The additive manufacturing industry, valued at a whopping $13.84 billion, is one of the fastest growing manufacturing technologies in modern times. Selective laser sintering (SLS) is one of the innovative 3D printing technologies that has driven its rapid growth and stimulated large-scale industrial implementation.
In this comprehensive guide, we discuss various aspects of SLS printing, including workflow, advantages/disadvantages, material properties, and some of its applications.
What is selective laser sintering 3D printing?
SLS 3D printing uses a guided laser as a source of thermal energy to sinter layers of raw material particles into a strong, durable component. When heated, the particles fuse together without melting. For this reason it is called a sintering process and not a fusion process.
SLS 3D printers are also available in various sizes, from desktop to large format industrial printing. They also differ in factors such as laser type (fiber, diode, CO2 laser, etc.), laser power, cost, and manufacturing accuracy.
This technique is known for its suitability for producing functional parts and rapid prototyping. The desirable material properties of SLS 3D printed parts are a promising factor for manufacturers and product developers.
Let's delve deeper into selective laser sintering and detail the workflow for better understanding.
SLS 3D printing workflow
SLS 3D printing takes place in several steps. Below is a step-by-step process that is standard for most SLS 3D printers.
1. CAD model preparation
It all starts with a CAD model of the part to be printed. The finished model is transferred to the SLS printer's integrated user interface or slicing software in file formats suitable for 3D printing. The most common file format is STL.
Slicing software prepares the CAD file for 3D printing and generates commands (called G-codes) that the printer controller can interpret in relation to the laser's scanning motion. The software also includes custom settings such as layer thickness, laser power and resolution, which are crucial factors in final print quality.
At this stage, the software also predicts important information such as printing time and material consumption.
2. Pre-printed checks
SLS 3D printers are sophisticated machines that require training and care to operate. Before starting printing, some checks are carried out to ensure that the printing process runs smoothly.
The build chamber (powder bed) is filled to the correct level and with the correct material. Furthermore, professional operators also perform some CAD checks before printing to ensure that there are no gaps that affect printability.
The galvanometers, i.e. the mirrors that direct the laser beam along the desired printing path, must be calibrated. The servomotors that drive them are also moved to their home positions to minimize errors. Calibration plans for this are often recommended by manufacturers in manuals.
3. Preheat the powder
The powder in the build space is preheated to just below the melting point before printing begins. The thermal energy of the laser is only used during sintering. This allows for fast 3D printing.
4. Print
In fact, the printing process itself is quite simple and ingenious. The laser scans the top layer of the part (previously defined by the slicer) above the surface of the powder bed.
The laser power is precisely adjusted to achieve a sintering depth that matches the layer thickness. The typical layer thickness range for SLS 3D printing is 0.05 to 0.15 mm.
The bed then drops one layer, taking the structure with it. A new layer of SLS material is spread over the bed by the coating blade/roller. This time, the laser scans the second layer, which connects to the previous one.
This process is then repeated until the entire part is printed.
5. Parts recovery and post-processing
After printing is complete, let it cool in the chamber for a while. Once cooled, it is removed from the build chamber. Because the part is surrounded on all sides by unsintered powder, the extraction process is often called “depulverization.”
Advantages of SLS 3D printing
SLS 3D printing offers numerous advantages and is therefore one of the most sought-after additive manufacturing processes in the industry.
1. Robust parts
SLS 3D printed parts have advantageous properties such as mechanical strength, tensile modulus, heat resistance, and water and air tightness. This is due to the sintering process, which firmly bonds the individual polymer particles together.
Due to their robustness, SLS printers have become a common device for developing and testing functional prototypes that not only mimic the appearance of the test product, but also its functionality.
2. No supports required
3D printed structures with features such as protrusions or bridges require support geometry to support these elements during printing. Most 3D printing processes, such as FDM, waste a lot of time and material when printing these support structures.
However, this is not a problem with selective laser sintering. Because the entire component is enclosed in a pool of loose powder, it is well supported during construction. This not only saves time but also reduces costs.
