When looking into lead times for printed products, you often see print speeds like 18,000 pages per hour marketed. At the end of the day, it means the equivalent of nearly 50 trillion pages printed per year.
But what’s the biggest challenge in quality printing lead times? Believe it or not, it isn’t necessarily the speed ink can be applied to paper or paper feed times.
One of the biggest problems for getting your print jobs on-time and high-quality is drying times. How can different factors affect sheet drying times, and therefore the quality of your prints?
Keep reading to find out that and more!
Factors That Influence Drying-Times in Offset Printing
Drying times will change based on an almost unlimited number of factors, it seems. Sheets that aren’t given the proper amount of time to dry, typically between 5-8 hours, have issues with ink adhesion, bleeding, and smearing.
Several of the most impactful and common factors that have solutions are:
- Amount of ink being applied
- Ink drying component concentration
- Fountain solution dose
- Paper moisture content
- Fountain solution or paper pH value
- Ink composition
- Low temperature or high humidity in the printing room
- Ink and varnish viscosity
There are numerous solutions for each scenario, which we’ll get into later. Just knowing the solution isn’t good enough though, because we need to know in detail why these issues cause drying problems.
What is the nature of offset printing that causes issues with paper and ink drying, anyway?
One of the largest contributing factors comes next — design.
Design Dilemma: Color Conversions
The majority of designs today are actually designed with a “digital-first” mindset. That is, logos are designed to primarily look good on smartphone and computer screens, and secondarily on printed materials.
An exception to this case might be if there are physical products and packaging.
One of the biggest issues is converting color from an RGB colorspace to CMYK colorspace, or four-color process, used in offset printing. A lot of younger or self-taught designers do the majority of their work on digital media, without much print-specific experience or training.
Many don’t deal with CMYK enough to know the difference between various CMYK ICC profiles, their history, and uses. More on the fixes that designers can build into the document from the start to reduce drying times later.
However, a lot of it has to do with fundamentals of color theory and using checklists called a “preflight” to make sure of some of the more important aspects of the print process.
A print-experienced graphic designer will know to check things like overprint, bleed, and other concerns. As well, they’ll know the color profile specifies a certain amount of ink to be used on coated or uncoated papers and what equipment the printer uses.
Coated or Uncoated: That Is the Question
There are hundreds of different kinds of paper, between bonded paper, satin or matte, and other art papers. Who knew that paper could be so complex?
One of the greater differences is simply if it is “coated” or “uncoated” papers. In this case, the most common coating material is one called kaolin, a type of clay material.
Why does that matter? In the case of coated paper, the ink sits on top and hardens. Coated paper is heavier and stiffer because of the clay coating.
With uncoated paper, it absorbs into the paper more, including the oils and fountain solution. This affects dot gain, or how much the dot of ink will swell. Uncoated paper is almost always better for photos for this reason.
Because they hold different amounts of ink, Total Ink Limit or Total Area Coverage is used to express how much ink is acceptable in the design. Cyan, Magenta, Yellow, and Black ink is specified by mixing percentages to make desired color.
For example, “rich black” is generally C30/M30/Y30/K100, a TAC of 190%, well below a SWOP profile TIL or TAC suggestion of 300%. Going above the recommendations are possible, but will cause muddying of colors, and increase drying times as the paper becomes saturated.
In the case of coated papers, when too much ink is applied, it will just pool on top and affect drying times and reproduction quality loss.
Humidity and Temperature Control
Temperature and humidity are essential to control during the whole printing process. This ensures swelling and drying, cutting, ink flow, and color reproduction will be more consistent.
Paper needs to adjust to the ambient temperature and humidity levels. Ideal conditions are about 23°C or about 73-74°F. The relative humidity level should be between 50-60% without more than a 5% change during the print cycle.
To adjust to the temperature of the printing area, an uncoated paper may need only four hours, but coated paper could need at minimum 72 hours. A pallet of paper, or a roll, may need 72 hours to reach equilibrium.
This is one of the hardest points to control in-the-moment. Fountain solutions, or the water used for the inks, is usually the culprit in the case of acidity issues and drying problems.
If the range is out of 5.5-6.0 pH level, it could deactivate components in inks that allow it to dry through oxidation or absorption correctly. When this happens more water is needed, which could make them ink unevenly.
With uncoated paper, absorption and oxidation drying happen together. In coated paper, absorption happens less, because less of the ink goes into the paper.
Too much acidity could prevent the stock from absorbing the water and it will stay in the ink instead, slowing evaporation.
Fountain solution contains surfactants, pH regulators, and other additives than just soft water. These solutions play a vital role in the printing process by facilitating interaction between the printing plates and inks.
There are more and more advances in “waterless” offset printing as time goes on, however.
We’ve already talked about inks a little, and their acidity level, temperature, and humidity concerns.
All printing inks have three parts, which are the vehicle, which are usually oil-based. These oils can be vegetable oils, mineral oils, or triglyceride based. Absorption rates can be influenced greatly just by the type of oil used in the ink.
Pigment is the colored portion of ink, which also gives texture and gloss characteristics. Pigments can be opaque or transparent, white, black, or colored. Metallic powders are also added, but most pigments are made from minerals or coal tars.
Additives are the third part of inks and many of them are for increasing the speed of the drying process. Bodying agents change the viscosity of inks, which are thixotropic.
This means that when physical energy is applied to inks in the form of rollers, it goes from a viscosity of 100 times that of honey to more of what we expect from most liquids.
There are many additives that perform many jobs, which is why managing fountain solutions and ink interaction is crucial.
Low-energy UV-curable inks are one innovation gaining more and more ground as they dry faster, with less risk of ink transfer and other drying problems.
What Can Be Done To Reduce Drying Time
We’ve talked about a lot of factors which affect drying times, and a few solutions. For ease, let’s condense these a bit into an easy-to-read list, now that you know the major factors. These are the most common fixes for excessive drying times:
- Reduce ink quantity by keeping to recommended TAC
- Convert colors from RGB designs to CMYK using proper color profiles
- Calibrate and reduce fountain solution if possible
- See if increasing drying compounds can be done along
- Replace the paper that has too much moisture
- Don’t use old inks
- Control the humidity and temperature
- IR and Hot-Air Drying
We haven’t talked about UV-curing and IR or Infra-Red and hot-air drying. UV inks are now capable of using LED-based low-energy UV to harden inks, especially in coated paper using oxidation drying.
Many offset presses like the Heidelberg Speedmaster series have UV drying and hot-air drying processes built-in. Even older presses can be retrofitted for UV curing.
Hot-air drying processes use high heat and move air quickly across the “wet” sheets, dramatically reducing dry times. For each 6°C drying time is reduced by about 25%. It’s important that the compressors and vacuum pumps used are high-quality and reliable products, like those produced by Gast.
Air movement, whether cold or hot, is the greatest contributor to drying times and controlling relative temperature and humidity before and after print.
Better, Faster, Stronger, Drier
Drying times affect your bottom line in big ways. From saving runs from being trashed due to unwanted ink transfer or getting more runs through the door faster, managing dry times in offset printing is no small thing. It’s also no small feat.
RG Group knows this, and we’ve spent over 60 years collaborating with customers, like you, on pneumatic, electromechanical, hydraulic, and process technologies. We carry only quality products from trusted manufacturers, like Gast.
Our business is keeping your business moving forward with innovation and profit. We do this with passion by acting as your partner with value-added services such as training, repair, installation, and maintenance programs. These
Want to know more about how RG Group can help you? Get in touch today to see how RG Group is here for you!