Introduction to Direct Ink Writing (DIW) Printing Technology

We’re here with our latest installment in the “What is Printed Electronics?” series — a journey towards understanding the different types of printed electronics technologies, the pros and cons of each, and some of the unique applications that they have in the world of printed electronics and additive manufacturing. So far we’ve covered an introduction to screen printing, but today is all about direct-ink-writing (DIW) printing technology; also known as dispensing printing technology, or direct-write technology for short.

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There are many different printing options out there when it comes to printed electronics — and knowing which one to choose for your brainchild can be challenging. Each has its own benefits and drawbacks, so our goal is to make it simple for you to do the mental math, choose the right materials, and ultimately the right technology. Because we’re intimately familiar with the drawbacks to picking the wrong technology at the start of the project — the significant upfront investments that you can’t get back, not to mention those extended timelines to make your design work with another production technology. But let’s get to the nitty-gritty details of direct-write technology, shall we?

What is direct ink write printing?

Depending on what your background is, the term ‘dispensing’ could mean a few different things. The term is generally used to describe a measured or metered volume of fluid being released from a nozzle in a certain location or into a container. Although that meaning is correct, the term shouldn’t be used when discussing printing technologies because it implies static fluid handling rather than the precision patterning required for a functional device. When printing/patterning is involved, the term ‘direct-write’ is more appropriate.

How does direct ink write work?

Unlike inkjet or screen printing where a swath or complete pattern is printed all at once, direct-write (DIW) printing requires the nozzle to be moved relative to the substrate to draw and fill in the entire pattern feature-by-feature. Basically, the nozzle moves like a pencil over paper to leave the pattern on the substrate, one trace at a time.

While feature-by-feature printing is inherently slower than high-throughput, mass manufacturing methods like screen printing, direct-write printing excels in low volume prototyping and applications with high customization, where other non-digital technologies like screen printing struggle.

Direct-ink-writing (DIW) systems are flexible and can be configured to work with a wide range of materials that are accessible to screen printing while simultaneously allowing both design and material changes to be made on the fly. This is why some folks have started to refer to direct-write systems as “screenless screen printers” — a term we’re really big fans of. This kind of flexibility allows designs and projects to progress without the tooling delays associated with making new screens for each iteration, while also allowing them to scale with the same materials by taking advantage of screen printing infrastructure when they are ready to bring their product to market.

A common hurdle designers face is that in order to bring a product to market the designer has to change materials to conform to the existing screen printing production infrastructure. The chosen material is often selected for its specific performance characteristics and generally cannot easily be swapped with another conductive ink or substrate without impacting the rest of the design. These hurdles can be avoided with direct-write systems since the materials can be maintained throughout development and into production.

Types of direct ink write dispensing

Time pressure dispensing

This is a type of pneumatic direct-write dispensing that uses pressurized air on a reservoir of material to extrude that material through your nozzle. The amount of material that comes out of the nozzle is governed by the air pressure and the time under which you apply it.

PRO:

• Simple to build an air pressure dispenser system and seems straightforward!
• Can be a cheap technology to work with.
• Low cleaning requirements as many of the components are disposable.

CONS:

• There’s a lag between initiating the pressure and actually dispensing your material which can cause drooling of your material.
• The volume of material in the reservoir is a big factor in this technology because as you dispense material, the pressure you need to apply to extrude more, changes. Basically, you’re going to need to adjust your settings on the fly, over the course of every print — and if you already have problems determining your flow rate, this is going to be a real challenge.
• You have no direct control over the amount of volume displaced, only the amount of pressure that you apply — this makes the consistency of printing features a bit of a nightmare because you can’t really ascertain a true flow rate for your material without in-depth studies, which can take months.
• It’s really susceptible to viscosity variations, and because it uses air pulses to exert that pressure, it makes clogging a real possibility for things like solder paste.

Piezo-actuated direct ink writing

This is technically not considered a contact printing technology because it uses a piezo jet valve, where the fluid is held between the piston and the nozzle and jetted at the surface from a set height. That piston is connected to an actuator which, when an electric charge is applied, oscillates up and down. This kinetic energy ejects the fluid onto your substrate. It gets a mention though because folks are starting to incorporate these into direct-write systems!

