Everything you need to know about polymer powder bed fusion

The SLS process involves the use of a high-power laser to selectively fuse particles of polymer powder—typically nylon—according to computer instructions. Starting at the bottom of the part, an SLS machine sinters each layer in succession, with the build platform lowering incrementally to move from one layer to the next. The key components of an SLS system are the build chamber, powder delivery system, re-coater, laser and scanning mirror. The laser and mirror are positioned above the build chamber, so the first sintered layer is effectively the base of the part, with each successive layer built upon the last.

The process begins with the application of a thin layer of polymer powder onto the surface of the build area. Because there is always a full covering of powder in the build area, the printed part does not require support structures; it is supported by unsintered powder. The temperature of the enclosed chamber is raised, though the temperature must remain significantly below the melting point of the polymer so the part does not deform. Sintering of the first layer takes place as the laser focuses its laser beam onto the scanning mirror, which can move about two axes in order to direct the beam onto the right areas of the powder bed. The laser traces a 2D shape into the polymer powder, raising its temperature to just below the melting point, which causes the particles of the powder to fuse together. This creates a precisely sintered pattern in the powder: the first solid layer. When the layer is complete, the build platform lowers incrementally, and the next layer can be created. The heating of the particles allows each layer to fuse to the next.

When all layers have been sintered, the build chamber cools down gradually to prevent deformation of the part. An operator takes the parts out of the machine and removes any excess powder, either manually or using a depowdering system. Post-processing may involve blasting abrasive media onto the parts to improve their surface finish. Any remaining unsintered powder in the machine can be reused.

SLS parts are durable and robust, especially compared to other AM parts and even compared to molded parts. The process is therefore widely used for the production of end-use parts in industries such as automotive and aerospace. It is also widely used for mass customization of low-volume end-use parts.

Table 1 – Advantages and limitations of polymer powder bed fusion

Key evolutionary trends of laser sintering technology

German company EOS and American company 3D Systems remain the global leaders in industrial SLS 3D printing today; however, competition is increasing, especially since the most recent of the original SLS patents expired in 2014, paving the way to the development of the technology’s first low-cost systems. Today, the race for market leadership comes down to technological advancements. This applies in terms of machine size, with Farsoon’s HT1001P offering for the first time over a meter in build length capabilities, as well as process evolution and material science. Process and post-processing automation, via direct or indirect partnerships with external finishing technology providers, is also a major factor in accelerating SLS capabilities and adoption.

While EOS has promised (and continued development) a radically new approach to polymer L-PBF with the LaserProFusion technology, featuring up to a million laser diodes,  one of the most significant and practical new developments in the polymer L-PBF is 3DM’s semiconductor-based quantum cascade laser (QCL) system. This laser technology can replace the laser print head in any L-PBF system to adapt the laser’s frequency to a specific polymer material. This approach results in significantly lower printing costs, higher speeds and better part quality compared to existing technologies.

Key vendors of SLS (laser polymer PBF) systems

EOS

EOS has made important advances in the key areas of process, materials and automation, with the company’s most popular systems spanning three size categories. These systems include the entry-level Formiga P 110 Velocis, the mid-size EOS P 396 (one of the industry’s top-selling systems) and the larger EOS P 770. In 2017, EOS introduced the EOS P 500, its most automation-ready machine. The EOS P 500 features an innovative layering system, which applies and compacts the material at up to 0.6 m/s, with its two 70-watt lasers reducing the costs per part by more than 30%. The system is controlled by EOSYSTEM, with software integration in CAD systems provided by EOSPRINT 2 and connection to ERP systems provided by EOSCONNECT. Automated interfaces and optimized accessories serve to reduce cycle times.

The newest EOS system is the EOS P 810, an upgraded version of the P 770 with augmented capabilities for processing high-performance polymers and composites, particularly its newest composite, HT-23 PEKK. Filled with carbon fiber, HT-23 is inherently flame-retardant and UV-resistant, meeting the standards of the aerospace (FAR 25.853) and mobility (EN 45545) industries. It is also suitable for series production applications in the electrics and electronics sectors.

