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Twenty Questions and Answers about UV Curing and Related Concerns

Trying to adjust to the realities of working with UV screen-printing inks? Use this Q&A discussion to clear up any misunderstanding about the inks, the curing process, and other aspects of UV technology.

Whether your shop is just starting to work with UV inks or you need to train new employees in issues surrounding the technology, the information provided here can shorten the learning curve. Read on to get answers to 20 frequently asked questions related to UV inks and their use.

What is the relationship between the watts per inch setting on the curing-unit lamp and the watts or milliwatts per square centimeter reading that I get from a radiometer?

Watts per inch is a unit of power (wattage) for the lamp in the curing unit. This is based on Ohm’s Law: volts x amps = watts. Watts or milliwatts per square centimeter represent the amount of peak irradiance (UV energy) per unit area measured by the radiometer as it passes underneath the lamp (Figure 1).

Peak irradiance is determined, among other factors, by the wattage of the lamp. Peak irradiance is measured in watts because irradiance represents UV energy or power. Lamp power also is measured in watts because this represents the electrical energy the lamp consumes. In addition to the amount of electricity the curing unit receives, other factors affecting peak irradiance are the condition and geometry of the reflector, age of the lamp, and distance from the lamp to the curing surface.

What is the difference between millijoules and milliwatts?

The total amount of energy arriving at a given surface over time is measured in joules or millijoules per square centimeter. The total energy is affected by the conveyor speed, lamp power and number of lamps, age and condition of the lamps, and the geometry and condition of the curing system’s reflector.

The power of the UV energy or irradiance arriving on a surface is measured in watts or milliwatts per square centimeter. Higher UV energy arriving on the surface allows a higher amount of energy to penetrate the ink film. Milliwatts and millijoules must be measured in context with the wavelength sensitivity of the measuring radiometer.

How can I make sure that UV ink is properly cured?

Properly curing the ink film the first time through the curing unit is very important. Proper curing minimizes substrate degradation, over curing, rewetting, and under curing and optimizes ink-to-ink or intercoat adhesion.

For screen printers, determining production parameters is a must prior to any production run. To test curing effectiveness prior to production, use the production screen, equipment, and ink and start with the lowest possible conveyor speed that the substrate can take, then print and cure one or two good pieces. Set the lamp or lamps at the wattage specified by the ink manufacturer. For slower curing colors, such as black and white, set the lamps on high.

After allowing the print to cool, test for adhesion by using the cross-hatch tape method. If the sample passes, increase the conveyor speed by 10 ft/min, then print, cure and test adhesion again. Keep increasing the conveyor speed at 10 ft/min increments until the adhesion begins to fail. This normally would show as the ink coming off around 5-15% of the cross-hatched area. This is the marginal failure point of the ink film at the particular conveyor speed and lamp setting you used. To set production speed, decrease the conveyor speed 20-30% from the marginal failure point, depending on the ink system, or follow the recommendations of your ink supplier.

Should I be concerned about over curing if the colors do not overlap?

Over curing happens when the surface of an ink film is over exposed to UV energy. The surface becomes harder and harder with continued exposure and, depending on the ink system, this hardening may happen after just one pass or several passes under the curing unit. Over curing is not so much of a concern as long as printed colors do not overlap.

However, a major factor to take in-to consideration is the film or substrate being printed. UV energy affects most surfaces and certain plastics are sensitive to specific wavelengths of UV energy. This sensitivity to certain wavelengths in combination with oxygen in the air can cause degradation on the plastic’s surface. The molecular bonds on the surface may be broken, leading to adhesion failure when UV ink is printed and cured on the material. This degradation of the surface normally happens gradually and is directly related to the amount of UV energy the surface receives. The degradation on the substrate’s surface may reach a point at which ink adhesion is compromised.

What happens when the ink is over cured?

One theory regarding over curing is that the ink’s surface becomes harder and harder (more cross-linked) with subsequent exposure to UV energy. The surface becomes so hard that when another layer of ink is printed and cured over the previous layer, the top layer would experience poor adhesion to the bottom layer. Another theory is the possibility of photo-oxidation on the ink’s surface. Photo-oxidation occurs when UV radiation, in the presence of oxygen, breaks down the chemical bonds on the surface of the ink film. If the molecular bonds on the surface are degraded or broken, adhesion of another layer of ink would be compromised. Over cured ink film may become less flexible and substrate embrittlement also may occur.

Why do some UV inks cure faster than others?

UV inks are formulated to adhere to specific substrates and to meet other specifications required by the application. In UV chemistry, the faster curing an ink is, the less flexible it is once cured. Imagine the ink molecules connecting and forming chains with each other (cross linking) during the curing process until all the molecules are used up. Now imagine these chains of ink molecules with many, many branches, but these branch-es are fairly short. This type of ink would be faster curing but not very flexible. Now imagine a very, very long chain of molecules with very few or no branches. This type of ink would be slower curing but flexible.

