Find the differences between SLA and SLS technology

3D Printing Technologies: SLS vs. SLA

Posted By on Dec 9, 2015 | 0 comments

At Sculpteo we receive the question what’s the difference between SLA technology and SLS technology a lot, and today we’d like to share with you the similarities and differences of these 3D Printing technologies and materials.

SLS, SLA, FDM, FFF, ABS, PA, PLA…  Today more than ever, keeping up with all those acronyms representing all the different technologies, materials or processes, can be a bit of a challenge. Knowing not only what they mean but also precisely what you get from them is even harder and that’s why we want to offer you a quick but thorough comparison of two of the main technologies in the market right now. After this short not-too-long read, you will know exactly when and why you want to use either SLS or SLA technology for your next 3D printed project. If you are in a hurry you can go directly to the table at the end of this post!

Introduction: SLS & SLA 101

 

The acronym SLS stands for Selective Laser Sintering which is exactly what goes on at the heart of this kind of 3D printers. The key word here is Sintering which is, according to our dear wikipedia;

 

“…the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction.”

 

In short, inside these printers a thin layer of material powder is applied on top of the building surface, all inside a hot chamber with the temperature just under the material’s sintering point. Immediately after that, a powerful laser beam “draws” a 2D section of the object on that surface, increasing the temperature above the sintering temperature on a tiny spot (where it’s focused) and hence sintering the powder particles together. Next, a new layer is deposited on top of the first one and the process repeats itself until the last 2D section of the object is produced. The last step is just uncovering the solid object that is now buried within the unsintered powder, clean it and it’s ready to go.

SLS printers

On the other hand SLA or SL stands for Stereolithography and it was the first additive manufacturing technology to be theorized and patented back in the 80s. Today we have several slight variations of the original concept but the main idea remains the same: a near-UV laser beam is focused on a thin layer of liquid photopolymer resin and quickly draws a 2D section of the desired object (equivalently to any other 3D printing technology). The photosensitive resin reacts solidifying and thus forming a single 2D layer of the object. Applying a new layer of resin on top and iterating the process for each section of the object results in the complete 3D printed object. The last step is cleaning the final object which is soaked in liquid resin and -more often than not- removing support structures.

SLA printers

Differently to SLS printers that are mostly used at industrial level and require some serious dedicated space, SLA printers have long ago reached the consumer and prosumer level, with relatively affordable small printers.

Resolution

 

Although there is no intrinsic resolution determined for each of these technologies, there are real physical constraints that result in each kind of printer offering a resolution within a certain range. Both systems use motorized platforms which need to be able to resolve the desired resolution, which is usually not a big technical problem. Leaving that aside, both systems also use focused laser beams to solidify the building material but because they use completely different wavelengths (ultraviolet for SLA and infrared for SLS), their focus size can be also quite different, with the UV focus spot being significantly smaller than for a IR laser. Having a smaller focus is like drawing with a shaper pencil, you can resolve more than using a blunt pencil and consequently SLA printers usually achieve higher resolutions both horizontally and vertically.

 

Note: Bear in mind that we are still talking from tens to a few hundred microns!

Mechanical properties

 

Another important difference between objects printed with SLS and SLA printers is how they behave mechanically. Just as we said on Resolution, here we can only generalize and describe the most commonly used materials for each technology. SLS printers can use a wide range of materials, always in powder form, but most often they are polyamides (Typically PA12) that can have additives to change properties such as color, strength, flexibility, stiffness etc. You may think you don’t know this material but most definitely its commercial name will ring a bell: it’s just Nylon, a fantastic all-rounder that offers durability, strength and extraordinary abrasion resistance among others.

 

For SLA printers, typically the manufacturer will offer their own resins but there is a wide range of third-party options available that are usually cheaper. Generally speaking, they tend to be significantly more rigid and thus brittle than Nylon although new compounds are being release all the time, with softer and more flexible characteristics similar to rubber material.

 

One significant difference of resin objects compared to Nylon is their behaviour under load. Rigid resins tend to fail violently crashing in multiple pieces while Nylon has a wide elastic region under load (where can still return to its original shape) followed by permanent deformation and finally failure.

 

You can see this here (go to 7:27):

 

Surface

 

Here it comes one of the most noticeable differences at first sight. On a SLS printer the sintering process intrinsically creates a porous solid material, as the air that was originally within the powder goes to create microscopic air bubbles on the sintered material. Although this is nothing you can see with your naked eye, it feels slightly “rough” when touching it. On the other hand, polymerizing a liquid substance creates a solid homogeneous bulk of material.

