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Additive Manufacturing 101-7: What is vat photopolymerization?

(Image: 3D Hubs)

Vat Photopolymerization (Image: 3D Hubs)

  Mechanical Design Engineer and Additive Manufacturing Ph.D. student

This is the eighth article in a series of original articles that will help you understand the origins of the technology that is commonly called 3D printing. First an introduction, followed by the seven main technologies categories (binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, vat photopolymerization) and then a design philosophy for additive manufacturing.

Vat Photopolymerization

ISO/ASTM definition: “vat photopolymerization, —an additive manufacturing process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization.”[1]

Vat photopolymerization can also be known as (in alphabetical order):

➢ Continuous Liquid Interface Production or CLIP[2]

➢ Scan, Spin and Selectively Photocure Technology or 3SP[3]

➢ Solid Ground Curing or SGC[4]

➢ Stereolithography or SL[5]

➢ Stereolithography Apparatus or SLA® (3D Systems Corporation)

➢ Two-Photon Polymerization or 2PP[6]

Stereolithography was the first AM process to be invented. The first patent was filed in 1975 which described a two-laser 2PP process[7]. The first parts were made by Dr. Hideo Kodama of Japan using SL in 1981[8]. Additional patents followed in 1984 when in three different parts of the world, people patented the SL processes. First on May 23 in Japan by Yoji Marutani[9], then on July 16 in France by Jean Claude André, Alain Le Méhauté and Olivier de Witte[10], and lastly on August 8 in the United States by Charles W. Hull[5]. Chuck Hull was the first to commercialize the technology when he founded 3D Systems in 1986. In 1988, 3D Systems commissioned Alberts Consulting Group to create a file format that could be sliced, resulting in the STL file format[11]. In 1991, Cubital introduced the Solid Ground Curing process but later ceased operations in 1999. In 2015 Carbon3D introduced a novel concept named CLIP using an oxygen-permeable bottom plate to help speed up the printing process. As the original patents surrounding this technology have lapsed, many startups have emerged taking advantage of this original AM process.

vat photopolymerization

vat photopolymerization example

Figure 1: Vat Polymerization example setup[12]

This process involves using a liquid resin as the main type of material. Specifically, this liquid resin has the special property of being able to become solid once it is exposed to light. This light can be ultraviolet as in SL processes, or for 2PP, with two photons of near-infrared (NIR) light hit within a very short period of time (several femtoseconds)[12]. This liquid resin is held in a container or vat, in which a flat build platform is partially submerged. This platform starts near the surface of the liquid and then gets exposed to light. This light can be a UV laser(SL), a digital light processing (DLP) projector, a UV light bulb filtered through a printed mask(SGC), an LCD screen similar to home theater projectors(CLIP), or even from very quick pulses (femtoseconds in length) of near infrared (NIR) laser light tightly focused to a very small area(2PP). Once the resin is cured and made solid, the build platform either moves further into the vat, or partially comes out of the vat leaving the solid cured portion just under the surface and then the process is repeated. In the case of SGC, the uncured resin is removed and then replaced with a liquid wax that solidifies, and then both the cured resin and wax is machined flat using a cutter and prepared for the next layer. If the process involves the platform coming out of the vat the resin needs to be transparent or have the solidification process occur at the very bottom of a vat with a clear window or bottom. However, this can cause the resin to solidify to the bottom which would prevent the platform from moving, or cause it to solidify so closely to the bottom that when the build platform moves up, significant suction is created which results in very slow movements. Recent developments by Carbon3D and the creation of the CLIP process have resulted in very quick builds due to the clear bottom acting as an oxygen permeable membrane which inhibits solidification of the resin within a certain zone around the clear bottom of the vat, which eliminates this suction force. This has shown to increase build speeds from 25-100 times compared to other AM processes, including SL.

Advantages to this type of AM are that it is capable of very high detail surface finish, even down to the nano-scale level, it can be very fast compared to other processes in terms of pure volume, and it’s also able to be scaled up to build desk-sized objects in very large vats.

Disadvantages include a limited number of material properties found in UV curable resins, which are not the most robust materials in terms of durability, strength, or stability. These resins can change shape over time, potentially change colour, and usually need a post-curing UV light oven to cure the material fully in order to get the most strength out of them. Some resins are also toxic and special gloves need to be used to handle parts until they are fully cured. Depending on the geometry of the part, support structures are required and can be very complex and require manual removal afterwards.

References

[1] “ISO/ASTM 52900:2015(en), Additive manufacturing — General principles — Terminology,” International Organization for Standardization (ISO), Geneva, Switzerland, 2015.

[2] Tumbleston J. R., Shirvanyants D., Ermoshkin N., Janusziewicz R., Johnson A. R., Kelly D., Chen K., Pinschmidt R., Rolland J. P., Ermoshkin A., Samulski E. T., and DeSimone J. M., “Continuous liquid interface production of 3D objects,” Science, vol. 347, no. 6228, pp. 1349–1352, Mar. 2015.

