A range of technologies and machines are used in resin 3D printing, the two main ones being stereolithography (SLA) and digital light processing (DLP). Both types achieve the same goal in different ways. Simply put, the main difference is that SLA uses a laser light source to cure the resin, while DLP uses an LED light projection system.
In this article, Mohou.com will discuss with you the similarities and differences between SLA and DLP 3D printing, focusing on the principles, advantages and disadvantages of their respective printing processes.
Basic knowledge of resin 3D printing
Parts printed using SLA technology (Source: Ross Lawless via All3DP)
Among 3D printing processes, SLA and DLP are often considered technologies capable of achieving the highest standards of part complexity and precision. Both rely on the use of light, typically in the ultraviolet region of the spectrum (365-405 nm), although some printers use visible light to cure photoresists. Simply put, a laser or projector draws an image into the resin, causing the liquid to harden.
Before discussing how resin cures, it is necessary to discuss what resin is.
3D printing resins are typically made of epoxy or acrylic and methacrylic monomers that polymerize and harden when exposed to light. This process is called crosslinking. A solid object forms when light hits the vat of resin, creating the specific shapes or patterns that make up each layer. Different resin materials can have very different properties, from soft, rubbery materials to very hard or high temperature materials.
The main advantage of printing with resin is the incredible detail that can be achieved, with the process almost perfectly replicating the image required for each layer. The main disadvantage of resin printing is the resin itself, as it is more difficult to process than standard fused deposition modeling (FDM) materials. Due to its complexity, the range of materials that resin can print is much smaller than you would like, especially compared to FDM.
SLA3D printing process
SLA 3D printing process (Source: Ross Lawless via All3DP)
Developed in 1986, SLA is the original 3D printing technology. The term was coined by Chuck Hull, founder of 3D Systems. It was the first company to commercialize the printing process and today it is used by amateurs and professionals alike.
SLA printing uses a laser beam to pass through a resin surface to cure a layer. Early SLA systems typically placed the laser beam above the resin, which fired downward. This structure is often called top-down orientation. However, most modern systems use an upward orientation, in which the laser beam is pointed upwards, toward the resin in the vat.
Regardless, the SLA printing process uses mirrors called galvos in the X and Y axes to selectively harden and solidify cross-sections of the object, layer by layer. The laser is turned on and off by a computer-controlled drive, ensuring that the resin is struck by the light in the correct position. As each layer solidifies, it moves upward to make way for the next liquid layer. Typically, the laser paints the perimeter of the room, followed by the solid putty, or vice versa.
The power of the laser point must be sufficient to initiate the cross-linking process within the photopolymer, but this is easily achieved using solid-state lasers found in most modern systems. Overall, this process produces excellent results and is always reliable.
Advantages and disadvantages:
Example of precise details implemented via SLA (Source: mikeymakesit via Thingiverse)
The biggest advantage of SLA printing is the precision that the laser can achieve. Since it is an array of image layers, there are no gaps in the cured polymer. Rather, it is a continuous line of hardened material that produces a very smooth surface finish and high level of detail.
This raster drawing process is also its biggest disadvantage compared to DLP printing because it takes longer to cure each layer. Since lasers operate at specific wavelengths and the curing effect of the resin is wavelength dependent, this also limits the use of third-party materials to some extent. So most laser machines also come with their own range of materials.
DLP 3D printing process
DLP 3D printing process (Source: All3DP)
DLP printing technology was invented by Larry Hornbeck of Texas Instruments for visual projection systems in media applications and was later modified for photopolymer printing. The company created DLP in 1987, but the first commercial system didn’t appear until 1997, when a company called Digital Projection Ltd brought it to market.
This printing process does not use a laser, but a digital light projector to flash a single image of each layer. The light is again guided by a mirror, but instead of a galvanometer, a digital micromirror device (DMD) is used. The DMD sits between the light and the resin and manages the rotation of all the mirrors to form the correct image on the build surface.
