What is SLS 3D Printing and How Does it Work?

Selective Laser Sintering is one of the common 3D printing technologies in rapid prototyping. This technology uses a laser to interact with powder material and build parts layer by layer. This article explains the basic principle of Selective Laser Sintering (SLS) 3D printing, describes how the technology works, outlines the complete process, identifies suitable materials, and discusses its applications.

Overview of SLS 3D Printing

Selective Laser Sintering is an additive manufacturing technology that uses a laser to interact with powder material and build parts layer by layer. It usually uses a CO₂ laser as the laser source. According to the layered data input by the computer, it selectively scans and sinters each layer. SLS is a highly flexible and adaptable additive manufacturing technology. It breaks through the limitations of traditional material forming and subtractive manufacturing. It does not need molds or support structures. It adds material to form the part. It has the advantages of high design freedom, a short product development cycle, and low manufacturing cost. It can quickly produce complex polymer, metal, and ceramic parts.

Principle of Selective Laser Sintering

Before scanning, SLS technology needs to preheat the powder to a temperature below its melting point. This reduces thermal deformation and powder sticking problems during laser scanning, and also helps with bonding between layers. The computer software controls the laser operation, power adjustment, powder preheating, powder spreading roller, and powder cylinder movement.

After setting the laser process parameters such as laser power, scanning speed, scanning spacing, and layer thickness, the computer controls the laser to emit a high-precision laser beam. The laser selectively scans the powder layer according to the input three-dimensional layered model data. The scanned area on the powder layer absorbs laser energy, and the temperature begins to rise. When the temperature reaches the softening point or melting point of the powder material, the scanned powder starts to flow. The individual powder particles begin to contact each other, forming sintering necks and bonding together. The unscanned areas remain in powder form and act as support for the scanned areas.

After the laser finishes scanning the specified area, part of the heat is transferred to the lower powder layer through thermal conduction, which creates bonding between the current layer and the layer below. The remaining heat slowly dissipates through convection and radiation on the surface. The temperature begins to drop, and the powder particles gradually cool and solidify. The powder particles in the scanned area bond together to form the required contour.

After the laser completes scanning one layer, the build cylinder descends by one layer thickness, while the powder supply cylinder rises by a corresponding height. The powder spreading roller then moves and rotates toward the build cylinder, pushing the excess powder from the powder cylinder onto the surface of the build cylinder to form a new powder layer with the exact layer thickness. The next layer is then sintered. This process repeats layer by layer until the entire part is completed.

When all sections are sintered, the printed part is removed from the powder bed. The unsintered powder on the surface and inside complex structures is carefully cleaned away. Then, post-processing steps such as sanding and drying are performed to obtain the final three-dimensional solid part.

As a rapid manufacturing technology completely different from traditional subtractive manufacturing, SLS has many advantages in making parts:

• Wide range of material sources. In theory, any powder material that can achieve particle bonding after laser sintering can be used as an SLS material.
• Simple manufacturing process. The entire process is controlled by a computer. Only model design and raw material preparation are needed to form the part in the SLS equipment. The manufacturing process is relatively simple.
• Relatively high forming accuracy. The accuracy of the formed part depends on laser scanning precision and the size of the heat-affected zone, which can be adjusted by changing process parameters.
• Suitable for producing complex-shaped parts without support structures or molds. The unsintered powder in non-scanned areas remains and supports overhanging layers. No molds are needed, so even very complex shapes can be formed.
• High material utilization rate. Unused powder after one build can be reused, which improves material utilization and reduces cost.

Selective Laser Sintering Process

Laser sintering technology can now use many different powder materials to produce parts of corresponding materials. Because the process is mature, printed parts generally have good accuracy and high strength. However, the biggest advantage of SLS is that it can directly print finished metal parts. The printed components can directly meet testing requirements. Laser sintering technology can directly or indirectly sinter metal parts, and the final material strength is much better than that of other 3D printing technologies.

According to the SLS process principle introduced earlier, the specific process can be summarized as follows:

1. The entire print chamber is always kept at a temperature slightly below the melting point of the powder material during printing.
2. Spread the material powder on the upper surface of the already formed part and level it.
3. Use a high-intensity CO₂ laser to irradiate the layer section of the part on the newly spread layer. The material powder is sintered together under the high-intensity laser and bonds with the previously formed part below.
4. After one layer section is sintered, the powder spreading system spreads a new layer of powder material, and then the next layer section is printed.

