PUBLICATION CENTER

Keeping your printers running smoothly and consistently over the long haul with simple and regular maintenance. The final part of this three-part series addresses basic maintenance for UV-curable printers.

By Justin Sherry

In the first two parts of this series we covered some basic maintenance tips for aqueous and solvent printers. While the inks in these two types of printers are technically different, they still share many similarities in the way they dispense ink as well as the way their respective media receive ink.

In both cases, the liquid ink gets absorbed into an inkjet receptive coating, or in the case of solvent, directly into the vinyl itself. The liquid carrier is then flashed off, leaving the pigment behind. Just like solvent printers, UV-curable printers use piezo-electric printheads to jet ink onto the print media.

However, unlike aqueous and solvent printers, UV curable ink reacts to UV light and instantly cures directly to the print media. When the ink cures, it becomes a hard plastic that has formed a physical bond to the print media.

Protecting Your Nozzles

As with all inkjet printers, head strikes should be avoided at all costs. UV-curable printers bring a few new twists to the table when it comes to head strikes.

The first comes from the curing process of the inks themselves, while the second comes from printing directly to rigid substrates.

As mentioned above, UV curable printers use inks that react to UV light. When a head strike occurs, ink that was sprayed onto the media comes into contact with the nozzle plate of the printhead. This is most serious with UV curable printers as the ink that has already been exposed to the UV light has already started the curing process.

If partially cured ink finds its way onto the bottom of the printhead, it will continue that curing process. What this means is cured ink can completely block nozzles. When this happens, there is no chance of getting the nozzle to recover.

This is why it is extremely important to follow your printer manufacturer’s guidelines when you encounter a head strike to reduce nozzle clogs. While all printers are different, most manufacturers would recommend a cleaning cycle which will, in most cases, purge ink from the head, while wiping the head with the wiper blade to remove any unwanted debris.

In any case, whatever course of action your printer manufacturer recommends, it is imperative that it is done as quickly as possible after the head strike occurs.

Rigid Media

Few would argue that the biggest benefit for using a UV curable printer is that it can print directly to rigid substrates such as Foam Board, Coroplast, PVC, aluminum and wood, just to name a few. While this unique ability is what draws most shop owners to invest in this technology, it also significantly increases the risk of a head strike.

One important rule to remember is that most of the rigid substrates that UV-curable printers use were not originally intended for direct digital printing. Because no board is perfect (and some are less perfect than others), you must carefully inspect each board before you print it.

First, make sure that all of the edges of the board are cut square and that no corners are bent. It is not uncommon to see Coroplast delivered with some of the edges of the board frayed, with the edges sticking up within striking distance of the printhead.

One way to deal with this is to use a utility knife to trim off the problem area, or to take sand paper and shave it off. Also, a bent corner on Dibond or aluminum can instantly kill a printhead. Another thing to watch out for is media that is bowed either in the middle, or on one side. Bowed media is more problematic and little can be done with it.

While most of the high-end UV-curable machines have extremely powerful platen vacuums, which are capable of flattening some warped media, your typical entry-level UV printers have much weaker vacuums and will do nothing to flatten a warped board.

Ideally, you would always have an ample supply of media on hand, or you would order your rigid media far enough in advance so if you receive bowed media, you would have time to refuse it and reorder new media that is flat. This can be especially difficult when you have looming deadlines and a pallet of warped media that “might” work.

Most suppliers will accommodate if you specify media that is flat for digital printing. Finally, always store rigid media flat. If media is stored upright on its side, it may become bowed or warped.

Roll to Roll Media

While roll-to-roll media seems safer than printing to rigid, and in most cases it is, a head strike using roll media can cause just as much damage to a printhead, if not more. For example, if you get a severe head crash printing to a roll of adhesive vinyl, so much that it obstructs the carriage in mid-pass, the heat from the lamps dwelling over the media could scorch the media on the platen.

First and foremost, check the roll for any anomalies that would most likely cause a head strike. Next, when using a take-up reel for roll media, make sure the media is straight on the take-up reel. Loading media skewed onto the take-up reel may cause the media to buckle and possibly cause a head crash.

If you feel your media is the problem, and you have loaded the roll as straight as possible, try looking at the roll on the back of your printer before it reaches the platen. If the media is buckled before it reaches the platen, and it is loaded straight, the problem may be with the roll of media, not the printer.

Likewise, if the media is straight going into the printer, and has buckled, wrinkled, or warped coming out of the printer, try reloading the roll. Finally, store roll media upright on its side. A media rack is ideal, but just make sure that the roll is not lying flat, lengthwise on the floor. This will cause flat spots in the roll, leading to possible print problems.

I Can Print Anything!

A wise man once said, “Even if you could print to anything with a UV-curable printer, that doesn’t make it a good idea.” While it’s true that UV-curable print technology allows users to print to a wide variety of uncoated media, it doesn’t mean it makes sense to print to everything that will fit inside your printer. Here are a few things to keep in mind when deciding on media…

Don’t attempt anything that can’t take a lot of heat: If the selected media can’t take the heat of the lamps, the media will most likely warp causing a head strike. One prime example is polystyrene. While every printer is different, most UV-curable printers have a difficult time printing to sheets of polystyrene as the heat of the lamps causes waves in the media, which then results in a head strike. If you want to attempt running polystyrene, or other heat-sensitive media, try using a faster print speed, or create a delay between passes (if that’s possible on your machine). The key is minimizing the heat from the lamps over the media.

Avoid media with a really slick surface: This is more of a judgment call, but substrates like glass, polycarbonate, and acrylic usually do not form a very strong bond to the substrate and can easily be scratched off. One exception to this would be if the graphic was to be reverse printed from behind or laminated to protect the durability of the printed surface. Also, all printers and inks are different, so one printer may get better adhesion to acrylic than another.

Not all media is created equal: As you try different rigid media, you will find that different brands of like media perform differently. You may find that one brand of 3mm PVC gets better ink adhesion, cuts better, or has a consistently higher white point than another. If you question whether or not you are getting optimal quality on a given substrate, consult your dealer and ask them what brands of like media other UV printers are using and why.

Justin Sherry is LexJet’s technical support specialist.