Most modern light engines use light-emitting diodes (LEDs) to accomplish the actual curing of the photopolymer. The switching state of the light-emitting diodes can be controlled individually, thereby improving the XY resolution. As with all projection systems, an image can only be formed at a specific distance (called the focal length) between the projector lens and the projection plane. The greater the distance, the lower the curing ability of the projector.
Today, the quality of DLP machines varies widely, with prices ranging from $300 to $200,000, depending on differences in light source power, lens throughput, and DMD quality.
The difference between DLP and SLA 3D printing is that the pixels are projected into the resin all at once to form the entire image. This makes the printing process faster, but also affects image quality. However, with the rapid advancement of DLP lightweight engines in recent decades, this issue has become less important.
Advantages and disadvantages:
DLP 3D printed sunflower (Source: ChaosCoreTech via Printables)
The biggest advantage of DLP systems over laser SLA systems is the ability to cure the entire layer in a single pass. Print speed does not depend on model size, as is possible with SLA systems. Unlike SLA systems, where the entire print bed can be exposed to light from the projector, only one laser must pass through the cross section of the part. In SLA systems, the laser moves quite quickly, so small and medium-sized objects can be printed faster than DLP machines. However, for large models and full panel printing batch production, DLP is faster.
Another advantage of DLP systems is that they are generally more cost effective than SLA machines and easier to calibrate. In contrast, SLA machines often need to be sent to the manufacturer for repair.
However, DLP systems tend to be built in smaller quantities than SLA machines. This is because larger build volumes require larger distances, and too large distances can make the resolution of the DLP printer too low. While this is not a disadvantage when configured correctly, the fact that image quality is based on the projector projecting the image at exactly its focal length makes it easier to produce poor quality results. Fortunately, most systems don’t have this problem.
Since the system is pixel-based, the image quality depends on the resolution of the DMD. Depending on the system, image quality may be lower and less smooth compared to SLA devices. This is because the part is hardened in pixels rather than in continuous lines like a laser system. However, on modern systems you shouldn’t be able to tell the difference unless you look closely.
In DLP systems, it is difficult to obtain a constant energy density across the entire solidification plane. Sometimes layer images need to be edited first, take this into account. This is more difficult in DLP because the light source must cover the surface of the DMD chip, not just a point in space. Ensuring that every pixel receives the same light intensity is more difficult than maintaining a constant laser intensity. This part of the work is completed when the system is debugged at the factory, so there is usually no need for concern. However, this also makes image processing techniques such as antialiasing more difficult, as these techniques often change the brightness of the image to achieve a smooth appearance.
Whether it is a DLP or laser system, the movement of the machine is the same. These image sources only affect the image quality of each curing layer. Depending on the wavelength of the light source, DLP and SLA systems can use the same materials, although some systems are optimized for one or the other. Optimization mainly revolves around pixel size and light energy density. The post-processing steps of both processes are the same, cleaning first then curing, but the high-power DLP system requires a shorter post-curing time.
LCD based system
Photocentric Visible Light Curing Printer (Source: Ross Lawless via All3DP)
It should be noted that LCD (mSLA) printers are often compared to DLP machines. Indeed, they are also capable of curing entire layers at the same time and using LEDs as a light source. DLP systems are often considered superior due to their higher light transmittance than LCD displays. In other words, the projector can transmit more LED light than the light passes through the LCD screen.
Some LCDs can block up to 80% of LED energy, but modern systems using monochrome LCDs are more efficient because they do not have red, green, or blue filters. LCD systems are much cheaper, making them one of the most widely used technologies in hobby machines. Some LCD-based systems can cure in the visible light spectrum, allowing them to use standard LCDs to great effect, such as Photocentric’s Magna.
The main differences between DLP and SLA:
DLP is not as granular as SLA (Source: Reddit)
YEARS
A single laser passes through the cross section of the part
Provides finer printing
Build volume does not determine resolution
Generally more expensive
DLP
The entire print bed is exposed to the light source
Unlike SLA, print speed does not depend on model size
Larger build volume means lower resolution
Easier to use for amateurs
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