What is SLA 3D Printing?

We expect that most of our readers are very familiar with 3D printing using FDM technology. After all, it is the technology behind most of the super popular desktop 3D printers that you see just about anywhere nowadays. In spite of its popularity, FDM printing still has quite a lot of limitations which might make you wonder: what is the best alternative to FDM technology? This article goes over what may be a strong contender for the title in the form of Stereolithography, or SLA. Read on as we discuss what SLA is, its strengths and limitations, and why it may be the best alternative to FDM.

What is SLA?

Stereolithography, or SLA, holds the distinction of being the first 3D printing technology ever developed. Developed in the early 1980s and patented in 1984, it is a process that is based on the ultraviolet curing of a photosensitive polymer material.

Similar to FDM, SLA uses an additive manufacturing process. The similarities pretty much end there, as SLA uses a very different approach. Where FDM relies on material extrusion to build objects layer by layer, SLA uses a vat photopolymerization process to cure a liquid resin and turn it into hard plastic. This is also done layer by layer to form a solid object. It is the 3D printing technology of choice for applications where highly detailed prints with smooth surfaces are needed, areas where FDM printers are notoriously poor at.

How does SLA work?

As with other 3D printing processes, the SLA printing process can be better understood by breaking it down into functionally distinct steps.

Design

The file formats recognized by SLA printers do not really differ from any other file formats that are typically used with other 3D printers. However, the design itself of a part that will be printed using SLA needs to consider material behavior that is quite unique to SLA. As we briefly touched upon, SLA is based on the photopolymerization of a liquid resin material to form hardened plastic. This is an exothermic reaction, meaning a lot of heat is generated during the photopolymerization step.

The sudden temperature increase and the subsequent cooling right after polymerization can result in significant warping and deformation, especially in designs that have not properly accounted for this phenomenon. For this reason, there are various guidelines that need to be kept in mind when designing a part that will be printed using SLA.

Thermal expansion and contraction generate stress uniformly along all three axes. To avoid deformation or cracking of the finished print, it is crucial that the print be designed so that there is no buildup of the stress due to thermal expansion and shrinkage. This can be done in a number of ways, such as by designing walls to have a uniform thickness all throughout. Stress buildup can also be avoided by having smooth corners where two walls intersect, instead of sharp ones. Some designs also incorporate support structures such as ribs and gussets to prevent deformation at specific parts of the print.

In terms of resolution, SLA is capable of printing at a much higher level of detail compared to FDM. With its resolution only limited by the size of the laser spot that induces polymerization, SLA prints can have details as fine as 30 microns. Resolution along the Z axis, or the layer height, can also be smaller in SLA printing. Compared to the smallest layer height of 50 microns that most FDM printers are capable of, SLA 3D printers can produce prints with a Z-axis resolution of as low as 25 microns. Most users of SLA printers use layer heights values in the range of 25 to 200 microns.

Printing

Depending on the positioning of the ultraviolet light source, SLA printers can have either a bottom-up or top-down orientation. Most desktop SLA printers take a bottom-up approach, where the light source is placed under the resin tank and the print is built facing upside-down. The tank holding the resin has a transparent bottom that allows the ultraviolet light to pass through. After a layer has been printed, the platform moves upward to detach the cured resin from the bottom of the tank and to allow a new layer of fresh resin to enter the bottom of the tank.

The top-down orientation is often reserved for large-scale and industrial SLA printers. In a top-down approach, the light source is located above the resin tank. The build platform starts at the top of the resin tank and moves downwards incrementally as the print progresses.

Bottom-up printers are easier and cheaper to manufacture, which is why a majority of desktop SLA printers have a bottom-up orientation. However, a bottom-up SLA printer is severely limited in terms of build size. Building a print upside-down also limits how heavy it can be, as it can easily become detached from the build platform if it has too much weight. On the other hand, top-down SLA printers have no such limitations on build size but are much more expensive in terms of both the initial investment and operating costs.

How exactly does photopolymerization work? It all starts with the resin, which is a mixture of monomers and oligomers, any of which can be used to form long chain polymers. In polymerization, longer chains of polymers create more points of entanglement, providing rigidity and durability to the plastic. Basically, the goal of the photopolymerization process is to bridge the individual monomers and oligomers to form long chain polymers.

Upon hitting the resin solution, the energy from the ultraviolet light provides enough energy for a molecule at the terminal member of a monomer or oligomer to separate. The removal of this molecule creates a reactive end member, which another reactive member can connect to form a longer chain. As the reaction progresses, longer and longer polymer chains are formed until there is enough entanglement to produce a hard plastic. This whole process only takes a fraction of a second to complete.

Curing

What makes SLA prints especially durable and smooth is the fact that the curing process does not stop even when the printing of a layer is completed. Each layer remains at a “green” state, where there are still reactive end members left open for chain formation. As each layer is built, it further forms chains with the previous layer, making stronger layer to layer bonds.

This also means that an SLA print cannot be considered fully cured even after the printing process. To maximize the benefits of durability that SLA offers, it is necessary to do a post-printing curing process. This can be done in a dedicated UV-curing chamber for 1 to 2 hours, or over direct sunlight for 1 to 2 weeks.

Factors to be considered when printing using SLA

Compared to FDM, there are less process parameters that will need to be tweaked in SLA printing. Most of the extra concerns that need to be addressed in SLA are dealt with mostly in the design phase. This means that a well-made design is a critical ingredient to the success of an SLA print.

