Selective laser sintering (SLS), a kind of 3D printing technology, belongs to rapid prototyping technology. SLS(Selected Laser Sintering) powder material selective sintering using carbon dioxide lasers on powder materials (plastic powder, etc., and the binder of the mixed powder) selective sintering is a discrete point layer by layer stacked with the integration of dimensional entities of the rapid prototyping method.
I. Principle of SLS process
Selective Laser Sintering (SLS) is a process that utilizes the principle of sintering powdered materials (mainly plastic powders, wax powders, metal powders, coated ceramic powders, coated metal powders, and coated sands with binder on the surface) under laser irradiation, and then selectively sintering them according to the interfacial contour information under the control of the computer, and then stacking them in layers to form the model. Any fusible powder can be used to create a model, and the resulting model can be used as a real prototype component.
During operation the powder cylinder piston rises and the powder is spread evenly over the molding cylinder piston (working piston) by the powder spreading silver. The computer controls the two-dimensional scanning trajectory of the light beam based on a sliced model of the prototype, and selectively sintering the solid powder material to form a layer of the part.
Prior to sintering, the entire table is heated to a temperature slightly below the melting temperature of the powder to minimize thermal distortion and facilitate bonding with the previous layer. Once the powder has completed a layer, the work piston drops one layer thickness, the powder laydown system lays down new powder, and the controlled laser beam scans and sinter the new layer. So the cycle repeats, layer by layer, you get three-dimensional parts.
Specifically its work process can be summarized in the following steps:
1. Parameter selection
Layering parameters: layering thickness, part processing direction, scanning spacing, etc.. Forming sintering parameters include scanning speed, laser power, powder type, and powder spread thickness ......
2. Prototyping
The SLS prototypes were fabricated without additional support because the unsintered powder acts as a support.
1) The powder particles are stored in the powder supply bin on the left side. When printing, the elevating platform of the bin rises upward, pushing the powder above the printing plane through the powder spreading roller to the printing plate, forming a very thin layer of powder.
2) At this time, the laser beam scanning system will be based on the slice of the two-dimensional CAD path in the powder layer selective scanning, the scanned powder particles will be due to the high temperature of the laser focus and sintered together to generate a certain thickness of the entity of the sheet, the unscanned area is still maintained in the original loose powder.
3) A layer of sintering is completed, the printing platform according to the height of the slice down, the horizontal roller will again spread the powder, and then start a new layer of sintering, at this time the layer with the layer is also synchronized with the sintering together.
4) This is repeated until all layers are sintered. Remove and recycle the unsintered powder, and the printed solid model can be removed.
3. Post-processing
After removal from the molding chamber, the excess powder is removed from the part with a brush and a special tool. After further cleaning and sanding, the prototype needs to be further processed.
Ⅱ.Analysis of raw materials used in the SLS process:
1. Plastic powder SLS
Characteristics: Nylon, polystyrene, polycarbonate, etc. can be used as raw materials for plastic powder. Generally sintered directly by laser without subsequent treatment.
2. Metal powder SLS
Characteristics: The raw materials are various metal powders. According to the sintering process can be divided into direct method, indirect method, two-component method. Due to the metal powder SLS when the temperature is very high, in order to prevent metal oxidation, sintering must be gold willing to come through is a unified party to see the mold protection gas (nitrogen, argon, hydrogen, etc.) in the container. The process is also known as laser selective zone melting molding method, that is, SLM process, can be regarded as an important branch of the SLS process.
3. Ceramic powder SLS
Characteristics: Ceramic powders are sintered by adding a binder to the powder. There are 3 types of binder: inorganic binder, organic binder and metal binder.
III. Advantages and disadvantages of the SLS process
(1) Advantages of the SLS process
1) Diversity of molding materials and low price. It is the most significant feature of SLS. Theoretically, any material that can form atomic connections between powders after laser heating can be used as SLS molding materials. At present, the main commercialized materials are plastic powder, wax powder, coated metal powder, ceramic powder coated with binder on the surface, coated sand and so on.
2) There is almost no requirement for the shape of the part. Since the lower layer of powder naturally becomes the support of the upper layer, SLS is self-supporting and can produce any complex shape, which is not available in many RP technologies. Molding is not subject to the limitations of traditional machining in which the tool can not reach certain surfaces.
