SLA and SLS Explained
Initially, 3D printing was considered a complicated and costly process that was only achieved with large, complex equipment. Today, 3D printing technology has advanced to the point that the equipment is much more accessible for more industries, even to the point that people can begin using these machines as hobbies.
These printers have had a somewhat negative association that not everyone can understand and use the equipment. That these printers are so complicated, it takes specialized knowledge for them to produce successful products.
This negative association may have been warranted when 3D printers were in their infancy, but today’s 3D printers are much more user-friendly and accessible for all.
The process behind these printers is relatively simple.
- Stereolithography (SLA) uses a laser to harden liquid resin into a predetermined form. When cured, these resins can take the form of either rigid or more flexible creations.
- Selective laser sintering (SLS) uses a laser to sinter powder into the form you’re trying to create.
These are both simplified breakdowns of the process behind these printers. As we continue, we’ll dive more in-depth to determine what makes SLA printers more accurate, and some of the benefits that you achieve from each.
SLA Printing: Basics and Benefits
Inverted stereolithography, also known as upside-down, is the most common and well-known version of SLA printing.
The necessary steps during the inverted SLA process are as follows:
- A tank is filled with your chosen resin either by the user or automatically by the machine
- The printer lowers a build platform down until it meets the surface of the resin
- Galvanometers direct a UV laser to where the resin and build platform meet and begins to draw the first layer of your product while hardening it at the same time
- The product lifts from the resin tank, allowing the new resin to flow in
- The product is then lowered again into the tank for the next layer
- Steps 4 and 5 continue until the product is complete.
What remains is a finished product that is smooth but covered in support structures.
SLA 3D printers use CAD drawings that you load into your printer via USB. Once you upload the design, the rendering will show the support structures that need to be present for the design not to wilt under its weight. Some machines allow you to shift the frames manually.
The support structures come from the same resin as the rest of the product that you’re printing. These structures are at weak points, where without them, the product’s design could fail.
Besides ensuring that your product maintains its shape, making sure your designs are accurate, the structures are easily removable and will show little evidence once removed. As we will discuss later, SLS printing doesn’t require such support during the printing process.
Materials: A primary benefit of SLA printing is that the resin that you use are varied. You have the opportunity to use a resin that will end up being hard or soft when cured. Each type you use will also have inherent properties depending on the final application you want to achieve.
Common resins used are listed below.
- High Temperature
Isotropic: The Layers are Bonded on Both Planes: 3D printed objects are created layer by layer. Because of this, certain types of printing (Fused Deposition Modeling FDM) will show visible layer lines, and accuracy will be compromised. SLA products are created layer by layer, but instead of just stacking on top, the layers are chemically fused, which has a few benefits:
- It makes for a watertight product.
- The layer lines aren’t going to be prevalent, which creates a much more visually pleasing piece and increases accuracy.
Room Temperature Heating Environment: Other printing techniques melt the material to fuse it. With SLA printing, you’re using UV light to cure the resin. Since high heat isn’t involved, you don’t have any issues with heat expansion creating inaccuracies.
Finish: The most noticeable thing that you’ll find with SLA printing is the smooth printing quality. This smooth finish allows for speedy post-production, with a light sanding, mainly on the areas where the support structures were connected.
SLS Printing: Basics and Benefits
SLS printers have only recently become more accessible, thanks to technological advances. Before these advances, these printers were relegated to a limited number of high-tech corporations.
As we have very briefly touched on towards the beginning of this article, SLS printing uses an entirely different technique than SLA. The medium that these printers use is polymer or plastic powder. These printers heat the polymer powder in layers to fuse it into the desired shape.
Printing through SLS is a different process than SLA. The process is as follows:
- Upload your CAD drawing to your printer to begin the process
- The powder is released in a thin layer on the surface of the build platform
- The powder is then preheated just below the melting point of the polymer powder
- A laser then draws the first layer of the object or objects that you’ve preloaded. The preheating of the printer makes it possible for the laser to bring the temperature of the powder to the melting point so they may fuse
- The build platform lowers by a layer, and the 4th step repeats until the object is complete
- Once parts finish printing, allow them to cool
- Remove cooled objects and separate the final product from any excess material that may have remained.
Any excess powder that hasn’t fused can be reused, making this a reasonably sustainable printing option.
The final product that you’re left with after cleaning it with compressed air is accurate but slightly rough to the touch. The products compare to light grit sandpaper. You can easily distinguish a product printed with an SLS printer when compared to an SLA printer.
Support: The products printed from SLS printers do not require support structures, unlike products printed from an SLA printer. Added bolstering is unnecessary because the polymer powder acts as a supporting structure for the rest of the product as it cools. Not requiring support structures has an additional benefit of creating extra space within the printer to allow for more products to be printed at once.
Build Space: As we discussed above, because there is no use for supports, the build space within the printer is more significant than it is in an SLA machine. More build space means more products can be created at once, leading to an increase in productivity, especially if you need to produce many products.
Design: The designs that are possible from an SLS printer are highly intricate. Instead of merely creating a piece of your design, you may be able to make the entire structure, even when it features interior or interlocking pieces. They will all form as one unit.
SLA printers will only allow you to make an individual part or component, which you will then combine with one or several other pieces. SLS printers can make all of these pieces together, which dramatically cuts down printing time, saving time and money.
If you suddenly need to add more parts, you can add them while printing is in progress to avoid any added downtime.
