In a perfect world, you should be able to put ink in the press and simply run a job. Unfortunately, every year flexo and gravure printing operations waste ink, substrate and press time trying to get color right. Although advancements in technology have made it easier to achieve color accuracy, the variables that affect color still exist.
In this three-part series, we’re sharing over two dozen reasons your color might be wrong on press. We’ve already covered two important factors – Instrumentation and Standards & Ink. Today we’re looking at how the environment and your press can affect final color.
1 – Temperature & Humidity
Temperature and humidity can have a huge impact on how ink transfers to the substrate. This is especially true with solvent inks. Some printers even run a summer and winter blend of solvents to help control this problem. Inks that are transferring well will lay down a stronger, denser color. Ink that dries too fast can appear grainy and light. If your coverage is poor, the substrate can bleed through and affect the spectrophotometer’s readings. Look at your solid ink patches through a loupe to make sure you have adequate substrate coverage.
2 – pH that is too high or low
High pressroom temperature can also cause the important amines in water to evaporate, creating a too-low pH. The resulting poor ink transfer will cause foaming, pin holing, and an overall lighter print color.
On the other hand, a really high pH can burn out the pigments in your ink, like certain oranges that don’t have the permanency of other pigments. These ink dispersions can actually disappear from the ink in small amounts, creating burned out color.
3 – Changing the cylinder
Possibly the largest offender of incorrect color is not making adjustments for new anilox or gravure cylinders. A brand new gravure cylinder is going to put down a lot of ink compared to one with millions of feet on it. While a new gravure job might have very strong inks to start with, months later they will begin to fade.
Since the volume on your cylinder plays a key role in how much ink is delivered, formulate your inks to the cylinders in the press and document it to the approved job. The cylinder will put down less ink over time, so closely monitor jobs that run for a long period of time. You may want to consider hanging an anilox cylinder chart to track footage so you know when to send the anilox for reconditioning.
4 – A dirty or plugged anilox
A dirty or plugged anilox will result in a very light color and is a difficult obstacle for an ink tech to overcome. If you’re adding colorant over and over with little to no results, the chances are good that your anilox roller is plugged. For immediate results, try swapping out the anilox or soda blasting the roller.
5 – Viscosity
Viscosity can be tough to monitor in flexo and gravure printing because it can change quickly. Although many modern presses have automatic systems that monitor viscosity, they can malfunction or be accidentally shut off. If your color gets dramatically stronger, there’s a good chance the viscosity has risen. Keep in mind, ink will lose a few seconds of viscosity when first put into the press. This occurs from the shear of the fountain circulation since ink is a thixotropic liquid.
Once you achieve the approved color, it’s a good idea to record your viscosity.
6 – Impression
Impression is probably the easiest thing to fix. Look at the dot gain and density of the colors you’re printing. If everything is set correctly but you’re still running strong, consider backing off impression a bit. The reverse can be true if you’re too light. You may also see mottling. These uneven patterns can cause erratic readings. Try adjusting impression, opacity, or ink vehicle to smooth out inconsistencies.
7 – Dot gain and density
Process printing can be tricky because your density levels can all be in the correct range, but one process color is still too strong. If the dot gain is too high, the press operator might be able to back off the offending process color just enough to correct the overall print. Density and dot gain work together, and both can be checked using the eXact densitometer tool. In addition, after many years of relying on the Murray-Davies and Yule-Nielson as dot gain calculations, we are seeing a new calculation. ISO 20654 Graphic Technology - Measurement and calculation of spot color tone value (SCTV), which is primarily used to measure tone value (dot gain) for special colors. This calculation does not use density, but rather colorimetric or spectral data. The good news is the eXact supports this workflow as well.
8 – Plate and blanket durometer
A really old photopolymer printing plate can develop hardness over time. This will result in a poor transfer of ink and color that is too light. In some rare cases, the color transfer might be so light that you can’t adjust the ink enough to compensate. In this case, check the durometer of your plates. Your plate supplier can give you more information.
9 – Doctor-blade issues
A worn doctor-blade can allow too much ink to pass between it and the anilox roller or gravure cylinder, resulting in a very strong color. In some cases, a press can be running fine; then suddenly the density shoots very high. When this happens, check your blade. Chances are it needs to be changed.
10 – Plate backing material
Like the durometer of a flexo plate itself, the adhesive backing can also play a role in how a flexo plate performs. If you’re unable to get the look or color you need, consider changing the backing material.
Different levels of plate backing material (“sticky back”) can affect print characteristics, dot gain, and impression. For example, harder adhesive backing can yield better results on some substrates, medium performs well for process work, and soft has a lot of give and is forgiving with impression. This may be a good option if you’re having issues with gear chatter showing up in print.
Wrapping it up.
Achieving accurate color in flexo and gravure printing is a delicate balance between the spectrophotometer, ink, substrate, press, and the environment. If your color just isn’t right, try checking these areas first.
Sometimes your issues may be in your process. Get in touch to speak with a color expert, or consider one of our many eLearning, classroom, seminar or onsite learning opportunities to make sure everyone is working accurately, efficiently, and consistently.