I can't imagine asking a printer if they ever struggle with achieving color or solid ink density (SID) during a press set up, and they reply with a "no". They would probably be much more likely to roll their eyes and asked me if that was a rhetorical question. The fact is this isn't at all an uncommon occurrence during a press set up. It doesn't matter if the ink is a PMS color or a transparent process color, lacking SID is common and just part of life as a press operator. When I was a press operator my first thought would've likely been to add fresh ink or allow the viscosity to raise. Once it was a point that I wasn't comfortable raising the viscosity anymore, (out of what may be laziness on my part) I may ask the ink technician to over pigment the formula in order to achieve the color strength I desired. What would follow would be a small argument between us, and ultimately they would prove to me (once again) that you can only over pigment up to a point before the pigment is no longer soluble in the ink. Feeling dejected, I would be resigned to the fact that I was going to have to change the anilox to a higher volume roll. But I'm not gonna lie, I don't think ever once did I consider lowering the viscosity of the ink in order to achieve more color strength.
Basic Ink Chemistry
Just a quick review; inks are made up of three or four primary components. There is the pigment, which gives the ink color, and there is the vehicle, often solvent or water, which acts as a carrier to get the ink out of the bucket/kit, through the ink delivery system and onto the substrate. There is the resin, which adds performance characteristics, and in the case of water-based inks, there are amines which keep the resins soluble. The ink is delivered to the anilox roll, and from there is transferred to the printing plate and onto the substrate. I mentioned earlier that ultimately I would have to switch out the anilox roll to a higher volume (BCM) in order to achieve adequate color strength. When we think of the anilox roll, typically we just think of a couple of characteristics. The first being LPI, which is important when taking into consideration the LPI of the printing plate, and volume which is important for color strength or SID. However, we also need to keep in mind the transfer factor of the anilox. The transfer factor is often affected by the depth to opening ratio of the shape of the anilox cells. Here's where things get a bit more interesting.
Surface energy appears when like-particles strive to stay together.
Ink Transfer is a Tricky Thing
When we calculate or physically measure the volume of an anilox roll, we need to remember that never will 100% of that volume will be removed from the anilox and transferred to the printing plate. In reality, only 45% to 95% typically is transferred from the anilox to the printing plate. An anilox transfer factor is affected by the volume, the cell geometry, the metering which includes ink to plate settings and blade psi, type of plate material and surface energy, and one more very important thing... viscosity. Generally speaking higher viscosity inks (inks that have a higher resistance to flow) will transfer from the anilox with a 45% - 60% effectivity rate. Whereas lower viscosity inks will transfer 85% - 95% of the ink from the cells.
As the inks viscosity continues to raise it can actually have an adverse effect on solid ink density. In other words the ink can become thick enough that the plate is not effectively removing (pulling) the ink from the cells. If it helps you picture this; and anilox roll filled with maple syrup and delivering that syrup to the photopolymer plate. I think we could all agree that we would expect the anilox to be nice and sticky even after delivering ink to the plate, correct? I know this is a bit of an exaggeration, but you get the point. For example, if you're lacking SID and you're running your inks at 28 seconds on a number 2 Zahn cup and you continue to let the viscosity raise, eventually you will lose SID. However, there's a possibility that by reducing the viscosity of the ink it will then allow the printing plate to pull more ink from the anilox cells by achieving a higher ink transfer rate resulting in stronger color. The higher the viscosity and the lower the temperature, the more the ink components want to stay linked together creating a resistance to flow. The lower the viscosity and the higher the temperature the less likely the particles are to want to bond with one another because of decreased surface energy. So is it possible to get the same SID from the same ink formulation running at 22 seconds on the number 2 Zahn cup as I could running it at 36 seconds? Yes it is, and it may even deliver increased SID at the lower viscosity, while minimizing the often most expensive component of the ink; the pigment.
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