Working with Custom Inks
If you’re a NOVA user, you may be planning on printing an ink that Voltera doesn’t provide. This guide will help prepare you for working with new materials, so you know what to look for and how to ensure you get the best results possible.
If you’re trying to figure out whether or not a material will print with NOVA, the most important thing to remember is that NOVA works best for screen printable materials. Whether or not you have a datasheet, here are a few useful rules of thumb or reference points.
When you’re buying a printing ink or material, they’ll almost always be categorized by their printing technology. Screen printable inks are paste-like (high viscosity), while inkjet, gravure, flexo, or other inks are generally water-like (low viscosity). If the datasheet says it’s screen printable, you’re on the right track! If it doesn’t say, ask what printing method the material is intended for.
Let’s say a datasheet only lists a viscosity number, you just have a material in a jar. What then?
While it might be counterintuitive, you are actually probably a good gauge for viscosity just by handling the material.
Reference table for viscosity
Most screen-printable inks will fall somewhere in the bracketed range (between 1 - 1000 Pa.s, or 1000-1,000,000 mPa.s), but that’s still not really all that helpful.
Instead, start by tilting the jar. If the material doesn’t flow on its own, try moving the material with a stir stick. Watch and feel how it moves around, and then have a look at the chart and compare relative to other familiar materials, in terms of texture.
Does it flow like yogurt, honey, toothpaste or mustard? If so, you’re probably in the clear!
Does it flow like water, oil, or very loose adhesive? This is probably going to be a challenge.
Does it not flow at all, like rubber? This will absolutely not work. You can’t really break a smart dispenser by trying, but I wouldn’t recommend it.
We recommend taking viscosity numbers with a large grain of salt, for a number of reasons.
- 1.Viscosity values are not fixed. They can hugely vary depending on the method used for measurement (rheometer? viscometer?), the shape of the measurement device (cone, plate, spindle…), the measurement parameters (shear rate, holding time, even the measurement program and how you loaded your sample), and even the temperature in the room (which is why we heat the smart dispenser).
- 2.Viscosity units can be misleading. 1 cP (centipoise) is the typical viscosity of water. 1 kcP (kiloscentipoise) equals 1000 cP, and is often used for higher-viscosity (screen printable) materials. 1 kcP is also equivalent to 1 Pa.s (Pascal-second), which is the SI unit for viscosity.
- 3.Viscosity is only one of many parameters that determine flow properties of an ink. Among elasticity, yield stress, adhesion, thixotropy, and others, viscosity only tells part of the story.
If you must rely on a viscosity number, we’d recommend only using it as a rough reference:
- Is the viscosity reported in cP, not kcP or Pa.s? These are typically low-viscosity, water-like materials, usually intended for screen printing. Unless it’s in the high hundreds to low 1000’s of cP range, chances are this will not print like a screen printable ink. It doesn’t mean you can’t use it, but set your expectations accordingly.
- Is the viscosity reported in kcP or Pa.s, not cP? These are usually in the right range of screen-printable inks, until you get to the 1000’s of kcP (millions of cP) - though, even then, it could just be reported incorrectly.
- Is it between 1-1000 kcP / Pa.S? Different sources will give different numbers, so again treat this as a rule of thumb - but this is a reasonable range to expect screen printable inks to be reported in.
Before you can print a new material with NOVA, you will need to pack it into a compatible cartridge. NOVA accepts standard EFD 5cc syringe barrels, which can be sourced from multiple suppliers, or directly from Voltera.
If you want the best possible packing for inks, you need to minimize the amount of trapped air in your ink. The best way to do that is with a dual asymmetric centrifugal mixer (DAC) with a vacuum setting. But, if you don’t have access to this equipment, NOVA will still give you very good print results.
Nozzles direct the flow of ink as it exits the cartridge onto your substrate. They are a critical component of the dispensing system, and control many aspects of the quality of your output. For that reason, choosing the right nozzle for the job will help ensure you get the best possible print quality.
At Voltera, we supply tapered metal nozzles in 4 sizes: 50um (yellow), 100um (orange), 150um (blue), and 225um (black). For most screen printable inks, we recommend starting with our 150um nozzle (blue hub); it will usually give you a good balance of printing resolution at a reasonable dispensing pressure.
For other materials, it’s good to know what nozzle options are available to you. Read on for further details.
All nozzles have the following components:
- 1.The hub - the Luer fitting, usually made of plastic, that attaches the nozzle to the dispenser. These are usually just molded plastic.
- 2.The barrel - the portion that directs the fluid to the tip. It can usually be tapered, conical, or straight. The barrel will usually determine how much resistance there is to printing; straight or narrow barrels will require more pressure, while tapered / conical or wide barrels will require less.
- 3.The tip or orifice- in straight-barrel nozzles, this is just the end of the barrel. The inner diameter (ID) of the tip determines the minimum print size, but smaller ID’s will also limit the inks you can use depending on their particle size, and require higher pressures.