3) High material utilization. The unsintered powder can be reused.
4) The parts have good mechanical properties. The finished product can be used directly as a functional test or small batch use.
5) Realize the integration of design and manufacturing. Supporting software can automatically convert CAD data into layered STL data, automatically generate CNC code according to the level of information, drive the molding machine to complete the processing and stacking of materials layer by layer, without human intervention.
(2)Disadvantages of SLS process
1) High equipment cost.
2) The internal looseness and porousness of the parts, the surface roughness is large, and the mechanical properties are not high.
3) The quality of the parts is affected by the powder, and it is not easy to improve.
4) The maximum size of the manufactured parts is limited.
5) The molding process consumes a lot of energy and the post-treatment process is complicated.
IV. SLS Application Examples
SLS process has been successfully applied in many industries such as automotive, shipbuilding, aerospace, aviation, communication, microelectromechanical systems, construction, medical, archaeology, etc., which has injected new creativity into many traditional manufacturing industries and brought the breath of informationization. To summarize, SLS process can be applied to the following occasions:
1)Rapid prototyping.SLS process can quickly manufacture the prototype of the designed parts, and evaluate and correct the product in time to improve the quality of the design; it can enable customers to obtain intuitive parts model; it can manufacture complex models for teaching and testing.
2) The preparation and development of new materials. Using the SLS process can develop some new particles to enhance composite materials and cemented carbide.
3) Small batch, special parts manufacturing and processing. In the field of manufacturing, the production of small-lot and special parts is often encountered. Such parts processing cycle is long, high cost, for some complex shape parts, and even can not be manufactured. The use of SLS technology can be economically realized in small quantities and the shape of complex parts manufacturing.
4) rapid mold and tool manufacturing. SLS manufactured parts can be used directly as a mold, such as investment casting, sand casting, injection molding, high-precision complex shape of the metal model, etc.; can also be molded parts after post-processing as a functional part to use.
5) in the application of reverse engineering, SLS process can be in the absence of design drawings or drawings are incomplete and no CAD model, in accordance with the existing parts prototype, the use of various and digital technology and CAD technology to re-create the prototype CAD mold volume.
6) Medical applications: SLS sintered parts can be used in the manufacture of ergonomics due to their high porosity. According to the clinical studies conducted abroad on the artificial static prepared by SLS technology, the biocompatibility of artificial bone is good.
V. Technology Application and Prospect
For the current existence of SLS system speed, accuracy and surface rent roughness can not meet the requirements of industrial production, St.S equipment cost is high and laser process parameters on the parts of the quality of the mouse sensitive to the impact of the longer time needed to figure out and other issues, the current research hotspots of experts at home and abroad focus on the following aspects:
1) Research on new materials. Material is the key link in the development of SLS technology, which directly affects the molding speed, precision and physical and chemical properties of the sintered specimen. At present, the parts manufactured by SL.S generally have many disadvantages such as low strength, low precision, and the need for post-processing, which requires the development of a variety of laser sintering rapid prototyping of special materials.
2) SLS connection mechanism research. The sintering mechanism of different powder materials is very different, the sintering process of metal powder is mainly controlled by the transient liquid phase sintering, but the current research on the sintering mechanism has stayed at the microstructure theory level, and it is necessary to analyze the sintering process quantitatively from the study of SLS kinetic theory.
3) Optimization of SLS process parameters: SLS process parameters (e.g. laser power, scanning mode, powder particle size, etc.) have an impact on the quality of SLS sintered parts. At present, the relationship between process parameters and molding quality is a hot spot in the research of SLS technology, and a lot of research has been carried out at home and abroad.
4) SLS modeling and simulation research. Due to the complexity of the sintering process, it is difficult to carry out real-time observation, in order to better understand the sintering process and guide the selection of process parameters, it is necessary to carry out computer simulation of the sintering process.
The development of SIS technology will have a positive impact on the research and development and application of equipment, new processes and new materials, and will have a great impact on the manufacturing industry to environmental protection, energy saving and high efficiency.
Due this type of molding method has a simple manufacturing process, high flexibility, wide range of material selection, cheap materials, low cost, high material utilization, fast molding speed, etc., for the above characteristics make SLS method is mainly used in the foundry industry and popular be used to directly make rapid molds. The 4-cylinder engine generated using SLS technology etc.
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