THE LIFE AND TIMES OF AN SLS POWDER

Do you know what the first selective

laser sintering (SLS) materials were?

Sugar, salt and sand.

At least that’s according to the Godfather of SLS, Carl Deckard, who told TCT last year that he knew SLS powders would need to be granular, so he started playing with what he had to hand.

Fast-forward 30 years, and we’re processing Nylons, PEEK, TPUs, flame-retardant polymers, anti-static polymers. SLS technology can leverage thousands of specific mechanical property needs, depending on both the material and sintering process.

The most commonly used powder in SLS is Polymide 12 (PA 12) - a robust and resilient nylon material that’s as good for end-use parts as it is for prototypes.

Over the years the compound, the milling, the process, the refresh rates have been refined to a point where one can call SLS a mass production process. The steps involved in making a powder before it even sees an SLS machine are complex; it starts with the chemical compounding.

“There are many different processes to manufacture polymers,” explains Moritz Kügler, the Product Manager for Polymers at EOS. “The main source is often crude oil that gets refined and made into monomers. Through

the polymerisation process, these monomers are made up into polymers.”

At this point, the chemistry can be altered to add mechanical properties like flame retardancy or elasticity. Post-polymerisation the raw material takes many guises - pellets or extruded wire - but either way at this point in the chain, for use in SLS the polymer must be ground and milled into a powder.

BACK TO THE GRIND

Many chemical firms choose to mill themselves. However, there are companies like the Dressler Group who specialise in producing the highest-quality powders for additive manufacturing from some of the toughest materials to grind.

“Normally a customer comes to us when they have a material with unique properties for additive manufacturing and tells us that they have a problem turning it into a powder,” says Axel Dressler, joint CEO (alongside brother Jan) at Dressler Group. “We try to figure out what their material requirements truly are and then show them how to achieve that specification using our technology.”

Dressler Group has both an Innovation Lab and Technical Centre, where for the past five years, the company has developed processes for grinding previously difficult materials like TPUs and PEEKs into usable powders for SLS. A customer journey at the Dressler Group usually involves exploration in the lab and technical centre, where they can take small trials of the powder for testing, before returning for production quantities.

“We have a lot of machines in both our technical centre and on our production line,” explains Jan Dressler. “It is important that we select the right machine to make the right powder. Trying to make powder when you have compounded a material filled with glass or carbonfibre is a challenging process. It has taken us more than three years to develop a process which can be used for previously ungrindable materials.”

The Dressler Group will be showing the world this technological development for the preparation of additive powders at both K Show and the upcoming Formnext event in November. The sibling management team are very excited about the possibilities; after all, new materials equal new applications. 4

HIT REFRESH

Despite all of that front-loaded work behind the preparation of an SLS material, the powder is still not ready for processing in a machine.

The virgin powder requires mixing with material that has already been used to flow correctly on a device; the majority of powders have well-defined refresh-rate data sheets developed through years of trial and error. Take the typical PA 12 material; EOS recommends a refresh rate of 50% virgin powder mixed with 50% recycled powder to ensure sufficient part quality. EOS has some customers who are more focussed on cost than part quality; they bring that refresh rate down to 40% virgin powder. However, EOS’s Moritz tells us how crucial adherence to refresh rates can be:

“Refresh rates differ from powder to powder, but say we have, for example, a flame retardant material, where you usually have the flame retardant chemicals applied on top of the surface of the powder, these coatings can be volatile during the process. So if you were to recycle that material, you couldn’t guarantee that it’s still flame retardant.”

It is not just the machine and the powder that affects the optimum refresh-rates. EOS recommends that you increase the virgin powder ratio if, for example, you have a densely packed job, this may be slightly more expensive, but proper nesting can optimise other areas such as machine time. Managed correctly, cost increases are negligible.

Keeping track of the powder used has often been a problem, particularly for the super users who go through tonnes of powder a week. Like a lot of early additive

issues, the problems come when, for example, your one additive employee leaves. Without robust data management, it’d be impossible for the next incumbent to know what materials have been used and what materials are okay to be recycled. Errors in logging materials can result in tonnes of powder, and therefore money, wasted. The proper disposal of the powder also does not come cheap.

