What’s SLA Printing and Is It Worth Your Time?
Kyle Maxey posted on May 09, 2016 |
Stereolithography is the oldest and one of the most widely used 3D printing methods. But what is it ...
The first 3D printer.

The first 3D printer.

Stereolithography, also known as SLA, was the first commercial 3D printing process. Developed in 1983 by engineering physicist Chuck Hull, the SLA process cures liquid resin by selectively exposing a liquid polymer to UV light. Most remarkably, months after Hull began germinating the idea that would birth rapid prototyping, three French researchers working for the French General Electric Company (Jean Claude André, Alain Le Méhauté and Olivier de Witte) filed for a patent on the SLA process.

Unfortunately, their parent company abandoned the pursuit of the patent on the grounds that its business prospects were dim. Ooof.

On March 11, 1986, Hull was granted patent #4,577,330 for an “Apparatus for Production of Three-Dimensional Objects by Stereolithography.” Within the year, Hull would form the company 3D Systems, and the world of prototyping would change.

Since those early days of SLA technology, a number of other prototyping methods have given rise to an additive manufacturing industry that’s both powerful and growing. Today, we’ll take a look at the practical aspects of SLA printing and figure out if the technology is worth investing in or if it’s a dead-end prototyping system with little industrial potential.

How Does SLA Printing Work?

SLA, Plain and Simple

In its most basic form, SLA printing uses laser cures a photopolymer resin to form a solid. To build a model, SLA printers build layers of hardened resin upon one another, creating a unified whole. During the production of each layer, a SLA system’s laser will rapidly trace the path of a part’s geometry, curing the resin with extreme precision.

Once a layer has been cured, the build platform upon which the print is held is stepped a layer’s thickness away from the laser’s focal point, and the printing process begins again. For inverse SLA, where an object is built upside down by having a laser shine it’s beam on the bottom a resin tank, the same process occurs; however, in between the lasing and build platform stepping processes, a middle step is required. In this middle step, the model must be peeled from the resin tank’s base to make it possible to lift the build platform a layer’s thickness away from its previous spot.

Depending on user-defined settings like “layer thickness,” SLA users can build models with almost imperceptible layer steps. Because of the tight control of layer thickness and the precision of laser beams, SLA printers can build models at very high resolutions and include very fine features.

The DLP Process

Similar to the SLA process, the digital light processing (DLP) process cures liquid resin using light. In fact, DLP systems use a projector that’s similar to ones found in movie theaters of conference centers. However, unlike the SLA process, DLP systems don’t trace the geometry of a part layer by layer. Instead, DLP printers project the entire geometry of a part onto resin and have the ability to drive their light to a specific depth of a resin tank, making it possible to build a part voxel by voxel.

What’s a Voxel?

A voxel is the three-dimensional equivalent of a two-dimensional pixel and forms the standard unit of resolution for 3D objects. Like pixels, voxels define the position of geometric features of a component in 3D space by inferring their position relative to the voxels that surround them.

Because of its ability to print in three dimensions, DLP printers can build models with exceedingly high resolution with little to no visible layering.

A New Horizon with CLIP

The newest arrival to the SLA scene might also be its most important. Called continuous liquid interface production (CLIP), this reimagining of photopolymerization printing uses an oxygen-permeable composite window as the base of its resin tank. Because of its oxygen-permeable window, CLIP systems form a dead zone in the first layers of the resin. With a dead zone established, a DLP image of a part’s x and y cross-section is projected onto the photo-curable resin just beyond the dead zone. By rapidly firing these cross-sections, a 3D model can be constructed as the build platform is raised away from its material pool.

What sets the CLIP system apart from other SLA-style printers is the dead-zone layer it uses to isolate any model that it’s building model from the base of the resin tank. Because the model is suspended in the resin and not attached to the resin tank’s base, the CLIP system doesn’t require a peeling process to remove the model from the tank base. Without a peeling a process, prints can “grow” right out of the resin tank at speeds anywhere from 25 to 100 times faster than convention SLA printers. 
The lack of layering in a CLIP print is apparent at the microscopic level.

