[3D Printing News] Introduction to SLS 3D Printing Technology

【3D Printing News】Introduction to SLS 3D Printing Technology

In this introduction to SLS 3D printing, we will guide you through the basic principles of laser powder sintering technology. After reading this article, you will understand the basic mechanisms of the SLS process, as well as its advantages and limitations. Before we dive in, let's take a look at how it works: (Further reading: Beginner's Guide: Comparison and Principles of Common 3D Printing Technologies)



Table of Contents

What is SLS?

Selective Laser Sintering (SLS) is an additive manufacturing process belonging to the powder bed fusion family. In SLS, a laser selectively sinters particles of polymer powder, fusing them together and building parts layer by layer. The materials used in SLS are granular thermoplastic polymers.
SLS is used for prototyping and low-volume production of functional polymer parts because it offers a very high degree of design freedom, high accuracy, and produces parts with good, consistent mechanical properties, unlike FDM or SLA technologies. The full potential of this technology can only be exploited when the designer considers its main advantages and limitations.


SLS Printing Process

How does SLS work?

Here's how the SLS manufacturing process works:
I. First, the powder bed and print area are heated to just below the polymer's melting temperature, and a thin layer of powder is re-spread onto the print platform using a blade.
II. A CO2 laser then scans the contours of the next layer and selectively sinters (fuses together) the polymer powder particles. The entire cross-section of the component is scanned, solidifying this block.
III. Once this layer is complete, the print platform moves down, and the blade re-coats the surface. This process is then repeated until the entire part is finished.

Once printing is complete, the part is fully encapsulated in unsintered powder, and the powder bed must cool down before it can be opened. This can take a considerable amount of time (up to 12 hours). The part is then cleaned with compressed air or other blasting media and prepared for use or further post-processing. The remaining unsintered powder can be collected and reused (only 50% of SLA powder is recyclable).


Schematic of an SLS printer

Features of SLS

Printer Parameters

In SLS, almost all industrial parameters are preset by the machine manufacturer. The default layer thickness used is 100-120 microns. A key advantage of SLS is that it does not require support structures. The unsintered powder provides all the necessary support for the part.
For this reason, SLS can create free geometric shapes that cannot be achieved with other manufacturing methods.
When printing with SLS, the entire print volume is very important, especially for small-batch production.
The print time for a build chamber of a given height is roughly the same, regardless of the number of parts it contains.
This is because the recoating step dictates the total processing time (laser scanning happens very quickly), and the machine must cycle through the same number of layers for each build.
Build chamber packing can affect the delivery time of small orders, as operators often wait until the build chamber is full before starting a print.

Inter-layer Adhesion

In SLS, the inter-layer bond strength is excellent.
This means that SLS printed parts exhibit nearly isotropic mechanical properties. The mechanical properties of SLS samples printed using standard polyamide powder (PA 12 or Nylon 12) (the most commonly used material in SLS) are shown in the table below, compared with the properties of bulk Nylon.

 
SLS parts have excellent tensile strength and modulus, comparable to bulk materials, but are more brittle (they have higher elongation at break).
This is due to the internal porosity of the final part.

A typical SLS printed part is approximately 30% porous.

Porosity gives SLS parts a granular surface. This also means that SLS parts can absorb moisture, so they can be easily dyed in various colors in hot water, but special post-processing is required if they are to be used in humid environments.


Dyed SLS parts in various colors. Their porosity makes them ideal for hot water dyeing.

Shrinkage and Warping

SLS parts are prone to shrinkage and warping: as new sintered layers cool, they shrink in size and accumulate internal stresses, pulling up the underlying layers.
Typical SLS shrinkage rates are 3% to 3.5%, which machine operators account for during production preparation and adjust design dimensions accordingly.

Large flat surfaces are most susceptible to warping. This problem can be addressed by orienting the part vertically in the print bed, but the best practice is to create flat areas with minimal thickness and design in cutouts to reduce volume. This approach also reduces the overall cost of the part due to less material usage.

Over-sintering

Thermal radiation can cause over-sintering of the unsintered powder surrounding features during melting. This can lead to the loss of small details (such as slots and holes).
Over-sintering depends on the size of the feature and the wall thickness. For example, a 0.5mm wide slot or a 1mm diameter hole can be successfully printed on a 2mm thick thin wall, but these features will disappear when the wall thickness is 4mm or greater.
As a rule of thumb, slots wider than 0.8mm and holes larger than 2mm in diameter can be printed in SLS without worrying about over-sintering.

Powder Removal

Since SLS does not require support material, parts with hollow cross-sections can be printed easily and accurately.
Hollow sections can reduce the weight and cost of parts due to less material usage.
Vent holes are needed to remove unsintered powder from inside the part. It is recommended to add at least 2 vent holes with a diameter of not less than 5mm to the design.
If high rigidity is required, the part must be printed fully solidified. Another method is to create a design with a hole to omit vent holes.
This method involves embedding tightly packed powder into the part, increasing its mass and providing some additional support for mechanical loads, without affecting print time.
An internal honeycomb lattice structure can be added to the hollow interior (similar to infill patterns used in FDM) to further increase the rigidity of the part. Hollowing out a part in this way can also reduce warping.


Removing powder from SLS parts

Common SLS Materials
The most widely used SLS material is Polyamide 12 (PA 12), also known as Nylon 12. The price of PA 12 powder is approximately $50-60 per kilogram.
Other engineering thermoplastics, such as PA11 and PEEK, are also available but are not as widely used.
Polyamide powders can be filled with various additives (such as carbon fiber, glass fiber, or aluminum) to improve the mechanical and thermal properties of manufactured SLS parts. Materials with added fillers are often more brittle and may exhibit highly anisotropic behavior.