According to U.S. Patent 4,575,330 (“Apparatus for Production of Three-Dimensional Objects by Stereolithography”) — Stereolithography (SLA or SL; also known as stereolithography apparatus, optical fabrication, photo-solidification, or resin printing) is a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photochemical processes by which light causes chemical monomers to link together to form polymers.
In more simple terms … VAT Polymerization technologies utilize a photo-polymer resin in a vat, that is cured by a light source.
- SLA Printer Characteristics
- Common Problems with SLA Printing
- Dimensional Accuracy of SLA
- SLA Materials
- SLA Benefits & Limitations
Stereolithography can be used to create prototypes for products in development, jewellery moulds, dental guides & dental components, medical models and computer hardware, as well as in many other applications.
While stereolithography is fast and can produce almost any design, it is relatively expensive when compared to other 3D printing technologies.
VAT Polymerization Technologies
SLA is famous for being the original 3D printing technology.
The process uses mirrors to aim a laser beam across a vat holding a light sensitive liquid resin, curing and solidifying the resin.
Direct Light Processing (DLP)
DLP follows a near identical method of producing parts as SLA.
DLP uses a digital light projector screen, to flash a single image of each layer, all at once (or multiple flashes for larger parts).
As the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular shaped pixels called voxels.
DLP can achieve faster print times than SLA. This is because an entire layer is exposed all at once, rather than tracing the cross-sectional area with a laser point.
SLA vs. DLP
The key difference between SLA and DLP is the light source each technology uses to cure the resin.
SLA printers use a point laser, whereas DLP printers use a light image.
Standard DLP projectors typically have a resolution of 1024 x 780 pixels, while standard SLA printers use a laser with a 130 – 150 micron spot size (this can vary depending on the size of the machine).
SLA Printer Characteristics
Printer parameters on VAT Polymerization 3D printers are fixed and cannot be changed.
Typically, the only operator inputs are part orientation/support location, layer height and material, and these are all specified at the slicing stage.
Most SLA printers auto-adjust settings based on the type of material that is being used.
Layer height and light source resolution (spot size or projector resolution) govern the surface finish and accuracy of a part. Most VAT Polymerization printers produce parts with a layer height of 25 – 100 microns.
For very small, finely detailed prints, it can be possible to swap out DLP projector lenses to use a narrower beam. This allows the beam to print smaller layers at a faster rate and at a higher level of detail.
Bottom-up vs. Top-down
VAT Polymerization machines produce parts in two orientations — bottom-up or top-down.
Bottom-up printers have the light source positioned below the resin vat. The bottom of the vat is transparent.
Top-down printers position the light source above the build platform.
The build platform begins at the top of the resin vat with a thin layer of resin coating it. Once the first layer has cured, the build platform moves down 1 layer thickness, resin re-coats the previously cured layer and the process is repeated.
Once the build is completed the part will be completely submerged in resin.
Common Problems with SLA 3D Printing
By default, stereolithography (SLA) 3D printers create fully dense parts. When you’re printing parts that do not require a certain strength, hollowing out the design is a technique that can save a considerable amount of material and time.
VAT Polymerization parts require support structures.
The location and amount of support depends on the type of printer being used. For top-down printers, support requirements are similar to FFF 3D printing, with overhanging features and bridges requiring support structures.
For bottom-up printers, the support structure requirements are more complicated than top-down printers.
For either method of printing, support structures are always printed in the main build material, as there’s only one vat, and must be manually removed after printing.
Curling of large flat surfaces, is one of the biggest issues relating to the accuracy of parts produced via VAT Polymerization.
Upon exposure to the printer light source, each layer shrinks during solidification. When one layer shrinks on top of a previously solidified (pre-shrunk) layer, stress between the two layers exists. The result is curling.
Support structures help anchor at-risk sections of a part to the build plate and mitigate the likelihood of curling.
Part orientation and limiting large flat layers also mitigate these issues.
Resins that have higher flexural properties (are less stiff), are at a greater risk of warping and may not be suitable for high accuracy applications.
VAT Polymerization technologies use thermoset photo-polymers to produce parts.
The polymer comes in the form of a viscous liquid (resin).
The price of resin can vary significantly, depending on the application. For SLA/DLP resins, the number of colours available is limited.
Photopolymer resins also have a limited shelf life (typically one year, if stored properly).
When producing parts using VAT Polymerization, it is critical that parts are cured correctly under UV light after printing. This will ensure they achieve their optimal properties. Information on the optimal UV exposure times are provided on resin datasheets by their respective manufacturers.
Benefits and limitations of SLA 3D printing
VAT Polymerization printers are capable of printing fine detailed prints with feature sizes as small as 0.3 mm. One of the limitations of this technology is that most prints require support structures to be attached to the model.
These supports leave marks on the surface and create uneven surfaces. It is therefore best practice to place the supports on the least visible parts of the model.
With the correct post-processing, VAT Polymerization parts can be finished to a completely smooth surface, similar to that of an injection moulded part.
Much of this post has been adapted from the excellent book, ‘The 3D Printing Handbook‘ by Ben Redwood, Filemon Schöffer, Brian Garrett and 3D Hubs. As such this post is not nearly as comprehensive as each section in the book. We highly recommend this book for anyone wanting to develop their understanding of 3D printing and Additive Manufacturing.