UV Decorating and Color Management
UV BasicsUV inks consist of oligomers (resins), monomers (thin, reactive liquids), pigments and photoinitiators. After exposure to UV lamps, the photoinitiators absorb the UV energy and start a cross-linking chemical reaction between the oligomers and monomers that results almost instantly in a cured ink film. These systems are regarded as 100% solids because virtually nothing evaporates from UV inks. As a result, printed parts can be handled immediately after curing, so no time is wasted waiting for prints to dry. Other benefits of UV inks include:
- Color stability before and after curing. Because UV inks aren’t subjected to high temperatures, colors are consistent before and after curing. The inks don’t need to be fired and cooled to view the final color.
- An enhanced color gamut. Today’s ink suppliers select pigments that will enable printers to match a wide range of colors.
- Heavy metal free. UV inks are typically made with organic pigments, which do not contain heavy metals such as lead or cadmium.
- Good package stability. When UV screen inks were first introduced over 20 years ago, they had limited package stability. It wasn’t unusual to get just six months of usable life from the ink. However, UV chemistry has improved over the years, and today’s UV inks can have a usable life of 24 months or more.
- Enhanced color stability on press. Because UV inks are typically 100% solids systems, ink color stays consistent during a print run. Conventional solvent-based inks experience solvent evaporation that can have an effect on the final color.
- Metallic and special effects. Because high temperatures aren’t needed to cure UV inks, a wide range of pigments—including fluorescents, phosphorescents, metallics and thermochromatics—can be used to create special effects.
Turning Color into NumbersThree types of equipment generally used to measure color are densitometers, colorimeters and spectrophotometers. While each type varies in capabilities, accuracy and cost, they all quantify color, removing subjectivity.
Densitometers. These instruments simply measure the amount of light absorbed by color and are therefore good tools to measure relative strength differences between colors. The most common use for densitometers is in four-color process printing, when a printed color is measured against the target color-key. Chromaticity (color strength) in process printing is vital where the goal is to accurately reproduce a desired image. However, these instruments aren’t designed to measure differences in hue (color shade).
Colorimeters. These instruments measure reflected light through a series of filters and convert color into mathematical color space coordinates such as CIELAB. From there, color differences versus target colors can be assessed numerically. There are limitations to the accuracy of this equipment because of the filters and illuminants (light sources) employed. A typical use for colorimeters would be as a basic quality control tool to compare colors.
Spectrophotometers. The most sophisticated, accurate (and costly) of color measurement equipment, spectrophotometers use complex mathematical equations in conjunction with software packages to obtain formula predictions for new matches, correct matches when they need adjustment, and compensate for substrate color and ink deposit variables (see Figure 1). These systems can be obtained as bench-top models or hand-held units. QC software programs store batch data, as well as numerical pass-fail tolerances that can be customized to individual requirements.
In the End, The “Eyes” Have ItColor measurement equipment has been widely used for many years across various markets segments, and the equipment and associated software packages have become increasingly more accurate and versatile. However, while all of this equipment has merit and can be extremely useful when used properly, the human eye still plays the most important role. Numerical color tolerance doesn’t always carry a lot of weight with customers. Have you ever tried to tell a customer that a color is within a numerical specification, but they still aren’t happy with the visual difference they see? For this reason, communicating and agreeing on color tolerances before a project begins is key. The color measurement equipment can then be used throughout the process to ensure that the color continues to stay within the specified tolerances.
We’ve Matched a Color—Now What?Matching a color is certainly an important part of the printing process, but how do you reproduce that color when you need it again? How do you keep track of ink inventory? How do you schedule what colors need to be made or estimate how much ink you need? All of these questions directly relate to color management, also known as ink room management.
Consider the screen printing process—we often use automatic equipment to coat and reclaim screens, and we use fully automatic presses, but the ink room is usually the last place we consider for automation. Recently, however, “smart scales” and automatic ink dispensers have been successfully introduced into the screen printing market. Both tools are designed to streamline color management.
Programmable scales or “smart scales” are controlled by software designed with various features, including:
- Electronic storage of formulas. Users can enter formulas into the software so they can be reproduced at a later date.
- Formula editing. Taking into account all the variables in the screen printing process, a color formula to simulate Pantone 185 red, for example, might work on one press through one mesh but won’t match under a another set of conditions. In these cases, the user can edit the existing formula in the software or store a new formula for those conditions.
- Inventory management. After entering an ink supplier’s base colors into the system, the user can enter current inventory levels, set minimum inventory levels, and enter cost and mileage data. This data can then be used to determine the cost of an ink blend and can also act as an early warning system for low ink inventory.
- Ink quantity estimation. Using data such as print quantity, print size, ink mileage, mesh count and percent coverage, the software calculates how much ink is required for a given job.
- Mathematical rework of formulas. If a printer makes an overage of a specific color, it can be entered into inventory and the system can be asked to use the color in another formula, thereby reducing the amount of blended colors in inventory.
Automatic dispensers aren’t new to the paint, paste ink and liquid ink industries, but they are relatively new (within the last seven years) to screen printing. These devices are controlled by software with the same features as the programmable scales. Gravimetric dispensers, which dispense by weight rather than by volume, can be accurate to 0.1 gram. Dispensers can dispense one gallon of UV screen ink in three to five minutes, and dispensed sizes can range from 1 pint to 5 gallons, depending on the printer’s requirements.
The biggest benefit of automatic dispensers is typically cost savings. Because the systems can dispense color on demand in three to five minutes, they significantly reduce the required number of pre-blended inks in a print shop, thereby providing a savings in inventory. Labor savings can also be achieved when comparing the automatic dispense time to the time it would take to manually blend the same color.