The lack of layering in a CLIP print is apparent at the microscopic level.

Though the CLIP system is still in a nascent stage, it could prove to be a new method for mass manufacturing some type of parts. In a recent TED talk, the inventor of the system, Dr. Joseph DeSimone, stated that CLIP systems can “connect the digital thread all the way from design to prototyping to manufacturing.” DeSimone said this because CLIP systems can currently build models using a wide variety of strong and elastomeric materials. In the future, DeSimone said that if CLIP systems can be made to build objects even faster (on the order of 1,000 times faster than conventional additive systems), the heat exchange occurring in the build process will make it possible to build objects in a number of new materials, including biocompatible materials, silicones, investment casting materials and more.

Finally, the CLIP system is also a departure from traditional SLA machines because it isn’t relegated to building objects in distinct layers. Because it grows parts so rapidly, models produced by CLIP machines resemble injection-molded parts and display their mechanical properties as well.  

In the end, each SLA process has its pros and cons, as Tim Caffrey, a senior consultant at Wohlers Associates, pointed out: “Vat photopolymerization (SLA) has a lot of variations in how the machines accomplish the task of photo-curing liquid resin to make a part. Each variation has its advantages and disadvantages. DLP is faster but doesn't scale well for large build areas. Top-cured means you need a vat with a lot of resin, in some cases an enormous volume of resin. Bottom-cured can experience issues with the cured layer adhering to the optical window, especially if that layer is a large x-y area. All in all, that's a lot of different methods for the vat photopolymerization process.”

Post-Processing an SLA Print

One of the biggest drawbacks to SLA-style printing is the fact that models can only be built from one material at a time. For most SLA printing processes, that means that any support material required by a print will be built from the same material as the model. In short, that means there’s no chemical method for removal of supports. For 3D printing users accustomed to the convenience of automated support removal through chemical baths, the support removal process associated with SLA can be a bit daunting.

Most support removal workflows begin with snipping support structures from the face of a model. While cutting support structures away from a build can remove nearly all support material, small support remnants can remain. To remove these elements, users will have to sand their prints, stepping up sand paper grades a number of times. Aside from support removal, SLA prints can require a number of finishing techniques depending on the material being printed and the properties that the final product must exhibit. Most notably, clear materials require a significant amount of sanding and clear coating to achieve an optically pure finish.

Though SLA support removal can be tedious, the results it can produce are astonishing. In fact, when combined properly, SLA printing and dedicated post-processing can deliver clear materials of optical quality and opaque models that exhibit extremely fine detail resolution.

What Are the Pros and Cons of the SLA Process?

Traditional SLA’s most obvious advantage is its ability to print high-quality models at relatively high speed, especially when compared to other printing methods like fused deposition modeling. When it comes CLIP systems the speed and quality of SLA printing is ramped up significantly. In addition to quality, SLA printers are also capable of printing translucent models, making it an ideal technology for prototyping optics, transparent covers like automotive head light covers, and even architectural models.

One of the biggest downfalls for SLA technology has to be how expensive the systems can be to run. Currently, SLA are quite expensive, and when an SLA system requires several liters of material to fill its print tray, the cost of material alone can balloon to thousands of dollars. Aside from cost, SLA systems don’t have a diverse portfolio of materials that display differing properties. For that reason, SLA printing is relegated to a smaller set of industries.

Furthermore, it has to be noted that SLA systems do produce plastic parts, and those parts are subject to degradation over time. “I’m no chemist,” said Caffrey, “but I do know that all plastics degrade at least a little with time and exposure to light. This is one reason why SLA and other photopolymer parts are good for prototyping and master pattern applications, but (generally) not for final part production. This is the ongoing challenge for photopolymer additive manufacturing: Can the material properties of parts be stable? Carbon has made mention of its development of UV-stable materials, which would expand CLIP’s potential applications to include final parts.”