The future of laser-PBF technology at EOS is represented by the company’s most ambitious project yet: EOS Laser ProFusion technology. In this new polymer L-PBF AM process, the material is melted by almost a million diode lasers to create the desired part, making the build process productive enough to compete with injection molding in many applications. In terms of post-processing and process volution, EOS systems can work with a variety of providers. However, via its AM Ventures fund, the company is a leading investor in DyeMansion, one of the leaders in AM-specific post-processing and workflow automation hardware, making these systems particularly compatible with EOS machines for part handling, finishing and coloring.

3D Systems

3D Systems’ principal SLS systems are the sPro series (140, 230) machines, which cater
to different size requirements. Its newest and most automated industrial-scale system is the SLS 380, which is intended to be the first of a new generation of machines. The SLS 380 is a high-throughput SLS system comprising software, material handling, and post-processing solutions (provided by AMT) tailored to cost-effective batch production. The machine features closed-loop process controls that enable high levels of repeatability across multiple parts, builds, machines and sites. In addition to a new water-cooled laser, the system utilizes a custom-developed 3D Systems algorithm to manage, monitor and control in real-time the thermal uniformity within the build chamber.

An important part of the SLS 380 solution is the material quality center (or MQC), designed to control, blend and deliver material on demand for an optimal ratio of fresh and recycled powder. There are two MQC options. The MQC Single is designed to connect to one SLS printer, whereas the MQC Flex is optimized to deliver material to up to four printers simultaneously, minimizing waste and eliminating operator intervention. The MQC Flex automatically blends fresh and recycled powder according to any specified mix ratio and includes a recycled powder bin that provides convenient and immediate storage for unused powder post-printing. Both versions of the MQC feature fully automated material feeding and an integrated breakout station for post-processing.

Rise of Chinese companies

The most intense competition for EOS and 3D Systems in the industrial SLS segment comes from Chinese companies establishing or consolidating their presence in Western markets. In some cases, these companies offer machines that are significantly larger than any Western hardware provider. Farsoon was the first to emerge. The company was founded in 2009 by Dr. Xu Xiaoshu, who served as a technical director at the industry’s first SLS laser sintering company, DTM Corporation (now 3D Systems), in the early 1990s. Farsoon’s expansion into Western markets began when it formed a partnership with French 3D printing company Prodways in 2015 through the “Prodways powered by Farsoon” brand. In 2021, the two companies officially parted ways, and Farsoon has now established a direct presence in Europe (Germany) and North America (USA).

Like EOS and 3D Systems, Farsoon now provides three different sizes of machine: the entry-level eForm, the mid-size 252P series and the larger 403P. Farsoon’s newest, top-of-the-line industrial system for mass production is the giant HT1001P, featuring a 1000 x 500 x 450 mm build volume (the first SLS system to ever go up to 1 meter in length) and dual 100-Watt lasers. The systems features CAMS – Continuous Additive Manufacturing Solution – capabilities which means it was developed to address future models of production within the Industry 4.0 framework by offering vertical scalability and modularization capable of integrating into any industrial manufacturing facility. Designed with a comprehensive materials system featuring closed-loop powder handling with increased automation, the system’s throughput is also enhanced by a high-efficiency top-feed system as well as fully digital multi-laser scanning capabilities.

Eplus3D is the latest serious contender from China to enter Western markets. The company produces SLS as well as SLA systems. Founded in 2014 by Feng Tao, who presided over the development of China’s first SLS equipment in 1993, EPlus3D initially formed a partnership with Chinese 3D scanner specialist SHINING3D to market its systems in Europe and North America. The company provided the machines that were sold under the SHINING3D brand in a deal similar to Farsoon’s arrangement with Prodways. In 2021, Eplus3D amicably parted ways with SHINING3D and opened a new facility in Germany to market its systems directly. Eplus3D has two facilities in China—in Beijing and Hangzhou—and exports machines to more than 40 countries, including Japan and South Korea as well as countries in Europe, America and Southeast Asia.

