Materials have been and will continue to be key elements of any additive manufacturing or 3D printing operation. New materials are emerging all the time. As vendors explore and develop materials, new information emerges about the best ways to use additive equipment.

We recently interviewed Davide Marini, Inkbit CEO and co-founder. Davide is a mechanical engineer among other titles and has worked in the additive manufacturing industry. Here are highlights of the interview. Inkbit develops multi-material additive manufacturing platforms.

Inkjet uses photopolymer-based chemistries. “Most of the materials used today in the photopolymer-based printers you see in the market traditionally come from the coating industry,” notes Marini. “They were developed decades ago. They are typically acrylics and methacrylics. They are inexpensive, but we believe that they do not allow the type of mechanical properties that are suited and needed for manufacturing applications.

Ensuring materials deliver on mechanical properties
The 3D printers Inkjet developed eliminate mechanical flattening, a result that occurs in many inkjet printing processes. Mechanical flattening affects the mechanical properties of the material in the sense that you cannot use chemistries other than acrylics.

Here’s how it occurs: The traditional way in which an inkjet 3D printer has been built, after each layer is deposited, there is a step where the roller or scraper essentially scrapes away the top, typically 25% of each layer, to make it flat. This is done to obtain the geometric accuracy needed for the part.

The disadvantages of this is that it slows down the process because you have to roll every layer, plus it wastes material. Says Marini, “Most importantly, because you have a mechanical part that is in touch with your materials, you can’t use all the materials that you want. Here’s an example, I could mention the epoxy resins that we use in our own inkjet machine. Epoxy is a continuous curing chemistry, unlike acrylics. So even when you turn off the UV light, the material keeps curing.

“In a polyjet 3D printer, the epoxy material tends to stick to the mechanical scraper and jam the machine. But because our system is contactless, we don’t have a mechanical flattener. Our machine is different from a traditional inkjet machine. It is still based on inkjet, but the design is different in the sense that after each layer is deposited, there is no mechanical flattening. Instead, we have a vision system that scans each layer at the voxel resolution. We create a three-dimensional topographical map of each and every layer, and we don’t touch it. We just take those data into account before depositing the next layer. So, it’s a faster process. It’s more accurate, and we can use better materials.

The use of a mechanical scraper is one of the reasons some inkjet systems cannot work with soft materials. “It’s just difficult because if you’re trying to flatten and shave off a portion of a soft material, it’s just quite difficult.”

Because the Inkbit printers eliminate mechanical flattening, the company can expand the type of chemistry used for materials beyond acrylics.

Trends in materials
Marini sees the following specific trends in materials based on customer conversations:
–People want production grade materials, which translates into high temperature resistance, high chemical resistance, high impact resistance, et cetera.
–People are becoming more aware of the importance of multi-material design. Being able to combine different materials in the same part is important.
–The need for food grade 3D printed material, which requires significant regulatory compliance. Many materials are not suitable for building parts that will be used in food processing applications.
–The need for sustainable materials. For example, we have designed a support material for a printer that is meltable and non-toxic, it is a wax like material that can be even used in the process

Obstacles to the development of materials more suitable for the AM printing process
The use of inkjet style additive systems has challenges but also opportunities.
“At a purely technical or scientific level,” notes Marini, “the most important obstacle in bringing the inkjet architecture into production is that you have to use low viscosity inks because a print head works by jetting droplets of materials at very high frequency through small nozzles, typically 10-15 microns in diameter. So you can’t jet very viscous materials. And the problem is viscous materials is ideally what you want because they have long chain molecules. These higher viscous materials give rise to more interesting mechanical properties. So, that is the reason in my opinion that as of today inkjet systems have correctly been associated with prototyping parts.

“We started Inkbit because we wanted an inkjet architecture, which has a lot of advantages such as multi-material capabilities, precision, and proven scalability in factories, but we wanted production grade resins, production grade chemistry. So if you want that, you need to essentially play some tricks with chemistries. Basically, what you want is to jet low viscosity resin, which means short chain molecules, and find ways to polymerize those after the fact into long chain molecules so that you have the beautiful properties that are needed. The chemistry challenge is significant and very exciting. We are exploring all sorts of new chemistries beyond acrylics.”

Most of today’s inkjet additive systems use nozzles based on jetting ink onto paper. Naturally, nozzles that could handle higher viscosity materials, “would be a game changer for us,” notes Marini. “We are always, always looking for printheads that can jet high viscosity. We are in discussion with one manufacturer for precisely such a printhead. To give you a sense of range, as of today we can print resins maybe up to about 15 centipoise, but this manufacturer claims that they can make a printer that can go up to a hundred, which for us would be fantastic.