Options for SLA Printing

The build volume of SLA printers can vary. Left: Formlabs’ Form 2 (145 × 145 × 175 mm). Right: 3D Systems’ ProX 950 (1,500 x 750 x 550 mm).
The build volume of SLA printers can vary dramatically. Left: Formlabs’ Form 2 (145 × 145 × 175 mm). Right: 3D Systems’ ProX 950 (1,500 x 750 x 550 mm).
Since many of the patents protecting SLA technology have expired in the last few years, a number of startups have begun bringing high-quality desktop SLA printers to the market. By far the most popular manufacturer in this segment of the SLA market is Formlabs. Since its founding, Formlabs has introduced three SLA printers to the market, though its initial offering, the Form 1, has been relegated to second-tier status by the Form 1+ and the newer, larger Form 2. All Formlabs systems can produce very high-quality models, though the print times can be a bit long due to the peeling process that happens in between the curing of each layer.

While Formlabs has been doing some tidy business in the desktop SLA market, it will eventually be overtaken by companies like Carbon if they don’t offer a CLIP-style system. Since Carbon’s debut, large partners like Ford Motor Company and Kodak have cozied up to new technology, lending it more legitimacy as a true successor to traditional SLA and DLP printing. Unfortunately, Carbon has yet to release a consumer model of their machine.

In addition to the smaller players in the field, additive manufacturing giant 3D Systems also offers a wide range of SLA systems. In fact, 3D Systems has everything from a desktop model aimed at jewelry designers to the ProX 950 that can print entire engine block models. Unlike most desktop SLA machines, 3D Systems’ SLA machines build their models by photo-curing geometry via laser from above the resin tank. Once a layer is cured, the model’s build platform is lowered and another layer is cured. This process dramatically reduces the amount of support material used in a print, making it a more cost-effective printing method and reducing model post-processing time. That’s a major differentiator for 3D Systems and should be for anyone considering large-scale SLA part production.

Finally, when it comes to DLP printers, EnvisionTec runs the show, although Autodesk has decided to jump into the fray with their Ember desktop DLP system. EnvisionTec offers a host of printers that use DLP technology to build models of outstanding quality. Currently, the company offers no fewer than 12 printers that cater to diverse markets, including aerospace, dentistry, jewelry and others.  

The Final Verdict

Being the oldest additive manufacturing technology on the market, SLA printing has proven itself to be a more than adequate method for prototyping models and creating short-run parts. In fact, Caffrey has said, “SLA has been the gold standard of overall appearance and ‘cleanness’ of models from the onset of the industry. [T]he service provider industry has used SLA for the bread-and-butter technology for more than two decades. As the incumbent technology, it is/will be replaced only by a technology that is clearly superior in cost, value or part quality. In our annual survey of service providers, one question is asked: ‘Which additive manufacturing technology is making your company the most money?’ 3D Systems’ SLA has ranked first every year.”

Currently, SLA-style printing is undergoing a bit of a renaissance with the development of CLIP systems. Not only are CLIP systems revolutionizing the speed at which 3D printers can operate, they also open up greater opportunities for material development in the SLA space. If the promise of CLIP systems does come to fruition, SLA-style printing could represent a new method of mass manufacturing.

What’s That Mean for SLA Users Today?

Familiarizing yourself with the idiosyncrasies of SLA technology and post-processing could be of incredible value in the near future. While knowing how to use an SLA system is important today, that institutional knowledge of post processing, proper part orientation and general SLA quirks might be transformational for your company if CLIP-style systems are ever commercialized.

So Is It Worth It to Invest in the SLA Market?

With the entry cost of SLA systems plummeting over the last three years, investing in SLA technology can cost as little $5,000 for a complete desktop setup. That being said, production-grade printers like 3D Systems’ ProX 950 can cost hundreds of thousands of dollars. Whatever the budget, SLA technology is an excellent prototyping tool. For anyone looking for quickly printed, high-quality models, SLA should not be overlooked. Furthermore, learning how to leverage SLA technology could be a game changer in the long run, as it might just represent a large part of manufacturing’s future.

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