By Nathan Bosket February 26, 2024 at 4:46 PM PT

It all took off when Charles Hull created an advanced liquid polymer substance, that when struck with a light source of a specific wavelength, would cure and become solid. It would soon play a crucial role in one of the most precise methods of additive manufacturing today, vat polymerization. When it comes to technology, there is almost always more than one way to accomplish the task it is designed for and 3D printing technology is no different. 3D printing takes many forms, each an attempt to print the object quicker, print with different materials, print with less mishaps, or reduce prices without reducing quality. Vat polymerization, which uses a light to strike UV-curable resin layer-by-layer to form an object, is among the most effective of these technologies, and in recent years, it has come great strides in each of these areas of performance. And just like other technologies, this form of 3D printing has its many different styles of operation, each with their pros and cons.

Stereolithography Apparatus (SLA) Printer

SLA 3D printing was made practical by the mid 1980s and it was the first time 3D printing of any type had truly become a functional reality. This type of 3D printer can operate in various manners, but the basics are the same. A UV light-reactive liquid polymer resin is poured into a reservoir with a transparent film on the bottom, and when the printing process begins, an upside-down build plate lowers into the resin and nearly presses against the bottom while leaving just enough space for a super thin layer of resin beneath. Then, an ultraviolet laser is beamed through a sophisticated mirror system that pivots a mirror and in turn directs the reflected laser around the bottom of the reservoir to cure the resin as it traces out the layer. Once a layer is completed, the build plate along with the just cured layer separates from the film and releases the suction created between the cured resin and the film that was generated by the curing process. Resin then flows from the sides and replenishes the area underneath the build plate, the build plate lowers, resin cures onto the next layer, and the process repeats.

This process can allow for unparalleled horizontal accuracy and does not suffer from print distortion at big print volumes, which can be a factor with other methods of vat polymerization. But, on the negative end, they are extremely slow, potentially taking multiple days to print very large objects, depending on layer size and material. While adding more lasers can multiply the rate at which it can complete the print, it currently just cannot keep up with the speed of other styles. Nonetheless, for applications requiring the finest of details, or where horizontal smoothness is a sensitive concern, it still holds its ground as the most precise vat polymerization technology available.

Digital Light Processing (DLP)

The DLP projector was patented in the late 1980s by Texas Instruments and was later applied in 3D printing at the turn of the millennium. Digital light processing systems act similarly to video projectors, except instead of casting a rectangular full color image on whatever it is pointing at, they cast a sharp “light silhouette“ at an extremely specific distance at a very specific wavelength. When applied to 3D printing, these advanced projectors operate by shining a high intensity UV light beam at a digital mirror device (DMD), which is a semiconductor embedded with thousands of microscopic mirrors, each one representing a voxel in the printing volume. These mirrors are selectively pivoted to manipulate the light and control its path, projecting a light silhouette representing the layer being cured, which then passes through the film at the bottom of the reservoir, curing the resin and the entire layer all at once, in just one second or so.

Where this process stands superior is its speed. It may take an SLA printer around a minute just to cure a single layer the size of a credit card, whereas a large DLP printer can complete the same layer in just a second or two. The time saved can really add up. Plus, the lateral size of the object being printed does not increase print time, whereas the size of the object on an SLA printer can multiply the print time massively.

However, this process is limiting at very large print volumes. Since the light coming from the projector expands out and away from the center at which it originated, it causes some of the light to hit the film at an angle, causing particularly large prints to become minutely distorted at the outer parts. Another issue with DLP is that the amount of mirrors in the DMD correspond to the resolution, which means if the printer has a low density DMD, the resulting prints will have a small amount of pixelization on the outer edges of the print. However, these issues have been improving as these printers have developed, and while they are not as precise as SLA printing, the DLP process is still one of the fastest and most durable methods of vat polymerization and it still possesses supreme capability in medium-format, industry-level 3D printing at extremely rapid rates.

Liquid Crystal Display (LCD) aka MSLA

Liquid crystal display (LCD) printing, also known as Masked Stereolithography Apparatus (MSLA), is a rather new technology that offers low cost, high precision, ultra fast resin 3D printing to the masses. This technology is similar to DLP in that each layer is cured all at once, but the way it works is much less complex. LCD printers use an array of very bright UV LED lights at the base of the printer that shine up through an LCD screen made out of millions of transparent sections, quite similar to pixels in a television display. These sections then have electricity selectively applied to them, which causes them to turn opaque, which collectively mask off the light source and form a “hole” in the screen that takes the shape of the layer to be cured. The UV light passes through the “hole,” which cures the entire layer all at once, hence the alternative name, masked stereolithography (MSLA).

This technique’s properties are quite similar to the DLP process, although its main drawback is that the LCD screen cannot withstand the heat generated from the light source as long as a DLP printer can, typically only lasting about 2,000 hours of use before it needs to be replaced. DLP printers, on the other hand, may last around 20,000 hours of use, and although LCD screens tend to be easier to replace than DLP modules, the cost of replacing them can add up.

But MSLA technology does have some great advantages, starting with the fact that these printers are relatively easy to manufacture, since LCD screens have gotten much easier to produce and their components are much less sophisticated than other technologies. This leads to them being much more affordable than other vat polymerization technologies. In addition, due to the properties of the light source, the light does not skew, which makes for no distortion, meaning there is theoretically no physical limit on how big LCD printers can get.