“All of the infrastructure that has been developed around the 2D inkjet printing industry is huge, is reliable, and has really been proven to be scalable. Italy has a very strong industry in the manufacturing of ceramic tiles and those are typically decorated by inkjet at very high speed, very large volumes. So we chose inkjet because we like the potential. And we see that there is no need to prove to a manufacturing company that inkjet is scalable to large volume production.

“Plus, inkjet technology lets you play with material features by dropping different kinds of materials in different places. We are on a very exciting journey with a combination of chemistry, hardware, innovative architecture, and we’re using machine vision, machine learning. But on the pure chemistry, I think the beauty of the challenge is the following. Let’s say, for example, we take a process like binder jetting. Typically, one starts from a powder that is basically a thermoplastic. The mechanical properties have been locked in by the manufacturer of the powder. Bring the powder together (in a binding process) and you have your part. But in inkjet, what we’re actually doing is we are creating the properties of the final molecule as we go during the process. This is both a big challenge and a huge opportunity because yes, it’s something that has not existed so far.

Meeting designers’ needs for data
Traditional materials used in machining and injection molding have a long history that designers rely on. Obtaining that same confidence with the ability to develop new materials can be an obstacle for some designers.

“This issue goes to the heart of the challenge that we face. It’s a problem of perception. Engineers in the production world have been used to essentially using resins that were developed in the 1950s and sixties, like ABS, polycarbonates, et cetera. If we, for example, tell them we now have ethylene formulations, they would be surprised. It’s important to provide data on the mechanical properties and to work with the engineers to ensure that they trust the technology. And what we’re doing is printing a lot of parts for them. And there will be a process of getting accustomed to new chemistries. We have a testing lab and I think that we have all sorts of testers and we’ll have an impact resistance testing and tensile stress testing and all sorts of temperature and chemical resistance testing. We do a lot of testing on materials.

Inkjet printing lattices
One of the features possible with lattices is the ability to change the mechanical properties, point by point. Inkjet additive manufacturing enables designers to create multi-material lattices.

“It opens up even more opportunities to explore lattices in an interesting way. As an example, we have been producing structures that need to have a soft skin, but there’s a rigid lattice inside. We could vary the structure of the lattice itself. Some of the parts that have generated the most excitement for our platform have been extremely fine scale lattices. We call them digital foams. Some of them go down in the cell size down to 200 microns, and those are softer.
“If you were to touch them, they would feel probably like an orange. But what’s interesting is that we can create the directionality in the mechanical properties. Of course, it can be completely digitally designed. Because apparently nobody else prints such fine structures in soft materials. Users can see the process through the vision system in the machine, so we don’t need any mechanical flattening. And we also have a measurable support, so we can remove the support material from very fine structures. We are excited by lattices and we want to take them one step further.

Lattices can also be designed with specific features in specific directions. “Let’s say I give you a cube. I could make a lattice such that if I squeezed in the X direction, it’s softer than when I squeezed in the Y direction.

Explorations
Marini and his team are exploring the use of materials for sound isolation. “With one of the manufacturers that we’re working with, they have the need to isolate sound from a piece of equipment that they’re making. And there’s a particular frequency that is disturbing to the ear. And we’re trying to see whether digital forms, when designed by using simulations of sound propagation, could we design from scratch a form that only eliminates a certain frequency, for example. This is very exciting.

If you see it, you can control it
The Inkjet system includes a vision system that facilitates the build process.
“At the high level,” says Marini, “the engineer will not even see the working of the machine, but the advantages would come after the fact, because we have a vision system integrated into the machine by design, the machine creates a topographical, three-dimensional map of every single layer. We stitch those map layers together into a stack and create what I would call the equivalent of a medical CT scan for the part. You can throw it away if you don’t want it, or you could use it, store it for quality control, for tracing, for tracking capabilities, et cetera.

“It’s an exciting capability, especially for the medical industry, where say you have a device that needs to be made in mass production volumes, but at the same time, it has to be personalized. So how do you go through FDA approval of something like that? Because typically FDA approval relies on the process, but in this case, every product is different. So we believe that having a file, a digital replica on each and every product, we believe is important. And it was communicated to us by a medical device company, that was the reason why they approached us for this exploration.”

On the touch screen, designer will see at every layer the deviation from the intended geometry. They will see the machine detecting the imperfections of every layer and correcting them instantly.

“The system tracks every single voxel deposited in real time and reacts to it. It’s a vision-based, full feedback control based machine. Nobody has built anything like this before.”