Corrugated Post-Print Best Practices

Today, all manufacturing industries face challenges that require changes from past business practices. Twenty, even ten, years ago the consumer was a passive actor who accepted market proposals. Now the end-user is an informed and sophisticated consumer who seeks differentiation, personalization. Today’s consumer has a clear understanding of the supply chain and the value it brings to key aspects of packaging, such as the environment and sustainability. Companies seek greater efficiencies and more productive use of resources. Because of this, Consumer Packaged Goods (CPG) companies must produce better packaging while controlling their manufacturing costs and minimizing time loss and materials.

Flexo's Evolution
The flexographic industry is no stranger to this “new” reality and strives to reduce response times and provide packaging with greater shelf impact at competitive prices. All this PLUS the trend towards shorter runs as CPGs require their brand identity be maintained regardless of the region, the printing method, or the substrate. Successful converters have adapted to this new reality, by increasing production by establishing standard procedures, assuring adequate tools are available and providing constant training to their production staff. This allows successful converters to increase production with minimal cost increases.
Why change what has been done for so many years? Because the needs of customers are changing!

Evolution of the Box
The corrugated box has also evolved from a means of product protection in shipping to an important sales tool. Even the simplest, one color box requires adequate legibility with barcodes or QR codes that are fundamental to the organization of inventories, selection, and distribution. With the great e-commerce boom, companies are forced to respond with customized boxes, while continuing to ensure highly efficient operations.

Considering the above, we will discuss some practices for the press floor that can be implemented to improve efficiency and reduce errors for the post-print corrugated market.

Before going to press
After proper plate processing (liquid resin, analog sheet, or digital sheet), it is necessary to inspect for defects and measure the plate with a micrometer to ensure the gauge and relief tolerances are within specification (Image 2). Any low point will cause printing failures and over-printing which affects the printed result AND the strength of the box. Additionally, the more consistent the gauge, the longer the life of the plate on press. The use of a Shore A calibrator will confirm that the hardness of the material is ideal for both the graphics to be printed and the substrate used (Image 3). Corrugated materials require low durometer from 25 to 40 Shore A. All instruments used must be properly maintained and calibrated following the manufacturer’s recommendations to ensure accurate and precise measurements.

Image 2. Digital micrometer for reading the thickness of the plates. The plate should be supported on a table perfectly parallel to the floor and with a sufficient area to prevent movement. Source: FLEXOGRAPHY Principles & Practices. 5th Edition.
Image 3. The Shore A gauge of durometer measures the penetration of its cone on a soft material. Source: FLEXOGRAPHY Principles & Practices. 5th Edition.

Prior to mounting the plate, check the work order to confirm measurements of the box design, number of colors, plates per color, and other specs. Cutting the plates, whether manual or automated, must be precise, clean, and without burrs that impair adhesion to the carrier. Beveling the edges of the plate is highly recommended (Image 4). Beveled edges improve the contact area with the sealant and reduce the risks of the plate lifting during the production run. The polyester backing, as well as, the carrier must be thoroughly cleaned to remove any traces of dirt that may affect the application of the rigid mounting tape. Typically, when there is poor adhesion between these components, there are high areas that print with more pressure on the cardboard. After the placement of each plate on the plate roll, it is important that constant pressure is exerted with a rubber roller to improve adhesion and avoid bubbles. The plate mounting must not only guarantee an exact registration but also an even impression of each color.

Image 4. Beveled edges and cut corners of a plate. 

There are different targets available for manual or automatic evaluation. The designer, mounting operator, and printer must agree with the type and location of the registration marks (Image 5) Everyone involved must understand how to use the marks to clearly communicate if a registration problem arises from design, mounting, or printing. Flexographic Image Reproduction Specifications & Tolerances (FIRST) supports the use of the Railroad Track Target which contains implicit measurements for lateral and circumferential adjustment thus avoiding the use of a measuring tape reducing the time required to achieve aligned images. The goal is to place the colored triangle on the centerline. These targets along with the traditional crosses should be placed in the four corners of the sheet.

Image 5. The orientation of the register targets on the sheet. Source: Cordes Porcher 

On press
On press, organization and cleanliness are fundamental and necessary. A lean manufacturing mentality must be maintained following the basic principle of the 5s which focuses on how best to organize the workspace to maximize efficiency: sort, set in order, shine, standardize, and sustain. The tools and supplies should be easy to see, easy to obtain, and easy to return, guaranteeing only the materials needed for the print run will be on the press.

A program of preventive maintenance should be agreed upon for the press with the personnel dedicated to this task: inking systems prepared and adjusted to provide the thickness of the desired ink film to the plate; rollers under concentricity, conicity and parallelism tolerances confirmed by the manufacturer; bearings and bushings in perfect condition and properly greased; register and material guidance controls; and drying units operating efficiently with the airflow itself. Negligence in maintaining these items and delayed replacement of worn items will result in problems of quality and productivity.

