Powder Bed Fusion – Comparing SLS, MJF and SAF 3D printing

David Boström3D-skrivare, SLS, TipsLeave a Comment

Powder Bed Fusion comparison

Anyone entering the world of Additive Manufacturing (AM) enters a world of many three-letter abbreviations. The best known are FFF / FDM since these technologies have become so affordable that there is a huge consumer market at least for the FFF 3D printers. The main issues with the FFF / FDM technologies are that the prints will break easily, you can only print one object at a time, and you need to print support structures that take time and can be challenging to remove. All of these problems are solved with Powder Bed Fusion (PBF) 3D printers. On the flip side, you enter a completely different price range.

There are three different technologies within polymer PBF – SLS, MJF, and SAF. They are all used in applications like functional prototyping or in the health sector to print orthotics. So what are the pros and cons of each? Let’s start with the commonalities.

What do SLS, MJF, and SAF have in common?

All three technologies are variants of Powder Bed Fusion technologies. This means that they print one or several objects in a printing chamber, which has a pre-heated build platform that is lowered very precisely to allow for layers of very fine plastic powder to be distributed one at a time. The whole chamber is pre-heated to a temperature just below the sintering temperature.

The objects are printed layer by layer, by heating only those sections of the layer that are to make up the printed objects. The result is a printed object which is strong in all directions, unlike FFF or FDM prints which are only strong in the X and Y directions, but weak between the layers in the Z direction. The unmelted powder remains untouched, forming a powder bed that supports the printed objects. No separate support structure needs to be printed.

Once the print is done, the printed volume forms a so-called cake that is hot. It will have to cool down slowly to avoid unwanted shrinkage and warping of the printed objects. When the cake has cooled down, the unsintered or unfused powder is removed, and the printed parts are cleaned from any residual powder. The unsintered powder is collected, filtered, mixed with virgin powder, and used again.

All three techniques will produce functional prototypes, to the benefit of any company wishing to speed up the R&D and development processes. All three techniques can also be used to produce small to medium series of geometrically complex components, parts or products.

Next, let’s focus on the differences.

SLS 3D printing

SLS process model

SLS process model

In 1988, the first patent ever to be filed within what today is called PBF was for a technology called Selective Laser Sintering (SLS). Since the patents have run out, there are several companies using this technology and there is a broad range of SLS 3D printers to choose from, all price competitive. (Not cheap, but value for money).

The SLS 3D printing technology is based on a laser heating up the powder wherever the printed part is to be built. Unlike the other technologies, the SLS 3D printer sinters the powder in a single point, offering precise control over the temperatures. This means there is no need for spraying fusing or detailing agents, which eliminates the need for inkjet heads. The running cost will hence always be lower.

By varying the speed and the energy level of the laser beam, the optimal parameters for different types of powder can be set. This makes it relatively easy to add another powder to the range of printable powders.

Different producers of SLS 3D printers have selected different types of lasers. A machine operating with a narrow laser beam will be able to print finer details, but will also take more time to finish a print.

Lasers can operate at different wavelengths. Some SLS 3D printers are based on lasers that operate in the visible spectrum (Formlabs, Sintratec, Sinterit and more). Others have lasers that operate at completely different wavelengths (Wematter and others). The advantage of choosing a laser operating at an “invisible” wavelength is that the 3D printer much easier can handle powders of different colours, types and properties.

MJF 3D printing

The MJF technology

MJF process model

Multi Jet Fusion (MJF) is trademarked by HP and is technically a powder bed fusion 3D printing technology although it has similarities to binder jetting. MJF was introduced to the market by HP in 2016. The technology gets its name from the multiple inkjet heads that carry out the printing process.

Unlike SLS, MJF requires an inkjet head to run across the powder and deposit both a fusing and a detailing agent onto it. The fusing agent is very dark (black), and the detailing agent is typically some kind of coolant. Next, an infrared heating unit moves across the print. The dark dyed powder will absorb energy faster than the white powder (also cooled down by the detailing agent), thereby melting together with the underlying layer of powder wherever the fusing agent was added. The powder in the areas with the detailing agent remains unfused.

For the fusing agent to have any effect, the energy from the heaters needs to have a wavelength that enables the darker colour to absorb more energy. MJF does not work with a laser but rather with heaters. Hence, all MJF 3D printers operate with wavelengths in or near the visible spectrum.

SAF 3D printing

The SAF technology

SAF process model

Selective Absorption Fusion (SAF) is the latest abbreviation in powder bed 3D printing technology. Developed by the Danish start-up company Xaar in the years 2017-2021, the technology is so far only used in the 3D printer developed by Xaar; the H350. Ever since Stratasys bought Xaar in October 2021, the SAF trademark is owned by Stratasys.

SAF is in principle very similar to the better-known MJF technology. The main difference is that it only uses the fusing agent, not any detailing agent.

Pros and cons of SLS, MJF and SAF 3D printing

Developments in additive manufacturing are fast. Below we have listed the differences that you should be aware of when shopping for a 3D printer.

Please note that this article is written as objectively as possible. The aim is not to make you choose one technology over the other, but to guide you as correctly as possible. In our experience, this saves time for everyone involved, including our sales force.

It should be noted that the Gravity 3D printer from Wematter is based on SLS technology. To learn more about the differences between these three technologies, please download our guide via the link below!

Polymer Powder Bed Fusion (PBF) 3D Printing Comparing SLS, MJF and SAF technologies Download pdf

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