Colorwise, polyamide powder comes basically in two flavors, black and white. A third option could be gray by mixing those two, but one could say that the offer is rather limited, with no option for translucent or transparent SLS prints. On the contrary, resins allow for virtually unlimited colors because the user can generate them using base white or transparent resins and color pigments. As objectively as the author of this post can be, translucent resin looks really, really cool.

Support

 

For objects that either do not have a clear standing position (e.g. a ball) or they do but they have overhangs ( e.g. the shade of a table lamp) support material or structures are required for almost all 3D printing technologies, with SLS printers being one of the few exceptions, thanks to the unsintered powder acting as support material all around the object being printed. Depending on each particular case, sometimes the user can dismiss or minimize the use of support at the expense of quality on the trickier areas, but this is a trade-off that is always there when using SLA printers. When support structures are used, the printed part will require some manual cleaning time and even then, is not uncommon to end up with “scars” on the spots where it was removed.

PreForm Support designed by Formlabs to minimize its impact on the print. Credits: Formlabs.

PreForm Support designed by Formlabs to minimize its impact on the print. Credits: Formlabs.

Unfortunately, using a secondary support material is not an option with SLA as all the building material comes from a single vat, resulting in both the object and its support structure made out of the same material.

Post processing

 

Although both SLS and SLA prints have post processing options, they are usually different because the prints are different by themselves.

 

  • Polishing: SLA prints are smoother and shinier, so polishing is mostly unnecessary. For SLS prints, it can be an interesting option if a smoother, shinier aspect is required.
  • Coloring: SLA resin objects are non-porous and impermeable, so dyeing is not an option. Painting can work, but depending on the object’s shape it may be pretty challenging given how complex a 3D print can be. On the other hand, dyeing is a great option for SLS prints for the exact opposite reasons and it’s actually widely used to produce colorful 3D printed plastic objects.
  • Trimming/Cutting/Gluing: Both polyamides and resins are suitable for some hand manual work, always using the right tools. You can learn more about this on our blog.

Food/Chemical compatibility

 

Food containers are probably not on the Top-10 list of 3D printed objects but being food compatible and biocompatible as is the case with our plastic (polyamide 12) material definitely does no harm (remember that this may not be the case for dyed polyamide!). On the SLA world, biocompatible or food approved resins are not that popular, specially not for consumer-grade printers, but it is still possible to find and buy them from specialized providers (generally related with medical applications).

 

SLS Nylon objects can also withstand some solvents like Acetone, some types of alcohols among others, you can check the list on our plastic material page. For resins you will need to refer to the specifications of each individual product as it can vary significantly.

Costs

 

Because SLA printers are “cheap” in comparison with a SLS printers, one could think that SLS prints should be more expensive too. In reality, thanks to the fact that Nylon is such a common material outside the 3D printing circle and that SLS printers can often output extremely large quantities in a single run, SLS prints are usually cheaper than SLA prints. That is, of course, if you don’t have to buy the printer, otherwise SLA would most likely be cheaper, even if consumables are relatively expensive afterwards.

Conclusions

 

Conclusion time comes and there is not a winner here, because this comparison wasn’t intended to find one. Each technology has its pros and cons and it will depend on the project requirements which options is best suitable for the job. For parts or projects that are focused on functionality, an SLS Nylon print may be the way to go. Both SLS and SLA qualities are among the highest in the market anyway, and Nylon’s robustness excels in almost every case. When superior quality and appearance is required and/or the part needs to be translucent, then a SLA print will be the right choice, always taking into consideration its mechanical limitations.

 

SLA SLS
Material Photosensitive resins Usually Polyamide (Nylon)
Achievable quality Superior High
Surface texture Smooth, often shiny Slightly rough
Colors (no post-process) Virtually unlimited Opaque and translucent White, Gray, BlackOpaque
Support (complex shapes) Required Not required
Mechanically Strong and brittle, new flexible compounds Strong and flexible
On mechanical failure Almost no deformation until sudden fracture Gradual deformation until fracture
Abrasion resistance Variable Superior
Post-process Polishing (rarely needed)Painting PolishingDyeingPainting
Food Only with special resins(can be expensive) Yes
Chemicals Not defined Highly resistant (Nylon)
Cost Printer relatively inexpensiveResins can be expensive Printers seriously expensiveMaterial inexpensive

 

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