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Computational technique advances colour 3D printing process

25 May 2015

Researchers have developed a method called computational hydrographic printing that physically aligns a surface colour texture on a 3D surface with unprecedented precision.

Various objects decorated using computational hydrographic printing that physically aligns a surface colour texture on a 3D surface with a precision never before attained (photo: Changxi Zheng, Columbia Engineering)

Working with researchers at Zhejiang University in China, Changxi Zheng, assistant professor of computer science at Columbia Engineering, has developed a technique that enables hydrographic printing - a widely used industrial method for transferring colour inks on a thin film to the surface of manufactured 3D objects - to colour these surfaces with an alignment of unprecedented precision.

Using a new computational method they developed to simulate the printing process, Zheng and his team have designed a model that predicts colour film distortion during hydrographic immersion, and uses it to generate a coloured film that guarantees exact alignment of the surface textures to the object.

"Attaining precise alignment of the colour texture onto the surface of an object with a complex surface, whether it's a motorcycle helmet or a 3D-printed gadget, has been almost impossible in hydrographic printing until now," says Zheng. "By incorporating, for the first time, a computational model into the traditional hydrographic printing process, we've made it easy for anyone to physically decorate 3D surfaces with their own customised colour textures."

Used in mass production for transferring repeated colour patterns to a 3D surface, hydrographic printing can be applied to various materials including metal, plastic, wood, and porcelain. The process uses a PVA film with printed colour patterns placed on top of water.

An activator chemical is then sprayed on the film, softening the colour film to make it easily stretchable. Next, a physical object is slowly dipped into the water through the floating film. Once the film touches the object, it gets stretched, wrapping the object's surface, and adhering to it. 

Throughout the process, the colour ink printed on the PVA film is transferred to the surface. But the process has a fundamental limitation in that it is almost impossible to precisely align a colour pattern to the object surface, because the object stretches the colour film. With complex surfaces, the stretch can be severe and even tear the film apart.

"So current hydrographic printing has been limited to transferring repetitive colour patterns," Zheng explains. "But there are many times when a user would like to colour the surface of an object with particular colour patterns, to decorate a 3D-printed mug with specific, personalised images or just to colour a toy."

Building upon previous work on fluid and viscous sheet simulation also done at Columbia Computer Graphics Group, Zheng has developed a new viscous sheet simulation method to model the colour film stretch during the hydrographic printing process.

This model predicts the stretch and distortion of colour films and creates a map between the locations on the film and the surface locations to which they are transferred. With the map, he can compute a colour image for printing on the PVA film and then, after the hydrographic immersion, it forms the desired colour pattern on the object's surface.

To prove that this simulation works, Zheng and his collaborators in China used off-the-shelf hardware to build a calibrated system in which a mechanical apparatus precisely controls the object immersion, and a 3D-system measures the object orientation and its dipping location. With the incorporation of Zheng's simulation model, they were able to compute a colour image to feed into their hydrographic system for precise texture registration.

To avoid severe film distortion and the danger of the film tearing, they introduced a multi-immersion design: the object can be dipped multiple times, each with a different orientation and a film printed with a different colour pattern. The computation of colour patterns allows the transferred colours from individual immersions to be combined into the desired 'final' surface decoration.

"This system is easy to set up for personal use and it's quite inexpensive, less than 40 US cents per printing," Zheng says. "And it works for a wide range of complex surface geometries and materials. This was a challenging but fun project. I'm very interested in the interaction between the virtual world and the real world, and this research is a good example of how virtual-world computation can work hand-in-hand with a real-world manufacturing process and significantly improve the production quality."

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