Although laser sintering technology has very obvious advantages, it also has disadvantages. First, powder sintering causes a rough surface that needs later sanding treatment. Second, it requires high-power lasers, which means higher equipment and maintenance costs, plus supporting protection and control components. The overall technical complexity of the equipment is high, and manufacturing is difficult. Ordinary users cannot afford it, making it hard to promote on a large scale.

Selective Laser Sintering Process Parameters

Good SLS printed parts need enough accuracy and strength. If the accuracy is insufficient, the final part cannot meet the requirements. If the strength is too low, it cannot maintain complex shapes or undergo post-processing, leading to part damage or insufficient strength. Only when powder particles are fully softened and bonded can the strength of the formed part be improved. This requires enough heat in the laser sintering area to melt the powder particles. However, too much laser energy density will create a large heat-affected zone through thermal conduction, causing bigger dimensional errors and poorer forming accuracy. Therefore, the effect of laser energy density on strength and accuracy must be considered together to set reasonable process parameters.

Laser Power

The output of laser energy is mainly determined by laser power. The heat from the laser acting on the powder has three paths: absorbed by the powder in the selected area, conducted to surrounding areas, or lost to the air through convection, radiation, and reflection. The laser beam is a moving heat source, and its interaction time with the powder is usually only a few to tens of milliseconds. So the powder heats and cools quickly. During heating, thermal properties such as laser absorption rate, reflectivity, and thermal conductivity of the powder material change with temperature. The temperature at each point inside the powder also changes constantly. This is a very complex, unsteady heat transfer process.

Scanning Speed

In the laser sintering process, when the laser scans across the working plane, the powder particles melt, flow, and bond. The laser scans point by point into lines, then line by line into surfaces, and finally layer by layer into a solid. When scanning speed decreases, laser energy density increases, and the energy absorbed by the material near the scanning point also increases. This enlarges the width and depth of the melting zone, which helps increase the strength of the formed part. Because the width and depth of the melting zone greatly affect scanning spacing and single-layer thickness, scanning speed must work together with these two parameters. Lower scanning speed reduces manufacturing efficiency. Also, when the laser scans boundaries, the increased melting zone width enlarges the heat-affected zone, which reduces the accuracy of the formed part.

Scanning Spacing

Scanning spacing is the distance between two scanning lines. During laser scanning, only the powder in the scanning line areas needs to bond together. The temperature field in the scanning area should affect the surrounding powder as little as possible. Scanning spacing is usually chosen to be slightly smaller than the laser spot diameter. This creates a small overlap between adjacent scanning lines without creating obvious bonding boundaries in a single layer. It makes the single-layer bonding more uniform and keeps the temperature field from affecting surrounding areas too much, ensuring better dimensional accuracy of the green part.

Single Layer Thickness

Single-layer thickness refers to the powder spreading thickness, which is the height the build cylinder descends each time. The distribution of laser energy density decreases in the thickness direction, so the sinterable layer thickness is very limited. Too large a layer thickness will cause weak bonding between layers, and the part may delaminate or have reduced strength in the height direction. Too small a layer thickness will cause some already sintered powder to be re-sintered, affecting forming quality. The powder spreading roller applies downward pressure on the powder, which helps increase powder packing density. Therefore, the smaller the single-layer thickness, the higher the density of the sintered part. There is also a horizontal force that can cause a slight offset between layers and reduce accuracy. Especially for parts with curved surfaces, laser sintering produces stepped surfaces that cannot transition smoothly, reducing surface and shape accuracy.

For parts with curved surfaces, the error produced during laser sintering is related to both the slope of the curve and the single-layer thickness. Increasing layer thickness makes the staircase effect more obvious, increasing errors in volume, shape, and size between the actual sintered part and the designed part. Therefore, when sintering parts with curved surfaces, the layer thickness should be appropriately reduced, and the processing direction should be carefully chosen to achieve higher accuracy.

Laser Spot Diameter

When the laser beam hits the powder surface, it forms a spot of a certain size. When sintering powder with the laser beam, there is a deviation between the contour line of the formed part and the scanning path of the spot center, causing the outer contour of the part to appear slightly larger. In addition, the spot can round off sharp corners of the part, affecting shape accuracy. The influence of spot size on forming accuracy partly masks the influence of powder particle size. Laser spot diameter also has a big effect on forming efficiency. At the same scanning speed, a larger spot diameter improves the uniformity of energy density distribution, allows larger scanning spacing, and helps increase efficiency. A smaller spot diameter helps improve bonding strength between layers and the mechanical properties of the part. Using variable spot technology allows small spot scanning on boundaries and large spot scanning inside. This improves scanning efficiency, reduces deformation, and still produces high-strength parts.

