chemical cleaning, or water flushing. The mold removal process requires careful operation to prevent damage to the already formed casting. The mechanical vibration method is suitable for situations where the shell is easily separated from the casting; chemical cleaning can thoroughly dissolve the shell residue, but attention should be paid to the erosion effect of the agent on the casting itself; the water flushing method is simple and convenient, but sometimes it may not be able to remove some stubborn shell residues.
  • Post-processing
      Surface treatment of metal models, such as sanding and polishing, is carried out to improve the smoothness and dimensional accuracy of the casting surface. Further machining can be performed, such as drilling and milling, to meet specific design requirements. At this stage, the dimensional accuracy, density, and other mechanical properties should also be tested to ensure that the final casting meets Quality Standards.

Key technologies in the 3D printing casting process

(1) High-precision 3D printing technology

  • In the 3D printing casting process, materials are deposited layer by layer through 3D printing technology to manufacture molds or investment molds for casting. Different 3D printing technologies will affect the accuracy and surface quality of the molds. For example, SLA technology has high molding accuracy and can produce fine structures, which is suitable for the manufacture of small precision molds. SLA uses liquid photosensitive resin and solidifies layer by layer under ultraviolet light irradiation, which can achieve high resolution and detail expression, so that the molds manufactured can accurately restore the designed shape and help produce high-precision castings.
  • The accuracy of 3D printing is also related to printing parameters, such as layer height and printing speed. The smaller the layer height, the higher the accuracy of the printed parts in the vertical direction, but it may increase the printing time. Printing too fast may cause problems such as unstable material bonding, which needs to be optimized and adjusted according to specific printing materials and 3D printing technologies. In addition, correct maintenance and calibration of 3D printers are also crucial for producing accurate printing. Because 3D printers are usually composed of many small and complex parts, if the printer’s nozzle is blocked or there are deviations in mechanical parts, it will affect the accuracy and quality of printing.

(2) 3D printing materials suitable for casting

  • When choosing 3D printing casting materials, the materials must have high temperature resistance and fluidity. During the casting process, the mold or investment mold needs to withstand the initialization and filling of high-temperature liquid metal. If the material has poor temperature resistance, it will cause deformation or even damage to the mold. For example, ceramic materials have the characteristics of high melting point and high hardness, which can be used to manufacture molds in some high-temperature casting environments. At the same time, the material should have good fluidity so that it can be smoothly stacked and formed layer by layer during the 3D printing process. Some special resin materials have been developed based on such requirements for use in the 3D printing casting process.
  • 3D printing casting can also use a variety of metal materials, such as aluminum alloy, titanium alloy, stainless steel, etc., to meet the requirements of different industries for casting performance. The molds or directly printed models made of these metal materials have guaranteed mechanical properties such as strength and toughness, which is conducive to casting parts with excellent performance. Moreover, different metal materials have different physical and chemical properties, which have great advantages in meeting the diverse needs of casting parts. For example, the aerospace industry may require lightweight and high-strength titanium alloy castings, while the automotive industry has a greater demand for aluminum alloy castings because aluminum alloy has the advantages of light weight and relatively low cost.

(3) Model design and simulation technology

  • 3D printing casting technology is closely integrated with Computer Aided Design (CAD). Through CAD software, complex geometric shape models can be designed, which is the basis for realizing personalized and customized parts production of 3D printing casting. CAD software can accurately control the size, shape, structure and other parameters of the model, so that the designed model can accurately meet the casting needs. Moreover, by using the 3D visualization function of CAD software, designers can observe the shape and structure of the designed parts in space in advance, so as to timely discover unreasonable parts in the design and make modifications.
  • Finite element analysis (FEA) also plays an important role in the design process of 3D printing casting models. Through FEA technology, engineering analysis can be performed on castings, such as simulating the stress distribution and deformation of castings under external forces, temperature changes, and other working conditions. Based on the analysis results of FEA, designers can optimize the design of the model, reduce material usage, reduce weight, and improve mechanical properties. In this way, the design scheme can be evaluated and optimized before casting to ensure that the final casting performance meets the requirements.
  • The simulation technology of the casting process is also a key part. After the model (which can be a mold or investment mold) is manufactured by 3D printing, a computer program can be used to simulate the casting process. For example, the flow of liquid metal inside the model can be simulated. If there is a risk of poor flow or porosity, the casting port position and pouring system of the model can be adjusted. This helps to reduce waste generation, reduce production costs, and detect potential problems in advance to ensure that the produced casting parts meet the requirements.