Rise of entry-level SLS

As SLS requires the use of high-powered lasers, it can be very expensive. The cost and potential danger of SLS printing mean that the home/desktop market for SLS printing is never going to be as large as the market for other additive manufacturing technologies. However, the January 2014 expiration of the most recent patent for Deckard’s SLS technology has opened up the market to multiple competitors and expedited the introduction of lower-cost systems, which are usually referred to as “benchtop” systems (meaning they are best suited to be used in a workshop).

The current market leader for benchtop SLS technology is Polish company Sinterit, which was one of the first to introduce a low-cost system (alongside Swiss company Sintratec and Italian company Sharebot). Sinterit was founded in 2014 by former Google Software Engineer Konrad Glowacki and his partners, Michal Grzymala-Moszczynski and Pawel Szczurek. After developing the original Sinterit Lisa system, the company recognized the need to scale up with a professional management team, bringing in new figures in senior management, sales, marketing and R&D, along with experts in support, production, finance and legal. The company’s top-selling system today is the LISA Pro, which retails for around €12,000 and is part of a complete solution that includes material handling and part cleaning hardware.

Formlabs, the company that created the affordable/prosumer SLA technology segment, is attempting to replicate that success in the SLS field. Entering the market with some delay following its original announcement, the Fuse 1 system did not enjoy the same rapid success as Formlabs’ stereolithography 3D printers. However, the company is now building momentum and a large installed base, bringing more credibility to the entire benchtop SLS segment. Priced below $15,000, the Fuse 1 features patent-pending Surface Armor technology, printing a semi-sintered shell around the surface of the part to ensure mechanical properties and a surface finish competitive with higher-priced systems. In addition, the system offers an efficient powder recovery system and a removable build chamber for continuous manufacturing.

Sintratec has made significant progress on its benchtop SLS technology, supported in part by investments from SLS market leader EOS via the AM Ventures technology fund. Starting with a self-assembly kit costing under $5,000, the company developed the Sintratec S2, an all-in-one solution consisting of a Laser Sintering Station (LSS), Material Core Unit (MCU) and Material Handling Station (MHS). More recently, Swedish company Wematter has emerged as a serious contender in the low-cost SLS market. In 2021, Wematter launched the Gravity 2022 SLS 3D printer, built for extremely hot and cold working environments, and the Density post-processing station that uses water blasting and compressed air processing to clean parts, even in office environments.

Less popular but also relevant as an intermediate solution between industrial and benchtop is the entry-level industrial segment of SLS 3D printers. These are systems priced between $50,000 and $150,000. They are offered as an entry-level solution by Chinese manufacturers such as Farsoon and Eplus3D, while the lowest-priced systems from 3D Systems and EOS both come in at closer to $200,000 or above. French company Prodways markets an internally developed entry-level SLS system priced at $100,000, the ProMaker P1000. In Asia, low-cost system manufacturer XYZprinting has introduced the MfgPro230 xS, priced at $60,000.

Thermal powder bed fusion (MJF, HSS, SAF)

With the launch of its Multi Jet Fusion (MJF) technology at the end of 2016, HP signaled the start of a new era in terms of powder bed fusion productivity. By integrating certain elements of binder jetting technology and implementing a thermal (infrared-based) fusion process, HP’s MJF technology brought significant benefits in terms of speed and productivity, similar to the way continuous DLP processes affected the vat photopolymerization market segment. HP has since built up a significant leadership position in the use of polymer PBF for final parts production; however, new competitors such as voxeljet (HSS technology) and Stratasys (SAF technology) are now entering this profitable and rapidly growing market segment.

Key vendors of thermal powder bed fusion systems

Understanding thermal powder bed fusion processes

HP Multi Jet Fusion technology enables high build quality up to 10 times faster than most laser polymer powder bed fusion 3D printing solutions on the market—and at a lower cost. These breakthroughs in quality and speed have accelerated the adoption of 3D printing to create a digital transformation of manufacturing.

Built on three decades of HP investment in inkjet printing, jettable materials, precision low-cost mechanics, material science and imaging, MJF technology has the unique ability to produce parts with controllable physical and functional properties at each point in a part. By jetting HP functional agents using HP print heads, material in the working area can be fused, detailed and transformed point-by-point.