Selective Laser Sintering Materials

SLS technology is a powder-bed-based additive manufacturing technology, so the characteristics of the powder material have a great influence on the performance of SLS parts. Among them, powder particle size, particle size distribution, and powder particle shape are the most important. SLS technology has a wide range of forming materials. Many SLS materials have been developed at home and abroad. According to material properties, they can be divided into: metal-based materials, ceramic-based materials, polymer materials, etc.

Polymer Materials

One outstanding advantage of SLS technology is that it can process a variety of materials, including polymers, metals, and ceramics. Compared with metals and ceramics, polymer materials have the advantages of lower forming temperature, smaller sintering laser power, and higher accuracy. They were the earliest and most successful SLS printing materials and still occupy an important position. The diversity of their types, performance, and various modification technologies provide broad space for their application in SLS. SLS technology requires polymer materials to be made into solid powder with an average particle size between 10 and 100 μm. They must melt (or soften, react) and bond after absorbing laser energy without severe degradation. Currently, the main polymer materials used for SLS are thermoplastics and their composites. Thermoplastics can be divided into crystalline and amorphous types.

Amorphous Polymers

Amorphous polymers begin to have active molecular chain movement at the glass transition temperature (Tg), and the powder starts to bond with reduced fluidity. Therefore, during SLS, the preheating temperature of amorphous polymer powder cannot exceed Tg. To reduce warping of the sintered part, it is usually set slightly below Tg. When the material absorbs laser energy, the temperature rises above Tg, and sintering occurs. Amorphous polymers have high viscosity at Tg, and the sintering rate is inversely proportional to viscosity. This results in a very low sintering rate, low density and strength of the sintered part (porous structure), but high dimensional accuracy. In theory, increasing laser energy density can increase density, but in practice, excessively high energy density often causes severe decomposition of the polymer, reducing density instead. It also intensifies secondary sintering and reduces accuracy. Therefore, amorphous polymers are usually used to make parts that do not require high strength but need high dimensional accuracy. Common amorphous polymers used in SLS include polycarbonate (PC), polystyrene (PS), high-impact polystyrene (HIPS), and polymethyl methacrylate (PMMA).

Crystalline Polymers

The sintering temperature of crystalline polymers is above the melting temperature (Tm). Above Tm, crystalline polymers have very low melt viscosity, so their sintering rate is high, and the density of sintered parts is very high, generally above 95%. Therefore, when the material has high inherent strength, crystalline polymer sintered parts have high strength. However, crystalline polymers have large shrinkage during melting and crystallization, and the volume shrinkage caused by sintering is also very large. This makes them prone to warping deformation during sintering, resulting in poorer dimensional accuracy. Currently, nylon is the most commonly used crystalline polymer in SLS. Other crystalline polymers such as polypropylene, high-density polyethylene, and polyether ether ketone are also used in SLS technology.

Industrial thermoplastic polymer products are usually granules. Granular polymers must be made into powder before they can be used in the SLS process. Polymer materials have viscoelasticity. When crushed at room temperature, the heat generated increases viscoelasticity, making crushing difficult. The crushed particles may also re-bond, reducing crushing efficiency and even causing melt drawing. Therefore, conventional crushing methods cannot produce powder suitable for SLS. The main method for preparing micron-level polymer powder is cryogenic grinding. This method uses the low-temperature brittleness of polymer materials to prepare powder. Common polymer materials such as polystyrene, polycarbonate, polyethylene, polypropylene, polymethacrylates, nylon, ABS, and polyester can all be prepared into powder using cryogenic grinding.

Ceramic-Based Powder Materials

Because ceramic materials have very high melting points, it is difficult to directly melt them with lasers. Ceramic parts are also made using indirect methods. During SLS forming, the laser melts the binder, which bonds the ceramic powder together to obtain the desired shape. Then, post-processing steps such as infiltration or isostatic pressing are used to give the ceramic part enough density and strength. If too little binder is added, it is difficult to bond the ceramic particles, easily causing delamination. If too much binder is added, the volume fraction of ceramic in the green body becomes too small, easily causing cracking, shrinkage, and deformation during debinding. Binder addition methods mainly include mechanical mixing and coating methods. The coating method is usually achieved by dissolution, precipitation, or solvent evaporation.

