Establishing Process Control with Direct Print Corrugated

With customers demanding consistently higher-quality print as well as tighter tolerances, the flexographic print industry has been challenged. However, we have accepted this challenge and stepped up to the plate (no pun intended).

New developments and technologies with prepress, plates, inks, substrates and presses have all contributed to our successes. Even with the latest and greatest technologies, process control continues to be the make or break for flexography. A good process control system gives printers the opportunity to satisfy a customer’s demand for consistent products day after day.

Direct print corrugated introduces its own unique challenges with harsh printing environments, inconsistent/compressible board, large widths/repeats and soft, thick plates, along with the numerous mechanical challenges that come with flexography. In order to keep this to an article and not a book, let’s break down process control into four categories: materials, maintenance, control targets and training.

Materials

Before we look into anything else, we need to make sure we have consistent materials being used on press. There are three main materials that have the greatest impact on a print: substrate, ink and plate.

One of the biggest struggles for direct print corrugated is the appearance of fluting, as seen in Figure 1. Boxes need to be strong in order to protect the product inside, so the flutes are critical. Unfortunately, these flutes become an eyesore when printing halftone dots. Dot gain varies with the valleys and peaks of these flutes as the plate comes in contact with the board. This variation in dot gain gives us the fluting appearance that can take away from the intended communication of the graphics. Precise impression and optimized flat top dot plates will help reduce fluting, but having a consistent and smooth board is paramount.

[perfectpullquote align=”right” cite=”” link=”” color=”#00FFFF” class=”” size=””]Direct print corrugated brings its own unique challenges, but with the proper training, wise use of control targets, habitual maintenance and consistent incoming materials, you can achieve great results.[/perfectpullquote]

Out of the three ink types, water-based is the most difficult to work with, from my experience. It becomes even more difficult when the environment it is used in is constantly changing. The surfaces of corrugated board (clay coated, mottled white, #1 white, #3 white, kraft, brown) are normally not the smoothest print surfaces and vary in absorption characteristics. Therefore, they require thicker ink films to cover absorbent substrates and thinner ink films to cover nonabsorbent substrates. Inks need to be formulated to achieve the desired strength required with the anilox roll used, while still drying at a rapid rate and achieving the customer’s end use requirements. Inks need to be color matched before hitting the press, so the only variables press operators are expected to adjust are pH and viscosity.

Plates are another material component that need to be consistent by the time they hit the press. With most corrugated plates coming from their makers, it is important to verify you have the right example, with the correct prepress applied and mounted on the correct carrier. Using a benchtop micrometer to measure the plate package caliper ensures the plate is correct for your press undercut and has the proper relief for your process. The plate supplier should be using plate making control targets to ensure the proper bump and press curve are applied to your file, as well as making sure its plate making process is in control.

Maintenance

Now that we have consistent incoming materials needed to run the press, it is vital to determine the press itself is in a condition suitable for running consistent product. Maintenance schedules recommended by your OEM are always the best starting point. It is important to stay on top of maintenance even when you feel like the timing isn’t best. Downtime from broken equipment is always worse and longer than downtime for scheduled upkeep—maintenance that can prevent breakdowns. Both scheduled maintenance and autonomous upkeep habits are important expectations to instill in any printing culture.

Since most direct print corrugated presses do not have a quick and easy method for removing anilox rollers, their cleaning and inspection are more difficult, but still need to be accomplished. An easy way to check for a damaged anilox roll or metering issue is to run a 100 percent solid coverage plate that is larger than the repeat of the anilox. Any damage to the anilox will be visible in the printed solid, as seen in Figure 2. Establishing a scheduled doctor blade change is also important for consistent ink metering, as well as preventing damage to the anilox.

As with all printing methods, cleanliness is very important. Direct print corrugated is no exception to that fact, even though the environment can be more challenging. The key to keeping a printing press clean is quite simple: Develop good habits and schedule time to clean. Treat cleanliness as a maintenance item and stay on top of it daily. Water-based inks do not like to rewet without the use of harsh chemicals and force, so cleaning them while wet is much easier than after they have dried. Work with your ink, anilox and plate suppliers to find an appropriate press wash for your specific environment.

Control Targets

As stated in Flexographic Image Reproduction Specifications & Tolerances (FIRST) 6.0, “It is impossible to control what is not measured.” In order to make sure the print is in control, you need to have a control target that gives the operator feedback. While some targets give visible feedback, others require measurement devices such as a densitometer or spectrophotometer. Direct print corrugated printing requires a few additional targets that are not normal for other flexographic printing.

Slur and impression targets are necessary to gauge impression on press. Both the hexagon and flower targets are common, with the hexagon giving the most feedback. When the hourglass pattern appears, this tells the operator there is excessive plate-to-substrate impression. When the bow tie pattern appears, this tells the operator there is excessive anilox-to-plate impression. It is important to note that appearance changes in slur targets can also be the result of numerous issues, such as improper ink metering, high ink viscosities, a loose plate, etc.