A key innovation is that MJF is a planar process: dual carriages scan across the working area in perpendicular directions: one carriage recoats the working area with fresh material, and the other prints HP functional agents and fuses the printed areas. This separates the pro- cesses of recoating and printing/fusing so that each process can be separately optimized for performance, reliability and productivity.

After job completion, the build unit is rolled into an HP Jet Fusion Processing Station for cooling, unpacking the parts and recovery and refreshing the build material. While those processes are completed, a build unit that has been refreshed by the HP Jet Fusion Processing Station can be rolled back into the printer for continuous production.

Multi Jet Fusion technology uses scalable HP thermal inkjet technology to make print bars of different widths by stacking print heads across the width of the scan. Just as this capability allows HP to scale its 2D printing solutions from the desktop to more than 100 inches wide, HP can create a range of HP Jet Fusion 3D printing solutions with working areas of different sizes. HP print heads can also be stacked along the scan direction to add more nozzles for speed, functionality and nozzle redundancy for dependable printing quality.

The build begins by laying down a thin layer of powdered material across the working area. The material recoater carriage scans from top to bottom. Next, the printing and fusing carriage with a thermal inkjet (print head) array and energy sources scans from right to left across the working area. The leading energy source preheats the working area immediately before printing to provide consistent and accurate temperature control of each layer as it is printed. The print heads now print functional agents in precise locations onto the material to define the part’s geometry and its properties. The printing and fusing carriage now returns left to right to fuse the areas that were just printed.

At the ends of the scans, supply bins refill the recoater with fresh material, and service stations can test, clean and service the print heads on the printing and fusing carriage as needed to ensure reliable operation. After finishing each layer, the surface of the work area retracts about the thickness of a sheet of office paper, and the material recoater carriage scans in the reverse direction for optimum productivity.

HP’s vision for Multi Jet Fusion technology is to create parts with controllably variable mechanical and physical properties within and across a single part or among separate parts printed simultaneously in the build unit. This is accomplished using additional “transforming” agents to control the interaction of the fusing and detailing agents with each other and with the material to be fused. Depositing transforming agents voxel by voxel across each layer allows the Jet Fusion 3D printers to produce parts that cannot be made by other methods.

Even before HP presented its plans to launch MJF technology, a team from Loughborough University in the UK had been developing a process referred to as High Speed Sintering (HSS) using Xaar print heads. Since 2014, the subsequent global rollout of HSS processes, under license to multiple companies, has contributed to accelerating the shift toward digital manufacturing. The HSS process involves depositing a fine layer of polymer powder followed by an infrared-absorbing fluid directly onto its surface in the required 2D pattern. An infrared lamp is shined onto the build, causing the printed fluid to absorb the energy, then melt and sinter the underlying powder. This process is repeated layer by layer until the build is complete.

The use of digital inkjet printing makes the process faster than point-based laser processes, as a layer can be sintered in a single pass. However, like other powder bed processes, High Speed Sintering is a self-supporting process, and support structures are not required. The speed of HSS is not determined by the ink throughput of the print head as it is with material jetting, in which all of the part-building material has to pass through the print head. With HSS, less than 5% of the manufactured part passes through the print head, the rest being the powder that gets sintered by the energy-absorbing fluid. Powder lay down, therefore, affects print speeds more than print head lay down. HSS is about 10 times faster than SLS and able to produce parts with mechanical performance equal to or above SLS, with both technologies typically using PA12 material. HSS can achieve dimensional tolerances of ±200 microns and feature resolutions of below 0.5 mm. SLS parts have a typical layer height of 100 microns, whereas HSS can create thicker layers if sufficient IR energy can be absorbed. This is possible when using highly loaded and viscous droplets.

There are only a few minor differences between voxeljet’s HSS technology and HP’s MJF. In HSS, as with MJF, an energy absorber Is used. Selected areas of the powder bed fuse together while the unprinted powder remains loose. A second cooling fluid is not necessary with HSS, because the temperature of the printed and unprinted powder material can be controlled independently using two different IR emitters with different wavelengths. The unprinted powder can be recycled, reprocessed and fed back into the process cycle, just as with MJF. The voxeljet VX1000 HSS is larger than any of HP’s current printers, while voxeljet also allows open and unrestricted material use.

