Resin 3D Printing (SLA, DLP, LSPc)

Stereolithography

Stereolithography is among the first rapid prototyping technologies. The AM technique leverages a layer by layer structure fabrication. In this process, a laser beam targets a free surface of a photosensitive liquid to enable polymerization of the liquid on that area and transform it into a polymerized solid.
Two processes are crucial to enable the production of an object via SLA: curing and recoating. As far as curing is concerned, it should be noted that SLA harnesses UV-assisted photopolymerization of liquid monomers. A UV laser is scanned over a layer of the liquid monomer to cure the monomer in selected surfaces as determined by the tool paths. Once a layer is complete, another layer of resin is coated on top of the cured layer. This technique is called recoating. Both recoating and curing processes are repeated until the end of the manufacturing.
3D Systems developed this AM process in 1988 based on the work of Charles Hull. Over time, other companies have developed the technology further and expand the utilization of SLA-based 3D printers. SLA delivers components with high accuracy and a wide range of materials compatible with this process are becoming available.

Fig. shows two different types of SLA process, which is commercially available now.
Two types of stereolithography techniques for rapid prototoyping of ceramics.
(A) Top-down system with scanning laser on top, (B) bottom-up systems with digital light projection

A) A top-down system with scanning laser on top

Top-down SLA systems use low-power, highly focused UV laser beam to scan successive cross-sections of a 3D Printed part in a vat of liquid photosensitive polymer. 
As the laser traces the layer, the polymer solidifies, and the excess areas are left as a liquid.
"After the platform sank down for the next layer, a blade is moved across the surface to smooth it before scanning. The platform is lowered by a distance equal to the layer thickness (25 to 50 μm), and a subsequent layer is formed on top of the previously completed layers. This process continues until it completes the buildup. The part is drained above the vat. The excess polymer is swabbed or washed away from the surfaces." (Additive Manufacturing: Materials, Processes, Quantifications and Applications. By Jing Zhang, Yeon-Gil Jung)
In most cases, the operator achieves a final cure while placing the part in a UV oven. At the end, supports are removed from the object and surfaces are polished, sanded, or otherwise finished. 

B)  A Bottom-up system with digital light projection (DLP)

Bottom-up systems have become trendy for several applications. 
A light source (an LED lamp) combined with a deformable mirror device (DMD) presents an entire layer of photopolymer. An SLA-based system displays the photopolymer from below through a transparent window, therefore facilitates the part removal from the vat. By integrating new features in SLA 3D Printers, manufacturers have been able to improve the speed of the process —without any need at all to stop the operations between layers for recoating.
In addition to avoiding the use of expensive lasers, SLA delivers a wide range of benefits:

  • It does not require a large vat of photopolymer which means that the material can be provided as it is used. Researches show that, "a large vat of photopolymer as in conventional stereolithography can cost thousands of dollars and the unused material is subject to contamination and degradation." 
  • It delivers excellent resolution and fast operation.