Selection of materials

An area in which FDM and SLA differ so vastly is in the type of materials they use. In contrast to the solid filaments that FDM use, the raw materials used in SLA are liquid resins. It is also worth noting that the resins used in SLA are thermosets, as opposed to the thermoplastics that FDM uses. This means that SLA prints cannot be re-melted to form new prints. Liquid resins are also generally more expensive than filaments, costing $50 to $400 per liter.

Depending on the application, different liquid resin alternatives are available. For general use, a standard resin or a clear resin may be sufficient. For a print that needs to be a little more durable, a tough resin that mimics the strength of ABS may be used. There are also more specialized resin products, such as a high temperature resin, a rubber-like resin, and a dental resin for making high quality dental molds.

Support structures

Much like FDM, support structures are an essential part of an SLA print. Since there is no concept of composite prints in SLA, support structures will have to be made of the same resin material and removal will have to be done by hand.

Support structures are especially important for prints using bottom-up SLA printers. In such cases, the object is often oriented at an angle with the goal of minimizing the cross-sectional area of each layer. This is done to avoid detachment of the print from the build platform during the peeling step, as each new layer essentially bonds to the bottom of the resin tank as it is built.

Warping

As already touched on earlier, warping or curling is also an issue in SLA printing. This happens due to shrinkage of the part after the curing step has started. This is an inevitable circumstance, but the effects of warping or curling can be minimized by using a few design principles. The goal of these design principles is to avoid the buildup of stress in certain points of the print due to shrinkage, thereby preventing deformation.

Layer adhesion

Layer adhesion is a less critical element in SLA than it is in FDM. The presence of reactive points between successive layers creates very strong bonds, resulting in a print that has more uniform tensile strength. Post-printing curing is an important step in ensuring maximum layer adhesion, as it guarantees total completion of the curing process.

Finishing

Depending on the look that the user may be targeting for the SLA print, a variety of finishing options can be done. All of these methods start with the removal of support structures. Since these support structures are made from the same material as the rest of the print, there is no choice but to remove them manually by pliers or any other sharp tool. The nibs left by these support structures can then be sanded smooth using dry or wet sanding process. This results in a surface that is sufficiently smooth with just enough texture for an application of paint, if desired.

It is possible to sand the surface of an SLA print to a point where it is transparent. Application of a polishing compound can give it a sheen, making the surface so clear and shiny that it almost looks like glass. This obviously takes much more time, but the results are visually striking. Another finishing method is to spray the SLA print with a UV-protective acrylic paint. This prevents excess UV exposure of the object, which may eventually degrade its mechanical integrity.

What are the benefits of SLA?

SLA excels where FDM does not, making it the perfect alternative. Objects printed using SLA come out with a smooth finish that is just not possible with FDM. Using a fine laser point also gives FDM the capability of reproducing highly detailed designs that FDM cannot. In terms of appearance and level of accuracy, SLA prints are definitely superior to FDM prints.

In terms of choices for materials, SLA is also no slouch. Aside from standard and clear resins, there are now available resins for more specialized uses such as rubber-like resins and extra durable resins. The selection is not quite as wide as the choices of FDM filaments, but there is potential in this particular area.

Next to FDM, SLA is the second most popular technology for making desktop 3D printers. The level of popularity of desktop SLA printers is far below that of FDM, and this is most likely due to the price point of even the cheapest SLA printer. Still, we can foresee SLA technology improving in the future, which may lead to the production of cheaper alternatives.

What are the limitations of SLA?

The primary reason holding back the mainstream popularity of SLA is its cost. Most of the popular models of desktop SLA printers have a price tag upwards of $3000. There may be a few alternatives that are within the $1000 to $2000, but these are still expensive compared to desktop FDM printers which can be bought for as cheap as $300.

On top of the high initial investment in desktop SLA printing, the cost of buying the resin is also significantly higher than the typical FDM filaments. The cheapest SLA resin is probably priced at about $5 per liter, which will only be enough to make several small prints. For the same price, you can probably buy a spool of cheap PLA filament which you can use several times over. In any case, the costs associated in buying an SLA printer and the upkeep with regards to materials is significantly higher than in FDM.

Although the prints made with SLA look so much more better than with FDM, SLA prints still are not as durable as one would want. Being made using thermoset plastics, SLA prints are a little more brittle and are prone to cracking. Moreover, prolonged exposure to sunlight degrades SLA prints in terms of both aesthetics and mechanical strength. It appears that, similar to FDM, prints made using SLA are best for display items and proof of concept demonstrations. They cannot be relied on to be truly functional.

The future of SLA

SLA technology has just started to make its way into desktop-sized models. Right now, there’s only probably a handful of manufacturers of desktop SLA printers. With the trend of continuously improving and evolving technology, we believe it’s only a matter of time before desktop SLA printing technology lifts off. The main hurdle for this is the cost – if manufacturers could make cheaper SLA printers, then we see no reason for it to not be a worthy rival to FDM in terms of widespread popularity.

Final thoughts

As an alternative to FDM, SLA printing technology holds the distinct advantage of creating objects with superior aesthetics. With a smoother finish and finer details, SLA prints are perfect for visually pleasing prototypes and display items.

Although there are a few desktop SLA printer models out there in the market, it has not quite picked up popularity in the same way that FDM has in the last couple of years. This is likely due to the high initial costs associated to it, as well as the similarly high costs of upkeep and materials. Hope is not lost, though. With rapidly evolving technology, it may only be a matter of time before SLA technology becomes affordable. When such time comes, SLA just might turn out to be the new frontier of desktop 3D printing.