In Part 1 we talked about the differences between offset and digital printing. In Part 2 I’m going to talk about how our digital operation runs. The HP Indigo 5500 is a powerful machine that’s capable of some pretty amazing printed work and we’re very pleased to have it as part of our team. I’m going to break this post into three main parts: Substrates (or the paper and synthetics we print on), Ink, and Operation.
For there to be printed work, there has to be something to print on. Some machines can print on anything from wooden doors to 200lb rolls of vinyl. We have a very specific set of substrates that we’re able to print on, but we always like to push the limits. All substrates have a few characteristics that we use to easily define them. Size, thickness, and grain are all measurable traits of a substrate.
Size is pretty straightforward, as I mentioned in our last post, we’re able to print on anything from 8.5”x11 to 13”x19”. This is the standard for digital printers, but offset machines can run sheets up to 40” across. Artwork is imposed, or placed, on the sheet by our software to fit the largest number of pieces per sheet. For example if we have a standard business card (2”x3.5”) we can print 30 business cards per sheet to minimize our paper waste. If the artwork has a bleed, which is where the artwork goes right up to the edge of the card, we have to account for that by printing .125” extra on each side which gets cut off in the finishing process. This makes it easier to avoid any unprinted paper from showing up on the edge of the card but forces us to only print 24 business cards per sheet of paper (assuming 13”x19” sheet size).
Unlike most printers, we will alter the imposition to retain maximum rigidity for the printed piece – another example of our crazy attention to detail.
That brings us to our next characteristic: grain. This is very similar to the grain of wood flooring, it’s simply the direction that the fiber runs on the paper. Grain can be one of two things, long or short. Long grain paper has the fiber running along the long edge of the paper, which makes it fold more easily along the short edge. Short grain is just the opposite. Going back to our business card example: we know how many business cards we can fit on a sheet of paper, but we also need to make sure the grain of the paper matches the direction of the business card. If you take a look at the two imposition examples, one will result in a business card going with the grain, adding to the rigidity, and one will print against the grain, causing it to feel thin and floppy. Unlike most printers, we will alter the imposition to retain maximum rigidity for the printed piece – another example of our crazy attention to detail.
The third main characteristic of a substrate is the thickness, which can be measured in far more ways than is practical. Here at Moxy Ox, we use two main classes of substrate weight: text weight and cover weight. Text weight substrates are very thin and when you think of regular printer paper you’re thinking about a 24lb text. Cover weight substrates are often referred to as “cardstock”, but there are many different cover weights. We typically measure text in pounds and cover in points. The Upper limit of our Indigo Press is 18pt. We push that limit a little bit with some of our more unique substrates.
When we want to add a substrate to our selection there are a few things we look for. We need to make sure it’s been certified by RIT, and we also look for unique textures and finishes. Once we have selected a stock, we order samples in order to observe how well the substrate accepts the ink from our press.
HP Indigo has a special type of ink called ElectroInk. It’s very different from any type of digital toner or offset ink. The ElectroInk that we use is made up of small (1-2 microns) colored particles mixed with an oil solution. All digital printers have the advantage of being able to apply ink or toner digitally, instead of manually via plate. The advantage that ElectroInk brings is that the colored particles are much smaller than toner particles which allows them to penetrate the surface of the substrate more consistently. The result is beautiful, rich colors, texture consistency and fantastic color accuracy.
When the press is running, the particles are mixed with Isopar L (a type of oil) to form a solution that can easily be applied in exact locations by the printer. Once the ink mixture is applied, the press heats the substrate to steam off excess Isopar L, leaving only the colored particles remaining. The Isopar is then recycled by the press and mixed back into the ink, ready to be applied to the next sheet. The ink is applied one color at a time in a halftone pattern. The print, when seen from a distance, appears as one color.
So, we’ve covered substrates and ink and we’re ready to start printing. The HP Indigo 5500 press can be broken into 3 different sections: the substrate trays and intake, the drums, and the finishing area.
The substrate trays are essentially large drawers that the paper is placed in until it’s time to it. We tell the press which trays hold certain substrates, so it knows which one to pull from automatically. When we’re ready to start printing, the press uses a system of vacuums and belts to pull one sheet at a time and feed it into the main body of the press.
This entire process takes only a few seconds, and the press can output up to 8,000 sheets per hour.
Once the substrate is fed into the body of the press, it is grabbed by two suction cups and wrapped around a cylinder called the Impression Cylinder. On the other side of the press, we have a different cylinder called the PIP, or print imaging plate. This PIP is held at a constant negative voltage and writing heads, called scorotrons, apply a positive charge to the PIP in the areas that the ink needs to be. The ink is attracted to the most positive charge on the PIP so when it is applied it only sticks to the areas that the scorotrons have charged. The ink is then transferred from the PIP cylinder to the blanket cylinder. Now that the ink has moved from the PIP to the blanket, the PIP returns to a flat negative charge and is ready to have ink reapplied. The ink is then transferred from the blanket cylinder directly onto the substrate, one color at a time. This entire process takes only a few seconds, and the press can output up to 8,000 sheets per hour.
The HP Indigo is an amazing piece of technology; it allows us to revive print and transform it from a boring necessity to a luxury, at a price that everyone can afford. But it’s not the only weapon in our arsenal against boring print, and later we’ll go over some of the other tools we use. Stay tuned and thanks for reading Behind the Print!
Technology & Research