HP MJF 3D Printing

MJF & SLS: what is the difference?

HP's Multi Jet Fusion (MJF) and Selective Laser Sintering (SLS) are two industrial 3D Printing technologies that belong to the powder bed fusion family. In both processes, parts are built by thermally fusing (or sintering) polymer powder particles layer-by-layer. The materials used in both MJF and SLS are thermoplastic polymers (usually Nylon) that come in a granular form.

The main difference between MJF and SLS is the heat source. SLS uses a laser to scan and sinter each cross-section, while in MJF an ink (fusing agent) is dispensed on the powder that promotes the absorption of infrared light. An infrared energy source then passes over the building platform and fuses the inked areas.

Both MJF and SLS are capable for producing functional parts with good dimensional accuracy, smooth surface and free-form geometry.
The physical properties and mechanical behaviour of SLS materials is well-documented, but comprehensive data for MJF are not widely available.
The cost of MJF and SLS is similar, but the MJF lead time is on average 3 days faster.

MJF is a combination of the SLS and Binder Jetting technologies.

Since MJF and SLS create parts that are very similar, it is important for a designer to understand what slight differences should be expected when placing an order with either process. In this article, we compared the two technologies in terms of workflow, accuracy, materials, cost and lead time.

Fabrication process & workflow

Here is a summary of the fabrication process for the two technologies:

1.Fabrication process & workflow

Selective Laser Sintering:

A thin layer of powder is first spread over the build platform. A CO2 laser then scans each cross-section, sintering the powder. The platform then moves downwards one layer and the process repeats until the job is complete. The whole bin has to cool before the parts can be removed from the powder.

Multi Jet Fusion:

A thin layer of powder is first spread over the build platform where it is heated to a near-sintering temperature.

A carriage with inkjet nozzles (which are similar to the nozzles used in desktop 2D printers) passes over the bed, depositing fusing agent on the powder. At the same time a detailing agent that inhibits sintering is printed near the edge of the part.

A high-power IR energy source then passes over the build bed and sinters the areas where the fusing agent was dispensed while leaving the rest of the powder unaltered. The process repeats until all parts are complete.

Similar to SLS, the printed parts are encapsulated in powder and need to cool down before they can be removed.

2.Dimensional accuracy

Both MJF and SLS are industrial technologies that offer high dimensional accuracy. However, MJF has a small edge over SLS as can be seen in the table below.

This is mainly because MJF printheads deposit material at 1200 DPI (or approximately 1 dot every 0.022 mm), while the typical laser spot size of SLS system is approximately 0.3 - 0.4 mm in diameter. Also, the detailing agent used in MJF helps with printing small features and sharp edges.

Notably, neither technologies require support structures, allowing the creation of free-form models without any support removal marks, and are both susceptible to warping, so large flat areas must be avoided. In our testing, we experienced that MJF parts were more prone to warping than parts printed with SLS.

* : The recommended minimum wall thickness of structural areas for both technologies is 1 mm.

3.Appearance & Surface Quality

The fusing agent currently used in MJF systems is black in color because dark materials absorb radiation more effectively. As a result, MJF parts have a light grey appearance. An optional post-processing dyeing step can be applied to achieve an uniform black finish.

SLS parts are usually printed white and can be dyed to any color. Grey SLS nylon powder is also available and parts printed in this material have a very similar look to MJF parts but are smoother to the touch.

Parts printed in both technologies have a grainy surface finish, but can be post-processed to a very high standard. If aesthetic appeal is the main requirement, dying is highly recommended.

4.Material Properties

The main material used in both processes is PA 12 (nylon). When printing in this material, MJF parts have superior strength and flexibility and more homogeneous mechanical properties compared to SLS parts, which are weaker along the print direction.

However, SLS printers offer more material options, such as carbon filled PA, aluminium filled PA (alumide) and flexible TPU. Moreover, SLS is a well-established and well-studied process. The properties of SLS materials are thoroughly documented by both manufacturers and independent researchers.

On the other hand, only PA is currently available in MJF and important engineering properties, such as impact strength and creep characteristics, are not widely available. Of course, this is expected to change as the technology matures and because MJF is an open material platform, allowing for faster material development.

5.Cost & Lead Time

To assess the two technologies in terms of cost and lead time, we run a small experiment on the 3D Hubs platform.

We requested quotes from 3D print services in the US for each technology, for a prototype (1 part) and for a small-batch production run (10 and 100 parts). The model used in this test was the bracket pictured below. It has a volume of 23.8 cm3 and bounding box dimensions of 101.9 x 45.0 x 18.0 mm. 

MJF has the upper hand compared to SLS when it comes to pricing. It is consistently more cost-effective by approximately 15% to 30%. The price difference is smaller for larger quantities. This is probably due to the streamlined post-processing workflow (cooling, powder removal etc) available for MJF, which allows the service providers working with these systems to drop their prices.

So, is MJF the superior process? Not always. SLS offers a larger material selection including carbon and glass filled nylon, as well as TPU and PP.