Material: Injection molding is one of the most popular ways to produce some of your favorite products. Nylon, which is a common polymer used in SLS printing, is an alternative to injection molding. It can be much less expensive to use a printer than injection molding, especially when creating prototypes or one-off creations.
Material Reuse: Another subject that we’ve touched on is the unused polymers’ ability to be reused for your next project. Any particles that fused aren’t reusable, but those that haven’t are reusable. Keep in mind that any heated polymers will be slightly compromised, so they need to be added to new, unused material.
SLA Vs. SLS: A Direct Comparison
Now that we have a basic understanding of both 3D printing methods and some of the benefits each feature, we will compare them based on several factors directly.
SLA printers are significantly more expensive than their SLS counterparts. Typically, you will find that SLS printers can be about 2.5 times as expensive as an SLA printer of similar quality.
- The tech is still new. Some of the price discrepancies come down to the fact that the technology that has made this product more readily available is relatively new. As technology catches up and is reproduced, you can expect the price of printers to lower accordingly.
- These are complicated machines. The other reason for such a significant price difference is SLS printers are more complex and specialized. With these machines, you can expect that you’ll need a certain level of training to become proficient.
An example here is an SLS and SLA desktop printer from the same manufacturer to demonstrate this price difference.
SLA printers have a higher resolution than SLS printers because of the materials they use to make their products. SLS printers use polymer powders, which create an inherently granular effect on the finished product.
While the process is accurate, it doesn’t allow you to see the detail you would with SLA printers. These printers create a very smooth surface so that you notice the smallest detail upon completion.
The edge also goes to SLA printers when you judge them based on versatility. Resin allows you to create products that are hard or soft, heat resistant, or impact resistant. These products will all also have an inherent water-tightness that is crucial for specific builds.
SLS printers create designs that just aren’t possible for SLA machines. Not only is there no need for supporting structures, but the designs themselves can be much more complex. Whether you need geometric patterns, interior pieces, or parts that work together, SLS printers can handle it all.
Factors That Determine 3D Printing Accuracy
Though both systems of printing are considered highly accurate, and in some cases, people might consider them to be equally accurate, there are a few reasons that SLS products reign supreme.
This is the first factor in achieving accuracy. When you can attain a higher resolution, you can produce more detail within your product.
Both printers are isotropic, meaning that the layers bonded chemically so that you won’t notice the levels between layers, even in highly detailed sections.
Then what is creating this resolution gap?
It all comes down to resin vs. powdered polymers. Resin, being liquid, can make a very smooth finished product, even before you do any of the post-printing finish work.
The powder is granular, so even when it reaches its melting point and bonds together, the product’s exterior is left with a rough finish. The uneven nature of the product leads to a lack of resolution. If you think of it as a TV, resin creates more pixels per square inch than powder can create. More pixels equal better image quality.
This is the second-most important part of accuracy in 3D printing. We have explained earlier in this article how these two printing types adhere to each layer together.
- SLA printers cure each layer of resin together with UV light, which doesn’t require much heat to be successful.
- SLS printers use heat. Both the printer itself preheats the polymer material, and the laser adds another heating element to fuse each layer.
Although this adhesion method is highly successful for SLS printers, even creating isotropy, it does have its downsides.
The Heat of SLS Can Affect Accuracy
Since the temperatures involved in SLS are so high, the product is more prone to uneven heating and expansion and contraction. Though the object you’re printing may be made to very minute specifications, it may expand outside of the parameters you’ve set as it is heated to fuse new layers.
The opposite can take place as it cools. If an area cools faster than another, then it can shrink.
These units do a masterful job of heating and cooling, but these things can sometimes be unpredictable.
These expansions may not be extreme, but any movement away from the parameters set aren’t ideal. To put it in perspective, think about it like hardwood floors in your house:
The wood in your home is continuously moving throughout the year. As the climate becomes more humid, there is more moisture in the air. This moisture is absorbed in the flooring, causing them to swell slightly and lock tightly together.
When the humidity drops during the winter, this moisture is sucked out of your hardwood floors, and you may begin to notice some gaping from the floors shrinking.
Hardwood floors contain a bevel around the edges, which creates the illusion that they aren’t moving as much as they really are.
Unfortunately, there is no tolerance allowed by a bevel in 3D printing. Your product’s dimensions aren’t supposed to change, so when your piece expands or contracts, the accuracy is compromised.
SLA printers don’t face this problem because the temperature never rises high enough to melt its printing medium. Using low heat from UV light to cure versus using high heat to fuse layers is a very different process
The two factors mentioned above are the leading reasons that SLA printers have an edge in accuracy.
Both SLA and SLA Have Strengths
Throughout this article, we have discussed SLA and SLS printing basics and some of the advantages that you’ll receive from each. When compared directly, SLA printers can produce higher-resolution products, with more material versatility, for generally less money.
The SLS printer can produce more complex products that contain multiple different pieces working together. They also require no support structures, which allows them to make many products all at once.
SLA Wins Out for Accuracy
Both printers are considered highly accurate products with specific strengths and weaknesses inherent.
Overall, the printing medium of the SLA printers lends to a more precise printing process. Not only do they create a smooth finish, but they also do not have issues with expansion or contraction, which can compromise the parameters that you’ve set for your products.
Perhaps more important than either one of these printers’ accuracy is how you plan on using the printer. Since both are highly accurate, your decision might depend on the products you’re printing. If you require highly complex products, then there’s realistically only one choice, SLS. If versatility is more crucial, than you may prefer an SLA printer. Both printers will provide you with a highly accurate base for your creations.