PUBLICATION CENTER

An overview of the advantages and disadvantages of variable-drop and fixed-drop printheads, plus solid advice on keeping this crucial hardware running at optimum performance.

By Russ Brown

When you break down the individual components of a UV-curable printer, the printhead is arguably the most vital component in the equation, and it’s usually the most expensive. It’s also the piece that requires the most attention, both in the production process and as you consider purchasing new equipment.

Variable or Fixed?

Wide-format printheads are either variable or fixed. As the terms imply, variable means the printhead can fire variable-sized dots, which can range from 10 to 80 picoliters. Fixed-drop printheads, on the other hand, fire a single-sized drop.

The Legend 72HUV, for instance, utilizes a Spectra Galaxy 256 fixed-drop printhead that fires 50-picoliter drops, and only 50-picoliter drops. The HP Designjet H45500 UV-curable printer, on the other hand, fires drops that can vary between 10-30 picoliters.

The primary advantage of variable-drop technology in UV-curable printing is that it allows finer text and better flesh tones at close viewing distances since it can vary the drop size from small to large. The problem is that the variable drop printheads tend to be as much as three times slower than a fixed drop. This variance in speed can be compensated for by adding more nozzles, which adds to cost of the printhead, and thus the overall cost of the printer.

The firing frequency when printing small drops is very high (so the carriage can move fast) but this requires a high-pass print mode, which slows down production. The firing frequency for large drops in a variable-drop printhead is low, slowing down the entire printing process. And, when you mix and match dot sizes within a print, you slow the printer down to the lowest common denominator.

If the printhead can fire three different-size drops – small, medium, and large – the carriage then has three speeds:  fast, medium, and slow. If you use the small and medium drop in one image, you print at the medium carriage speed.  If you use all three, then you print at the slowest carriage speed.

Basically, as you consider the printhead equation, you need to consider the type of output you’ll be producing. If most of your work involves small text and close viewing distances (five feet or less), and speed is not a factor, variable-drop might be a good fit.

However, if most of your output is designed for viewing distances greater than three to five feet and you want to maximize production, a fixed-drop printhead is the best bet, especially since most production work, from interior to outdoor signs and graphics, fits into this category.

Protecting Your Investment

As mentioned earlier, printheads are the heart of the printer, and generally the most expensive part, and should be treated and maintained accordingly. This is especially true with UV-curable printers.

Never touch the nozzle plate of the printhead with anything other than what the printer manufacturer recommends.  This includes materials like rags and chemicals. Using the wrong material can scratch the nozzle plate and cause drop misdirection. Using chemicals can harm the printhead, and if a chemical is not compatible with the ink it can cause other major problems, such as gluing the nozzles closed, which is a bad thing.

When wiping the printheads (with materials recommended by the manufacturer, of course) always wipe in one back-to-front direction, and only once. Failure to do so can do more harm than good, as you may actually wipe dirt into the nozzle, effectively blocking it, or causing drop misdirection.

After a head strike, always and immediately run the printhead maintenance routine. A head strike can push partially cured ink into a nozzle, and this partially cured ink can continue to cure and block a nozzle. Run the printhead maintenance routine prior to shutting it down overnight or for shutdowns of more than four hours.

If not printing for more than 30 minutes, turn off the UV lamps. All UV-curable printers have scattered UV light. When printheads are parked on the carriage with scattered UV light, it provides an opportunity for ink to cure on the nozzle plate.

Keep the printer clean and free of dirt. If the inside of the printer is dirty from being in a dirty/dusty environment, the dirt will migrate into the printer and eventually get in the ink lines or on the nozzle plate. In addition to venting the printer with a portable air scrubber to remove gases like ozone created by the lamps, a good idea is to run another air scrubber or a similar HEPA filter in the room to keep dust and dirt particles to a minimum.

Religiously and consistently change ink filters per the manufacturer’s recommendation. For the Legend 72HUV, Digital Equipment Company recommends every three months. If the printer is a bulk feed printer, clean the ink tanks out once every six months. Gunk will accumulate in the bottom of the bulk tanks and you do not want to suck that gunk up into the printheads.

Russ Brown is vice president of Digital Equipment Company.