SLA,  DLP, LCD 3D printing, what does it mean and what is the difference?

Let’s shed some light on what are the different photopolymerization 3D printing techniques and how do they work.

Lately the resin 3D printing scenario has become a little confusing with the introduction of new LCD technology and while before it was quite clear when a 3D printer was using SLA technology and when DLP, now you can find in the market 3D printers using LCD or, more appropriately MSLA, technology, being sold as DLP or SLA.

However, each of these acronyms stands for a specific photopolymerization technology and are not interchangeable. Each technology works in a different way, with its own features and its pros and cons.

We, at Lumi Industries, have explored all these photopolymerization technologies and decided that it was time for users to get some clarification, to fully understand what they are buying and using.

The base: what does photopolymerization means?

Photopolymerization is a technique that uses light (visible or ultraviolet; UV) to create a chemical reaction which cause a liquid material called polymer (a network of smaller molecules) to become harder through a “curing process”. This is exactly what happens when we 3D print with resins.

We take a liquid material, we flash it with light and it becomes solid. What changes is the kind of light and light source we use to flash it and the way we guide this light to flash and harden the liquid material exactly where we want to, leaving remaining material around, in a liquid state.

Here comes the acronyms we have been speaking about, SLA, DLP, LCD or MSLA.

What is SLA o Stereolithography?

It is the mother of all resin based 3D printing technologies, patented in 1984 by Charles Hull, 3D System founder, it implies to have a UV laser which draws an object layer on the liquid resin, curing it.

Normally you keep the laser fixed and use moving mirrors (galvanometers) to aim the laser beam exactly where we need it. In our Lumipocket LT All-in One 3D printer, we tried a low-cost alternative, we moved the UV laser itself on a Cartesian plan, same moving system of common FFF printers.

A 3D printer resolution on Z axis always depends on the precision of the Z axis mechanical system, the firmware and software controlling its movement. However resolution on XY axis, on SLA 3D printers depends on the laser beam diameter.

SLA 3D printers may be a little slower since the polymerization of one layer becomes a path the laser has to go through in a way or the other. The bigger or more complex is the object to print, more time the laser will take to complete a single layer.

What is DLP?

DLP  is a technology invented by Texas Instrument and applied on projectors for high speed, efficient, and reliable spatial light modulation.

When talking about DLP 3D printing, it means we use a digital projector to project a single image of each layer. The light emitted by these kind of beamers, when specifically created for 3D printing, can be UV. However we, like many other companies, have been using also commercial DLP projectors, because, although not pure UV, when not too much shielded, the light emitted had the right spectrum to cure UV photopolymers (resins).

This is what we used in our LumiFold, Lumipocket and LumiForge 3D printers.

In DLP 3D printers case, the resolution on XY axis depends on the square pixel size.  So on one side the higher is the projector native resolution, higher is the resolution of the projected layer.

However the focal distance between the projector lamp and the 3D printing area is also important. The more we move away, the more the printing area gets bigger, but also its pixels. This is why, in DLP 3D printers you might have different resolutions corresponding to inversely proportional printing areas.

What is LCD or MSLA (Masked Sterolithography)?

We have been testing this idea of technology back in 2015, but at that time LCD screen were not meant to be used for 3D printing. They had a backlight difficult to remove and tended to filter UV light more than what they do now, so it was very difficult to print with them. FEP film were not easy to source at that time and using a glass resin vat with silicone (PMDS) layer, proved to be too thick to allow the creation of the kind of detailed image we were looking for.

In the past five years , technology advanced very fast and now printing with an LCD screen has become common use.

It is called MSLA, Masked Stereolithography because the LCD screen, works like a mask over a UV light source.

LCD screen is made of tiny pixels which being active or inactive, create our object layer image, letting the UV light pass through them or not.

DLP and LCD technologies tend to be faster than SLA, because they project one full layer at the time. No matter how big or complex the object to print is, only its height and selected Z resolutions, will affect the working time.

A MSLA or more commonly LCD 3D printer resolution on XY axis depends on the LCD screen pixels size. Higher screen resolutions, means higher XY resolution of the 3D printed object.

The light source can be a single UV LED array or can be a more complex structure including lenses which are able to focus the light in a more direct way so to light LCD pixels in a more accurate way, increasing resolution

With out latest 3D printer Lumi³, LumiCube we are finally exploring again LCD or MSLA 3D printing technology, we paused in 2015. We are using a high resolution 2K LCD screen proving a possible 47.3 microns minimum size on XY axys together with a UV light parallel system, which is able to convey the light in a way that fully exploit the LCD screen resolution to attain the maximum accuracy. Mechanical resolution on Z axis reaches up to 20 microns, below this level it is useless to go, because photopolymers (resins) at the current state of the art are still not able to go below 25-20 microns. What we made sure instead, is that this performance is costant, repeatable and therefore time-saving, thanks to a sturdy high precision linear system on the Z axis, which does not need continuous adjustment and maintenance to work properly.

Additional patent pending solutions we applied to fully support the end user, are going to introduce a new disruptive way to develop resin 3D printers.

Conclusions

This guide is not meant to be scientific, fully comprehensive of all possible 3D printing aspects, or comparative. This guide is just meant to give you a clearer idea of what is what.

We just wanted to disclose what we realized from our long experience and expertise in these different resin 3D printing technologies: SLA; DLP. LCD.

If the 3D printer is properly developed and built, with quality components and a real understanding of the final user, there is not a technology which is better than the other. It always depends on what you need to 3D print and what suits better your need!