The principle of the SLS process
The principle of selective laser sintering molding method, powder particles are stored in the feed silo on the left, the powder silo lifting platform rises when printing, and the powder higher than the printing plane is pushed to the printing plate of the printing silo through the powder roller, forming a very thin and flat powder layer; at this time, the laser beam scanning system will selectively scan the powder layer according to the two-dimensional CAD path of the slice, and the powder is scanned The end particles will be sintered together due to the high temperature of the laser focus, resulting in solid flakes with a certain thickness, and the unscanned area still retains the original loose powder; after the completion of one layer of sintering, the printing platform drops according to the height of the slice, and the horizontal roller flattens the powder again, and then starts the sintering of a new layer, at which time the layers are also sintered together at the same time; and so on until all layers are sintered. Remove and recover the unsintered powder to remove the printed solid model
Second, the advantages of SLS process
Advantages of SLS 3D printing technology
1, SLS can use more materials, the materials that can be used include polymer, metal, ceramics, gypsum, nylon, and other powders, but due to the refinement of the market, metal materials will be called SLM, and because the material used in SLS on the market should account for 90% of nylon materials, so we usually default to SLS is printing nylon materials.
2. Accuracy, now the accuracy is normal to achieve a tolerance of plus or minus 0.2mm.
3. No need to support, it does not need to support the structure, the suspended layer that appears in the lamination process can be directly supported by the unsintered powder, which should be the biggest advantage of SLS.
4. High utilization rate of materials and high utilization rate of materials. Because it does not need support and does not need to add a base, it has the highest material utilization rate among several common 3D printing technologies, and the price is relatively cheap, but it is more expensive than SLA.
Of course, SLS 3D printing technology has more or less some limitations and disadvantages.
(2) Disadvantages of the SLS process
1. Shrinkage of printed products
Nylon and other powder materials shrink in three dimensions after sintering. Shrinkage generally depends on many factors, including the type of powder used, the laser energy used to sinter the particles, the shape of the part, and the cooling process. Note that the part does not shrink symmetrically in all directions. Generally, software is used to cut models and processes for SLS. Incorporate shrinkage into the calculation of manufactured high-quality parts. However, an engineer should know the shortcomings of the printer he uses and prevent it from happening in a particular location.
2. Post-printing
Excavating new SLS prints is similar to the work of archaeologists. After the printing process, the powder has enough time to cool down and the material blocks (which may weigh several kilograms) can be removed. Experts need to dig out parts from blocks of material, vacuum excess powder and even use the wind to sweep away the remaining particles. Once all the work is done, the sintered prints can be heated and cured, increasing the strength. Other finishes may also take time to sand, stain, or paint the surface.
3. Color change/moisture absorption
If the print is dyed, painted, or coated, the porous structure is advantageous and can show problems with the finished product. They can absorb large amounts of dust, oil, or water from the air, changing color (e.g. from white to ivory) or losing weight. The print plate needs to be fully loaded. SLS printers don’t matter if you want to print small parts or not – they need to be loaded to the height of the part, and they also need to keep the X and Y axes embedded in the powder. Even if the material does not touch the laser, the remaining material will be destroyed after each use. That’s why SLS printers can try to process as many items as possible at once.
4. Processing and material loss
SLS printers wear and tear. The “cheap” desktop version costs around $7,000 – and only for printers. If someone wants to handle the entire process including cleaning, post-treatment, finishing, and curing, they will need a lot of money. Materials are also expensive – a kilogram of PA 12 costs $100-120 at the average manufacturer, but the equipment “eats” several kilograms in a single use. If you plan to complete an industrial grade, you will need to prepare more than $800,000 for a high-grade system.
5. Use fresh powder every time
Most sintering printers preheat the powder before touching the laser, so the material is damaged due to temperature changes. Some materials can be reused (recyclable), however, printing with damaged powder will not yield good results – manufacturers recommend mixing it with unused materials. Depending on the material and equipment type, more than 85% of the powder can be recycled, but several kilograms of damaged and unsintered material may be left behind.
Third, technology applications and prospects
Given the current SLS system speed, accuracy, and surface roughness can not meet the requirements of industrial production, St.S equipment cost is high, laser process parameters are sensitive to the influence of parts and rats, and it takes a long time to explore, the current research hotspots of domestic and foreign experts focus on the following aspects:
1) Research on new materials. Material is a key link in the development of SLS technology, which directly affects the forming speed, precision, and physical and chemical properties of sintered specimens. At present, SL.S manufacturing parts generally have many shortcomings such as low strength, low precision, and post-processing, which require the development of various special materials for rapid prototyping by laser sintering.
2) Research on the SLS connection mechanism. The sintering mechanism of different powder materials is completely different, and the sintering process of metal powder is mainly controlled by instantaneous liquid phase sintering, but the current research on its sintering mechanism has stayed at the level of microstructure theory, and it is necessary to quantitatively analyze the sintering process from SLS kinetic theory.
3) SLS process parameter optimization research. The process parameters of SLS (such as laser power, scanning method, 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 research hotspot 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, real-time observation is difficult, to better understand the sintering process and guide the selection of process parameters, it is necessary to perform a computer simulation of the sintering process.
The development of SIS technology will have a positive impact on equipment research and development, application and research, new processes and new materials, and play a huge role in promoting the development of the manufacturing industry to environmental protection, energy saving, and high efficiency.
