A well-known keyboard manufacturer has done this test: printing a keyboard with a plain gravure printing ink (epoxy resin/clear curing agent, also called two-component ink) and printing a keyboard with UV ink (without a transparent curing agent) In the test, a “mechanical†finger was used to repeatedly simulate the keystroke effect of a human finger on the keyboard on the keyboard, and it was determined how many keystroke frictions the seal on the keyboard can withstand.
In this abrasion resistance test, the pad printed with ordinary inks began to fall off after it took about 100 to 200 million keystroke rubs, while the UV ink printed keyboard could withstand 12 million keystroke rubs. The abrasion resistance of UV ink prints is not only more than ten times that of the epoxy resin/clear curing agent, but also the print drying cycle is reduced from the original 7 minutes/component to 15 seconds/component, and the time is reduced by 96%.
UV inks can also provide excellent solvent resistance after curing. This is partly due to the high cross-link density (high molecular weight) of the ink film after UV ink curing, and the large degree of cross-linking between molecules, so that between the molecules does not Easy to separate, the ink film will not dissolve in the solvent.
For comparison purposes, let's take a look at the changes that have occurred since the transition from two-component (two-component) epoxy inks to UV inks.
1. No need to dispense ink before using ink (unless it is necessary to match colors); naturally, long-term curing cycle of two-component ink is replaced by curing cycle of UV ink curing almost instantaneously, which of course will save a lot of time; At the same time, there is also a benefit to reduce the possibility of damage to the substrate due to heat; the space required for the UV curing system is only a fraction of the space occupied by drying equipment for conventional solvent-based inks.
2. The two-component epoxy resin ink has only a short "activation period" after mixing. After staying on the printing machine for 4 hours, the ink will be stuck in the ink hole of the printing machine. This will delay the printing production because it will stop. Remove the ink in the ink hole. The “activation period†of UV inks is virtually unlimited, and printing is sometimes stopped just to adjust the amount of solvent in the ink, but it does not affect print production. Table 1 shows the performance and cost comparison of UV inks and two-pack epoxy inks.
Of course, there are still some issues to consider when it comes to UV ink pad printing. For example, it will face new costs, UV light curing systems, and UV measurement equipment. In the following sections we will discuss this issue. If the substrate is a low surface energy material such as polyethylene (PE), polypropylene (PP), etc., also need to consider the flame treatment or corona treatment to ensure that the substrate and UV ink can have sufficient adhesion . It may also be necessary to add some new print quality control measures to ensure the quality of UV ink prints to meet the printing customers' requirements.
Table 1 Comparison of UV gravure printing ink and solvent type gravure printing ink
The photocuring of gravure printing UV inks requires that the gravure pad printing images are generally smaller than the screen printing images, which means that the gravure printing UV light curing device is smaller than the typical screen printing UV light curing device. In most cases, the curing system has a protective cover to protect the UV lamp and reflector assembly.
If the facility already has UV-curing equipment that supports screen printing, it may also be suitable for curing drying of gravure printing jobs. Just make sure it provides the correct UV lamp and UV wavelength output characteristics. Many gravure printing inks can be cured with a medium pressure mercury lamp or halogen lamps. The input power of the UV lamp must be at least 300 watts in order for most of the input energy to output UV light at wavelengths of 365 nm and 254 nm. The reflector can be used to focus and disperse the energy of the UV lamp. For substrates with poor heat resistance, dichroic mirrors that absorb infrared light are recommended.
Depending on the power of the UV lamp, it may be necessary to adjust the speed of the curing unit belt (transmitting the print) to ensure that the print is cured with sufficient UV light. If measured with a radiometer, the UV light energy provided by each UV-curing unit at a wavelength of 320-390 nm is about 2 J/cm2. If the substrate is capable of withstanding heat, the print is preheated for a period of 5 seconds prior to UV curing, which can further increase the cross-link curing speed of the ink. (To be continued)