3. Fast
These devices are considered fast 3D printers, with commercial printing speeds of up to 48 mm/h. This means it outperforms a whole range of alternative 3D printing processes.
Furthermore, although the workflow of this process is divided into several steps, it is quite time-efficient. Laser scanning is almost instantaneous, the build chamber can be removed and replaced with another when the first one has cooled, and there is also the option of batch printing, which we will discuss below.
4. Batch printing
3D printers are available with different build volumes, with larger machines up to 750mm in one direction. A productive approach is to efficiently use the volume of the build space by adjusting the stack pressure.
This means that as many parts as possible fit into the build chamber as a “batch”. This makes sense because laser scanning itself is very fast. With batch printing, layers of multiple parts can be sintered with a single scan instead of just one, saving hours of printing time.
5. High dimensional accuracy
SLS produces very precise parts. As already mentioned, the layer height can be as low as 0.05 mm. Additionally, most 3D printers are capable of producing tolerances of +/- 3%. The packing pressure created by the powder bed also helps minimize deformation.
For this reason, parts produced with SLS 3D printing have a near-final shape and do not require extensive post-processing.
Disadvantages of SLS 3D printing
Like any manufacturing process, SLS has its advantages as well as its disadvantages. We highlight some points that you should consider before choosing SLS.
1. Limited raw material options
There is no extensive catalog of materials for selective laser sintering. Aside from some types of nylon, thermoplastic elastomers, and TPU (thermoplastic polyurethane), SLS-compatible materials are somewhat difficult to find. The appearance is also limited to grayish metallic surfaces.
Although scientific research is expanding the material scope of SLS printing, it is currently at a clear disadvantage.
2. Grainy surface
Although the final product is dimensionally accurate, it does not have a smooth, shiny surface. In many cases, additional surface treatments may be required to meet manufacturing requirements. Competing techniques, such as stereolithography, have a much better surface finish.
We will discuss some of the common post-processing methods that help resolve this issue in the following section.
3. Expensive printers
SLS printers are generally expensive devices. For cheaper versions, you should expect costs in excess of 5,000 USD. High-end versions cost more than $100,000.
However, it should be noted that the initial investment is easily offset by comparatively lower material and printing costs. Additionally, a new wave of innovative desktop 3D printers are hitting the market that aren't that expensive but have decent specs. Learn more about the cost of 3D printing services.
4. Health and environmental risks
SLS materials are fine powders with a particle size of just 20 microns. Particles of this tiny size can easily enter the respiratory tract and cause health problems. Sintering can produce vapors that escape the work environment and can have a negative impact on the environment.
It is recommended that operators use appropriate personal protective equipment, such as masks and gloves, when handling these materials.
Materials suitable for SLS 3D printing
Sinterable powder polymers are the main class of SLS materials. Some of the most important materials in this category are summarized below.
1. PA12
PA 12 is also commonly known as Nylon 12 and Polyamide 12. It has high tensile strength, toughness and flexibility. PA 12 is also chemical and heat resistant, making it an ideal candidate for functional products.
After 3D printing, it has a slightly grainy matte surface. Of course, it has a white color that can be easily colored into other colors.
2. PA11
Polyamide 11/Nylon 11 is biocompatible, strong, flexible, heat resistant and chemical resistant. It is known for its isotropic behavior, which proves advantageous in certain load-bearing applications such as vehicle interiors, prosthetics and medical devices.
3. Alumida
Alumide is nylon filled with aluminum powder. This mixture provides greater strength and rigidity. Additionally, aluminum gives the final product a shiny metallic appearance that is not as grainy as a pure nylon material like PA12 or PA11.
4. PA-CF
Carbon fiber reinforced nylon is a combination of high performance with excellent strength and impact resistance. It is remarkably light, as is characteristic of carbon fiber products. Common applications include fixtures, fittings and light automotive parts.
Surface Finish Options for SLS Printed Parts
Post-processing has already been covered above, but we have not highlighted the different techniques used by experts. However, the surface quality of dust-free parts is not ideal. Depending on manufacturing requirements, different surface treatments can be applied.