Most UV chemistry is optimized for certain applications. For example, an ink that is formulated for membrane-switch overlays would need to adhere to the various substrates used for membrane overlays, including polycarbonate, primed polyester, and sometimes vinyl. In addition, the cured ink film must be compatible with laminating adhesives and be flexible enough to die cut and emboss. Also, the chemistry must such that the ink will not react with the substrate’s surface, which could lead to cracking, shattering, or delamination. Such inks are typically slower curing. In contrast, an ink that is formulated for cardstock or rigid plastics for point-of-purchase displays must adhere to these substrates, but may not need to be very flexible. Inks for these applications are generally faster curing.

Regardless of whether the ink is fast or slow curing, the important point to consider is the end application. An-other important factor to keep in mind is the curing equipment. The ink may be fast curing, but if the curing equipment lacks the efficiency to cure, the ink will be slow curing or worse, will not cure at all.

Why does polycarbonate film become yellowish when I use UV inks?

Polycarbonate is sensitive to UV wavelengths of less than 320 nanometers. The yellowing on the surface is caused by the breakage of the molecular bonds caused by photo-oxidation. UV energy is absorbed by the molecular bonds of the plastic and generates radicals. The radicals react with oxygen in the atmosphere to generate appearance and physical changes within the plastic.

How can the yellowing on polycarbonate’s surface be avoided or even eliminated?

If UV inks are used to print polycarbonate, the yellowing on the material’s surface may be minimized but may not be completely avoided. The use of additive curing unit bulbs, such as iron and gallium, has been proven to minimize this yellowing. These bulbs emit less UV energy in the shorter wavelengths detrimental to polycarbonate. In addition, curing each ink color properly to minimize exposure of the substrate to UV energy and keeping the curing unit well maintained will go a long way in reducing surface degradation of polycarbonate materials.

Are UV inks considered green, and if so, why?

UV inks are green compared to solvent-based inks. UV-curable inks are consider-ed to be 100% solids, meaning that just about everything in the ink is reacted into the final ink film. Solvent-based inks evaporate solvents into the atmosphere as the ink dries. Solvents are considered volatile organic compounds and not environmentally friendly.

What is the unit of measure for the density numbers a densitometer provides?

Optical density numbers are unit-less. Densitometers measure the amount of light reflected or transmitted by a printed substrate (Figure 2). An electronic photocell eye is connected to a meter that mathematically converts percentage of reflected or transmitted light into logarithmic density values. The use of logarithmic scale compresses large measurement numbers into small and manageable values. The basic mathematical formulas are as follows.

What factors affect density?

In screen printing, variables that can greatly affect density values are ink-film thickness (determined by mesh, tension, off-contact, and floodbar and squeegee settings), color, size and number of pigment particles in the ink, and the color of the substrate. Optical density combines the opacity and thickness of the ink film. Opacity is affected by the number and size of the pigment particles and their light-absorbing and light-scattering characteristics.

What is dyne level?

Dyne per centimeter is a unit of mea-surement used to quantify the amount of force created on a surface. The force is caused by the intermolecular attraction of a particular liquid (surface tension) or solid (surface energy). For the sake of practicality, this measurement is referred to as the dyne level.

The dyne level or surface energy of a particular substrate is an indication of the material’s wettability and ink adhesion (Figure 3). Surface energy is a physical property of a substance. Many films and other substrates used in the printing industry have very low dyne levels, such as polyethylene with 31 dynes/cm and polypropylene with 29 dynes/cm, and need to be treated.

Treatment raises the dyne level of a particular film, but only temporarily. So factors such as the amount and time since initial treatment, handling, storage, humidity, dust, and contamination will affect the dyne level when you’re ready to print. Since dyne levels change with time, most printers find it necessary to treat or retreat these films immediately before printing.

How does flame treatment work?

Plastics, by their very nature, are non-polar and have essentially inert surfaces (low surface energy). Flame treatment is one of several methods of pre-treating plastics in order to raise the dyne level of the surface (Figure 4). This method is commonly used in the bottle-printing industry but is also used in the automotive and film-converting industries. In addition to raising the surface energy, flame treatment also helps eliminate surface contamination.

In flame treatment, the high temperature effects changes on the surface of the substrate by oxidation and breaking down hydrocarbons (from propane or natural gas) to free radicals, ions, and other charged particles. This changes the polarity of the plastic’s surface, which enhances ink adhesion and wettability.

What is corona treatment?

Corona discharge is another method for treating plastics to increase dyne level. Ionized air is created by running high voltage through a dielectric covered roll. The substrate is passed through the ionized space, which causes molecular bonds to break on the material’s surface. Treatment is affected by the air gap distance to the part and the speed of the material as it passes through the system. This method of treatment generally takes just fractions of a second. Corona treatment is commonly used on thin films for web printing applications.

How do plasticizers affect ink adhesion on PVC?