Key evolutionary trends

With the introduction of the first HP Jet Fusion 4200 3D printer at the end of 2016, additive manufacturing entered a new era of productivity. Today there are three companies offering thermal powder bed fusion capabilities, each presenting a different approach to digital additive mass production.

Since the launch of MJF technology, HP 3D Printing has targeted high-throughput production of parts with break-even (compared to injection molding) at about 50,000 units. To achieve these capabilities, HP worked on developing an optimized workflow, starting from the very begin- ning with the introduction of a Processing station (sold with the 3D printer) to rapidly cool down the Build Units and recycle unused powder. As production rates grew, so did the complexities of the workflow, whose evolution can be seen in the next-generation HP Jet Fusion 5200 systems.

Today, the HP offer has expanded to include the newest HP Jet Fusion 5400 series, which can produce white parts. Workflow automation capabilities have also expanded to include multiple hardware systems such as Natural Cooling Units and the Automatic Unpacking Station (from AM Solutions). Software has also advanced significantly to enable high-volume production via the HP 3D API for factory integration, a Universal Build Manager (from Dyndrite) and a Process Control system.

At the same time, the company has worked to build the HP Digital Manufacturing Network, a network of AM service providers which now includes over 50 certified providers of AM production services. The company is also working with AM adopters that look to leverage 3D printing via a pay-as-you-go system referred to as 3DaaS. This service includes automatic replenishment of HP 3D supplies and HP 3D Printing Care Services (including remote and onsite support as well as an online dashboard for easy, convenient tracking of billing and usage). Overall, MJF systems are now producing millions of parts per year, with single companies, such as SmileDirect, 3D printing as many as a million parts per year on their own.

Voxeljet and Stratasys, each working with their own versions of thermal powder bed fusion technology, are now the main competitors in the polymer additive mass production segment. Voxeljet introduced the VX200 HSS in 2017, based on the architecture of its sand binder jetting system of the same size. The 290 x 140 x 180 mm build volume of the machine can be used for fast polymer AM production, using nylon 12 as standard material, though the company also supports PP, TPU, PEBA and EVA via partnerships with materials manufacturers Covestro, BASF and Evonik. Voxeljet has now also launched the VX1000 HS, which offers by far the largest build platform of any high-speed PBF system, with a build volume of 1000 x 540 x 400 mm. Providing automation and high production rates, the machine is designed for continuous use in industrial production, with minimal personnel requirements, virtually no interruption and extreme cost-efficiency. The VX1000 HSS can also be networked and flexibly expanded via a modular automation concept that is adaptable to varying production requirements.

In April 2021, Stratasys introduced the Stratasys H350 3D printer, the first system powered by technology developed jointly with Xaar. Representing the culmination of more than 10 years of research and development, these SAF 3D printers were first installed at Stratasys Direct Manufacturing and various beta customer sites in Europe and the USA.

One of the unique selling points of the Stratasys H350 is that is uses High Yield PA11, a bio-based plastic from Arkema made of renewable raw materials derived from sustainable castor oil. Compared to PA12, PA11 has a lower environmental impact, superior thermal resistance and is less brittle. It has passed initial tests including ISO 10993-5 for cytotoxicity and UL94 HB for flammability. Stratasys ultimately plans to support a wide range of certified polymer materials.

The architecture of SAF technology enables part packing in the build volume to a standard density of 12%, compared to the 6–10% density typical of powder bed fusion 3D printers. Moreover, Stratasys has been able to demonstrate support for packing densities of up to 23.5% in real-world conditions. Packing density directly translates into either more parts per build or a faster build time.

To optimize workflows and scale production, H350 customers can integrate with manufacturing floor systems through the MT Connect standard. Customers are also able to utilize software applications such as Materialise’s Magics, Siemens NX and PTC Creo using Stratasys Build Processor. Stratasys will soon provide GrabCAD Print support for build preparation. To close the circle at the end of 2021, Stratasys acquired all remaining shares of Xaar 3D Ltd (the company had already acquired a 45% stake in the joint venture) from Xaar.