The raw materials commonly used in SLS technology are powders of plastics, waxes, ceramics, metals, and their composites. Wax can be used as a precision casting wax mold, thermoplastics can be used as a lost mold, ceramics can be used as casting shells, cores and Ceramic parts, metal can be used as metal parts. This technology is less expensive and can produce complex shaped parts, but the forming speed is slower, resulting in lower precision and lower strength due to the lower density of the powder coating.
Fused Deposition Modeling (FDM) is a process in which a wire is liquefied and deposited into a complex shape by a nozzle. The FDM process replaces the laser with a liquefier. The key to the technology is to obtain a melt with a low viscosity, easy deposition, and a controlled stable "road" width.
The wire used in FDM technology includes paraffin, plastic, low melting point metal and ceramics, and can directly prepare metal parts and various models. FDM technology is less expensive, while forming at a slower speed and with lower precision.
Three-Dimensional Printing (3DP) works like an inkjet printer. It uses a nozzle to spray a liquid binder onto a specific area of ​​a pre-paved powder layer, and then sprays the powder and sprays the adhesive. Get the shape of the part you want. It is also possible to spray the ceramic slurry directly layer by layer to obtain the desired shape, the technical key of which is to prepare a suitable binder.
Materials for 3DP technology include ceramic, metal, and plastic powders. The 3DP technology has lower cost and faster forming speed, but the looseness of the powder layer affects the strength and precision.
2 RP technology to produce ceramic parts
Due to rapid layer-by-layer forming, no mechanical processing, and no mold, RP technology has great appeal for the production of complex-shaped ceramic structural parts that are difficult to manufacture and difficult to process. It can also manufacture functionally graded materials, functional materials such as capacitors. , circuit boards, thin film thermocouples, stress sensors, etc. The RP technology that has been used to prepare ceramic parts is described below.
2.1 LOM
Donald A. Klosterman et al. [5-7] of Dayton University used LOM technology to prepare ceramic parts, ceramic matrix composites and resin-based composite materials. Their ultimate goal is to produce high-density ceramic-based composites with almost no residual processing.
The first step is to produce single-phase ceramics, which have been prepared with single-phase SiC and AlN ceramic parts. First, a ceramic film having a width of 20 cm, a length of 1 m, and a thickness of 0.15 mm and 0.3 mm was formed by a standard casting process, and the film was composed of 60 vol% ceramic powder and a binder. Three powder systems were studied: coarse SiC (30 μm) powder + graphite powder; crude SiC (30 μm) + fine SiC (2 μm) powder + graphite powder; and sinterable AlN (2 μm) powder. Since the ceramic film is short and cannot form a continuous roll and is low in strength and is not sufficient to withstand the feed motion, the placement of the film depends on manual operation. The body produced by the LOM process is soft and easily deformed, so that the body is hardened by a partial degreasing process to make it easy to handle. The partial degreasing process requires strict control of the heating to volatilize the plasticizer while maintaining the original adhesion. Prior to sintering, a complete degreasing process was developed based on the thermal analysis data. The sintering process is carried out by liquid or gaseous infiltration of Si, and the free Si reacts with the previously added C to form SiC. The SiC content is increased, so that the part has no obvious shrinkage. The case of the AlN ceramic member is not described, and the main problem is also the sintering shrinkage and deformation. The room temperature four-point bending strength value of the SiC member is 160 MPa, which is improved as the lamination technique is improved.
Improvements in the LOM system include supplying an inert gas over the cutting zone to avoid oxidation of C or N, redesigning the feed mechanism to reduce waste and automate the costly ceramic membrane.
Because of the limitation of the layer thickness, the surface of the green body is not smooth and thus boundary polishing is required. The boundary treatment can be performed by two methods of cross grid cutting and surface polishing. The surface polishing method has less chips and a smoother surface. Both methods require adjustments to the software.
At the beginning of the study, a heat roller was used to provide pressure and heat source to increase the interlayer bonding. The temperature of the hot roller was as high as 120-180 ° C to melt the binder to achieve bonding. Due to the temperature gradient, there is a discontinuous bond between the layers resulting in cracks. The strength is lower due to interlayer cracks and pores. By spraying glue, the bonding force between layers can be improved, and the temperature of the heat roller can be lowered. The disadvantage is that the difficulty of boundary polishing is increased, and a stricter degreasing process is required. In addition, post-forming pressurization treatment can also improve interlayer bonding. Through the process improvement, the four-point bending strength of the SiC parts is 200-275 MPa.
