Guide to Resin 3D Printers: SLA vs. DLP vs. MSLA vs. LCD

Resolution in 3D printing can be difficult to characterize and standardize between machines and manufacturers. Ultimately, the quality and accuracy of a print depends on many factors, namely: the size, shape, and power distribution of the light curing the resin, and the scattering, bleeding, and polymerization characteristics of the resin. For example, some resins scatter light more than others, which can cause extra resin to cure and create less detailed features in a print.

Resolution — once referred to as ‘dots per inch’ or DPI on paper-and-ink printers — is easily understood as ink covering the X and Y axes. When 3D printing became more widely adopted, the addition of the Z-axis further complicated the definition and measurement standards for 3D printing resolution. 

To determine the highest resolution 3D printing technology and manufacturer, both the ability to specifically trace a part in the XY planes and the minimum Z-axis layer height should be considered. The ability to specifically trace the intended pattern, however, relies on several factors. 

Resolution for laser-powered SLA 3D printers is determined by four factors: spot size of the laser, the power distribution of the laser spot, the precision with which the galvanometers position the light in the XY plane, and the Z-axis minimum layer height.

Laser spot size, at first glance, seems to indicate the minimum feature size possible — but that is not the case. Though it’s true that if the entire part consists of a single, round dot or sphere, then the minimum feature size is determined by the laser spot size — it cannot cure something smaller than itself. However, a dot or tiny spherical part is not realistic. For realistic parts, the laser can trace shapes and features that are smaller than its spot size, because it can move in the XY plane in increments smaller than its own spot size — tracing the ‘outside’ of a very small feature. 

As an example, Form 3+ has a laser spot size of 85 microns, but an XY resolution of 25 microns. Here, XY resolution refers to how precisely the printer can draw in the XY (horizontal) plane.

Resolution of DLP 3D printers is determined by the pixel size (both X and Y measurements), the power and uniformity of the light, the use of anti-aliasing, and the Z-axis minimum layer height, similar to laser-powered SLA printers. 

The smaller the pixel size, the greater the resolution — similar to familiar technologies such as TVs or handheld electronics. In DLP 3D printing, XY resolution is defined by the pixel size, the smallest feature the projector can reproduce within a single layer. This depends on the resolution of the projector, the most common being full HD (1080p), and its distance from the optical window. As a result, most desktop DLP 3D printers have a fixed XY resolution, generally between 35 to 100 microns.

DLP 3D printers’ resolution diminishes as build volume grows bigger, because there are no projectors available with significantly more pixels. Therefore, manufacturers have to increase the distance from the light source with the same number of pixels, leading to decreased resolution and print quality. 

Resolution of MSLA 3D printers is determined by pixel size of the LCD, the collimation and uniformity of the light source, the use of anti-aliasing, and the Z-axis minimum layer height. 

Similar to DLP 3D printers, the smaller the pixel, the greater the resolution. But smaller pixels are only one factor. MSLA printers need to have highly collimated (meaning the rays are parallel) light that does not spread when passing through a pixel in the LCD screen. The light must also be extremely uniform so the resin is cured with the same optical power through every pixel in the LCD screen.

Luckily, Z-axis minimum layer height is less complicated — most resin 3D printers have a standard layer height of between 25 and 200 microns. For inverted resin 3D printers, layer height is determined by the vertical depth of the resin squeezed between the build platform and the bottom of the resin tank, so effectively the height of the resin cured in each layer. 

Long story short, does resolution actually matter in resin 3D printing? The answer is yes, but resolution in itself is often just a vanity metric. It offers some indication, but it doesn’t necessarily correlate directly with accuracy, precision, and print quality. Higher resolution (smaller pixels or laser spot size in the XY dimension and smaller layers in the Z dimension) also generally comes with significant trade-offs in terms of speed or even reliability (as more layers can introduce more possibilities of error).

Because 3D printing is an additive process, each layer introduces an opportunity for dimensional inaccuracy. As each layer forms, minuscule deviations from the intended shape of the cross-section can compound, affecting overall accuracy. Accuracy depends on many different factors: the type of 3D printing process, hardware design, resin properties, print settings, and post-processing workflow. In general, resin 3D printers the most accurate and precise 3D printing processes. Differences in accuracy and precision vary more in regards to model and manufacturer than technology. 

In resin 3D printing, the first major factor impacting accuracy is the optical performance of the printer – specifically the size, shape, and uniformity of light curing the resin. Professional laser-powered 3D printers, such as the Form 3+ and Form 3B+ use high-quality galvanometers and extensive calibration to ensure that the laser’s light hits the resin at precisely the intended spot, and follows along the laser path without deviation. 

The accuracy of DLP 3D printers is affected by the resolution and power of the light projector and the quality and calibration of the micromirror devices. DLP printers are often limited in their accuracy by pixel distortion at the edges of the build where the micromirror devices have to project the light further from the light source. 