Example analysis of 3D printing casting process

(1) Analysis of 3D printing sand casting

Taking the production of turbine industrial parts by a certain 3D printing sand casting as an example:
  • CAD design
      Firstly, use CAD software to accurately design the three-dimensional model and casting (including process information such as pouring system) of the turbine industrial parts. During the design phase, it is necessary to determine the size, shape, and structural details of each part according to the actual working requirements of the turbine. For example, the shape and angle of the turbine blades, the structure of the internal flow channel, etc. will affect its performance. Accurate digital models can be designed through CAD software.
  • Process simulation
      After the CAD design is completed, process simulation is carried out. The casting process is simulated using computer simulation software, including the flow of liquid metal in the sand mold, possible defects during solidification, etc. If it is found that there are areas where liquid metal flows poorly or shrinkage may occur during solidification in the simulation results, the original CAD model or casting process plan can be adjusted for these problems, such as adjusting the position or size of the pouring port.
  • 3D printing sand mold
      According to the results of the process simulation, the 3D printing sand mold is started. The process of 3D printing sand mold is to first lay a layer of sand, solidify it with adhesive, and then layer by layer. The material of the sand mold is quartz sand, and resin is used to bond the sand material together to form resin sand. During the 3D printing process, attention should be paid to parameters such as the thickness of the resin sand material and the amount of adhesive used to ensure that the performance indicators such as strength and air permeability of the sand mold meet the requirements. For example, if the layer thickness is too large, it will affect the accuracy of the sand mold, and if the layer thickness is too small, it may cause insufficient strength of the sand mold.
  • Casting
      After 3D printing the sand mold, the casting operation is carried out. The molten liquid metal is injected into the sand mold cavity through the casting system, and the formed turbine casting is obtained after the metal cools and solidifies. Since the size and shape of the sand mold have been accurately controlled during 3D printing, as long as the casting process parameters (such as casting temperature, casting speed, etc.) are properly controlled, the quality of the casting can be guaranteed.

(2) Analysis of 3D printing wax mold casting

Taking the production of parts in 3D printing wax casting as an example:
  • 3D printing wax mold
      After receiving the customer’s digital file, send the file to the 3D printer for wax model printing. The 3D printing technology that is compatible with the wax material can ensure the accurate shape of the printed wax model. 3D printing technology can effectively manufacture complex structures of wax models, such as parts with fine structures or special shapes inside, which can be accurately manufactured. Traditional wax model production processes may have difficulties in manufacturing such complex wax models.
  • Planting wax trees and hanging pulp
      The printed wax mold prototype can be used for planting wax trees, and then pasted on the wax mold. The process of planting wax trees requires combining multiple wax molds according to certain layout rules for subsequent casting operations. The paste used for pasting should evenly cover the surface of the wax mold to form a uniform film, which determines the quality of the shell formed later.
  • Dewaxing and preheating
      Put the wax mold combination after hanging paste into the heating furnace for dewaxing operation. Since a permeable investment casting shell is used, the wax used for investment casting can flow away through the shell. After dewaxing is completed, it is loaded into the preparation furnace and preheated before pouring. The parameters of the dewaxing and preheating steps need to be accurately controlled, such as the temperature and time of dewaxing. Improper operation may cause wax residue or shell damage.
  • Pouring and post-treatment
      After the preheating is completed, the molten metal is poured in. After the metal cools down, the outer shell of the metal is removed and the casting is separated from the wax tree. Finally, some post-processing is performed, such as cleaning the surface and testing the dimensional accuracy, to ensure that the casting meets the quality requirements. By 3D printing wax mold casting, the order-to-delivery time of the casting can be shortened from 6-12 weeks to 2-5 days, greatly improving production efficiency, and the same component can be cost-effectively iterated multiple times to obtain a better final product.

