Variations on a Droplet Theme

As the boundaries of the possible continue to expand in the world of wide-format digital printing, so does the amount of jargon that goes with it. And for a guy who simply wants to make banners and other printed large-format projects, it becomes an ever-increasing challenge to keep up with an industry that has a tendency to change faster than Clark Kent in a phone booth.

But who wants to learn all that jargon? Does a guy in a sign shop really care whether a snazzy new piezo printhead can depict a pixel as small as 140 µ x 140 µ (micrometers), or that it has a an ejection response of up to 52 kHz (kilohertz)? No, not really. But there is some jargon that you, as a regular-guy large-format printer user, should indeed be familiar with.

Likely you already know an image printed at a resolution of 1440 x 1440 dpi (dots per inch) will look better than one printed at 300 x 300 dpi. And chances are that you also realize that when it comes to digital printers, there’s usually a trade-off between speed and resolution. The question usually comes down to this: Do you want speed or quality?

However, many of today’s digital printers have a feature that sounds jargony, but if present, can help shave off some of the trade-off between speed and quality. The term is “variable dot,” and in essence it refers to a printer that can produce ink droplets of different sizes using the same printhead. It’s a simple idea, but not simply explained.

Variable dot, as with most things technical, is a bit of jargon that comes with even more jargon attached to it. Okay, bear with me here. Printheads are built with hundreds of tiny nozzles where the ink is shot out in tiny droplets. The volume of ink in each droplet is measured in picoliters—one picoliter is about 10-12 liter. The nozzle size will determine the “native drop size” that the printhead can produce.

In the digital printing world, six picoliters is considered a very small drop size, 20 picoliters is considered average size and 50 picoliters is pretty large. Each time the piezo crystal inside the nozzle chamber is stimulated, it pulses and a droplet of ink is forced through the nozzle. Micro-pulses of the piezo in variable-dot printheads will expel multiple drops that coalesce into a larger single drop in mid-flight before hitting the substrate.

Each nozzle in a variable-dot printhead can produce different size drops depending on how many micro-pulses are made. Each micro-pulse delivers the amount of ink equivalent to the native drop size. So, a four-pulse variable-dot printhead with a native drop size of six picoliters could potentially produce drop sizes of 6, 12, 18 or 24 picoliters. A four-pulse variable-dot printhead with a native drop size of 20 picoliters could produce drops of 20, 40, 60 or 80 picoliters. Printheads that can produce more than four pulses per drop offer an even wider rang of drop-size options.


So, why is all this technical gimmickry so dang important? Well, when it comes time for you to shop around for a new wide-format inkjet printer, you’ll want to know what cool-sounding terms like “variable dot” really mean. And if you do, you’ll know the right questions to ask in order to find out if that printer is right for you.

If you’re looking for a printer to do a lot of fine detail work, look for a printer with variable-dot printheads with a small native drop size. If you’re looking to do high volumes of huge banner work, then go for a larger native drop size printhead.

Larger dots allow for faster printing with good color density with fewer passes, while smaller dots allow for finer detail and/or crisp fine-point text. Variable-dot capability allows for efficient print speeds and fine, crisp text or image detail when needed. The main benefits are these: decreased print time with enhanced quality, finer detail with crisper colors in the mid-tones and shadows, less graininess and smoother gradations.

The advent of this technology actually changes the dpi/resolution numbers game. When compared to single-dot printhead images, variable dots offer finer detail with faster print speeds at the same resolution. For example, a 720 dpi image created with variable dots will look finer and will print faster than 1440 x 720 dpi image from a single-dot printer. And a variable dot resolution at 1440 x 720 dpi will actually look better than a single dot image at 2880 dpi.

So, what’s the downside? Well, some people say that very small drop sizes are more difficult to control and could result in unwanted banding, but for printheads with a quality build, this is not a problem. Variable dot technology has been around a few years, is proven in the field and definitely a desirable feature for any printer.