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Additive Manufacturing Essentials

2.2 Vat-based Processes

Additive Manufacturing Essentials2.2 Vat-based Processes
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Learning Objectives

By the end of this section, students will be able to:

  • Provide a baseline description of vat-based AM processes.
  • Differentiate based on the nuances of SLA, DLP and LCD technologies.

Stereolithography (SLA) is the most common vat-based process. It has traditionally held the title of being one of the oldest 3D printing technologies still on the market and continued to be used daily throughout medical, industry and educational sectors. The SLA vat size ranges from 100mm3 to 2100 x 700 x 800 mm.

A group of oval chain links in the print bed area of the 3D printer. Connected to the chain links is an extensive lattice network of much thinner structures.
Figure 2.3 SLA parts with supports SLA parts constructed with support material. The thicker oval chain links are the parts being created ,and the thinner lattice structures above the chain links are the support materials. (credit: Modification of “A Functional Chain Printed on a Formlabs Form 2 SLA 3D Printer” by Formlabs/Flickr, CC-BY 2.0)

Derivative technologies to SLA include Digital Light Projection (DLP) and Liquid Crystal Display (LCD) masking systems. DLP is an AM process where a projector screen is used to cure photopolymer resin by projecting pixels of light using a digital micromirror device. LCD, also known as masked SLA, uses a liquid crystal display to mask projects one full layer at a time using UV light emitting diodes as a light source. Resolution on the XY axis depends on the LCD screen pixel size. We will highlight each of the major characteristics of each 3D printing subsystem to distinguish the difference among the processes (SLA, DLP, LCD).

Material Feedstock

Generally, SLA, DLP, and LCD use similar materials: ultraviolet-sensitive photo curable liquid resins. The UV sensitive liquid material is filled in a tank, or vat, which holds the material. The type of liquid resin depends on the size of the machine and the machine’s UV laser wavelength energy. For example, the small desktop SLA systems have a laser wavelength of approximately 395 nm.

Material options include:

  • ABS-like
  • Polypropylene-like
  • Epoxy
  • Polyurethane
  • Silicone
  • Cyanate Ester
  • Urethane Methacrylate

Once the laser discharges into the material feedstock, a solid structure is formed layer upon layer. The resultant part is considered a “green part,” whereas it is not at full material strength. Green parts are removed from the vat and post-processed in a UV light chamber to reach full material strength.

DLP energy systems use a wavelength ranging from 400 nm up to 680 nm. Due to the differences in wavelengths, the UV sensitive liquid that is held in the vat must be compatible with the DLP system.

Similarly, LCD energy systems use a wavelength from 405 nm for UV-based versions and royal blue 450 nm for daylight versions.

In summary, SLA, DLP and LCD all use a vat-based material feedstock system with similar material types, however, the feedstock itself must be compatible with the light wavelength for each respective technology to produce optimal parts.

Energy Sources

SLA: As discussed in the material section, for SLA systems, the energy source is a UV laser. The energy from the laser partially cures the liquid material, and the specific wattage varies from the large industrial SLA machines that can have up to 800mW to the small desktop machines of 400mW. The laser spot size governs how small of features one may be able to print and is steered in the X-Y plane by a set of galvanometer mirrors

DLP: The DLP systems cure resin with a much different approach than the laser SLA systems. The digital light projector flashes images of layers onto the vat. A digital micromirror device (DMD) selectively directs the light. DLP printers produce layers made up of “voxels” the 3D equivalent of pixels”

LCD: LCD technology is similar to DLP, with the light flashing images with LEDs. In this case, a screen reveals only the pixels necessary for the current layer. The screen eliminates the need for a special device to direct the light.

Process Architecture

SLA: The platform lowers by a layer thickness and a vacuum pumping smooths out any disturbances in the surface of the liquid material using a metal re-coater blade. There are two distinct approaches to the build plate. Some SLA technologies have a platform dropping, top-down laser approach, other SLA technologies have a platform lifting, bottom-up projection. For clarity regarding the distinction, reference the images below.

In part A, a pivoting galvanometer mirror is suspended above a platform that drops into a square container called a photopolymer resin vat. On the platform are the previously exposed layers, and a gap between the platform and the top of the resin vat contains the resin that will be printed using the laser. In part B, the pivoting galvanometer mirror is suspended below the platform that rises into a square container called a photopolymer resin vat. On the platform are the previously exposed layers, and a gap between the platform and the top of the resin vat contains the resin that will be printed using the laser. A layer of transparent glass is at the bottom of the resin vat to hold the material but allow the laser to pass through.
Figure 2.4 (a) SLA platform dropping approach (b) SLA platform raising approach

No matter if the build platform raises up during fabrication or lowers, the basic machine anatomy is made up of:

  • Laser/Light Source
  • Mirror/Scanning Galvanometers
  • Build Platform and Motion System
  • Vat (Liquid Resin)
  • Computer / HMI

The laser beam is columnated through a series of optics and steered directionally into the vat of liquid resin using scanning galvanometers as shown. The liquid resin is temporarily cured using a process known as photopolymerization.

DLP and LCD: DLP and LCD processes exhibit the same processing architecture as SLA machines and may also be platform dropping or platform raising. However, the major difference between SLA and DLP/LCD is the energy source, which is less expensive to manufacture for DLP and even less expensive for LCD. As highlighted in the DLP energy sources subsection, DLP technology uses tiny mirrors to project an image, while the LCD machines project LED based light energy.

Part removal

SLA, DLP and LCD: All SLA based processes may require sacrificial support material to be designed into the part to support features of a part. This supporting material will be in either tension for platform lifting machines or in compression for platform dropping machines. In either condition, extreme care must be taken to gently remove the support material after curing.

The actual parts fabricated during the SLA, DLP or LCD process may go through a rinsing system after fabrication to remove all loose resin from the part before being placed in a UV light booth for final curing.

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