Plasticizers are chemicals that are added to plastics during processing to make the finished material soft and flexible. Plasticizers are especially prevalent in polyvinyl chloride (PVC). The type and amount of plasticizer added to flexible PVC or any plastic material depend on the mechanical, thermal, and electrical properties expected from the plastic.

Plasticizers have the tendency to migrate to the surface and affect ink adhesion. Plasticizers on the surface are contaminants that have the ability to lower the surface energy of the surface. The more contamination, the lower the surface energy and greater the risk of poor ink adhesion. Substrates affected by this problem can be cleaned with a mild solvent before production to improve their printability.

How many lamps do I need for curing?

Depending on the ink system and substrates to be printed, a one lamp curing system may be sufficient. However, if the budget allows, a two-lamp curing unit will provide the benefit of a faster cure, especially with slower-curing colors, and it will allow for faster overall production rates. Two lamps are always better than one because a two-lamp system would provide more millijoules or total energy at the same conveyor speed and lamp settings. The main factor to be taken into consideration is whether the curing unit will be efficient enough to cure the inks on any substrate at reasonable production speeds.

How does the viscosity of the ink affect printability?

Most inks are thixotropic. This means that the viscosity changes with applied shear (stirring, floodbar and squeegee action), time, and temperature. Viscosity also decreases with increase in the rate of shear (e.g., speed of floodbar or squeegee). When the temperature gets warmer, the viscosity becomes lower as well. Screen printing inks are formulated to print well on press. However, depending on the press set up and prepress modifications to the ink, printability issues may occur. The viscosity of the ink on press will not be the same as it was in the container.

Ink manufacturers determine a specific range of acceptable viscosities for a particular product. Viscosity measurement is controlled within a specific range of shear force, time, and temperature. With thinner or low-viscosity inks, thickening agents may be added. For thicker or high-viscosity inks, thinners may be added. Contact your ink supplier for specific products and processing information.

What affects the stability or shelf life of UV inks?

One factor that influences stability is ink storage. UV inks normally are packaged in plastic containers instead of metal because plastic containers are permeable to oxygen. Also, an air gap is desirable between the surface of the ink and the container lid. This air gap—specifically, the oxygen in the air—helps in minimizing any premature cross linking within the ink. In addition to the packaging, the temperature at which ink containers are stored plays a big part in maintaining their stability. High temperatures will cause the ink to react and cross link prematurely.

Modifications made to the original ink formula may also affect shelf stability if the remaining ink is stored. Additives, especially catalysts and photoinitiators, may lead to more limited shelf life. Finally, exposure to stray UV light from windows and lighting when a container is left open can affect stability.

What is the difference between in-mold labeling (IML) and in-mold decorating (IMD)?

The basic principles behind IML and IMD are the same. A decorated film is placed into an injection mold and then melted plastic resin is injected to form a part with specific geometry such as an ice cream container or cellular phone cover (Figure 5). In IML, the labels may be printed using various printing technologies such as gravure, offset, flexography, or rotary or flat screen printing. These labels are generally printed on the top surface so the unprinted side would come in contact with the injected resin. The injected resin would normally be the same type of plastic as the label material or a compatible substitute.

In IMD, which is used to create more durable parts, the printing generally is done on the second surface of transparent films. Screen printing is the printing technology of choice for these applications. Compatibility of the film and a UV ink to the injection resin is a must. In this sector, IMD typically means in-mold or insert-mold decorating.

What happens if I use a nitrogen curing unit to cure colored UV inks?

Curing systems that cure prints in a ni-trogen atmosphere have been around for decades. These systems are mainly used for texturing and curing clearcoats printed on the top surface of membrane-switch overlays. The main purpose of the nitrogen is to replace oxygen because oxygen is a cure inhibitor. However, due to the very low irradiance from the bulbs in the nitrogen chamber, pigmented or colored inks may not cure very well in these systems.

Most nitrogen systems still being used have a texturing or germicidal lamp installed before the nitrogen chamber. As its name implies, the main function of the texturing bulb is to create a texture. Clear UV inks used for this application are formulated with photoinitiators that absorb in the shorter wavelengths. When these clears are exposed to the texturing lamp, the surface cures faster than lower levels of the ink film, which causes the ink surface to wrinkle and become textured. Depending on the chemistry, colored UV inks may also texture when they’re exposed to a texturing lamp.

Getting more answers

Beyond the questions addressed here, UV ink technology has many other unique characteristics that can pose challenges to screen printers. Your best resource in overcoming ink performance issues and finding the right formulation for particular applications is your ink manufacturer. Take advantage of the company’s knowledge and expertise, and your presses will keep churning out high-quality UV prints.

 

Bea Purcell is Nazdar’s market-segment manager for the membrane-switch overlay, in-mold decorating, industrial, and container markets. She has extensive experience in formulating UV inks, technical support, training, and sales in UV-curing technology. Purcell holds a bachelors degree in chemical engineering from the University of San Carlos and a masters degree in education from the University of Southern California. She also is a member of the SGIA Membrane Switch Council.

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