SLA stands for Stereolithography. This was the first type of 3D printing to be invented, by Chuck Hull, was back in 1984. SLA is characterised by the use of a laser to cure resin, housed in a resin tank, from a liquid to a solid. Typically, a low powered UV laser beam is used for this.
Like all 3D printing, or additive manufacturing, SLA works by building up parts in layers. Each layer cycle begins when laser light is used to precisely cure a liquid photopolymer, in a pattern, that represents the cross-section of a 3D design. Once the layer is complete, the build platform, upon which the part is affixed, moves up a little. Fresh liquid resin runs into the gap left behind, and the process can repeat.
The DLP process is a little different. DLP stands for Digital Light Processing, referring to how DLP printers use a digital projector, coupled with a digital micromirror device, to cure the resin material. Photopolymers can be formulated to react to just about any light source, and so DLP printing processes were invented shortly after SLA ones.
The DLP print cycle is very similar to the SLA one. The construction of DLP machines and SLA machines is almost identical, with the exception of the light source. DLP machines project an image, representing the cross-section of the part, onto the resin surface. This bright light is focussed on the resin tank above, which cures the entire layer at once, unlike an SLA printer which only cures one spot at a time.
Note: There is also another popular and very similar process, known as MSLA. In this process, an LCD screen is used to selectively 'mask' areas of the build plate that don't require material, and leave open those that do. A general light source shines from behind the masked screen, so that the unmasked areas cure the resin where its needed, and the LCD panel blocks the rest. MSLA and DLP are such similar 3D printing technologies from a designer's and operator's point of view, that we will treat them as one technology here, and refer to both as DLP going forward to avoid confusion.
Both types of resin 3D printing have excellent resolution, especially when compared to other processes such as FDM 3D printing. Both SLA and DLP processes produce parts with an outstanding surface finish - which is similar to an injection moulding - making them perfect for creating production prototypes. Both can also create accurate models, functional prototypes, and end-use parts for a number of different industries.
Material options are fairly similar between the two technologies. Photopolymer resins have the same advantages and drawbacks when formulated for either technology, because of their very similar chemistry. Both printing technologies are compatible with a wide range of durable, stiff, flexible and coloured resins to meet users needs.
Designing for SLA and DLP is very similar. Both technologies require supports, which should be considered early in the design stage. The most important areas of the design, from an aesthetic point of view, should face away from the build platform for optimum results.
The differences between DLP technology and SLA technology are mainly a direct consequence of the way that they process and focus the light which is used for the photopolymerisation reaction.
The laser beam, used in the SLA process, has a defined 'spot size', which is typically around 100 microns, although it varies a lot between manufacturers. This effectively represents the smallest area of resin that can be cured at a time. The position of the laser beam is controlled by moving mirrors, very precisely, using a galvanometer.
In some designs, controlling the laser source in this way results in a little it of distortion at the edges of the build volume, as the far side of the laser spot is projected a little further than the near side when at a large angle. Some more advanced SLA 3D printers (such as the ones we use) avoid this issue by ensuring that the incident laser beam is perfectly perpendicular to the build platform.
DLP 3D printing doesn't have this issue, as an LCD screen is used to cure the resin rather than a laser. This means that each pixel of the screen is directly underneath the voxel of resin that it cures.
The effective 'spot size' however, can be larger or smaller depending on the resolution of the screen, and the size of the build volume. For example, a 1080p screen has a resolution of 1920 x 1080 pixels. If this was used on a DLP printer that had a 1920mm x 1080mm build area, then the effective spot size would be one square millimetre. Larger DLP printers therefore, have an inherently lower resolution that smaller ones, all else being equal.
Generally, SLA technology is considered more accurate due to the precision of the UV light source, but both options will produce high quality prats with characteristically smooth surfaces.
As SLA printers cure one spot on the resin vat at a time, they generally print slower than DLP 3D printers, which cure an entire layer at a time. There is a particularly marked difference in print speed when the build plate is relatively full. When only one or two components are being printed at a time, printing speed is comparable, and an SLA printer will sometimes even be quicker.
There is a significant difference between hobby, desktop and industrial printers that print using professional quality resins. More expensive machines are often significantly faster, and a high quality SLA 3D printer will usually outperform a cheaper DLP one.
Large format 3D printers are available on the market in both SLA and DLP sectors. Large format DLP printers require extremely high quality LCD screens or projectors however, due to the aforementioned problem with relative 'spot size'. As screens are so common in modern life however, good quality screens can be sourced relatively easily and cheaply - especially when compared to lasers. The cheapest resin 3D printers on the market tend to use the digital light processing method for that reason. More high quality resin printers tend to use stereolithography.