[3] Groth C., Kravitz N. D., Jones P. E., Graham J. W., and Redmond W. R., “Three-dimensional printing technology.,” Journal of Clinical Orthodontics : JCO, vol. 48, no. 8, pp. 475–85, Aug. 2014.

[4] Levi H., “Accurate rapid prototyping by the solid ground curing technology,” in Proceedings of the 2nd Solid Freeform Fabrication Symposium (SFF), Austin, Texas, USA, 1991, pp. 110–114.

[5] Hull C. W., “Apparatus for production of three-dimensional objects by stereolithography,” U.S. Patent 4,575,330, 11-Mar-1986.

[6] Maruo S., Nakamura O., and Kawata S., “Three-dimensional microfabrication with two-photon-absorbed photopolymerization,” Optics Letters, vol. 22, no. 2, p. 132, Jan. 1997.

[7] Swanson W. K. and Kremer S. D., “Three dimensional systems,” U.S. Patent 4,078,229, 07-Mar-1978.

[8] Kodama H., “Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer,” Review of Scientific Instruments, vol. 52, no. 11, p. 1770, 1981.

[9] Marutani Y., “Optical Shaping Method,” Japanese Patent 60,247,515, 07-Dec-1985.

[10] André J. C., Mehaute A. Le, and Witte O. de, “Device For Producing A Model Of An Industrial Part,” French Patent 2,567,668, 17-Jan-1986.

[11] Allison J., “Re: History of .stl format,” [Online email], 15-Jan-1997. [Online]. Available: http://www.rp-ml.org/rp-ml-1997/0091.html. [Accessed: 05-Feb-2016].

[12] Gibson I., Rosen D. W., and Stucker B., Additive Manufacturing Technologies. Boston, MA: Springer US, 2010.

What’s the Difference Between Stereolithography and Selective Laser Sintering?

By Jeff Kerns Machine Design

Depending on the application, one must weigh the pros and cons of a liquid- or powder-based approach to optimize 3D printing.

With the rapid growth of 3D printing, some people may confuse the different types of printing processes. In fact, the misunderstanding is so great that organizations are creating standards to facilitate better communications when talking about additive-manufacturing (AM) processes. We’ll explore the differences between two of these AM processes: stereolithography (SLA) and selective laser sintering (SLS).
Related

First, there’s a difference between ultraviolet (UV) lamp cured and UV laser cured. The lamp-cured processes such as multi-jet and some fused-deposition-modeling (FDM) processes are not SLA, and should not be confused with it.

SLA Process

The SLA laser travels through the liquid photo-resin, building the part layer by layer.

SLA is a photopolymerization process in which a build tray is submerged 0.002 to 0.006 in. (0.05 to 0.15mm) in a basin of liquid photosensitive material. This depth can vary based on laser strength, material, or tolerance desired. A UV laser (not lamp) solidifies one slice of the part onto the build tray. The tray then submerges, another 0.002 to 0.006 in., and the laser solidifies the next slice of the part. The thickness of the layers can affect the quality of print and tolerances. An industry average tolerance is around 3.9 × 10-3in. (0.1 mm). The laser travels the entire path of the part’s cross-section as it builds up each layer, so speed becomes an important consideration.

Commercial SLA printers generally cost a few thousand dollars.

SLS Process

After the part is formed in the powder-bed process, it’s removed and cleaned for any post processing. The powder that’s left un-sintered acts as a support material; after the process, it can be sifted and reused.

In powder-bed SLS, a layer of powdered material is carefully laid down by a leveler or roller on the build tray. A laser then sinters the cross-section of the part. Subsequently, the tray drops another 0.002 to 0.004 in. (0.05 to 0.10 mm) and the process repeats. Similar to SLA, layer thickness varies based on laser strength, material, or tolerance desired.

A variation of SLS is called the cladding process. One way to clad is via powder jet, whereby a print head sprays powder to form layers as a laser sinters each layer. The powder-jet cladding process may be used to fix broken or damaged parts. Users can even align the grain structure of the new material with that of the existing part, an important consideration for corrosion resistance. Often times, cladded parts require post machining.

Some 3D printer companies offer a three-step process in which one machine clads (puts one material over another), measures, and grids the part to the desired tolerance. These processes often use fiber lasers, rather than CO2 lasers, to lower operating cost and make control simpler.

SLS economical printer can cost anywhere from $12,000 to $33,000.

SLS vs. SLA

One major difference between SLA and SLS revolves around material selection. SLA works with polymers and resins, not metals. SLS works with a few polymers, such as nylon and polystyrene, but can also handle metals like steel, titanium, and others. SLA works with liquids, while SLS uses powders that raise safety concerns. Breathing in fine particulates of nickel, for example, can be harmful. Breathing apparatuses and ventilation should be considered, depending on the type of powder… more

SOURCE – Machine Design