Large Format Printing: Heat, Ink, and Media

Heat is an integral part of modern large format printing for both eco-solvent and latex printers. All large format outdoor printers use heat. Heat helps ink penetrate media and helps printers dry the ink. This produces prints that can withstand long term exposure to UV radiation from the sun.

Heat is as essential an ingredient in large format printing as it is in baking. And, just as in baking, it must be added in the right amount. Use just enough and you have a delicious concoction. Dial in too much and you have an overcooked mess. How do you know when to add more heat to your large format recipe? What happens when it's overcooked?

Heat, Ink, and Media: Eco-Solvent Ink

As discussed in this article, eco-solvent inks like EnduraINK PRO and MUTOH Eco Ultra have been subjected to rigorous industry testing and are rated for up to three years outdoors without lamination. 

Because of the properties of the ink, you don't need a lot of heat in order to accomplish this. The temperature range of the heaters in the PrismJET VJ54 Gen 2 is 30 - 50°C (86 - 122°F).

The PrismJET VJ54 Gen 2 has three heaters; a pre-heater, platen heater and a dryer (Fig 1). The pre-heater and platen heater warm up the media and accelerate ink absorption. The dryer of course helps dry and cure the print. Since the heat is absorbed into the media from the bottom, the optimal temperature will depend in part on the thickness of the media.

With self-adhesive vinyl, the heat travels through the Kraft or silicone paper release liner and upward into the face film. Thicker products like PrismJET 222 6 mil vinyl work best at slightly higher platen temperatures than 2 or 3 mil vinyls. The same is true of 13 oz banner media relative to 10 oz banner. But banner media doesn't have a release liner so it tends to absorb heat more easily than vinyl.

SignWarehouse recommends lower platen temperatures for scrim banners than we do for calendared vinyl. Why not just turn them all the way to maximum to ensure the fastest possible ink absorption?

Well, to return to our baking analogy, too much of a good thing is not good. Too much heat in your oven will burn the bread. With vinyl and banner media, too much heat causes ripples in the face film that lead to head strikes. A head strike occurs when the print head makes direct contact with the print media. This results in ink smears on the print that generally ruin the print job, and, in extreme cases, may damage the print head. The goal is to warm the media enough to accelerate ink absorption and shorten drying time without producing cockling and head strikes.

So how do you know how much heat to use for various media? Fortunately, you don't have to guess. The FlexiSign & Print or LXI RIP software that comes bundled with the PrismJET 54Gen2 and other MUTOH-made printers includes optimal heater settings in the ICC Profiles.

When we build ICC profiles, we take the optimal media temperature into account. When you use that profile, the software sets the printer's heaters to the right temperature. The printer will not begin the print job until those platen and dryer temperatures are reached. All you have to do is enable the heater options from Production Manager before you send the print job.

It's a good idea to do this in the Default Job Properties tab so that it's automatic on every job you send (Fig 2) so the right recipe is baked into the profile. This is one more reason why it's important to use the right ICC Profile with your PrismJET large-format printer. The temperature is set during ICC Profile generation because it can affect print quality. As noted above, the eco-solvent ink needs heat to penetrate the media. Too low of a temperature will result in excessive dot gain, mottled prints, and incorrect ink limits. Too high of a temperature can result in ink drying too quickly as it contacts the media. This can cause insufficient dot gain resulting in washed-out color.

Using a large-format outdoor printer with eco-solvent ink and the correct ICC Profile is a proven recipe with just the right amount of heat for serving up tasty outdoor-durable prints.

Heat and Media: Latex Ink

Heat is also an integral component in large-format printers using Latex ink. Latex is a water-based ink that relies on a high heat source and some optimizer to cure and encapsulate ink on the surface of print media. The goal is to protect prints from UV radiation and extend print life. Aqueous inks used in other large format inkjet printers generally last only about 6 months in outdoor applications because the ink sits outside the topcoat on the media where it's exposed to UV radiation and subject to abrasion.

HP, the largest manufacturer of Latex inkjet printers, makes no specific claims about how outdoor durable their ink actually is. They merely state that it's as durable as eco-solvent. Because the base ink is water-based and sits on the surface of the media, the Latex recipe requires quite a bit of heat to encapsulate and cure the ink. Because part of its job is to evaporate the water from the ink, the heat in a Latex printer comes from a high-voltage curing lamp above the media. The lamp temperature ranges as high as 125°C (257°F). That's more than twice as much heat as that needed in eco-solvent printers.

As you might expect, that much heat directed at print media can cause a myriad of problems, from potential burns to the danger of spontaneous combustion of aerosol fumes (see HP Latex 110 User manual). If you're using a Latex inkjet printer, a ball of fire from an aerosol spray can is probably not your biggest worry. The more pertinent question is what kinds of problems can be caused by directing that much heat at your media. The answer is…plenty. Here's a short summary of heat-related issues from the Latex 110 User manual.

• Ink Smears: As noted above, excessive heat can cause scrim banner and self-adhesive vinyl to warp. This cockling effect raises the media off the platen into the path of the print head causing head strikes and ink smears. It only takes one to ruin a print job and require an expensive and time-consuming reprint. And, since Latex printers use multiple heads, a head strike can knock one or more heads out of alignment. Realigning the heads adds another time-consuming aspect to the fix. Speaking of time-consuming fixes, HP's suggested solution for media cockling is to reduce the temperature on the heater. This requires you to increase the number of print passes, which slows the print speed and retards productivity.
• Warped images: The HP Latex manual refers to a problem called "bow deformation". This is a warping of the entire media surface caused by overcooking the media. It affects calendared vinyls and cellulose-based photo papers. Bow deformation is especially problematic for the production of contour-cut decals because the vinyl can warp so much that accurate print and cut alignment is impossible.
• Shrinking prints. Any sign maker worth his or her squeegee knows heat causes vinyl to shrink. This normally happens over a period of years as the vinyl ages in the sun. With Latex printers, it can start before you ever deliver the product. According to the HP Latex 110 User manual, exposure to the intense heat of the curing lamp can cause some substrates to shrink right after the image has been printed. This is especially problematic if your image needs to be tiled because the tiles can't be aligned correctly.
• Color Variations: Pages 93 & 94 of the 110 manual warn that the process of evaporating the water out of the ink can cause undesirable color shifts. This is especially problematic for vector graphics with spot colors. Ink variations in a raster image with lots of detail may not be that visible. But heat-induced color shifts in spot color graphics can be hard to hide.
   

If you choose a more advanced large format outdoor printer like the PrismJET VJ54 Gen2, you will get a refined recipe that delivers the benefits of heat and eco-solvent ink without over-cooking your media. The media temperature is already optimized in the ICC Profile so your prints look sharp and crisp, and your vinyl and banner graphics are dry and ready to use right off the printer. You'll have fewer head strikes and ink smears. And you don't have to worry about vinyl warping so much that you can't tile your prints or contour-cut your decals.

All large format outdoor inkjet solutions use heat. If you pick the right one, you can enjoy the benefits without getting burned.

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