Q&A: UV Inkjet vs. Traditional Analog Methods of Decorating
Engineered Printing Solutions
by Tim Scully
When comparing the differences between traditional printing methods – such as pad and screen printing – and digital inkjet printing, we like to use the analogy of comparing a 1969 Camaro to a 2014 Tesla. They're both cars, and they can both get you where you are going, but the methods of how they work and the cost of operation are quite different. Traditional printing always has been done with direct contact of the inks via a vehicle such as a pad, screen or blanket to the product being printed. Inkjet deposition is done through a non-contact form of printing with specified offsets of the heads to the substrate being printed, thus eliminating pads, screens, plates, type set and stamps.
Traditional forms of printing always have been a mechanical process, and inkjet is a computerized process. The biggest single advantage of the digital process is the ability to change from one graphic to the next with the click of a mouse. No longer is there the need to change screens, pads, plates and registers between the colors for each individual print process. For companies printing short runs or dynamic data, the times savings are dramatic. However, there also are advantages to traditional forms of printing.
Question: How does ink adhesion differ?
Answer: Traditional forms of printing have been around for years, so there is a vast historical knowledge base. Inks have been tested on different substrates, so we know what pre-treatments, type of inks, additives and post-curing processes need to be applied for different print scenarios. By contrast, UV-curable inkjet printing is rapidly developing, but with little historical knowledge.
The main concerns when using pad or screen printing inks have always been adhesion and drying time. Certain materials, such as olefins, generally require a pre-treatment to obtain the proper surface tension for adhesion. Occasionally, static and surface contamination can warrant some adjustments to the inks.
Unlike traditional pad/screen print inks, adhesion is not the only concern with the UV-cure digital inks; wettability (dyne level) plays a big role in obtaining a quality print. The wet out of the print is what provides a clear, smooth, quality image. If the printed dots encounter surface tension, static or contamination, the dots will bead up and create a grainy image. Temperature is used to control inkjet ink viscosity versus analog inks where additives (thinners) are used. Environmental effects on all ink systems are a factor to consider, and humidity control and temperature play a vital role in print quality in both inkjet and other older technologies.
Question: Are there color limitations with digital inkjet?
Answer: When using digital CMYK Lc Lm inks, it is important to realize that there are limitations to color matching. Large brands have specifically formulated colors that represent their products. These are easily matched in traditional forms of printing by mixing inks to create a desired color. However, the use of custom colorants on digital devices is very rare because of their cost, the development time and the difficulty (if not impossibility) of changing colors and cleaning ink feed systems after use without contaminating the next color. It's been estimated that only 30 to 40 percent of Pantone colors can be achieved using a standard digital CMYK. That leaves a significant portion of brand colors outside of the achievable gamut, whether they're specified using Pantone or not. Most brand owners are aware of these limitations and understand that a perfect match for certain high chroma colors is unachievable and accept that close is close enough.
A so-called "HiFi" space using a larger number of colorants – such as CMYKOGV (i.e. CMYK + orange + green + violet) – greatly increases the color gamut; even this, it's still estimated that only around 80 percent of Pantone colors can be printed accurately in this way. However, these inks are very specialized and are not universally compatible with all printheads and print processes. Commonly available UV-curable CMYK ink sets that include light magenta and light cyan can achieve 55 to 65 percent of the Pantone Matching System (PMS) spectrum, but again, a six-color printer is significantly more expensive than a four-color printer (more heads, ink management modules, electronics, etc.). There are on-board color controls as part of the RIP package on some digital printing equipment that mimic the abilities of Photoshop, allowing for quick color adjustments on the fly. Once made, these tweaks are saved and retained for all future print jobs using that image data set.
Question: What are the differences in ink curing?
Answer: Although there are UV-cured inks in other print processes such as pad and screen printing, the majority of ink curing usually is done by running the items through an oven to complete the flashing off of solvents. Pad printing inks are printed wet on wet. The solvent, depending on the ink and print substrate, usually flashes off within seconds and can be handled almost immediately after printing. Originally, the UV digital inks were designed to be cured by high-pressure arc lamps. It was a complete process with cure time measured by the time exposed under the lamp. Cooling the bulbs always was a concern.
There were many positives realized with the development of the UV-LED inks. The cost of operation went way down, extravagant lamp cooling systems were eliminated and there no longer was ozone created by the high-pressure arc lamps, thus eliminating the need for ozone deconstruct systems built into the print exhaust. With the new curing lamps, there also were challenges that had to be overcome. The UV-LED lamps remain constant, so there is no longer a concern if it is properly curing the ink. The newer lamps bring new concerns, including dialing in the correct speed with specific substrates and setting the optimum power levels of the lamps. The offset of the lamps to the product all have to be tested and formulated to ensure proper wet-out, adhesion and curing.
Question: How does substrate shapes affect print quality?
Answer: Pad printing has been the ideal solution for printing on odd-shaped items with compound curves. The transfer of the ink image with the silicone pad has been a proven method for decades. Accuracy and repeatability of the print is excellent. Even difficult images on extreme shapes can be adjusted with artwork distortions so that when the print is transferred, it adjusts the distortion created by the stretch of the pad. Opacity is usually excellent, even on dark substrates. Plates can be etched to different depths to allow for transfer of additional ink, and different durometer pads can be selected to achieve an improved image transfer.
Digital inkjet differs because there are limitations to the amount of offset the heads can be from the print surface. The further the drop of ink has to travel, the lower the print resolution will appear. The ideal offset for printing with most digital heads is a maximum 1.4 millimeters from head to print surface. The size of the dot, resolution, time of flight of the ink and travel speed are all taken into consideration when printing on a slightly curved surface. Inkjet systems originally were designed to print on a smooth flat surface. With adjustment to the speed of print, UVLED curing speeds and electronic settings and by isolating air turbulence, the ability to print on curved surfaces with greater offset distances continues to evolve.
Question: How do the consumable costs compare?
Answer: When potential customers ask us how much it costs per part for the decoration, calculations are done based on consumable costs. In pad printing, take the cost of ink used for a specific length run; factor in waste, and determine the per-piece cost for ink. The pad usage also is easily calculated by dividing cost against the realized usage life. We also can calculate the operation costs based on parts produced per hour and include setup time, average time of change over from job to job and clean up.
When calculating costs for the digital inkjet system, look mainly at ink consumption. The PC running the system automatically will calculate the ink consumption and provide a per-piece ink cost. There is virtually no changeover time involved going from print job to print job. The completed artwork can be sent to the system using a USB drive or directly via LAN. Remember, this all makes servicing via the Internet an excellent alternative to flying in a technician.
Question: What questions can be asked to help make a decision between analog and digital inkjet systems?
Answer: When faced with a decision, evaluate the pros and cons of traditional analog systems and new digital inkjet systems.
- How often does graphic change?
- What is the required production rate?
- What are the color requirements?
- What is the cost of the equipment?
- What is the potential value added with the personalization and/or traceability that can be realized with new digital inkjet equipment?
- What type of personnel do you have at your disposal?
- How much are you spending on expensive supplies?