PROS:

• It’s really fast — it dispenses at approximately twice the speed of pneumatic jet valves. Because it doesn’t need to move in the z-axis at all, there’s no height sensing and repositioning that needs to occur. Those things typically only take fractions of seconds, but over the course of your print — they add up!

CONS:

• Doesn’t typically work with things like solder paste, or other materials with ultrafine metal particles in them because they need to evenly jet the material.
• The cost! If your application requires high speed and a high degree of accuracy, then the piezo-actuated direct ink writing machine might be worth the investment, but if not…
• Requires a lot of cleaning and maintenance to keep in tip-top shape.

Auger valve dispensing

This technology uses an auger that turns inside a cylinder that is fed material by a syringe, similar to the way that plastic extrusion screws on injection molding equipment works.

‍PROS:

• Because of how this system works, it’s pretty good at dispensing very small and very accurate amounts of material, and it’s less sensitive to viscosity variations.

CONS:

• There are going to be materials that have problematic particle size issues! This is one of those times when size really does matter…
• Non-newtonian fluids (all conductive inks and solder paste are non-newtonian fluids!) are thixotropic — which means that as you shear them, their viscosity drops and it takes time for the viscosity to recover. This can cause agglomeration of particles that leads to nasty clogging.

Positive displacement

This term is a bit of a catch-all term for technology that employs the use of a motor-driven piston to extrude material through your nozzle — and it’s the technology that the Voltera brand is centered around. Generally, if you’re pushing the plunger at one rate, your flow rate will be the same on the other side. Think about how a syringe would work!

PROS:

• Positive displacement is conceptually more simple and more reliable as long as it’s applied appropriately.
• Doesn’t revolve around a pressurized air supply!
• Because it uses straight displacement and doesn’t rely on pressure build up to apply it, you don’t need to think about how that pressure will relate to the displacement of fluid, or how the pressure required to displace fluid will change as the volume in the cartridge changes — as long as you’re using a low viscosity fluid.
• Low cleaning requirements as many of the components are disposable.

CONS:

• The rheology of conductive inks does introduce some variables, like the fact that they’re thixotropic.*
• It uses a fixed volume reservoir of conductive ink, so there will be times where you will run out of material and you’ll have to reload your cartridge. This means downtime for material changeover.
• There’s also the possibility of trapped air in your fluid which is compressible and can alter your flow rate.

*This is the magic (I can hear my engineering folks saying, SCIENCE, not magic!) of the Voltera product line. We’ve managed to create positive displacement, direct-ink-writing tools that use high viscosity ink. There’s a lot of materials science knowledge, math, and fancy hand waving in the back end that makes this possible!

Materials and performance

Material waste

Another advantage direct-write printing has over screen printing is that the materials involved are contained until the moment they are deposited on the substrate. This means that there is virtually no cleanup and no material is wasted or contaminated after each use.

Feature resolution

There are several key aspects that determine the resolution of a direct-write system. These include nozzle size, pressure control, height control, and material properties, just to name a few.

By tuning these parameters, direct-write printing systems can generally produce features down to 100um with feature aspect ratios up to 1:1 with configurable print heights, but it is not uncommon for finer features to be produced. In the pursuit of higher resolution prints, both the nozzle and print height must be reduced to limit the amount of fluid that can be deposited. As a result, pressure and print height control become paramount to maintaining consistent flow rates and avoiding crashing the nozzle. In the event of a damaged nozzle, it can be quickly replaced with commonly available disposable dispensing tips.

As with any piece of equipment, there are costs associated with direct write systems. Because of the technologies involved, direct-write systems are more like laboratory inkjet systems in terms of cost than they are to screens, but with the ability to print high-viscosity screen printing pastes.

It’s important to evaluate both the short-term and long-term needs of a project before investing in equipment. Direct write technology in combination with screen printing is a powerful combination that can unlock the potential of printed electronics and additive manufacturing for a lot of innovative applications!

Conclusion

That’s the gist of it, folks. Have questions? Did we get something wrong? Drop us a line and let us know! We’re happy to correct ourselves based on the experience of those in the industry, and we love answering questions to help the printed electronics industry be more accessible.