3D printing technology, rapid prototyping technology, also known as additive manufacturing, is based on a digital model, using powdered metal or plastic bonding materials, through the layer-by-layer printing method to construct objects. 3D printing technology combines cutting-edge technologies in materials technology, digital modeling, information processing and other fields, breaking the traditional processing thinking mode and being regarded as “the most iconic production tool of the third industrial revolution”. [1] 3D printing technology is used in jewelry, industrial design, construction, automotive, aerospace, medical industry and other fields.
Ceramic materials have high strength, high hardness, high temperature resistance and corrosion resistance. They are widely used in biological and mechanical engineering. However, due to their hard and brittle characteristics, ceramic molding is difficult, the processing cost is high, and it takes a long time. The application of 3D printing technology to the production of ceramic products will greatly reduce the production cycle and production cost of ceramic products, and promote the utilization of ceramic products.
1.3D printing ceramic technology
At present, ceramic 3D printing technology mainly includes laser selective sintering technology (SLS), fused deposition molding technology (FDM), layered solid manufacturing technology (LOM), three-dimensional printing technology (3DP) and inkjet printing technology (IJP).
1.1 Laser Selective Sintering Technology (SLS)
The laser selective sintering technology (SLS) is mainly realized by the three structural components of the pressure roller, the laser and the table. The specific principle is that the powder is laid on the workbench by a pressure roller, and the computer controls the laser beam to scan the powder of the prescribed range, and the binder in the powder is melted by laser scanning to form a layered structure. After the scanning is finished, the workbench is lowered, the pressure roller is coated with a new layer of powder, scanned again by the laser, and bonded to the previously cured sheet-like ceramic, the same step is repeated, and the finished product is finally printed. [2]
The main advantages of laser selective sintering technology are wide printing materials, high molding efficiency and material utilization, and low cost. Due to the need for laser introduction during the molding process, the powder needs to be preheated and cooled, the molding cycle is long, and the subsequent processing process is complicated. At the same time, since the raw material powder to be used needs to be bonded under the action of a laser and completely fired at a high temperature, the types of products that can be prepared are limited.
1.2 fused deposition molding technology (FDM)
The raw materials for the fused deposition molding technique are hot-melt ceramic materials, and most of them are made into a filament shape which is convenient for storage and transportation. The fused deposition printing device is mainly composed of three parts: a feeding roller, a guide sleeve and a nozzle. At the beginning, the hot-melt filament material enters the guide sleeve through the feeding roller and under the joint operation of the driven roller and the driving roller, and the friction coefficient of the guide sleeve is low, so that the filamentous material enters the nozzle accurately and continuously. The material is heated and melted in the nozzle and printed according to a digital model of the computer output. [3]
The fused deposition molding technology does not require the help of laser technology, has the advantage of low cost, and is easy to use and maintain. The disadvantage is that the printing process requires a support structure. During the process of stacking printing, as the height increases, the mass of the upper portion increases, and the strength of the lower material is insufficient to support and fix the upper material. In particular, when printing a product having a complicated shape, the upper printed matter tends to be larger than the underlying printed matter, and in order to prevent the ceramic article from collapsing during printing, an external support structure is required.
The fused deposition molding technique is simple in principle and the process is relatively easy to control, but the printing process requires a relatively high temperature to melt the printing material, which requires the material to be easily decomposed after heat fusion and maintain proper fluidity. In order to meet the structural performance requirements of the product, the printed material must have a certain compressive strength and a certain rigidity. In order to ensure the dimensional accuracy of the material, the shrinkage rate of the material during solidification molding cannot be excessive. Therefore, ceramic fused deposition molding technology has been greatly restricted.
1.3 Layered Entity Manufacturing Technology (LOM)
The layered entity is manufactured by laser cutting ceramic film sheets, using a film sheet coated with hot melt adhesive on the back as a raw material, and the layers and layers are heated and pressed together, and the shapes of the layers are cumulatively added to form a solid piece. The hot melt adhesive contains a resin, an organic binder, etc., and is sent to the surface of the adherend by a hot melt adhesive, and the hot melt adhesive is cooled to complete the bonding. [4] Layered solid manufacturing technology uses the ceramic chip to cut and form, directly from the face to the body, omitting other techniques from point to line, from line and surface processing, which is a layered entity manufacturing technology Advantages compared to other 3D printing technologies.
The ceramic flakes used in the layered solid manufacturing technology can be prepared by the casting method, and the foreign technology for preparing the ceramic flakes by the casting method is relatively mature, and the raw material is very convenient to obtain. The layered entity manufacturing technology has a fast forming speed, and the preliminary preparation work is simple, but the material utilization rate is low. The molding principle is simple, the working space is large, and it is suitable for processing large-sized parts, but the mechanical parts produced by the layered solid manufacturing technology have poor mechanical properties and low precision, and are not suitable for processing precision parts.
1.4 3D printing technology (3DP)
The three-dimensional printing technology uses a computer to control the precision nozzle to first spray the binder solution on the flattened ceramic powder according to the shape of the part interface, and then bond the powder together to form the contour of the part, so that the layers are stacked and finally processed. Parts are required. [5]
The three-dimensional printing technology has a simple molding principle and can be adapted to print a variety of ceramic materials, such as zirconia ceramics, zircon sand, aluminum oxide, silicon carbide and silicon oxide. Since the process is bonded by means of a spray-bonding agent, the choice of the binder and the proportion of the proportion are very important. Adhesives that meet the requirements must have proper viscosity and surface tension. To meet this requirement, it is sometimes necessary to add a certain amount of additives such as dispersants and active agents to the binder.