As for the ink film thickness, it is essential that the pressman knows their anilox. They must know the linescreen and volume, and the condition of the anilox. Although the change of anilox in a corrugated printing machine is relatively infrequent, a routine cleaning program must be carried out. Equipment that incorporates the recommended cleaning chemical is available to complete the cleaning on the machine. Likewise, an inventory audit must be carried out, relying on the manufacturer expertise.

Another critical element to control on press is the ink. Those used in the corrugated industry are water-based and require an alkaline pH (pH> 7.0) to keep the resin in solution. This is achieved through the incorporation of chemical substances such as amines that tend to evaporate during the run which reduces the pH and increases the viscosity. This causes variation in the color density, dirty impression as the ink accumulates on the plate due to a poor redisolubility, excessive generation of foam, and accumulation of paper fibers on the surface of the plate. To maintain consistency in the control of the color during printing, both pH and viscosity should be measured frequently and pH corrective solutions should be added to maintain stable ink properties. The addition of water reduces the pH and should be used with great care as the color strength can also be affected. Before making an adjustment in the viscosity, one must ensure that the pH is in the range recommended by the supplier (pH 8.5-9.5) (Image 6). Any measurement carried out must have adequate and regularly calibrated equipment and the ink temperature recorded.

Image 6. The pH should be maintained in a range of +/- 0.3 during the print run to ensure a stable behavior of the ink. Source: Flexographic Image Reproduction Specifications & Tolerances

Once the plates are placed in the press, the next step is to confirm adequate inking through the appropriate adjustment of the pressures: anilox-to-plate and plate-to-substrate. First, verify the ink flow to the anilox is at its optimum level, the printing plate must be completely inked without excesses through the slow approach of the anilox. Then move the whole assembly towards the cardboard substrate. Confirm that the substrate has printed the entire plate and stop the adjustment at that moment. The anilox adjustment should be done first and last to avoid excess ink on the plate. The objective is to apply the lowest possible pressure that allows a consistent impression; excessive pressure will cause greater dot gain, premature wear of the plate, and considerable crushing of the board. Drag Targets can be useful to visually evaluate excess pressure depending on the pattern shown in each one (Image 7). If a more objective evaluation is desired, the inclusion of control strips with different dot percentages will allow the reading of the dot gain experienced by each: a minimum point patch, quarter-tone, halftone, shadows, and solid block would be a good sample for process control. Regardless of the target used, they must be placed on both sides of the press to ensure the parallelism of the rolls. The values must be correlated with those established during the press fingerprint test to confirm if a certain value read with the densitometer confirms or rules out an over impression.

Image 7. Impression targets: circular (anilox/plate) and hexagonal (plate/substrate). Source: Flexographic Image Reproduction Specifications & Tolerances.

Plate Cleaning
During the press run and at the end of the run, cleaning processes must be carried out on the plates which can extend their useful life. Immediately after the production ends, clean the plates to prevent the ink from drying completely. A brush with soft and fine bristles (nylon or natural fiber) or a suitable sponge must be used for this operation; never use brushes with metal bristles or rough cloths. Moisten the plate with a cleaning solution prior to brushing it; a solution of lukewarm water with an alkaline pH (> 9.0) works well to remove the ink from the surface. When in doubt contact the supplier to develop a swell test under the established protocols and determine the variation of caliber and hardness using a small sample of the plate. It is not necessary to aggressively brush the plate as excess pressure can damage the surface. During brushing, apply generous amounts of the cleaning solution to keep the surface moist and avoid scratching. Improperly cleaned plates can stick together, develop surface cracks, and become more difficult to clean for reuse. After cleaning, dry the plate with an absorbent soft paper or forced air to remove residual water.

Plate Storage
Finally, store the plates vertically. The storage area should not exceed 100°F (38°C) and should be located away from ozone generating sources such as motors that generate electrical discharges and energy sources. Keep the plates in an environment free of dust, keep them dry, and at room temperature. Protect plates from exposure to direct light sources that can cause softening, brittleness, and discoloration.

Image 8. The protection of the plates to UV light will prolong their useful life. Avoid direct exposure using filters.

Undoubtedly, there are more areas that can be considered but the most important thing is to understand the variables, control them, and rely on suppliers to define standard operating parameters in each of their areas of responsibility. Knowing the capabilities of the printing system is critical through press fingerprint tests to make consistent designs. However, the capacities of the personnel of each area and the processes that are carried out day after day must also be reviewed. The best printing equipment may not provide the expected results if employees are neither properly trained nor empowered to implement procedures to make their work more efficient. The objective is not to work harder but to work smarter, and this is where the company that adapts best manufacturing practices will maintain a differentiated position in the market.

To learn more contact MacDermid Graphics Solutions. Email us today.

Written By: Ivan Rozo, Business Development Manager, MGS Latin America

Ivan Rozo is the Business Development Manager for Latin America. Ivan is a Chemical Engineer with a Master Degree in Business Administration with an emphasis in marketing. He has more than ten years of experience in the Flexographic industry working in roles of sales and technical support in which he has led optimization projects. Ivan's is responsible for attracting new business and consolidating strategic accounts at MacDermid in Latin America.

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