Beijing University of Chemical Technology: Photo-Curing in 3D Printing—Benefits & Ongoing Challenges

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Beijing researchers are examining SLA, DLP, LCD, CLIP and MJP techniques in the recently published ‘Photo-curing 3D printing technique and its challenges.’ As 3D printing continues to rise in popularity and progress in capabilities available to users, there are still a wide variety of obstacles to overcome and different processes to refine—especially as the structures being fabricated become more complex.

Although photo-curing is one of the first known 3D printing techniques, based on the use of photosensitive resin, challenges remain. There are great benefits to photo-curing, such as speed, precision, and surface quality, making the technology attractive for applications like dentistry; however, users continue to battle problems as parts and prototypes may emerge brittle, deformed, with little resistance to weather, and poor biocompatibility—a requirement much more in demand today as tissue engineering continues to be a major focus.

This study reviews ‘relatively mature and commercialized photocuring 3D printing techniques,’ along with discussing pros and cons, problems, and potential applications in the future.

Stereolithography (SLA) is obviously very well-known in the digital fabrication realm, patented by Charles Hull, and presented to the world upon the inception of the technology—evidenced by the first 3D printer now resting at the National Inventors Hall of Fame in Alexandria, VA.

3D printed bust of Hull and first-ever 3D printer that Hull invented, the SLA-1. Image: National Inventors Hall of Fame, from ‘You Can Now See the First Ever 3D Printer — Invented by Chuck Hull — In the National Inventors Hall of Fame.’

“Commonly, the wavelength of lamp used by SLA machine is 355 nm laser beam, the laser beam is above the resin tank and the exposure direction is from the top, the liquid resin is solidified when scanned by laser beam. A platform is lowered into the resin; thus, the surface of the platform is a layer-thickness below the surface of the resin. The laser beam then traces the boundaries and fills in a two-dimensional cross section of the model, after a layer of resin cured, the platform descends a distance with one layer, solidification is repeated through layer by layer until a solid 3D object is produced,” state the researchers.

Photo-sensitive resin is coupled with cationic photopolymerization or a hybrid photopolymerization mechanism. One of the main goals is to avoid volume shrinkage as it is naturally not an advantage during the 3D printing process, and cationic photopolymerization is suitable due to little or no issues in that area. The researchers also point out that because the laser beam can cover such an expansive space, large-scale models can feasibly be printed. There is a drawback, however, due to slowed speed when increasing the size of the build.

Other disadvantages with SLA include a limited amount of resins to be chosen from, and issues with low resolution as well. It is however still quite useful for applications like dental, molds, and auto and aerospace components.

Digital light processing (DLP), functioning with a projector for creating the image in resin, has been in existence for around 20 years now. Centered around the innovative DLP chip, an ‘advanced optical switching device,’ this technology allows users to create small objects with excellence in precision.

Free radical photosensitive resin is generally used in DLP, and while accuracy and precision are granted, there are disadvantages due to size limitations—and the fact that DLP 3D printers tend to be expensive. The researchers recommend it for applications like jewelry casting and dentistry.

Liquid crystal display (LCD) 3D printing is centered around the use of a liquid crystal display, offering high resolution for users.

“However, during the electric field switches, small numbers of liquid crystal molecules cannot rearrange, resulting in weak light leakage. This causes the precision of LCD printing technology to be inferior to the DLP,” explain the researchers.

“In addition to the printing accuracy, the major difference between DLP and LCD 3D printing is the light intensity. It is well known that light intensity is an important factor for photopolymerization which determines the speed of printing and curing degree. Therefore, only if increase the amount of initiator or extend the exposure time, the photosensitive resin for DLP 3D printing could be used in LCD 3Dprinting.”

LCD is affordable and offers great resolution; however, LCD technology must be maintained and replaced regularly. Light intensity is weak, there is the potential for ‘light leakage,’ and the tank must be cleaned often. Still, the authors recommend LCD for the fabrication of toys, jewelry, and dental products.

Continuous liquid interface production (CLIP), made famous by Carbon, entails the use of what is essentially a more progressive version of DLP:

“The basic principle of CLIP technique is not complicated, UV projection at the bottom makes photosensitive resin solidify, while the liquid resin at the bottom of the tank maintains a stable liquid area due to oxygen inhibition, thus ensuring the continuity of curing. Special window at the bottom allows light and oxygen to pass through,” state the researchers.

One of the greatest benefits of CLIP technology is that objects are produced exponentially faster—as much as 25 to 100 times so—with the potential for printing 1000 times faster than DLP.

Multi-jet printing (MJP), also around for about 20 years, offers an increasingly efficient system for industrial users, featuring multiple nozzles spraying liquid photosensitive resin. Numerous materials in numerous colors can be printed simultaneously, and with great accuracy. Not only that, there are no limits on print size! Drawbacks, however, include lack of affordability in equipment and materials. The researchers recommend MJP today for ‘precision medical’ applications and jewelry casting.

The characters of different photocuring 3D printing techniques.

In conclusion, the researchers state:

“… the limited performance of the photosensitive resin and the bottlenecks of 3D printing technology restrict the application of photocuring 3D printing. Once the technical problems such as rapid curing, large size and high viscosity resin printing was solved, as well as the development of high-performance materials, biocompatible materials and degradable materials, the photocuring 3D printing will have a broad prospect.”

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[Source / Images: ‘Photo-curing 3D printing technique and its challenges’]

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