1.5 inkjet printing technology (IJP)
Inkjet printing technology is developed from the three-dimensional printing technology, which combines ceramic powder with various organic substances and solvents into ceramic ink. The ceramic ink is sprayed onto the platform layer by layer by computer instruction to form the desired shape and Size ceramic body. [5] The preparation of ceramic ink is the key to inkjet printing technology. It requires ceramic powder to have good uniform dispersion in ink, suitable surface tension, viscosity and electrical conductivity, faster drying rate and higher solid phase. content.
Inkjet printing technology does not require laser technology to work, saving production costs. However, the current configuration of ceramic inks and clogging of inkjet printheads restricts the development of this technology. Therefore, in the future research, we should pay attention to the following problems: (1) Reasonably select the size of the inorganic non-metallic particle size and the viscosity of the binder in the ceramic ink; (2) Select the appropriate nozzle capillary according to the content of each additive in the ink. diameter.
2.3D printed ceramic material
Ceramic materials have the advantages of high temperature resistance and high strength, and are widely used in industrial manufacturing, biomedical, aerospace and other fields. The development of 3D printed ceramic raw materials has also become a major factor restricting the development of 3D printed ceramics. It is especially important to develop new 3D printed ceramic materials. The following are some of the 3D printed ceramic materials that are still under development.
2.1 alumina ceramics
Alumina is a widely used ceramic material. Alumina ceramics have a wide range of raw materials and low cost, and have become one of the most used raw materials in the ceramic industry. The traditional process of preparing alumina ceramics is complicated, time-consuming and labor-intensive. The 3D printing ceramic technology has the advantages of simple process, short time consumption and strong operability. The use of 3D printing technology to produce alumina ceramics can greatly shorten the preparation time, improve the precision of products, and expand the application field. [3]
In the ceramic 3D printing technology, in order to ensure good mechanical properties of the ceramic body, the alumina material is generally mixed with organic materials to form a slurry, a powder or a powder with other alloy powders.
2.2 Tricalcium phosphate ceramics
Tricalcium phosphate ceramics, also known as tricalcium phosphate, are widely found in human bones and are widely used as a good three-dimensional scaffold for bone repair in the medical field. They can also be used to prevent and treat calcium deficiency disorders. The chemical composition of tricalcium phosphate is very similar to that of bones, and it has the advantages of no variability, good biocompatibility, and can exert good bone conduction. After implantation, the good biodegradability of tricalcium phosphate itself helps the body to metabolize faster. Therefore, the development prospects of this material are very impressive and are closely watched by people. [2]
Research on 3D printing technology of calcium phosphate ceramics has been carried out abroad. G.A. Fielding et al. prepared a ceramic slurry by mixing calcium phosphate with ethanol and successfully printing. At the same time, domestic scholars also have deep research on the biological activity of calcium phosphate ceramics. For example, Lin Kaili added biologically active elements to the calcium phosphate ceramics to improve the biological activity of calcium phosphate ceramics, which is a 3D printed bioceramic technology organism. The improvement of function plays an important role. [3]
2.3 organic precursor ceramics
The technique of organic precursor synthesis of ceramics was invented in 1960. The preparation of ceramics via precursors can be used to prepare a variety of new ceramics from molecular scale design, network size forming, and low decomposition temperature and high temperature performance stability. The main principle is to thermally degrade organic precursor materials (polycarbosilane, polynitrosilane, polysiloxane, etc.) to prepare ceramics. The specific process is that organic small molecules form organic macromolecules through condensation reaction, and the macromolecules form organic-inorganic intermediates, ie precursors, under the conditions of heat or light, and then further thermally crack and sintering the precursors to form ceramics. .
T.A.Schaedler et al. combined UV curing technology with 3D printing technology to print precursor ceramics, which not only realized the complex shape and fine structure of ceramics, but also shrinked the ceramics by high-temperature sintering to prepare high-density ceramics. [3]
2.4 silicon nitride ceramics
Silicon nitride ceramics are excellent in high temperature engineering materials due to their high strength, low density and high temperature resistance. Its strength can be maintained to a high temperature of 1200 ° C without falling, does not melt into a melt after heating, will not decompose until 1900 ° C, and has a very high corrosion resistance, but also a high-performance electrical insulation material. Li et al. used a combination of three-dimensional printing and pressureless sintering to prepare porous silicon ceramic materials with a porosity higher than 70%. [1]
2.5 carbon titanium silicide ceramic
Titanium carbonitride ceramics have a layered hexagonal crystal structure and are widely used in biology and medical fields. The titanium carbonitride material has the advantages of high thermal conductivity, high electrical conductivity, good ductility, plasticity and high strength, stability, corrosion resistance and oxidation resistance of the metal material. Sun et al. used 3D printing and cold isostatic pressing technology to prepare dense carbon titanium silicide ceramics. [1]
3. Conclusion
At present, the domestic research on 3D printed ceramic technology is still in its infancy, and it lags far behind the level of the United States, Germany, Japan and other countries. There is still a lot of room for development. The application of 3D printing technology in the ceramic field is not yet mature. Considering the market, 3D printed ceramic technology is difficult to integrate with the market, and it is difficult to form economies of scale. In the future, the main direction of the industrialization of 3D printed ceramic materials in China is to strengthen the basic research of 3D printed ceramic materials, solve the mechanical properties of 3D printed ceramic materials and the shrinkage rate of sintered products, and develop serialized 3D printed ceramic materials and form Industrial production capacity.
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