The accuracy of LCD and MSLA 3D printers is affected by the uniformity and collimation of the light source, and the pixel size and quality of the LCD screen. Most LCD printers have lenses over their light sources, but most are very basic, and can lead to uneven light, dark spots, or distortion — creating inaccurate parts. The quality of the LCD screen is also crucial — greater quality means the LCD screen will respond to the software programming telling it where to cover the light and where to let it through.  

The Formlabs Light Processing Unit 4 (LPU 4) includes a custom LCD screen with a 50μm pixel size and pretuned anti-aliasing for excellent dimensional accuracy. Form 4’s Backlight Unit generates a uniform area projection of an ultra-high power light using 60 LEDs and a plano-convex lens array and optical baffle which creates highly collimated and uniform light, even at high power. 

The second major factor impacting accuracy is the forces that a part experiences during printing. All inverted resin printers have two types of print forces. Peel forces occur during the peeling process of each layer as the build platform is rising and the cured layer is separating from the resin tank before starting a new layer. Squish forces occur on each layer when the build platform is lowering and squishing the print into the pool of liquid resin. These forces can cause layers to shift and deform, ultimately leading to lower dimensional accuracy.

Some printers use a basic flexible film resin tank that gently separates from cured layers and minimizes peel forces. Although these tanks reduce peel forces, they have two key problems. They use a non-durable film that is easy to damage and has to be replaced frequently. They are also prone to suction forces, where the flexible film suctions onto the LCD screen and causes a spike in peel forces.

Form 4, a professional MSLA printer, uses a dual-layer flexible film tank that minimizes peel forces and is extremely robust and resistant to damage. Form 4 also features a proprietary, microtextured optical film, called the Release Texture, that provides airflow between the resin tank and LCD screen, preventing suction forces and increasing accuracy.

^{The Form 4 Release Texture provides airflow between the resin tank and LCD screen, preventing suction forces and increasing accuracy.}

The last major factor impacting accuracy is the properties of the liquid resin inside the printer. The process of curing liquid resin depends on many variables, including resin: temperature, homogeneity, scattering, viscosity, and more. Small changes to any of these variables can cause more or less resin to cure, which can cause up to hundreds of micrometers of deviation in the size or shape of cured layers. Most resin 3D printers do not control these variables, resulting in frequent and unpredictable changes in dimensional accuracy.

Form 4 addresses this problem in several ways. First, Form 4 precisely controls the temperature and homogeneity of the liquid resin using precision heaters, infrared temperature sensing, and a high-speed resin mixer. Formlabs also develops and manufactures its own materials, and tightly controls the reactivity, viscosity, and scattering properties of every material. Moreover, Form 4 uses advanced slicing software, printer hardware calibrations, and validated print settings to automatically compensate for the properties of individual materials.

Once again, the accuracy and precision of a 3D printer are not solely dependent on the technology, but the exact hardware design, liquid resin properties, print settings and calibrations, and post-processing workflow.

Before the introduction of inverted SLA 3D printing, resin 3D printers had large build volumes and were very expensive. Inverted resin 3D printing then made the technology more accessible, but these machines were often much smaller as large builds would have been extremely difficult to print successfully due to the high peel forces. 

Now the combination of inverted SLA (which allows for just as much resin as you need instead of large vats), and new peel technologies makes it possible to print larger build volumes without spending $100,000+ on a traditional large-format resin 3D printing system. For industries like consumer goods product design and healthcare, large-format resin printers are an ideal solution. Their human-scale parts and high precision enable applications that require extremely tight tolerances based on body scans. 

Those looking to evaluate which size resin printer is best for them should first consider the application. There are three main product categories for laser-powered resin printers — desktop, benchtop, and industrial-sized systems. 

Industrial-sized systems were more common in previous decades for large corporations who could afford both the high cost of purchase and maintenance as well as the higher square footage and infrastructure requirements of top-down stereolithography printing. 

Desktop-sized resin 3D printers were enabled by the invention of inverted SLA 3D printing, and as the technology was adopted by more manufacturers, smaller printers with build volumes of around 15 x 15 x 20 cm, like the Form 3+, became common. 

Benchtop resin 3D printing similarly was made possible by the combination of inverted SLA technology and improved peeling techniques. Benchtop 3D printers such as the Form 3L, which offers a 33.5 x 20 x 32 cm build volume, have the ability to do human-scale prototyping of consumer goods, as well as methods to create end-use products through the use of 3D printed rapid tooling. 

With DLP 3D printers, there is a direct trade-off between resolution and build volume. The resolution depends on the projector, which defines the number of pixels/voxels available. If one moves the projector closer to the optical window, the pixels get smaller, which increases the resolution, but limits the available build area. Some manufacturers use higher resolution 4K or 8K projectors, but these are very expensive, inflating the costs of the end product.