Metal-Based Powder Materials

SLS indirect forming of metal powder by uniformly mixing metal powder with polymer powder (binder). The laser energy is absorbed by the powder material, causing a temperature rise that softens or melts the polymer binder into a viscous flow state, bonding the metal powder together to form the initial metal green part. Then, debinding, high-temperature sintering, metal infiltration, or resin impregnation processes are used to obtain the final metal part.

In addition, another method uses low-melting-point metal powders such as Cu and Sn as binders to prepare composite metal parts. This type of binder remains in the green part after forming. Because the low-melting-point metal binder itself has high strength, the green part has high density and strength, so high-performance metal parts can be obtained without debinding or high-temperature sintering steps. With the development of SLM technology, research on preparing metal parts using SLS is becoming less common.

Applications of Selective Laser Sintering

Investment Casting Patterns

Large and complex precision investment casting patterns and sand molds can be formed in a few days or even hours using large-platform SLS equipment. During forming, under a preset preheating temperature, a layer of powder is spread on the worktable with the powder spreading roller. Then the laser beam, under computer control, scans the powder in the solid areas according to the cross-section contour information of the pattern or sand mold. This raises the powder temperature to the melting point, melts the particle boundaries, and bonds the powder together. The powder in non-sintered areas remains loose and supports the workpiece and the next layer. After one layer is formed, the worktable descends by one layer height, and the next layer of powder spreading and sintering is performed. This cycle continues to form the three-dimensional pattern and sand mold. The rapidly formed patterns and sand molds are then used in investment casting and sand casting to produce key components for important fields such as aerospace, military, shipbuilding, automotive, and machine tools in China. This reduces process steps, shortens cycles, lowers costs, and achieves the goal of “halving both cost and cycle time” in casting, improving traditional casting technology. Therefore, using SLS to manufacture coated sand cores has broad prospects in casting.

Biomanufacturing

Using SLS to form biopolymers for personalized medical implants and tissue engineering scaffolds is currently one of the research hotspots in the SLS field. Through computer-aided design, SLS technology can produce three-dimensional porous tissue scaffolds and personalized biological implants with controllable structure and mechanical properties. It can effectively control porosity, pore shape, pore size, and external structure, thereby promoting cell adhesion, differentiation, and proliferation, and improving the biocompatibility of the scaffold. Currently, biopolymers suitable for SLS are mainly synthetic polymer materials, including poly-L-lactic acid (PLLA), polycaprolactone (PCL), polyether ether ketone (PEEK), polyvinyl alcohol (PVA), etc. They are often combined with bioactive ceramic materials such as hydroxyapatite (HAp) or β-tricalcium phosphate (β-TCP) to obtain good bioactivity.

Polymer Functional Parts

Polymer parts formed by SLS have good performance and can be used directly as plastic functional parts. The materials used for SLS forming are mainly thermoplastics and their composites. Thermoplastics can be divided into crystalline and amorphous types. Because crystalline and amorphous polymers have completely different thermal properties, there are huge differences in their laser sintering parameter settings and part performance.

Tolerance and Capacities

At Getzshape, our custom 3D printing services cover four main technologies, which are SLA, SLS, SLM, and FDM. Our SLS 3D printing tolerance and capacities are listed below.

Surface Finishes for SLS Printed Components

Dyeing: As the most efficient and economical coloring method, dyeing is primarily utilized to enhance the aesthetic appeal of SLS components. It ensures uniform color penetration without altering part dimensions.

• Compatible Materials: PA12, PA12-GF, TPU
• Color Palette: Black, Pantone, and RAL ranges

Spray Painting: For applications where dyeing is inapplicable or specific finishes are required, spray painting serves as a high-versatility alternative. It delivers precise color matching and superior surface coverage for a premium look.

• Compatible Materials: All SLS materials
• Color Palette: Black, Pantone, and RAL ranges

Vapor Smoothing: This process refines and seals the surface of SLS parts through a chemical vapor treatment. By eliminating surface porosity and reducing crack initiation points, it significantly boosts mechanical performance—specifically elongation at break, impact resistance, and fatigue strength.

• Compatible Materials: PA12, TPU