SLA and SLS — Which Technology is Right for You?

SLA and SLS have similar-sounding names, and they both use lasers. That’s where the similarities end — they are different 3D printing technologies that serve practically opposite purposes. It’s critical for any additive manufacturer to pick the right technology for …

Ile Kauppila

October 7, 2021

SLA and SLS have similar-sounding names, and they both use lasers. That’s where the similarities end — they are different 3D printing technologies that serve practically opposite purposes.

It’s critical for any additive manufacturer to pick the right technology for them. The wrong choice can make your parts mechanically inferior or just plain unsuitable for their intended application.

In this article we’ll explore what is the difference between SLA and SLS printing, and provide a detailed comparison so you can choose the best technology for you.

Overview of SLA

Stereolithography (SLA) is one of the older additive manufacturing technologies, with the first commercial printers dating to late 1980s. The fact that SLA printers — like Formlabs Form 3 — are still popular is a testament to its capabilities.

SLA uses a single UV laser and liquid UV-curable resin materials. The printers focus the laser on one point at a time on the print bed, either reflecting it around through mirrors or moving it with a gantry, to harden the resin and create the part’s geometry.

Unlike with solid material-based technologies, SLA printers lower the print bed into the resin vat and slowly raise it as the layers harden. This is why your print will always be upside down when you open the print chamber.

SLA can offer extremely high detail accuracy and fast print speeds, and the desktop-sized printers are affordably priced. However, the parts require extensive post-processing, including washing, curing, and support removal.

Pros:

  • Accurate details
  • Fast print speed
  • Cheap cost of entry

Cons:

  • Extensive post-processing
  • Requires supports
  • Limited print volume
Formlabs Form 3 – Desktop SLA 3D printer

Overview of SLS

Selective Laser Sintering (SLS) isn’t quite as old as SLA, but it’s still a venerable industry veteran in its own right. The first SLS printers showed up on the market in the early 1990s and the technology has advanced greatly since then.

Materials for SLS printers — such as Formlabs Fuse 1 — come in the form of powder. The machine uses its high-powered laser to fuse and sinter the powdered material together into a solid object.

Depending on the power of the laser featured in your machine, you might have an incredibly wide range of materials at your disposal. On top of thermoplastics, SLS printers may be able to print in metal and ceramic powders.

The printer used a roller to continuously deposit a new layer of powder onto the print platform. As such, SLS has the significant advantage of not requiring supports, and lets producers create parts with highly complex geometries.

But despite the intensity of the laser, SLS doesn’t create completely solid bonds between the material layers, which means the parts aren’t strong enough to use without additional firing in an oven or kiln. SLS printers are also high-end machine, which makes them expensive to purchase.

Pros:

  • Great design freedom
  • Wide material range
  • Large print volumes

Cons:

  • Lower detail accuracy
  • Expensive printers
  • Parts need firing before use
Formlabs Fuse 1 – SLS printer

Detailed Comparison of SLA and SLS

We’ve now covered the basics of SLA vs. SLS printing. But such a skin-deep overview can’t give you a complete understanding of how to pick the right technology. Let’s get down into the nitty-gritty and look at various categories in greater detail.

Process

Both SLA and SLS work in a roughly similar manner, using a laser to harden the feedstock material. The lasers aren’t the same, though — SLS machines feature much more powerful lasers since fusing a powder takes more energy than curing liquid resin.

This means that the SLS laser is potentially dangerous to humans, and therefore SLS machines use totally enclosed print chambers. They require internal cameras for you to be able to view the print process The SLA laser isn’t as powerful, so you will usually be able to see the print through a tinted glass.

SLA printing requires you to design your parts to be self-supporting or to add in additional support structures. SLS eliminates supports as the unfused powder surrounding your part provides it with all the support it needs. This allows SLS to create more complex parts than SLA.

Both SLA and SLS require post-processing, although the SLA process is more laborious. SLA parts require washing, post-curing, support removal, and potential sanding. With SLS, you can get away with de-powdering and firing the parts in most applications.

Finally, SLS printers can generally print larger parts. SLA printers are usually desktop or benchtop-sized machines, while SLS printers are larger and aimed at heavier industrial applications.

Materials

Liquid resins for SLA have one major downside to them — many of them are toxic. This adds another level of complexity to SLA post-processing, since you won’t want to touch the liquid with your bare hands. SLA materials are also more expensive.

That’s not to say SLS powder can’t be pricey, but you usually get a better price to quantity ratio. It’s also easier to clean up the powder than the toxic goo in an SLA process, although you should still definitely use a filter mask.

Part Strength and Properties

The final parts are what matter most in additive manufacturing, and SLA and SLS shine in different areas. In terms of part strength, SLS is the clear winner since it can print extremely strong metals and ceramics. Although there are exceptionally tough resins, generally SLA printed parts are brittle, inflexible, and not suitable for mechanical applications.

However, SLA produces a much better surface finish and detail quality, nearly at the level of injection moulding. SLS printers can’t produce as fine details simply due to the nature of the powdered materials, which leaves the surface slightly grainy.

Speaking of SLS surfaces, they will be slightly porous even after post-processing. This means the surface can absorb small amounts of moisture. The insides of the part will be solid, but it’s good to be aware of the surface porosity.

Cost

SLA materials might be more expensive, but the printers themselves are not. In fact, they are some of the most affordable 3D printers on the market. You can get a decent SLA printer for around £1,000, and a professional-grade machine for a couple thousand pounds more.

SLS printers, on the other hand, easily go above the £10,000 price point. Material costs and the higher part output of SLS machines usually level the playing field in the long run.

SLA vs. SLS — Pick the Right Technology

SLA and SLS are both powerful 3D printing technologies, but they shine in different areas. Which you need depends entirely on your project.

As a general guideline, SLA is the way to go if you need superior detail and surface quality, and mechanical properties aren’t a consideration. On the other hand, if you require mechanically strong parts that don’t have to look that pretty, SLS will deliver.

Consider these properties when you decide on which technology to use. Then again, you could just invest in both an SLA and SLS printer and get the best of both worlds.

Learn more about the differences between SLA and SLS from our team at SolidPrint3D. Call us on 01926 333 777 or email us at info@solidprint3d.co.uk.

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