However, most corrugated printers use a stepped impression target since the hexagon slur target is best used for fine-tuning impression. Using liquid polymer to make thick plates gives the ability to create this stepped impression target slug. This target will have stepped print heights in increments of 0.002-in. to 0.005-in. that will only print when over-impressed that amount in both anilox to plate and plate to substrate. The step printing at perfect impression would read 0.0-in., where the step printing at 0.010-in. over-impressed would read 0.010-in. Ideally, you would want 0.0-in. impression, but this is not practical in some direct print applications.

It is imperative press operators are trained to not try to get the entire impression gauge to print when setting impression. This instantly equates to an over-impressed plate and premature wear on the impression gauge, which then leads an untrained operator to impress even more to get the worn down target to print, thus over-impressing the plate even more.

Railroad Track Targets as seen in Figure 3 are commonly used for corrugated printing, since they give the operator the exact amount of misregister and an easy visual to correct. This reduces the amount of adjustment needed to get the print in register and also minimizes the need to constantly use a tape measure during a pressrun to maintain register. The scale and tolerance of these targets should be determined in the fingerprint press trial and will often need adjusting to get a scale properly built for your printing environment.

Measuring tone scales used in process control targets for corrugated printing ideally requires larger patch sizes and devices with larger apertures. As mentioned earlier, the flutes cause inconsistent dot gain depending on whether the dots find themselves on a flute tip or flute valley. Because of this periodic high and low, you need to capture a larger area to achieve more usable data. It is recommended to have solids and tone scales two-to-three-flutes wide for the most accurate measurement. Densitometers and spectrophotometers will work best with either 0.157-in. or 0.236-in. aperture sizes on corrugated. The larger aperture allows the device to measure more surface area for a more accurate measurement across flutes.

Ink trap targets are also very important to use in direct print corrugated. When using larger volume (bcm) anilox rollers or printing on nonabsorbent substrates with water-based inks, the wet ink traps often behave poorly. The print sequence, drying rate of ink, ink pH/viscosity, dryer settings and press speed can all have an effect on the wet ink trap. In order to produce the best quality print, you need to ensure that the ink film is transferring to the substrate, laying down smoothly and drying even when printed on top of other inks.

Training

Before knowing how to evaluate and measure control targets, the press first must be set up correctly to print the first sheets to pull. Every press variable and job component needs to be clearly defined in the job recipe/ticket. This ensures the job is set up in a repeatable manner. Also included in this job recipe is that specific job’s targets and tolerances.

[perfectpullquote align=”left” cite=”” link=”” color=”#00FFFF” class=”” size=””]Even with the latest and greatest technologies, process control continues to be the make or break for flexography.[/perfectpullquote]

Even after having the correct control targets printed for operators to use, if they do not understand how to measure or evaluate the targets, then it was a waste to print them. Thorough and consistent training for all operators is important, especially when it comes to how they will use control targets. For instance, there are many variables that can affect physical dot gain on press and the operator should be educated on these. The operator should be trained to work through a list of possible reasons why the 50 percent control target is measuring 82 percent when the target was 72 percent. Many times, there are time-consuming press changes happening that are not necessary when all that is needed is to adjust ink viscosity into tolerance.

Every manufacturing process has targets and tolerances. It is vital these targets and tolerances are specified and easily understandable. Most of the main components to the flexographic print process can have a measurable number applied to them, which needs to be communicated clearly. These numbers need to be included in each job recipe or ticket. Below is a basis list of these numbers for direct print corrugated:

• Ink viscosity and pH
• Measured dot area of specific tint
• Solid ink density, color (L*a*b*, L*C*h°)
• White opacity
• Feeder setup
• Impression starting points

The blood of the flexographic print process is the ink. With water-based inks used in corrugated print, the pH and viscosity is expected to be adjusted by the press operator. The pH and viscosity should be measured and recorded on a scheduled basis to ensure it stays in control throughout the pressrun.

Foam or microfoam is common in direct print corrugated, so its cause and resolution need to be communicated to the operator. Water-based inks foam when air is pumped into them through the return. The most common way to reduce excessive foaming is to make sure the return line is submerged in the ink and not just dropping the ink from the top of the bucket. However, even when the pump and return are set up correctly, air can enter the ink system through the empty cells of the anilox as it rotates back to collect more ink. Defoamer is used to reduce foaming and should only be used in small amounts in a squirt bottle. Press operators should be trained on proper ink management and basic ink problem solving.

You Can Do It

As with any flexographic printing, process control is critical to produce repeatable and consistent products. Direct print corrugated brings its own unique challenges, but with the proper training, wise use of control targets, habitual maintenance and consistent incoming materials, you can achieve great results. Work with your suppliers to leverage their knowledge and resources to aid in establishing good process control. FTA also offers great resources through FIRST 6.0, online training and FIRST Individual Certifications that will educate operators on how to establish and maintain process control through every step of your process.

About the Author: Brad Gasque graduated in 2011 from Clemson University with a BS in graphic communications, and then was hired at Clemson’s Sonoco Institute of Packaging Design and Graphics.  During his four years at Clemson, he managed the Advanced Print Technologies Lab as well as R&D projects, and taught university classes and industry seminars.

Brad joined DuPont Advanced Printing in 2015 as part of the Technical Service Team. He supports sales representatives and customers with troubleshooting plate/print issues, performing audits, and teaching hands-on training.