Optimization method of 3D printing casting process

(1) Optimize printing parameters

  • The influence and adjustment of printing parameters on quality
      Printing parameters including layer height, printing speed, printing temperature (for specific printing materials), etc. have a critical impact on printing quality. For example, unreasonable layer height setting may produce a step effect that affects the smoothness of the model surface and reduces dimensional accuracy. If the layer height is set too high, the surface will present obvious layers, affecting the appearance and dimensional accuracy of the casting; if the layer height is too small, although the surface accuracy is improved, the printing time will increase significantly. In terms of printing speed, if the speed is too fast, the material may not be able to solidify or bond in time, resulting in insufficient strength of the model or even structural defects. For different 3D printing materials and printer types, it is necessary to determine a suitable layer height and printing speed range through experiments and experience to balance printing quality and efficiency. Similarly, printing temperature has an impact on the fluidity and curing effect of materials. For some hot-melt materials, high temperature may cause structural deformation and dimensional errors due to excessive material flow, while low temperature may prevent smooth extrusion or incomplete curing.
  • Parameter optimization based on learning algorithm
      Reinforcement learning-based methods can be used to optimize printing parameters. For example, by collecting a large number of printed models’ printing parameters to train a q-learning model, the optimal printing parameters of the printed model are obtained and stored in the trained model. When facing a new model to be printed, it is matched with the printed model, and the optimal printing parameters of the printed model with the highest matching degree with the model to be printed are used as the actual printing parameters of the model to be printed. This method can continuously optimize printing parameters during the 3D printing process, replacing the traditional 3D printing method where printing parameters mostly rely on a large number of process experiments, which can significantly improve the quality of 3D printing, shorten the printing cycle, and reduce costs. As the number of 3D prints increases, the sample size of the printed model increases, and the optimal printing parameters will be continuously optimized, thereby improving the quality of 3D printing.

(2) Material performance improvement and selection optimization

  • Improving the performance of 3D printing materials
      For existing 3D printing casting materials, their performance can be improved by adding certain additives or special treatments. For example, adding reinforcing fibers to resin materials can increase their strength, making them more suitable for manufacturing casting molds that withstand greater stress. For metal materials, their fluidity and high temperature resistance can be improved by optimizing their microstructure or adjusting the alloy composition. Adding a certain amount of titanium element to aluminum alloys can improve their strength and high temperature resistance.
  • Choose the appropriate 3D printing material
      In 3D printing casting, it is crucial to choose the most suitable material according to different casting requirements. For investment casting of small jewelry with extremely high manufacturing accuracy, resin materials with high resolution and low contraction rate may be more suitable. However, for casting of large industrial parts, such as automobile engine cylinder blocks, it is necessary to choose metal materials with high strength, high temperature resistance, and reasonable cost, or high compression resistance sand mold materials (if it is 3D printing sand mold casting). The cost, processability, environmental friendliness, and other aspects of different materials are also factors that need to be comprehensively considered when choosing materials.

(3) Model design optimization

  • Design simplification and optimization based on functional requirements
      During the model design stage, it is necessary to fully consider the functional requirements of the casting, remove unnecessary complex structures or optimize the structure to reduce the difficulty of printing and casting. For example, if there are some structures inside a casting that do not affect the actual function but increase the manufacturing complexity, they can be simplified. At the same time, the design should follow the characteristics of 3D printing manufacturing, such as setting up support structures reasonably (if necessary) to ensure the stability of the model during the printing process. Although the support structure needs to be removed after printing, if the design is reasonable, it can effectively prevent the deformation or collapse of the model during the printing process. For example, for some cantilever structures or model parts of hanging structures, appropriate support structures can ensure printing quality.
  • Optimize design using simulation and analysis tools
      The analysis function of CAD software and specialized finite element analysis software are used to analyze the performance of the model during the design process. For example, the stress-strain situation of the casting under actual working conditions (such as external force loading, temperature changes, etc.) is analyzed, and the structure and shape of the model are adjusted according to the analysis results. By using simulation software for the casting process, the flow process of metal in the mold can be simulated, possible defects (such as pores, shrinkage holes, etc.) can be predicted, and the design optimization of the model or casting process (such as pouring system, cooling system, etc.) can be carried out in advance to improve the success rate of casting and the quality of castings.
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