Knowledge sharing by Guangdong Shunde Teamwork Model Technology Co., Ltd, whom with over 20 years rapid prototyping experience.
Email: ken@gdtwmx.com
Website: www.gdtwmx.com
Powder Sintering Laser Rapid Prototyping System (SLS)
1 Introduction
Laser Rapid Prototyping (LRP) is a new manufacturing technology that integrates advanced technologies such as CAD, CAM, CNC, laser, precision servo drives, and new materials. Compared with traditional manufacturing methods, the prototype has high duplication and interchangeability; the manufacturing process has nothing to do with the geometry of the prototype; the processing cycle is short, the cost is low, the general manufacturing cost is reduced by 50%, and the processing cycle is shortened by more than 70%; Technology integration to achieve design and manufacturing integration.
The recent developments in LRP include: stereolithography (SLA); selective laser sintering (SLS); laser cladding (LCF); laser near-sharpening (LENS); laser thin-film laminating (LOM) technology Laser induced heat stress forming (LF) technology and three-dimensional printing technology.
2 Laser Rapid Prototyping Technology
2.1 Stereo Optical Modeling (SLA) Technology
SLA technology is also called photo-curing rapid prototyping technology. The principle is that the computer-controlled laser beam scans the surface of the photosensitive resin as a raw material point by point. The thin layer (a few tenths of a millimeter) of the resin in the scanned area is cured by photopolymerization. Form a thin layer of the part. The workbench moves one layer thick distance down, so that the cured resin surface is coated with a new layer of liquid resin, and the next layer is scanned. This is repeated until the entire prototype is completed. Since the photopolymerization reaction is based on the action of light rather than on the effect of heat, a lower power laser source is required during operation. In addition, because there is no thermal diffusion, coupled with the chain reaction can be well controlled, it can ensure that the polymerization reaction does not occur outside the laser spot, so the processing accuracy is high, the surface quality is good, the utilization rate of the raw material is close to 100%, and the shape can be manufactured. Complex and delicate parts with high efficiency. For larger parts, they can be fabricated by first forming a block and then bonding.
The United States, Japan, Germany, and Belgium have invested a great deal of human and material resources in researching this technology, and new products have been introduced. Xi'an Jiaotong University in China has also successfully developed a three-dimensional light molding machine LPS600A. At present, more than 10 factories around the world produce this product.
2.2 Selective Laser Sintering (SLS) Technology
The SLS technology is similar to the SLA technology except that the liquid photopolymer is replaced with a powdered raw material and applied to the powdered material with a certain scanning speed and energy. The technology has the advantages of wide selection of raw materials, easy cleaning of redundant materials, and wide application range, and is suitable for the manufacture of prototype and functional parts. During the forming process, laser working parameters, powder properties, and sintering atmosphere are important parameters that affect the quality of the sintering process.
2.3 Laser Cladding (LCF) Technology
The working principle of LCF technology is basically the same as that of other rapid prototyping technologies. It is also through the numerical control of the work table that the laser beam scans and claddings the powder and finally forms the part with the required shape. The results of the study show that the following factors are relevant: the part slicing method, laser cladding thickness, laser output power, spot size, light intensity distribution, scanning speed, scanning interval, scanning method, powder feeding device, powder feeding amount, and powder particle size. The precision and strength of the formed part have an effect.
The difference from other rapid prototyping technologies is that laser cladding can be used to make very dense metal parts. The strength of the laser cladding can reach or even surpass parts produced by conventional casting or forging methods, and thus has a good application prospect.
2.4 Laser Near Lens (LENS) Technology
LENS technology combines SLS technology and LCF technology, and maintains the advantages of both technologies. The metal powders used have three forms: 1) single metal; 2) metal plus low melting point metal binder; 3) metal plus organic binder. Due to the powder application method, the laser-sintered metal is low in density, porous, and low in strength regardless of the type of powder used. To increase the strength of sintered parts, post-treatments such as impregnating resin, low-melting-point metals, or hot isostatic pressing must be performed. However, these post-processing will change the accuracy of metal parts.
2.5 Laser Thin Film Laminate Manufacturing (LOM) Technology
LOM technology is a new type of rapid prototyping technology commonly used to make molds. The principle is to use a high-power laser beam to cut the metal sheet first, then superpose the multi-layer sheet, and gradually change its shape, and finally obtain the stereoscopic shape of the desired prototype.
The production of the die by LOM technology costs about 1/2 less than the traditional method, and the production cycle is greatly shortened. For the production of composite molds, thin molds, progressive molds, etc., the economic benefits are also very significant. This technology has been widely used abroad.