In order to achieve the ultimate goal of preparing ceramic matrix composites, Al 2 O 3 /SiC w , Al 2 O 3 /SiC f , SiC/SiC f were used as the research system. The problem encountered is first of all a problem of uniform distribution of whiskers or fibers. The solution is to use a layered method in which a single-phase SiC film is alternately stacked with a SiC f /resin pre-formed film, and the resin serves the dual function of providing strength and carbon source. This method avoids the damage caused by the friction between the fibers and the powder, and at the same time increases the volume content of the fibers in the product. The pre-formed film has a thickness of 0.25 mm and a volume content of 50%.
Lone Peak Engineering's E. Alair Griffin et al. [8] used LOM technology to prepare ZrO 2 and Al 2 O 3 ceramic parts. The selection principle is that ZrO 2 martensitic transformation has a reinforcing effect on the product. The film thickness was 116 μm and 58 μm, and the ceramic film material system was 12 mol% CeO 2 -ZrO 2 and Ce-ZrO 2 /Al 2 O 3 . The multilayer composite comprises a single phase Ce-ZrO 2 piece, a Ce-ZrO 2 and an Al 2 O 3 /Ce-ZrO 2 interleaved laminate. There is <1% pores in the sintered product, and there are no coarse crack defects in the green body and the sintered body. The thickness of the sintered layer is changed from 116 μm and 58 μm to 85 μm and 44 μm. In the Al 2 O 3 /Ce-ZrO 2 layer, Al 2 O 3 is uniformly distributed at a particle size of about 5 μm; since the Al 2 O 3 grinding ball is used in the preparation of the Ce-ZrO 2 slurry, the Ce-ZrO 2 layer is also A small amount of Al 2 O 3 is present . The interlayer bonding is also very good, the phases are evenly distributed, and the interface has no cracks. The strength of the single-phase ceramic member is 400 MPa, and the composite ceramic member is 500 MPa. The hardness of the Ce-ZrO 2 layer was 9 GPa, the hardness of the Al 2 O 3 /Ce-ZrO 2 layer was 15 GPa, and the hardness near the interface was 11.5 GPa.
The company's Curtis Griffin [9] and others used the LOM technology to prepare Al 2 O 3 samples and parts. The Al 2 O 3 film size was 10 cm × 15 cm × 0.015 mm. At the same time, samples were prepared by dry pressing using the same material for comparison. The comparison shows that the two sintered parts have the same sintered density and similar microstructure, but the density of the body, the weight loss and the amount of shrinkage are different, which is related to the difference in the binder content of the two methods, which also leads to LOM forming. The open porosity of the piece is high. Tests have shown that mechanical properties are independent of the forming method and test direction, and it is difficult for the microstructure to identify the boundary between layers. The strength is comparable to that of commercial products, the hardness is higher than that of commercial products, and the fracture toughness is the lower limit of commercial products, but this difference may be caused by different test methods, such as Vickers hardness for hardness and Knoop hardness for commercial products. The results show that the performance of the LOM forming parts is comparable to that of the dry pressing method, and the advantages are high efficiency and the ability to make complicated parts.
James D. Cawley et al. [10] of Case Western Reserve University used CAM-LEM (Computer-Aided Manufacturing of Laminated Engineering Materials) technology to make ceramic parts. The forming principle is the same as LOM technology. Suitable materials include engineering ceramics, composite materials, and metals. And alloy materials, plastic materials, etc. Three parts were made using Al 2 O 3 : a flange, a three-layer ceramic piece and a flow augmenter, which were practical. The results show that CAM-LEM is suitable for use as a sheet of engineering material that can be used for handling by a feed mechanism. The CAM-LEM system includes a 50W CO 2 laser, a 100mm x 100mm xy translation system, a four-node robot, an IPC workstation, a real-time control system, a pneumatic fixed platform, and a fixture. For parts with complex structures, a method of adding a temporary organic film can be employed.
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Hydraulic Pump classification:Pneumatic Hydraulic Pump & electric hydraulic pump & manual hydraulic pump
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