Interview by Jim Raymont, EIT
Editor’s Note: An excerpt from this in-depth interview with Nazdar’s team was published in Issue 3 2021 of UV+EB Technology. The full interview is found here.
Nazdar team members participated in a real-world roundtable discussion on UV inks and UV curing. This team has 163 years of ink, formulation, print and troubleshooting experience, and when they talk, I listen. Our discussion and their answers are reflected below.
How does Nazdar communicate cure specifications to its customers?
We almost exclusively use technical data sheets (TDS), which are posted on the website. The TDS for our ink lines designed for curing with broadband (mercury) sources provide very detailed specifications. The TDS for our ink lines designed for LED curing continuously are evolving and getting more detailed as LED inks are developed. We refer to manufacturer wattage ratings, band (wavelength) specifications and output energy recommended to cure an ink line. Just as it did for broadband (mercury) applications, it has taken time for both education and radiometers for LED cure applications to be widely available and accepted.
We initially communicated inkjet curing values by using general UVA values from our UV screen ink products (i.e., irradiance of 600 mW/cm² and energy density of 225 to 250 mJ/cm²). Users need to keep in mind that multiple passes are required with inkjet printing to obtain the desired energy density values.
We will work from the general to the specific for customer-based variables, such as the photoinitiator package. For example, we may revise a target UVA exposure to a minimum of 423 mW/cm² and 25 to 35 mJ/cm² for each pass of the UV source. This assumes that the customer has a radiometer with a fast enough sample rate to get the values from a single pass. Communication and establishing a relationship are key.
What challenges occur with a customer’s UV sources trying to meet ink specifications?
This answer depends on the type of print application.
- For broadband (mercury)-based screen print applications:
- Many screen printers have trouble meeting our minimum published requirements, even though they have some buffer built into them.
- Customers make do with multiple lamps, slower speeds, lesser ink deposit or … live with under-cured ink.
- There are multiple design considerations that go into a UV system to allow it produce stable, consistent UV energy. Screen printers are resourceful and self-reliant, and some have built their own curing units with a predictable wide range of results.
- Other challenges faced on a regular basis include:
- UV source-formulation mismatch (do not use “additive/doped” lamps to cure overprint clears)
- Availability of clean and consistent applied electrical power
- Proper preventive maintenance of the system, including cleanliness of lamp, reflector and cooling system; age and condition of the lamp; positioning of the lamp in relation to the reflector; height of lamp/reflector assembly above the substrate; condition of electrical components – capacitors; accuracy/repeatability of conveyor or scanning speed; and consistent deposit of ink.
- For broadband (mercury)-based flexography applications:
- The process is mature, and customers seem to be more familiar with both the process and required maintenance.
- When we do see problems, we look at the age of the bulbs, which can manifest itself with low cure at the edges of a wider print.
- Deeper through-cure can be a challenge when curing higher film thickness (e.g., heavier whites and coatings), particularly as speeds increase.
- Occasionally, very porous substrates contribute to cure challenges due to light scattering.
- For LED-based applications:
- LEDs can offer several advantages over broadband sources. These include better through-cure, higher ink film thicknesses, higher process speeds, less heat, lower energy cost-of-use, longer life and instant-on.
- With nearly every application, what can be done with UV also can be done as well or better with an LED.
- LED is a newer technology, especially in narrow-web applications, and there are unknowns compared to broadband technology.
- We have seen misconceptions and misunderstandings regarding LEDs that contribute to confusion among converter customers.
- Not all LEDs are created equal. We have seen “inexpensive” LED arrays provide acceptable cure performance, at least initially, before degrading rapidly in their output and ability to cure.
- With LEDs, the peak irradiance delivered (W/cm²) often is less critical than energy density (dose) delivered (J/cm²).
- For inkjet applications:
Many of the other items already mentioned apply to inkjet applications.
Additional challenges with inkjet applications include:
- Lack of UV measurement instruments (radiometers) by customers, field service technicians and even manufacturers that do not own or use them. They incorrectly believe that if the “light” is “on,” then the system is OK to use.
- Lack of a preventive maintenance program and schedule. This includes a lack of training, documented procedures/checklist and a backup plan when an employee is absent or leaves the company.
- Aggressive production schedule in which no time is set aside for preventive maintenance.
- Lack of knowledge and understanding of how the system works. What needs to be replaced beyond normal preventive maintenance items? How often? How does the applied electrical power impact the UV output on both broadband and LED sources?
What are some of the quality tests and checks that are performed to make sure an ink is formulated correctly and meets specifications batch to batch or lot to lot?
It depends on the printing technology and the end use application(s) for which the ink is designed. Many of the tests used to evaluate a batch of ink are universal, while others employed are specific to an ink type. Some or all the following tests are used to make sure an ink is formulated correctly:
- Ink tests: Viscosity, rheology, surface tension, filterability and check of the color.
- Print: Tests are performed to compare the ink to a known standard. This can include a split print: a side-by-side single-stroke print of the wet batch sample and wet QC standard sample. Inks are jetted and run through different print heads and cured under different sources and UV conditions, including speed.
- Post Printing: Once printed, the ink goes through additional QC tests that include checks of:
- Color: Visual appearance, dot gain blocking, color strength and hue.
- Physical: Pencil hardness, adhesion (standard thumb twist by pressing the thumb with force into the cured ink film to look for marring), gloss (look for poor curing clues) and smell (to detect un-cured monomer).
- Durability and stability: Tests at room temperature and in accelerated environmental or weathering chambers.
What is the process when customers contact you and state, “My ink is not curing”?
