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SLS – the cutting-edge in 3D printing

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If you want your company to bloom, to stay competitive, you need to take advantage of the best tools for the job. It’s a fact and no one questions it. It’s true for any job, any industry. If you aim to take advantage of a new technology, you’d better be prepared to get the real, cutting-edge thing, because you can be sure everyone else will be right on your toes… or even a few steps ahead of you.

The above is true for 3D printing as much as for everything else. An example? Imagine your company is in the business of creating plastic envelopes. Now imagine you can do it a lot faster, more efficiently, and with half of the hassle too. That’s good, right? That’s really good! And the thing that makes it possible, the one decision is choosing the right 3D printing technology for the job. To do that, you need to judge the advantages and disadvantages of competing options. Thankfully it’s pretty easy, because one of them is leaps and bounds ahead of others.

FDM, SLA, SLS – what does it all stand for?

These are all different techniques, different approaches to 3D printing. FDM stands for Fused Deposition Modeling – it’s a process during which the model is produced by extruding small flattened strands of molten material, forming layers as the material hardens immediately after extrusion. The SLA acronym describes Stereolithography – it employs photopolymerization, using light to link chains of molecules together, and form a polymer 3D structure. Both of them could be seen as forms of micro-scale sculpting, but they’re using different methods to “glue” the structure together.

Finally, there’s the SLS – Selective Laser Sintering, a fundamentally different technique. It makes use of a powder (most often it’s polyamide – a form of nylon), which is heated up, and then smelted with a laser in specific places, sintering consecutive layers into one, durable model. How does that work exactly? Imagine a miniature sandbox, with layers of polyamid “sand” being smelted in a couple of places and then covered with a fresh powder coating to allow the laser to reach another layer – which, once again, is smelted in carefully selected spots. After a fashion, the structure slowly emerges from the powder.

The importance of accuracy and support structures

Many people tend to think: the bigger things you can print, the better. In reality, it’s usually the complete opposite that’s true. What’s the point of printing a giant structure, if it’s only going to be a clumsy, uniform bloc? The Devil, as they say, lies in the details, and to create them you need accuracy. The higher accuracy you need– the narrower layers you can print – the more complex your models can become. This is where the SLS technique starts to shine. Thanks to the fact that it uses a polyamide powder, with an average granularity of 38[um], it can create layers as thin as 0,075  [mm] or less. For comparison: FDM printers usually manages to reach layer thinness around 0,100 [mm] and SLA, while doing a bit better in this regard, can still produce layers twice as thick as SLS, at best (0,15mm).

But it’s not all about accuracy either. There’s also the factor of having (or not having!) to use support structures. When you print a model it needs to be supported somehow, or it’ll just collapse – gravity, after all, is pretty unforgiving. In case of an FDM or SLA prints, special supports are made so that the model can stand or hang in place. These structures have to be later removed manually or chemically and the surfaces have to be smoothed. It all takes time, but the most important thing is that using support structures can actually limit your options when it comes to printing. Some things are simply impossible to print because of that. SLS doesn’t suffer from these limitations, offering you unparalleled freedom of form. All thanks to a pretty genius idea: structures are supported by the spare powder, that isn’t used during the print, but lies in the printer’s work chamber anyway. Remember the sandbox? During the printing of every layer, sintering only takes place in selected spots. So now you know what the rest of the powder is needed for. When the model is complete, you just remove the spare powder and voilà  – your structure is ready to be used.

Time and cost efficiency

Okay, printing detailed models is all great, but what if you need raw quantity, not complexity and quality? It’s the age-old question of choosing one over the other, right? But… Why not have them both? With SLS it’s entirely possible, thanks to a process called “nesting”. It’s pretty simple – since you always have to use more powder, than is actually required to print a certain structure (the actual quantity is defined by how high your model is) you can just fit more models into the printer’s work chamber, so they can be printed all at once. The laser will have more points to sinter, which will extend the printing process, but otherwise it works exactly the same way. Even with the added time, it’s still a lot faster than if you had to print each structure, one by one, not to mention the fact that you can get the whole complex model done at once, instead of printing it part by part. If you also factor in the time needed to remove the support structures and model assembly – something that’s absolutely necessary in case of FDM and SLA printing – then… yeah, you can do the math. It’s no surprise SLS is the fastest, currently known 3D printing process.

However, there’s a question that can arise in the mind of an attentive reader right now: if you have to use so much powder every time you print something, how cost-efficient SLS exactly is? It depends, but in general it’s a lot better than it sounds. SLS printers are usually optimized for mass printing, true, but that doesn’t mean printing single models is such a bad idea. What makes it a viable option is the fact that you can recycle powder that wasn’t used (sintered) during printing. You have to mix it with a bit of fresh material to make it work – in case of the Sinterit Lisa printer, the ratio sits at 7/3 – but in the end, the costs of using the printer aren’t as high as you’d expect them to be.

Durability and resistance without cutting corners

The materials used in SLS printing offer great durability of printed structures. When measured during the Charpy impact test, the PA 12 smooth material – one of the two commonly used types – reaches maximum fracturing energy of 5,23 [KJ/m2] in case of a U-notch type sample and 3,28 [KJ/m2] in case of V-notch type. It’s also worth mentioning that this kind of durability can be achieved without sacrificing the option to print movable parts in your models.

On the other hand, if you opt for using the TPU Flexa Black material, you can achieve impressive chemical, temperature and UV resistance, alongside great flexibility. Flexa Black has a melting temperature of 150 – 160 degrees Celsius, it can withstand UV radiation, no to mention chemical substances: acetone, glycerine, petrol, methanol and others.

Groundbreaking technology

As you can see, there’s currently one 3D printing technology on the market, that offers a clear advantage over the rest – Selective Laser Sintering. So why isn’t it everywhere, you might ask? Because up until now, it also had its disadvantages – it required a lot of space and was quite expensive. Because of these problems, it was used only in large-scale, manufacture size printers. But the technology has advanced, and the world still changes. Thanks to machines like the aforementioned Sinterit Lisa, the SLS technique of additive manufacturing is now available to everyone who’s got a free, medium size desk and a spare 5 000 euro. It really doesn’t get any better than that.