Oct 2, 2017 | By Benedict

Physicists at the Vienna University of Technology (TU Wien) have developed a new analysis method that could vastly improve the accuracy of resin 3D printing by identifying materials with suitable initiator molecules. The method uses ultra-short laser pulses and beam-splitting prisms.

Tu Wien's Aliasghar Ajami in the lab

When you think of high-precision 3D printing, you tend to think of machines that offer minimum layer heights in the sub-20-micron range and even finer resolutions on the X and Y axes. But printing a complete model the size of a dust particle? For most casual FDM makers, it’s hard to even contemplate.

That, however, is what researchers at TU Wien’s Institute of Applied Physics have been able to achieve, having developed a new analysis method for examining materials and their "initiator molecules.”

Initiator molecules are molecules in a 3D printable resin (and other materials) that are activated when they absorb photons from the 3D printer’s laser beam. This activation ultimately causes the resin to cure, forming a solid 3D printed object.

Professor Wolfgang Husinsky, part of the research team working on this 3D printing study, says that, for ultra high-precision 3D printing, you’re actually better off finding materials that don’t absorb photons when hit with a laser beam. Rather, you want to find a material whose initiator molecules are only activated when they absorb two photons at once.

“In order to achieve as high a resolution as possible, it is important that the initiator molecules are not activated by a single photon but rather are only activated when they absorb two photons at the same time,” Husinsky explains. “This two-photon process can only occur with the required probability where the laser light is at its strongest, i.e. exactly in the center of the laser beam.”

In other words, materials whose initiator molecules are activated by single photons do ultimately cure in a 3D printer, but they cure even at the edges of the laser beam. This effectively limits 3D printing resolution to the entire circumference of the laser spot, rather than a much finer point at the center of the laser spot.

So how do you ensure initiator molecules are only susceptible to double photons?

Interestingly, many materials are suited to this process of two-photon initiation—the only hitch is that, to find these materials, you need to work out the exact laser beam wavelength that will trigger molecule activation, something that has caused endless frustration to physicists in the past.

The Tu Wien study could lead to higher-precision laser 3D printing

“You had to carry out the same experiment over and over again with different laser wavelengths, and you would have to recalibrate the experiment set up from scratch each time,” says Aliasghar Ajami, the study’s lead author. “In practice, this is almost impossible.”

But the researchers’ new analysis method allows physicists like Ajami and Husinsky to analyze how molecules react to different wavelengths in one single measurement, vastly reducing the time it takes to match up materials with wavelengths.

The method involves the use of ultra-short laser pulses with a duration of a few femtoseconds.

“With these pulses as short as these, the wavelength is no longer strictly defined, so the laser beam no longer has one unique color, rather it is composed of many different wavelengths,” Ajami says.

Prisms are then used to split the laser beam into a two-dimensional “sheet” of light whose wavelength on top are different to the wavelengths underneath.

“If you move the sample through this laser light in an appropriate manner, you can analyze how the molecules react to different wavelengths in one single measurement,” Husinsky explains. “We are thus able to create a full two-photon absorption spectrum in one single working step.”

The TU Wien researchers believe their analysis method could enable higher-quality resin 3D printing using a range of new materials. That can only be good news for SLA and DLP 3D printing in general.

The researchers’ study, titled “Measurement of degenerate two-photon absorption spectra of a series of developed two-photon initiators using a dispersive white light continuum Z-scan,” has been published in Applied Physics Letters.

It involved the work of three TU Wien departments: the initiator molecules are created at the Institute of Applied Synthetic Chemistry, the 3D printers are built at the Institute of Materials Science and Technology, while the analysis method was developed at the Faculty of Physics.



Posted in 3D Printing Technology



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