Customers often confuse an “adhesion” issue with an “ink” issue. They use an ink not designed for a particular substrate and then say the ink is not curing when it’s just not adhering. Adhesion also can be related to the surface treatment (or lack thereof) of the substrate.
When we ask if the customer measured the UV conditions (irradiance, W/cm² and energy density, J/cm²), most times we get a “no.” We go over items that need to be maintained on their UV systems. This includes the last date of maintenance and when cleaning, rotation and/or replacement last took place on the reflectors, bulbs and other items, like quartz plates.
Other questions we ask to clarify the “ink issue” include:
- Is the ink agitated or has it separated (i.e., whites)?
- Did you install the right ink?
- Do you have the correct lamps in the machine? This includes verifying the lamp type (mercury vs. mercury-gallium) and verifying if a replacement third-party lamp purchased for $5 less performs as expected.
- Is your color management controlled?
- Is your ink film too thick? If so, print a gradation and see where the adhesion fails.
We sometimes ask a customer to add a significant amount of a mixing clear, probably 20% by weight, and make a print using the same print setup. Is it curing now? If so, the problem is a lack of UV energy, and preventive maintenance needs to be done on the equipment.
What information and values do you like to see when troubleshooting issues?
The information we need to troubleshoot can have both shared and unique information based on the actual print application. We often start with a list of what has been tried before we were contacted. Was just one process variable changed at a time? If maintenance personnel are available, they often are a good place to start. They may not understand a particular printing process but they but they do understand machines and work with a root cause analysis mindset.
We like to confirm that the reflectors/quartz plates have been cleaned and new UV lamps have been installed and are operating at full output. Our wish list of information includes:
- Ink product code and lot number involved
- Printing parameters in terms of speed
- If screen print, we ask for information such as the mesh count and squeegee durometer
- Curing parameters: Lamp make, model, power rating (Watts per inch), power setting (%) and speed
- Substrate type and color
- Irradiance (W/cm²) and energy density (J/cm²) values, along with instrument information
- For flexo applications, radiochromic films/strips are sometimes used to get an energy reading
- Additives and amounts used
- With both UV and LED, we make sure customers are not attempting to run an excessively high anilox volume (particularly with dark colors) that can negatively influence cure.
- Substrate surface tension (dyne levels) or receptivity that can mimic cure issues through low adhesion
What are you seeing when comparing digital (inkjet) printing to screen printing?
Looking back just seven to eight years ago, it was unusual to visit a screen shop and see an inkjet printer. Today, we can’t name a large print shop that doesn’t have multiple large-format printers. Most small print shops are inkjet first because the entry level costs are low.
Both print technologies are needed, and each has its specific applications where one is more feasible than the other. Inkjet also is diverging into markets other than graphics.
Screen printing is a science, and it was developed by individuals over time. Most experienced screen printers have a good understanding of the different process variables that need to be controlled. Screen printers use process control tools, including radiometers, to measure the UV output of their curing units.
Digital (inkjet) is simpler, with production often starting very quickly. The employee operating the inkjet may not have as much training and the same level of understanding of the different process variables that can impact the final product as the screen printer. This lack of understanding includes the adoption of process control and measuring of the UV sources for inkjet printers. It would make our jobs much easier if a radiometer was sold with each digital printer equipped with a UV source.
What kinds of speeds are you seeing on digital printers? Where are they expected to go?
Inkjet speeds are becoming faster every year, with many systems capable of speeds of 250 to 300 feet per minute. This translates on some presses to an output of 4,000 square feet or more per hour. It is common to see high-speed flat bed or single-pass machines replacing screen inline and offset machines. The speed is not just how fast the media is moving. Other factors to consider include head height, carriage or bed speed and travel of the carriage.
The speed that print heads travel, combined with making multiple passes across the ink, requires not only a scanning (profiling) radiometer, but a radiometer that is able to scan at an effective rate of 2,000 readings per second to get the full reading of how much UV the ink actually is seeing, including an accurate peak irradiance.
The process speed of narrow web flexo also is increasing, with today’s presses capable of speeds of 400 to 600 feet per minute for UV broadband sources and 1,000 feet per minute for UV LEDs. Many of our converter customers are currently running water-based or energy cure ink flexo presses around 400 feet per minute. The bottleneck often is inline processing vs. mechanical capability.
Digital speeds are rapidly approaching the capability of flexo when considering inline converting. As the speed gap narrows and consumable costs become more comparable, digital will become more attractive to the narrow web market.
What changes do you expect for your business for the next five or 10 years?
As inkjet increases in all areas of printing, there will need to be a combination of things to get inkjet printers to understand and troubleshoot their UV output and curing issues. Our wish list includes:
- Thinner UV LED reading devices or strips that can be used at low print head heights.
- Onboard printer UV output monitoring devices built into the machines.
- Education for print operators and print quality control personnel as to the importance of UV monitoring.
The next five to 10 years likely will see UV LED inks surpass both water and broadband mercury inks as the dominant process in narrow web flexography. And, as digital speeds increase and costs decrease, we see digital technology becoming more dominant in the narrow web space, with flexo becoming more of a support process when digital falls short for specific applications using hybrid presses.
For more information, contact Jim Raymont, director of sales at EIT Instrument Markets, at [email protected] or any of the interviewees from Nazdar:
- Travis Barcelona, Inkjet Product Development Manager, [email protected]
- Steve Noland, Technical Service Representative, [email protected]
- Tony Parsons, OEM Technical Services Representative, [email protected]
- Russ Petty, R&D Screen Lab Manager, [email protected]
- Bruce Ridge, Director Technical Service, [email protected]