Among 3D printing processes, stereolithography (SLA) and digital light processing (DLP) are typically seen as the technologies capable of reaching the highest standards in terms of part complexity and precision. Both rely on the use of light, typically in the UV region of the spectrum (380 – 405 nm), to cure a photosensitive viscous resin.
This resin, typically composed of epoxy or acrylic and methacrylic monomers, will polymerize and harden when exposed to light. As light shines on the vat to create specific shapes or patterns that compose each layer, a solid object can be formed and extracted from the otherwise liquid resin.
For the best 3D printers in both SLA and DLP, don’t forget to check out our list of the best resin 3D printers.
Stereolithography (SLA) is one of the oldest forms of 3D printing, dating back to the early 1980s. Using a laser, it delivers a concentrated amount of light to induce photopolymerization of resin monomers in a given spot. By sweeping the laser across a vat of resin, a layer of the print is drawn in accordance to the design specifications.
There are two basic designs of SLA printers. In the first, the laser is positioned above the vat and points down into the resin. More frequently, however, with the laser sits below the vat and points upwards into the resin.
Typically, the laser does not shine directly onto the resin but is in instead deflected off a rapidly moving mirror galvanometer that directs the beam to the appropriate point. When the laser finishes printing a layer, the platform holding the printed structure moves away from the laser by one layer height, allowing the next layer to be cured.
Digital light processing (DLP) as a 3D printing technology has its roots in an image projection technique born in the late 1980s at Texas Instruments. DLP projects each layer, creating an illuminated plane where photopolymerization will occur.
The moment the light hits the resin, it’s not restricted to a single spot as with SLA. Instead, the whole layer is formed at once. Here, patterning of the illumination is critical to achieve the desired shape for each layer. This is achieved with a “mask” produced by a digital micromirror device (DMD). It sits between the optical path of the UV-emitting lamp and the resin.
The DMD is a dynamic mask and is composed of an array of rotating micrometer-size mirrors that reflect the light into or away from the resin. This allows for the differential illumination (and polymerization) of the resin at different locations within the layer.
Modern DLP projectors typically have thousands of micrometer-size LEDs as light sources. Their “On” and “Off” states are individually controlled and allow for increased XY resolution. Some DLP printers replace the DMD with an LCD screen with noticeable impact on prices.
Given their shared basic mode of operation, it’s only natural to expect that their resins are very similar. Both require a photodegradable initiator substance (or a blend of different ones) that, upon interacting with light, forms highly reactive species (free radicals, cations, or carbene-like compounds). These in turn will activate the process of polymerization of monomer/oligomer molecules, which are also light sensitive and capable of cross-linking to generate a solid object.
However, SLA and DLP resins are not necessarily interchangeable. Note that the power density delivered during printing is very different between the two printing modalities, and the corresponding resins reflect that.
Nonetheless, in both cases the size of the monomer molecules will help to define the stiffness of the object. Short chain monomers typically result in harder objects, while long chain monomers allow for more flexibility.
One of the topics that’s usually discussed when comparing 3D printing with injection molding is the difference in mechanical properties. For example, unlike parts produced by injection molding, parts printed by FDM evidence mechanical anisotropy. That is, they display different mechanical performance when load is applied in parallel or orthogonally to the layers. However, unlike FDM, neither SLA nor DLP exhibit extensive anisotropy and their performance is more like that of injection molded parts.
Having learned about the two technologies, you may be wondering which is best. But the truth is, when it come down to DLP vs SLA, the answer is mostly dependent on your requirements.
If you prioritize accuracy and resolution above everything else, then you might hear that SLA should be your first choice, but things are changing. The market of SLA and DLP printers is quite heterogeneous, particularly for DLP printers. Nowadays you find consumer-level SLA printers that easily achieve a Z-resolution of 25 microns, while many DLP printers struggle to go below 50 microns. But recent advances are placing DLP as contender for the first prize. Companies such as Kudo3D and Gizmo3D Printers speak of high levels of accuracy in the X, Y and Z axes that are difficult to match even for good SLAs.
