Shape, color and shine.
These are the three most striking visual characteristics of an object. Currently, 3D printers can reproduce shape and color fairly well. However, shine remains a challenge. That’s because 3D printing hardware is not designed to deal with the different viscosities of the varnishes that give surfaces a glossy or matte appearance.
MIT researcher Michael Foshey and his colleagues may have a solution. They have developed a combined hardware and software printing system that uses standard varnishes to finish objects with realistic, spatially varying gloss patterns. Foshey calls the advance “a chapter in the book on how to make a high-fidelity display with a 3D printer.”
He sees a range of applications for the technology. It can be used to faithfully reproduce fine art, allowing virtually flawless replicas to be distributed to museums without access to originals. It can also help to create more realistic looking prosthetics. Foshey hopes that the progress will be a step towards visually perfect 3D printing, “where you could barely tell the difference between the object and the reproduction”.
Foshey, a mechanical engineer in the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), will present the paper at next month’s SIGGRAPH Asia conference along with lead author Michal Piovarči from the University of Lugano in Switzerland. Co-authors include Wojciech Matusik from MIT, Vahid Babaei from the Max Planck Institute, Szymon Rusinkiewicz from Princeton University, and Piotr Didyk from the University of Lugano.
Gloss is simply a measure of how much light is reflected from a surface. A high-gloss surface is reflective, like a mirror. A low gloss or matt surface is not reflective, just like concrete. Varnishes that give a glossy finish are usually less viscous and will dry to a smooth surface. Varnishes that give a matte finish are more viscous and look more like honey than water. They contain large polymers that, when dried, protrude randomly from the surface and absorb light. “You have a lot of these particles jumping out of the surface, giving you that roughness,” says Foshey.
But those polymers pose a dilemma for 3D printers, whose lean fluid channels and nozzles aren’t built for honey. “They are very small and can get clogged easily,” says Foshey.
The state-of-the-art way to reproduce a surface with spatially varying gloss is labor-intensive: the object is initially printed with a high gloss and with supporting structures that cover the areas where a matte finish is ultimately desired. The backing material is then removed to roughen the final surface. “There is no way to instruct the printer to produce a matte finish in one area, or a glossy finish in another,” says Foshey. So his team came up with one.
They designed a printer with large nozzles and the ability to deposit varnish droplets of various sizes. The varnish is stored in the printer’s pressure reservoir and a needle valve opens and closes to release varnish droplets onto the printing surface. A variety of drop sizes are achieved by controlling factors such as reservoir pressure and the speed of needle valve movements. The more varnish is released, the larger the drop deposited. The same goes for the speed at which the drop is released. “The faster it goes, the more it spreads once it hits the surface,” says Foshey. “So we essentially vary all of these parameters to get the droplet size we want.”
The printer achieves a spatially varying gloss by means of halftones. In this technique, discrete varnish drops are arranged in patterns that, when viewed from a distance, resemble a continuous surface. “Our eyes actually blend themselves,” says Foshey. The printer uses only three standard coatings: one glossy, one matte, and one in between. By including these varnishes in the pre-programmed halftoning pattern, the printer can deliver continuous, spatially varying shades of gloss across the print surface.
Along with the hardware, Foshey’s team produced a software pipeline to control the output of the printer. First, the user indicates the desired gloss pattern on the surface to be printed. Then the printer performs a calibration, testing different halftone patterns of the three enclosed paints. Based on the reflection of those calibration patterns, the printer determines the correct halftone pattern to use in the final print job to achieve the best possible reproduction. The researchers showed their results on several “2.5D” objects – mostly flat prints with textures that varied half an inch in height. “They were impressive,” says Foshey. “They definitely have more of a sense of what you’re really trying to reproduce.”
The team plans to continue developing the hardware for use on fully 3D objects. Didyk says, “The system is designed to allow for future integration with commercial 3D printers.”
This work was supported by the National Science Foundation and the European Research Council.