Digital light processing or DLP: when 3D printing meets cinema

The days when 3D printing was combined with rapid prototyping are long gone, in recent years a multitude of printing techniques have been born and printing materials are among the most diverse and constantly evolving. Let's focus now on DLP technology acronym for digital light processing , to understand its operating principle we have to think about the stereolithography process in which a UV laser light cures a photopolymer one layer at a time . The UV laser triggers the reaction by hitting the polymer in the liquid state, it must literally draw the piece to be produced one layer at a time as if it were a brush.

Digital light processing
Schematic representation of the printing process for stereolithography. Credits: Nanocoat Project "Innovative materials for 3D printing"

A drawback is certainly the slowness of this process, especially when the object is not hollow and entire areas have to be "colored", an idea could be to use more than one laser but what would happen if we took the concept to an extreme? This is the idea behind the DLP printing process where the laser spot is replaced by a projector , such as those for home theater, but with a lamp capable of emitting UV light, the resolutions are also those at which we are used to that is 720p 1080p and 4k , as we will see later this affects the printing results

How does the printing work? It can be considered literally upside down

The printer consists of a projector placed at the bottom, a tank with a transparent bottom, essential to allow the UV light to reach the polymer, and a mobile platform along the vertical axis also known as the build platform on which the piece will be formed.

Comparison of the two printing processes. Credits: Manufactur3D "The Difference between DLP and SLA"

The platform is immersed in the tank filled with resin until it touches the bottom, at which point the projector emits a frame for a sufficient time to polymerize a layer of resin equal to the gap between the platform and the bottom of the tank which represents the printing layer thickness . once the reaction has taken place, the platform is raised allowing the passage to the new resin and then returning to its position, everything is repeated as necessary. At the end of this process, the object as shown in the figure is obtained, upside down!

The great advantage is that the entire slice (frame of a section of the object to be obtained) is projected in a few moments and does not have to be reproduced by the passage of a small laser spot, as a result of which the DLP technology is almost insensitive to the amount of polymer to be crosslinked for a single layer .

Digital light processing
Some pieces just printed, the direction of growth is represented by the arrow. Credits: 3D-Expo.

Digital light processing: resins, process parameters and voxels

The process parameters of this technology are:

  • Layer thickness: as already mentioned it represents the thickness of the polymerized layer from time to time, typical values ​​can range from 15 ÷ 150μm
  • Critical Exposure: energy per unit of minimum area (eg mJ / mm²) to be supplied to polymerize the resin to a depth equal to the l ayer thickness; given these two values ​​it is possible to calculate the printing speed for a given resin, usually expressed in cm / hour and represents the time it will take to print a piece of a given height regardless of its section.

They are closely linked to the resin used , the various manufacturers offer a wide range of resins, each made ad hoc for a specific need. This is essential because a small variation even in the color of a resin, with the same chemical composition, would cause a variation of its characteristics in terms of absorption of light (and therefore also of UV rays) drastically changing the printing result and therefore are usually supplied by the resin manufacturer.

Digital light processing
Aliasing phenomenon, on the left the shape to be reproduced, on the right the result obtained due to the limited resolution. Credits: A. Luongo, V. Falster, MB Doest, MM Ribo, ER Eiriksson, DB Pedersen, JR Frisvad, "Microstructure Control in 3D Printing with Digital Light Processing"

On the other hand, a parameter linked to the printer is certainly the resolution of the projector , during the projection of the layer to be polymerized, or simply slice , the phenomenon of aliasing occurs just like for digital images; continuing to print the piece there is an overlapping of slices, each affected by aliasing, forming a solid voxel format or "three-dimensional pixels" this compromises the surface finish and represents the main limitation of this technology in many applications.

Digital light processing
On the right a detail of the slace that will be reproduced by the DLP projector, the "steps" effect is due to the pixels, on the left a sphere where it is possible to notice the aliasing effect. Credit: Liqcreate

Curated by Elia Dal Lago

The article Digital light processing or DLP: when 3D printing meets cinema was written on: Tech CuE | Close-up Engineering .