2.6 Laser induced heat stress forming (LF) technology
The principle of LF technology is based on the thermal expansion and contraction of metal, which means that the material is heated unevenly to produce a predetermined plastic deformation. This technology has the following characteristics: 1) No mold forming: short production cycle, high flexibility, especially suitable for the production of single-piece small batches or large-scale workpieces; 2) no external force forming: the root cause of material deformation lies in its internal thermal stress; 3) Non-contact forming: high forming accuracy, no tool and die wear, can be used for the manufacture of precision parts; 4) hot accumulation molding: able to form at room temperature difficult to deform the material or high hardness index metal, and can produce a cold hardening effect, The structure and performance of the deformed zone material are improved.
German scholars M.Geiger and F.Vollertsen have conducted extensive research in the laser processing and other processing methods of composite processing. At present, this technology has been applied to the flexible leveling of automotive parts and other shaped parts.
3 Laser Rapid Prototyping Technology Main Directions
3.1 Materials
Materials are the core of rapid prototyping technology. Any type of rapid prototyping machine is developed for specific types or materials with specific shapes. SLA for liquid photosensitive resins, LOM for paper, FDM for filamentary materials, and powdered materials SLS, now developing a new laser rapid prototyping machine, uses a gaseous material, an active gas, which decomposes under the action of a laser and deposits a layer of ceramic or metal on the work table. Form the workpiece.
In addition, people's greatest expectation for laser rapid prototyping technology is that they form directly the shaped parts that people need, such as plastic parts and metal parts. Even if it is done indirectly, appropriate material and process problems must be solved. The low cost of materials is also a research focus.
3.2 Equipment
The equipment includes not only the hardware, ie, the laser rapid prototyping machine itself, but also the corresponding software, how to slice the 3D CAD software directly, avoiding the approximate processing link of the STL format file, and the purpose thereof is to improve the forming accuracy and forming efficiency of the forming machine.
4 Laser Rapid Prototyping Technology
Laser rapid prototyping technology has become more and more widely used in various fields. The most important applications are the following:
4.1 Application in Product Design
SLA technology is used for automotive engine intake pipe testing. The shape of the intake manifold cavity is composed of a very complex free-form surface, which has a very important effect on improving the intake efficiency and combustion process. During the design process, airway tests are required for different air intake pipe schemes. The traditional method is to manually process the tracheal wood mold or plaster mold described by dozens of sections, and then use the sand mold to cast the air intake pipe. The understanding of drawings by wood molders and their own level of technology often lead to deviations of parts from design intent. Sometimes the effect of such errors is significant. Although the use of numerical control processing can better reflect the design intent, but its preparation time is long, especially when the geometry is complex. British Rover Company used the rapid prototyping technology to produce the outer and inner cavity modes of the intake pipe, and achieved satisfactory results.
The application of SLA technology in automobile body manufacturing. SLA technology can produce the required proportion of precision casting molds, thereby casting a certain proportion of the body metal model, using this metal model for wind tunnels and collisions and other tests to complete the final evaluation of the body to determine whether its design is reasonable. Chrysler Corporation of the United States has used the SLA technology to create a body model, put it in a high-speed wind tunnel for aerodynamic test analysis, and achieved satisfactory results, greatly saving the test costs.
The application of SLS technology in automotive lamp manufacturing. The shape of most automotive lamps is irregular, the surface is complex, and mold making is difficult. Through rapid prototyping technology, accurate product samples can be quickly obtained, providing a good reference for mold design CAD and CAM. At the same time, it is also possible to manufacture lamp dies quickly and accurately by means of investment casting using rapid prototyping technology.
Rapid production of prototypes, although differing from the final actual product in terms of material, but due to high shape and dimensional accuracy, can be used to evaluate the shape and size of the product, and can even perform partial performance tests compared to traditional prototype production. Not only saves time, but also saves money. Rapid prototyping technology is used in combination with three-dimensional coordinate measurement, three-dimensional laser scanning, and industrial CT, and reverse engineering (reverse) engineering can be quickly implemented.
4.2 Application in Mold Making
SLS technology is applied in the manufacture of automotive molds. The SLS technology studied by Texas State University has been commercialized by the American DTM company. At present, the company has developed the third generation of SLS2000 series products. The system can sinter wax, polycarbonate, nylon, metal and other materials. The steel copper alloy injection mold manufactured by this system can inject 50,000 pieces of workpieces.
In recent years, mold manufacturing technology based on RPM technology has grown from initial prototype manufacturing to rapid tool and mold manufacturing, and has become the focus of applied research and development at home and abroad. RPM-based mold manufacturing methods can be divided into direct molding methods and indirect molding methods.
The direct molding method directly uses the RPM technique to make the mold. The SLS method can be used to directly fabricate the metal mold in various methods of the RPM technology. The steel copper alloy injection mold manufactured by this method can have a life of more than 50,000 pieces. However, this method has a large shrinkage of the material in the sintering process, and the accuracy is difficult to control.
