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MIT Researchers Develop Photochromatic Ink for Recolouring of 3D Printed Objects

Photochromatic Ink (a) Before and (b) After UV Exposure
ColorFab Process developed by MIT researchers
Above: ColorFab Process developed by MIT researchers
(a) 3D Printing of object using photochromatic ink, (b) Users controlling the colour selection through the interface, (c) Same object coloured multiple times/Image Credit: MIT CSAIL

MIT researchers from Computer Science and Artificial Intelligence Laboratory (CSAIL) developed a new technologically advanced photochromatic material capable of changing colour on exposure to UV light. They developed a method named as ColorFab process which enables multi-colour changes in the printed objects.

3D printing has evolved from single coloured objects to full-colour objects. Multi-coloured objects can be easily printed and this has become a norm now. But once the object is printed, we cannot do much about it apart from colouring & dyeing. And every new change or iteration of the object requires new print leading to a big pile of unwanted prints.

Taking this problem into consideration, researchers at MIT have now developed a new photochromatic ink which is capable of changing colour on exposure to UV light. So far the tests were successful only for single-colour changes but the ColorFab process, enables multi-colour changes. The material can switch their appearance from transparent to colour and can be reversed back to its original appearance as many times as required when exposed to light of certain wavelength. The project is focussed on plastic materials but researchers believe it can be applied to various other materials and products.

Stefanie Mueller, co-author of the paper ‘ColorFab: Recoloring 3D Printed Objects using Photochromic Inks’ says, “Largely speaking, people are consuming a lot more now than twenty years ago, and they’re creating a lot of waste. By changing an object’s colour, you don’t have to create a whole new object every time.”

ColorFab System: The systems comprises for 3 components:

UV Light Projector: This is used for activating the colours.

Visible Light Projector: This is used to deactivate the colours

Rotating Platform: For a complete 360o exposure, the object is placed on the rotating platform.

ColorFab Process

Conversion

The team developed hardware and a software combination for the proper functioning of the system. The process starts with the users uploading their models in the ColorFab interface. The interface then converts the model for photochromatic printing process. Users select the desired colours from the supporting menu and then the software processes the model. The software converts the model into a voxel-material representation. This process basically splits the model into individual voxels, these voxels are assigned different colours, and the 3D model is divided into two .STL files (one for the printing colour and other for the infill).

Printing

Both these files are loaded into the printing software and the object is printed in full colour.

Colouring

Same object coloured multiple times with different colours
Above: Same object coloured multiple times with different colours/Image Credit: MIT CSAIL

Once the object is printed, it is placed on the rotating platform. The object is then flashed with UV light and all the voxels are activated and converted from transparent to colour.  Then, the visible light is flashed selectively (the selection can be controlled within the interface) to deactivate the unnecessary voxels from the final appearance. The object is rotated on the platform so all the voxels are sufficiently exposed and deactivated.

The activation takes around 5 minutes; the deactivation takes 17 minutes, so the overall process takes 23 minutes to complete.

Properties of Photochromatic Materials

Photochromatic Ink (a) Before and (b) After UV Exposure
Above: Photochromatic Ink (a) Before and (b) After UV Exposure/Image Credit: MIT CSAIL

Colour of Transition State: The photochromatic materials change form transparent to coloured state but the reverse is also possible where the coloured state is switched to transparent state.

Fading Out v/s Persisting: Some photochromatic materials return back to their original state when removed from the UV light, but certain specific materials shows colour persistence, i.e. it retains the changed state.

Activation/Deactivation: These materials are activated by a UV light source and deactivated by a regular visible light.

Saturation of colours: The saturation of colours is dependent on the amount of energy exerted by the UV light source. More the energy more will be the saturation.

Stefanie Mueller explained the implications of the technology, “the goal is for people to be able to rapidly match their accessories to their outfits in an efficient, less wasteful way. Another idea is for retail stores to be able to customise products in real-time.

Stefanie’s colleague & Co-author of the paper, Parinya Punpongsanon said, “This is the first 3D printable photochromic system that has a complete printing and recolouring process that’s relatively easy for users. It’s a big step for 3D-printing to be able to dynamically update the printed object after fabrication in a cost-effective manner.”

The key benefit of this method is that it does not require the user to reprint objects for every desired change, which saves time, reduces waste and does not require additional material.

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