On the other hand, due to the pixelated illumination of the layers in DLP, the technique consistently shows subtle artefacts at layer edges that look like “staircase steps”. While this is particularly true for some printers on the market today, companies such as EnvisionTec with their patented continuous digital light manufacturing (CDLM) technology, along with other forerunners of the DLP approach, are implementing anti-aliasing or pixel shifting corrections to mitigate this effect.
One of the biggest differences between SLA and DLP is speed. SLA, because of the highly localized nature of its polymerization approach, is typically be very slow. To mitigate this shortcoming, SLA 3D printers sweep the infill regions of an object faster than they do in the outer shells. This can save time in printing, but if you want your parts to be more mechanically stable, a post-processing stage of UV-curing is often advised.
The intrinsic advantage of DLP is that it allows for curing the whole surface of a layer at once. With no difference between the outline and the inner areas, post-curing is less of a requirement. As an example, a 30-minute-long print with DLP can take 4 hours with an SLA printer for the same 3D file.
Reliability of the system and consistency of printed parts are an important factor for those looking with these technologies as a means of production. DLP printers typically have fewer moving parts when compared to SLA machines. Because of this, DLP printers are sometimes thought to be less likely to malfunction and keep more constant levels of part quality. While this argument is intuitively sound, there’s no evidence clearly demonstrating this. As such, at least in this category, there’s no clear winner in the DLP vs SLA arena.
Nevertheless, these 3D printers are sophisticated devices, containing a significant amount of electronics and optical elements necessary for their operation. If poorly designed or assembled, they can lead to many headaches regardless of the technique used.
Maintenance and associated costs are also relevant points to consider. SLA machines, because of their complex architecture, require professional intervention should the laser require replacement, or if any of the optics fail. Calibration of the system is necessary, and often this can only be properly done by a professional, which can mean that the printer needs to be shipped back to the manufacturer. The advantage of DLP is that it has much simpler components. Should any of the parts fail, including the light source for example, a replacement component is easy to come by.
Finally, a word on cost. For the average consumer, DLP printers are typically cheaper than SLA ones. While a decent SLA printer will cost in the neighborhood of $3000 to $4000, you can buy a DLP printer for $500 to $1000 USD. Naturally, advanced machines on both sides of the court can easily have a $10,000 price tag attached.
Guessing what the future will look like in the world of tech is always a risky business. But a good place to start would be to see what’s currently being developed in laboratories around the world. It also makes sense to check out very high-end applications in the hopes that eventually they will trickle down to consumer products.
In current SLA and DLP printers, every photon irradiating the resin can potentially start the polymerization process. Currently, a new form of laser irradiation, called 2-photon polymerization (2PP), is being used mostly in research and high-end application purposes. This technique is somewhat similar to SLA, but instead of one it uses two lasers that pulse and are focused on the same point in space.
In 2PP, the energy required to start the polymerization reaction can only be harnessed at the intersection where the two lasers meet. Unlike what happens with standard SLA, where one photon is enough to start the polymerization, in 2PP as the name implies, two photons are required. Because of the nonlinear nature of the process, a resolution of approximately 0.1 µm can be realized. This is done by controlling both the laser pulse energy and the pulse frequency. The technology is nowhere near to being used by the average consumer, but this is how all things start.
Not everything is about resolution, advancement in 3D printing is also made at the expense of increasing the diversity of materials being used. This is where techniques such as SLA and DLP tend to lag behind compared to other techniques such as FDM.
In FDM, we can easily access a wide array of materials with different properties, such as optical, mechanical, biological, electrical and magnetic properties. By contrast, the photochemical process underlying SLA and DLP restricts the usage of materials.
But all is not lost. Increased functionality of parts printed with SLA and DLP can be obtained through the use of composites, where the resin is mixed with a filler made of particles with different properties. Fillers made of nanoparticles seem to be somewhat promising, notably with respect to improved mechanical and physical properties. Currently, significant research effort is directed at expanding the range of 3D printing resins to encompass polymers with electrical and thermal conductivity, magnetic and antibacterial properties while at the same time tuning their mechanical characteristics.
License: The text of "DLP vs SLA – 3D Printing Technologies Shootout" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
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