Indirect molding methods can be divided into: 1) the production of soft simple mold. Using silicon rubber, metal powder epoxy powder and low-melting alloy, etc., to accurately reproduce the prototype into a mold, or surface-treating the prototype, and applying a metal spraying method or a physical evaporation deposition method to plate a lower melting alloy to make a mold . The life span of these simple molds is 50-5000 pieces. Due to their low manufacturing cost and short cycle time, these simple molds are particularly suitable for small batch production at the trial production stage. 2) Production of steel molds. Combining RPM technology with precision casting technology enables rapid manufacturing of metal molds. Or EDM electrodes with high complex precision can be directly manufactured for the processing of mold cavities such as injection molds, forging dies and die castings. A medium-sized, more complex electrode can generally be completed in 4-8h, and the accuracy of the complex shape fully meets the engineering requirements. Ford Motor Company used this technology to manufacture automotive molds and achieved satisfactory results. Shanghai Jiaotong University has also used RP and precision casting methods to produce more than 80 sets of imported and imported molds for the automotive and automotive tire industries. Compared with the traditional machining method, the manufacturing cost and cycle of rapid die manufacturing are greatly reduced. China needs to import more than 800 million U.S. dollars annually for molds, mainly complex molds and precision molds. Therefore, the application of SLS technology in the automotive mold manufacturing industry in the future is very promising.
The paper mold made by LOM directly replaces the sand-casting wood mold by surface treatment, and its advantages are particularly prominent for high-precision molds with complex shapes. The paper mold produced by LOM is directly used as a forming mold of a low-melting point alloy casting mold, a trial-use injection mold, or a wax mold in lost-wax casting. The mold produced by SLS is directly used as a metal mold after copper infiltration.
Using a quick-formed part as a master mold to make a soft mold. Soft molds are molds made of materials such as silicone rubber, epoxy resin, and polyurethane. The use of a rapidly-formed part as a master mold to cast the above material forms a soft cavity, forming a soft mold, which can be used for small batch production, such as injection molded parts, low-melting-point alloy centrifugal casting parts, and the like. For plastic parts, in addition to the usual thermal
Injection Molding (injection molding machine), reactive injection molding can also be used for small batch production. The advantages of reactive injection molding are: the use of liquid raw materials at room temperature, low pressure injection molding, mold requirements are not high. The polyurethane parts obtained after curing have properties similar to those of industrial thermoplastics obtained by an injection molding machine, and articles of different colors can be obtained. The equipment needed is simple, cheap, easy to operate, and the price of the materials used is not high.
Use a quick-formed part to make a hard mold. Hard molds refer to metal molds, ceramic molds, plaster molds, and the like. There are two kinds of production methods: one is to make paper-based molds directly with LOM, and metal molds are made by surface metal arc spraying and polishing. The other is the so-called metal surface, hardback lining die, the production method is rapid prototyping parts (such as LOM pieces) as the mother model, the arc spraying method in the mother die surface to form a 1.6-6.4mm metal shell, The master mold is then removed and a metal-based synthetic material (liquid metal) or epoxy resin is injected on the back of the shell to form a so-called metal-faced, hard-backed die. The above hard molds can be used for sand casting, lost foam molding, injection molding, and simple non-steel drawing dies. The use of rapid prototyping technology to produce molds not only avoids complicated mechanical cutting, but also guarantees the precision of the molds. It can also greatly shorten the time for mold making and save the cost of mold making. For molds with complex shapes, the advantages are particularly prominent. However, there is still a shortcoming that the die life is relatively short. Even if it is a metal surface or a hardback die, its service life is not as good as that of a real die. Therefore, the rapid die is more suitable for single-piece and small-batch production.
4.3 Application in Orthopaedic Medicine
Each person's bones and teeth are different. In orthopaedic medicine, there are often problems with repairing or copying parts of the body's bones or teeth. How to customize it is a difficult problem that has not been solved in the past, and often causes problems for doctors. The patient brings pain.
The laser rapid prototyping technology brings a turning point to this situation. Using the principle of reverse engineering (reverse) engineering, this problem can be solved quickly and easily. Recently, a successful application was carried out at Peking University Hospital. A patient with pelvic cancer needs to replace the damaged right hip hemipelvic with an artificial hemipelvis. In the past, this type of surgery required measurement and grinding adjustment during the procedure, and it was difficult to achieve the desired shape. This time it was performed with CT. The patient's pelvis was scanned, a CAD model was obtained, and then the healthy left hemipelvis was mirrored to obtain the right hemipelvic model. The ideal shape was thus created. Once the operation was successful, the operation time was greatly shortened.
5 Conclusion
Since the advent of the first laser rapid prototyping machine and now just over ten years ago, the domestic research history is only about ten years. The industrial application has only begun in recent years. It can be said that it is in the ascendant, I believe laser rapid prototyping technology Research and application must have broad prospects.
August 06, 2020