1. Manual grinding
A slightly finer surface can be obtained using manual techniques. Operators use multi-grit sandpaper or grinders to perform a uniform surface treatment and reach all corners of the geometry.
2. Lightning and falls
These methods achieve excellent surface quality. Glass beads are often used in sandblasting, while ceramic chips are often used in tumbling. In both cases, these particles collide with the surface of the SLS part to smooth it.
However, the disadvantage is that some material is also removed, changing the dimensions of the part. Additionally, sharp edges can also be rounded, which may be desirable but not necessarily.
3. Chemical treatment
Chemicals like acetone can react with the surface and smooth it. This is not a popular method, but it works in some cases. When selecting the chemical, attention should be paid to reactivity.
4. Coating
Various coatings can be applied to achieve specific physical properties. Metallic coatings such as gold or copper are sometimes used to increase strength and aesthetics. Because 3D printed parts are porous, waterproof coatings such as silicone can be applied to improve performance.
5. Coloring/Varnishing
A simple solution to improve surface quality is to apply a layer of paint or varnish to the rough surface. This improves the look of the piece, adds a touch of shine and doesn't take much time.
SLS 3D printing applications
Due to its numerous strengths, SLS is used in several important industries.
1. Rapid Prototyping and Manufacturing
The mechanical properties of SLS 3D printed parts are comparable to parts manufactured using other processes. However, production is faster and more convenient. For this reason, SLS parts are often used for rapid prototyping and manufacturing functional products.
Thanks to high print speeds and part functionality, product developers have the freedom to quickly perform design iterations.
2. Aviation and Automotive
Both industries are constantly looking for lightweight, impact-resistant parts – properties characteristic of SLS printing. This is why 3D printing is becoming increasingly popular for producing individual, stylish parts for car modernization or decoration.
Flame retardant components in aircraft engines are another application where the specific properties of SLS materials come into play. Furthermore, the process is also frequently used to produce air ducts and prototypes.
3. Medicine
In the medical industry, functional and individually designed prosthetics and orthoses are now manufactured using 3D printing. 3D scanning technology allows these parts to be adapted precisely to the patient's body structure and SLS helps to achieve the necessary mechanical properties.
WayKen SLS 3D Printing Services
SLS is a popular additive manufacturing technology used in many industries due to its precision, accuracy and productivity. However, mastering this technology can be challenging. Therefore, it is important to work with a reputable service provider who has the experience and knowledge necessary to deliver high-quality results.
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Selective laser sintering has undoubtedly had a major impact on the additive manufacturing scene in recent years. Its potential and use are constantly growing as it attracts the attention of more and more industrialists.
Finally, we would like to recommend that our readers use this guide when making the decision to 3D print. It is still a relatively new market and you need to be well informed before finally deciding on a 3D printing process and manufacturer.
Common questions
What types of SLS 3D printers are there?
SLS 3D printers differ in several ways:
-Size : desktop printers with build volumes of 100 x 100 x 100 mm up to high-end industrial printers with build volumes of up to 500 x 500 x 500 mm.
-Types of laser : SLS printers can be equipped with a CO2 laser, fiber laser, infrared laser or diode laser.
-Print specifications : Specifications such as printing speed, minimum layer thickness, material capacity, etc. vary from product to product.
What is the difference between selective laser sintering (SLS) and selective laser melting (SLM)?
In SLS, the material particles are joined together by sintering. This process occurs below the melting temperature of the material and the particles simply melt together. In SLM, the work is carried out above the melting temperature and the particles are completely melted so that they solidify into a homogeneous part.
SLM produces parts with better mechanical properties and is suitable for very high quality applications. SLS also produces functional parts, but is not as stable as SLM.
What materials are commonly used for SLS 3D printing?
Thermosets and thermoplastic polyurethane are commonly used materials in SLS 3D printing. In particular, materials such as Nylon 12 and Nylon 11 are the most popular. Some specific applications also use a mixture of these materials with metals (aluminum) or fibers (carbon fiber, fiberglass).