Large-format DLP printers are still mostly top-down technology, instead of inverted. This requires large amounts of resin to be available at once. 

As a result, desktop, small-format DLP 3D printers are generally optimized for specific use cases. They have a smaller build volume and offer a high resolution to produce small, detailed pieces like jewelry, while others can produce bigger parts but at a lower resolution. 

Because LCD 3D printers rely on an array of LEDs and a similar-sized masking LCD screen, the accuracy and precision benefits remain the same as build volume gets larger. A large-format LCD or MSLA resin 3D printer will have the same accuracy as a smaller version, provided they are using an LCD screen with the same pixel size and a LED light source with the same uniformity and collimation. 

Unlike laser-powered SLA 3D printers, however, this does slightly increase the cost for every added LED and square cm of LCD screen added. A laser-powered resin printer can use the same laser for any size build volume, because its galvanometers can simply direct its path to farther reaches of the platform for larger sizes. However, in the interest of print speed, many benchtop large format laser-powered resin printers, like the Form 3L, add a second laser unit, which increases the cost similarly to how a larger LED array and LCD screen might.

Resin 3D printers are known for creating parts with smooth surface finishes that require minimal post-processing to achieve the look of injection molded plastics. Resin 3D printing, whether using a projector, series of LEDs, or laser, can cure liquid resin with much better surface finish than an FDM printer can extrude filament. Depending on the design, all resin and FDM printers might need to print parts with supports, which can leave behind touchpoints or artifacts on the surface that may need to be lightly sanded or clipped. 

Resin 3D printers’ layers are chemically as well as physically bonded to each other in the Z-axis, so the differentiation between vertical layers is very hard to detect, leading to a smoother appearance.  

For clear materials, such as Formlabs Clear Resin, this smooth surface translates to better transparency of the parts themselves, making it an ideal technology for parts requiring optical translucency or showcasing internal features. 

Laser-powered 3D printers like the Form 3/B/+ are popular for their almost perfectly smooth surfaces. Because a laser has a round spot size, curved edges are truly rounded, and the overall appearance of the parts is very smooth. 

DLP and LCD 3D printers also generally have incredibly smooth surface finishes and require very little sanding or polishing to look like injection molded plastics. However, both DLP and LCD printers are projecting light through voxels or pixels, which are rectangular. Historically, these voxels would cause a visible ‘stepping’ or ‘aliasing’ artifact in the X and Y axis, typically called voxel or pixel lines. 

Recently, DLP and LCD 3D printers have started using high-resolution projectors or LCD screens with very small pixels, plus anti-aliasing techniques where certain pixels are partially turned on. As a result, modern DLP and LCD 3D printers deliver surface finish that is mostly indistinguishable from laser-powered SLA printers.

As more businesses turn to 3D printing for production as well as rapid iteration, 3D printing speed becomes a greater consideration when choosing a technology. The right 3D printer is one that produces high-quality parts quickly, without compromising on accuracy, reliability, or materials performance Despite numerous advances in FDM and SLS technology, resin 3D printing is still the fastest additive manufacturing process. New print engines like LFD take that even further, and can produce most parts in just a few hours, or small parts in minutes of cycle time.

For small parts, laser-powered resin printers are comparable in speed to DLP and LCD 3D printers — when the laser doesn’t have to cover a large area to cure the resin, it can quickly accomplish each layer. However, for medium to large parts, or batch production, laser-powered 3D printers can be significantly slower than their DLP or MSLA counterparts. 

DLP printers have been popular in industries where fast printing has an effect on the bottom line, like dental labs. But they are more prone to optical distortion across the build area and require advanced calibration for high accuracy. They are also typically more expensive than LCD 3D printers for similar performance.. 

Advanced MSLA technology, like the Form 4, offers the fastest and most reliable 3D printing solution overall. Curing each layer all at once is the biggest factor driving that speed, but other factors, such as the Form 4’s dual-layer flexible film resin tank, Release Texture, high-speed automated resin filling, and fast resin heating, also decrease overall print time.

When considering MSLA printers for production, the quality and reliability of the components are important considerations. Most LCD printers have a reputation for a short lifetime, as generic LCD screens are not made for the type of conditions found in 3D printers. On Formlabs Form 4, the Light Processing Unit has a lifetime of 600,000–1,900,000 layers — and both can be replaced by the user easily and at low cost.

Scaling throughput is easy with desktop resin 3D printers. Their accessible size simplifies the process of adding more units and scaling up to produce more parts. There are also options to add automation for some units. Formlabs resin printers offer advanced workflow tools such as Form Auto for the Form 3/B/+ workflow that automates part removal to enable 24/7 printing. Automation tools like the Formlabs Automation Ecosystem reduce the number of touchpoints necessary for the print process, making workflows where many smaller, mostly similar items (like dental or ortho models) need to be printed nearly continuously.