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3D printing technology: SLA vs SLS

Introduction

At Sculpteo we are often asked: what is the difference between SLA technology and SLS technology? Today we’d like to share with you the similarities and differences of these 3D Printing technologies and materials, in order to help you figure out what technology you need for your 3D printing project. 

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 read, you will know exactly when and why you want to use either SLS or SLA technology for your next 3D printed project.

Introduction: SLA and SLS technologies

The acronym SLS stands for Selective Laser Sintering which is exactly what goes on at the heart of this kind of 3D printers. The keyword here is Sintering which is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction.

Inside these SLS 3D printers, a thin layer of material powder is applied on top of the building surface, all of this, 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. 

SLS 3D printing

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. This SLS process is a layer by layer technique. 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.

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 thanks to UV light. 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 3D printing

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

 

Resolution of these additive manufacturing techniques

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 an Infrared laser (or 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: Keep in mind that we are still talking from tens to a few hundred microns! SLS technology can totally help you create very detailed models.

 

Different mechanical properties for SLA and SLS technologies

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 powdered materials, but most often they are polyamides (typically, Nylon 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 other characteristics.

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 released all the time, with softer and more flexible characteristics, quite similar to rubber material.

One significant difference of resin objects compared to Nylon is their behavior 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. The choice between SLS vs SLA 3D printing will totally depend on the nature of your project

What about the surface?

Here it comes one of the most noticeable differences at first sight. Using 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.

SLA printing

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

Colorwise, polyamide powder is offering basic option: black and white, for raw material options. 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. Resins printed using SLA machines allow for other options, while mixed with color pigments. 

Supports during the 3D printing process

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 during SLS process. 

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.

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

 

Post-processing for SLS and SLA technology

Although both SLS and SLA prints have post-processing options, they are usually different because the prints are different by themselves, but you get some really good surface finishes.

  • 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.
  • Colors: SLS 3D printing is offering a wide range of color possibilities on our online 3D printing service.
  • 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 3D printing blog.

 

Food and 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 (but remember that this may not be the case for dyed polyamide!). On the SLA world, biocompatible or food approved resins are not that popular, especially 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 resin materials, you will need to refer to the specifications of each individual product as it can vary significantly.

 

Cost

Because SLA printers are “cheap” in comparison with professional 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 afterward.

Conclusion: Let’s recap

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 option 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. One thing is sure, these printing techniques are offering many more advantages than traditional manufacturing techniques such as injection molding.

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.

 SLASLS
MaterialPhotosensitive resinsUsually Polyamide (Nylon)
Achievable qualitySuperiorHigh
Surface textureSmooth, often shinySlightly rough
Colors (no post-process)Virtually unlimited Opaque and translucentWhite, Gray, BlackOpaque
Support (complex shapes)RequiredNot required
MechanicallyStrong and brittle, new flexible compoundsStrong and flexible
On mechanical failureAlmost no deformation until a sudden fractureGradual deformation until fracture
Abrasion resistanceVariableSuperior
Post-processPolishing (rarely needed)PaintingPolishing Dyeing Painting
FoodOnly with special resins(can be expensive)Yes
ChemicalsNot definedHighly resistant (Nylon)
CostPrinter relatively inexpensiveResins can be expensivePrinters seriously expensive Material inexpensive


Did you like this battle of technologies between Stereolithography and Selective Laser Sintering? Check out this one, about 
FDM vs SLA vs SLS and discover more information about these 3D printing methods. 

At Sculpteo, we are offering you the possibility to print using the SLS process, with our professional 3D printers. Upload your 3D design right now, and choose your plastic 3D printing material and receive your parts in a few days. Get your prototypes and low volumes printed in the best quality thanks to our service!

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