A new technique to coat plastic particles for 3D printing overcomes material and colour palette limitations.
Powder Bed Fusion (PBF) or laser sintering is one of the most common commercial 3D printing techniques, where a layer of free-flowing polymer powder is deposited and melted into a desired shape layer-by-layer. Polyamide-12 (PA-12) is a strong plastic that is often used in during this process to print complex and detailed parts used in automotive and aerospace applications.
However, designers have thus far been limited to colour options at the printing stage, with grey or white powders requiring colour to be added afterwards in an additional step. Until now, that is, as a team of researchers from the University of Nottingham’s School of Chemistry and Faculty of Engineering have recently developed a method to coat plastic polymer particles to add colour and anti-mould and fungal properties to the printing process.
Eduards Krumins, PhD Student in the School of Chemistry, explains how the technique works: “The coating process was designed in such a way that it would be one-step, sustainable, and scalable. The key to the process is the use of supercritical CO2 as a solvent. This solvent has unique properties which allows us to coat each PA-12 particle in a uniform manner by adding a monomer and an initiator along with the PA-12 particles into our reaction vessel. Then, we achieve reaction conditions which are 3,000psi and 65ºC, at this point the polymer starts to form and simultaneously coats each particle. After the reaction is finished, we simply remove the supercritical CO2 and collect the polymeric particulate powder.”
For the coloured materials, the researchers used Isobornyl methacrylate and dye methacrylate monomers to form the coating, chosen because the resulting polymer is coloured and has similar thermal properties to PA-12. The simple yet effective approach provides added functionality to the coating, with the coloured shell polymer able to be designed to match the mechanical and thermal properties of the printing polymer.
“The coating process does not inhibit the printing process,” Krumins adds. “After our production process is finished we get polymeric powder that is dry and ready to use. In terms of printing resolution we think that we are within standard printing resolution, same with accuracy and time, hence our materials can be used with little to no change to the overall printing process.”
So, why does it benefit manufacturers to add colour during the printing process as opposed to afterwards?
“The dyeing process currently necessitates large machinery which can be expensive and can take up a large amount of space,” explains Krumins. “Additionally, the process can last anywhere from several hours a day, meaning that the user is adding considerably to their overall production time. Companies that do not have the machinery needed for their dyeing needs can send them to post-processing services, however this can take even longer considering the parts need to be posted. With our materials, these steps in the production process can be skipped.”
In addition to the aesthetic colour benefits for printed components, the new technique also delivers other desirable properties, in particular, anti-mould and fungal properties. Currently, objects made using PA-12 cannot be used in moist environments due to the growth of mould and fungi. The new shell coating can be used to develop coatings that prevent this from happening, opening up new possibilities for the use of 3D printed objects in new areas.
“The anti-mould and anti-fungal properties of our materials could open a wide range of applications for SLS,” Krumins explains. “For example, we think that this could be used in food packaging situations such as factory tooling, maritime applications like interior and exterior polymer-based parts in ships and buoys, and in healthcare. These would all have to be separately tested but currently PA-12 is readily ‘biofouled’ in many waterborne environments meaning that bacteria, fungi and so on grow on PA-12 very rapidly. Our materials will allow users to have all the benefits of SLS along with the knowledge that bacteria and fungi will grow to a much lesser extent on the parts that they produce.”
According to Krumins, the team’s coated materials can be easily used in commercial printers, with only very minor changes needed, such as laser power, which is expected. And, the next steps for the project are promising.
“We are currently in talks with a few potential industrial partners,” Krumins says. “The next steps towards commercialisation are to form an agreement or partnership with a materials or printer manufacturer, scale up production to pilot plant level, and to continue developing new functionalities for SLS materials.”