- 4.The material - while not strictly a labeled component on a chart, the nozzle’s material can make a big difference in performance. Material options are covered below in nozzle types.
As mentioned above, the most common barrel shapes are straight-barrel, tapered, or conical. When choosing a barrel type, consider the trade-offs below:
- Intended viscosity:
- Conical nozzles are specifically made for precision dispensing with higher-viscosity materials. The conical barrel provides a gentle gradient, allowing NOVA to dispense through a high-resolution nozzle without requiring extremely high pressures. Tapered nozzles are similar, but are usually plastic and have larger nozzle orifices.
- Straight-barrel nozzles are better suited for low-viscosity inks, where the added friction from the long barrel allows for better dispensing control. If you find your ink is extremely sensitive to pressure changes, even with the small nozzle orifice, try a straight-barrel nozzle!
- Robustness: Conical nozzles are typically quite robust, and as long as the tip is not damaged, can be handled easily and withstand interacting with NOVA’s XYZ calibration switch. Straight barrel nozzles are generally decently robust, but at smaller ID’s, they will start to deflect, bend, or break with little applied force.
The nozzle Luer hub material doesn’t have much impact on the print quality, but there are a variety of material options for barrels and coatings.
Barrel materials are usually either metal (such as stainless steel) or plastic (usually polyethylene, sometimes polypropylene). In some specialty cases, there are also glass, ruby-tipped, or ceramic nozzles, but we won’t cover these options in this guide. When choosing barrel materials, consider the tradeoffs below:
- Cost: Cost is dramatically different between plastic and metal. While plastic nozzles can cost a few cents, metal nozzles can range from less than $1 for simple ‘general purpose’ straight-barrel nozzles, up to $10 or even more than $100 for certain precision tapered nozzles.
- Nozzle orifice / ID: Plastic nozzles can have an inner diameter as small as 200um, while metal nozzles can generally be made available in sizes down to 50um. Specialty material nozzles can offer even smaller orifices if necessary.
- Barrel options: Plastic nozzles are mostly restricted to tapered barrels, while metal barrels can be straight, tapered, conical, or even follow other exotic profiles like oval-shaped or chamfered orifices.
- Solvent stability: Plastic nozzles don’t have the same solvent resistance as metal. It’s helpful to check solvent compatibility tables if your material has particularly aggressive solvents.
- Nozzle outer diameter and adhesion: Plastic nozzles usually have thicker walls than metal nozzles, which means they have a larger outer diameter (OD). This gives more surface area for ink to stick to during dispensing, and can lead to ink pulling up along with the nozzle, which we call ‘stringing’. This can still happen with metal nozzles, which is where coatings come in.
Coating materials are an option with some precision conical nozzles. The purpose of coatings is usually to reduce adhesion between the nozzle OD and the ink, allowing for smoother flow and eliminating stringing when lifting away from the substrate. Coating materials are usually some type of low-surface-energy fluoropolymer, like PTFE. While coatings will add cost, they can be worth the added cost if your ink is prone to stringing.
Precision nozzles are more niche and more expensive, while simple plastic or straight-barrel dispensing tips can be found from many suppliers. Some example dispensing tip supplier options include:
- Nordson EFD - for a variety of general purpose, plastic, and precision nozzles
- Jensen, Subrex - for precision nozzles
- Amazon, Fisnar - for plastic dispensing nozzles
Now that you know more about the types of nozzles available, you should be able to select an appropriate nozzle. Here are some helpful rules of thumb:
- 1.Nozzle diameters - the rule of ten (or, rule of six): If you know your material’s particle size distribution, choose a nozzle inner diameter so that the 6 particles at D80 particle size will fit across. If you don’t know the distribution, play it safe and make it 10 particles at the average (or known) particle size. While particle size isn’t always the limiting factor with high viscosity materials, this can help prevent clogs. For example - if you have an average particle size of 3um (common for silver flake inks), you should have a nozzle diameter of at least 30um.
- 2.For low viscosity materials use a straight barrel nozzle. The added wall friction will give more precise control over the flow rate when using more water-like materials.
- 3.For high viscosity materials, use conical or tapered nozzles. Conical nozzles let you dispense high-viscosity inks at high resolution; if a small nozzle ID is not required, tapered nozzles can give you a similar option at lower cost.
If you start with the wrong nozzle, you will usually run into issues quite quickly. Maybe no ink flows at all, or maybe it flows too much, or is too sensitive to small pressure changes to adequately get your target trace width, or you are seeing excessive amounts of stringing and need a coated nozzle.
In any case, don’t be afraid to try something new if your nozzle isn’t working — just make sure you calibrate again when changing nozzle types.
When starting out with a new material, using a different nozzle type, or even changing the smart dispenser preheat temperature, it’s always a good idea to lock in some settings with NOVA’s calibrate procedure.
Even if you eventually need to do some manual print setting adjustments, this process will usually give you some insight into how your print is going, and if you have to change out your nozzle type. Take your time, and you’ll be sure to find some good quality settings in short order.