The technologies used in laser stereolithography (SLA) and stereolithography projection (DLP) are very similar to each other, but can produce significantly different results. Understanding the nuances of each of the 3D printing processes allows you to understand what the user can expect from the final result and how to effectively maximize the potential of each type of machine.
The word stereolithography comes from the Greek words “stereo” which means “solid” and “(photo) lithography” which is a form of drawing on the surface of a material. By definition, UV-based SLA and DLP are varieties of stereolithography, but drawing a layer with a laser can be completely different from projecting layers as a flat image. Let’s go deeper and try to understand what exactly this means.
Differences between SLA and DLP.
Following industry terminology, we will refer to laser-based SLAs simply as “SLA”. For both SLA and DLP, the photopolymer tray is selectively exposed to light to form very thin, hard layers that overlap one another to form one solid and bulky object.
SLA 3D printers use two stepping motors called galvanometers (one on the X-axis and the other on the Y-axis) to quickly direct a laser ray through the printable area, hardeninig the resin as it progresses. This process, layer by layer, splits the part into a series of points and lines, which are transmitted to galvanometers as a set of coordinates.
DLP 3D printers use a digital projector screen to flash one image of each layer at a time across the entire platform. Since the projector is a digital screen, the image of each layer is composed of square pixels, resulting in the layer being formed from small rectangular blocks called voxels.
The fact that the main nodes of these two processes have different geometric shapes makes it difficult to compare different prototyping devices only in terms of numerical characteristics.
Print size and speed in SLA and DLP.
DLP printing can significantly reduce print time for some parts, as each layer is exposed immediately, rather than being pulled out by a laser.
This priority print speed applies in two cases:
- For large, dense prints where the print will fill most of the free bed space as in this case, each layer is exposed much faster.
- For very small prints with fine details, it is possible to replace the projector lenses in places according to the print area, using a narrow beam of light for the quickest creation of small layers.
Although the printing process on DLP printers is faster, there is a trade-off between print speed and resolution, as well as surface cleanliness. The bottom line is that DLP 3D printers cannot print high-resolution prototypes that take up most of the print bed. For example, a DLP printer is capable of printing one detailed jewelry ring, better and faster than an SLA printer. However, to replicate it and print many detailed rings, you need an SLA printer that is able to maintain a consistently high print resolution over the entire print area.
The resolution of DLP 3D printers depends on the projector, which determines the number of pixels / voxels available. For example, Full HD is 1080p. A projector in a DLP 3D printer must be focused on the image size to achieve the specified resolution in the X-Y axes. If you need small pixels, you have to limit the scale of the entire print object, thereby reducing the area of the print itself. That is, for detailed printing on a DLP printer, you only need to use a part of the printable area, and large models will have a coarse resolution.
Surface treatment: voxels and layer lines
Because objects in 3D printing are made up of layers, prototypes often have visible horizontal layer lines. Since DLP displays images using rectangular voxels, there is also a vertical voxel line effect.
In this image, you can see the vertical voxel lines that look natural on the left and then outlined on the right. Since the voxel block is rectangular, voxels generally affect the curved edges of the model. You can compare this process to creating a sphere from Lego bricks. No matter how hard you try, the edges will look jagged on both the Z axis and the X-Y plane.
Removing voxels from the surface of curved objects is done by grinding.
Once you identify and understand the differences in technology and printing results, it is much easier to find the printing solution that best suits you and meets your expectations. Let us summarize:
Better to print to DLP printers:
- Small and very complex models.
- Fast printing of large models without much detail.
Better to print on SLA printers:
- A lot of small and very complex parts at the same time.
- Detailed large models.
Benefits of SLA / DLP technologies
Surface and detail.
SLA has +/- tolerances. 004 ”per inch, and the resin surface is ideal for post-processing such as polishing and painting.
There are many different materials with special properties such as strength, heat resistance, flexibility, or imitation of finished products such as rubber.
Quality and availability.
One of the cheapest rapid prototyping techniques while maintaining excellent detail and surface quality.
Allows you to integrate multiple components, and create thin-walled and geometrically complex components that the traditional method is not capable of.
No special tools are required to print and apply SLA parts, which significantly reduces production times. Enables faster innovation and time to market.
Several of the same prototypes can be placed on the same platform at the same time, greatly improving production efficiency.
Disadvantages of SLA / DLP technologies
Photopolymers are unstable over time, and most materials do not have a balanced impact / heat resistance.
With the further circulation of the prototype, costs and time will not be reduced.