UV丝网印刷油墨问题与解答（英文）（一）

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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.

1、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.

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2、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.

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3、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.

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4、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.

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5、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.

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6、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.

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