Firms like Russell Finex are looking to close the loop on the issue with automated systems like its awardwinning AMPro Sieve Station. After extensive research and working with high-profile users like New Balance on its 3D Printed Midsole project, Russell Finex put together a technology that automatically evacuates powder from build chambers recovering powder and sieving with virgin powders.

THE PRINTING PRESS

Once you have that perfectly mixed powder, it is time for use in an SLS machine. SLS is, for a good reason, still a go-to method for high-quality part printing. The likes of EOS, 3D Systems and Farsoon have a vast industrial range of machinery, and then there are companies like Formlabs and Sinterit bringing the technology to the desktop.

What happens inside those machines is a variation of a theme; the mixed powders are sintered using a laser to form solid parts suspended in powder (meaning the process, unlike others, doesn’t require support material).

Once a job is complete, the parts head to the depowdering station, where a vacuum is used to reclaim reusable powder. The powders surrounding the parts have often come under stress from the heat source and therefore are waste.

The next step is dependent on the requirement for post-processing or not. After depowdering, SLS parts retain a powdery rough surface finish, but the mechanical properties are ingrained and for fit and form prototyping, postprocessing is often not required.

However, SLS printing is now a manufacturing platform for mass production. BMW Group use the technology for their Mini Yours Customized dashboard fascias, the Royal British Legion are using the technology for the mass-manufacture of one of its popular poppy badges.

Certainly for consumer projects the post-processing steps become essential. The most common forms of post-processing are bead blasting and colour dyeing. Companies like AMT in the UK and DyeMansion in Germany have taken steps to automate this previously labour intensive step, and automation is key to unlocking SLS’s potential as a mass manufacturing tool.

One of the most significant case studies for the mass manufacture of parts using SLS is a Chanel 3D Printed mascara brush, which is available in major retailers across the globe. Estimated figures show that the service bureau making the parts, Erpro, has made over one million mascara brushes for the French fashion giant. 4

“This is a single-use application where 3D printing is price competitive [with injection moulding],” says EOS’ Moritz Kügler. “We are very proud of it and we are working on this kind of application to multiply, where the price is not just competitive for a single part but competitive for mass-produced parts made 100,000 times. I think this will be the fastest-growing market for SLS in the coming years.”

WASTE MANAGEMENT It’s often said of AM that it is good for the environment as we only use the materials we need. This is not strictly true. As alluded to throughout this piece, there is going to be leftover powder to be disposed of. A source at a leading service provider, talks us through what that powder is and how they dispose of it.

“Powder reclamation is pretty simple - any powder that is stuck to the part is waste, any that falls off is good to [re]use. We give the parts a light tap to encourage the powder to fall off, but no more. This maintains a good balance for the 50:50 old:new powder ratios. A professional waste disposal company collects around 400kg of waste powder from us per month - this is a mix of glass from the depowdering machine and PA 2200. It is difficult to be certain of the mix, but as an estimate, we buy 500kg per month - 25% becomes a part, 25% is waste and 50% reused for refreshing the next build. I’d say that 125kg of the powder taken is PA 2200 and 275kg is glass.”

Dr Sören Griessbach, founder of GS-PRO, has a particular bee in his bonnet about the amount of waste SLS produces.

“In my opinion, there is too much waste, particularly with filled material like carbon-fibre, glass or aluminium filled, where only 10% of the build cake becomes parts, and the rest is dumped. Nylon 11 is also a mess - about 40% of the material is waste. The PA12 is slightly better and since HP released Multi Jet Fusion the amount of virgin powder required was reduced to 25%. That still means you create about 10% waste, which is roughly 2-3kg every day for each machine or 0.75 tonnes per machine a year.”

GSPro has been working on a mechanical treatment as a solution to this issue; its patented technology allows SLS users to reduce the virgin powder requirement to 15% while maintaining mechanical properties.

Dr Griessbach’s passion started at his father’s 3D printing service company, VG Kunststofftechnik GmbH, which stored four tonnes of waste powder. The company investigated ways to reuse it and Dr Griessbach spun his technology out to GSPro in 2012.

The world of SLS powders is fascinating and in the space of three decades we’ve moved from Carl Deckard printing shapes with sugar to the likes of BMW, Chanel and New Balance mass producing products with the technology.

WORDS: DANIEL O’CONNOR