Quality Control in Flexo Platemaking

Flexographic printing is continuously evolving, and the platemaking process is seeing this exponentially. Platemaking is evolving not only in the development of new and innovative materials but also in the technological improvements of imaging and processing equipment. Together these make a more efficient and consistent workflow to deliver high-quality plate that can consistently reproduce the information of a digital file in its finished structure. All of us have been familiarizing ourselves with terms such as high definition, high resolution, surface screening pattern, and flat-top dot, among others. The reality is that ALL suppliers in this field have achieved a synergy, bringing about a complete revolution making the flexo printing process more competitive compared to other technologies such as rotogravure and offset. However, in an environment that tends to become more automated, there are still uncertainties in the manufacture of the plates and even returns of the finished product. In this post, we will review the procedures, tools, and criteria that can be used to maintain an adequate environment in ​​digital platemaking based on the golden rule...“What cannot be measured cannot be controlled". What to measure? How to measure it? How often? Are some of the questions that will serve as a starting point to deliver a finished product that meets the user's expectations.

Quality Control

Broadly speaking, quality control includes a combination of procedures designed to monitor and validate effective "acceptance/rejection" criteria in key stages of a process:

1. First, identify the variables that have the greatest impact on the process or workflow. These will then become the key control points for the process.
• By finding the key control points we can ensure quality at critical stages in the workflow and reduce the opportunity for defects downstream in the process.
2. Define standards to use "acceptance / rejection" criteria.
• Due to the physical limitations of the process involved, tolerances must be established for these standards.
3. Measurements are taken at these key control points to verify the process and conformity of the product to these standards.

A benefit of establishing measurement standards is the increased opportunity for a consistent and successful process or product. In addition, the key control points ensure that any defective product is identified in the earliest stages of the workflow.

Key Control Points

Image 1. Digital Plate Construction

A digital plate is processed without the use of a negative film. On the surface of the sheet material is a thin carbon layer that is removed using a laser ablation technique in conjunction with a 1-Bit TIFF file. After ablation, the carbon layer acts as a negative or mask, blocking ultraviolet light during the exposure process. The processing stages of a digital plate are back exposure, image ablation, main exposure, processing (solvent or thermal), drying (solvent), and post-exposure/finished plate which can be grouped into three key control points:

1. Ripped Digital File
2. Imaging (mask ablation)
3. Processed/Finished Plate

Ripped Digital file

Once the digital file is ripped, it becomes the digital negative and cannot be manipulated. All information of the image, tone curve, and metadata is contained within the file and is used directly by the imaging device. Therefore, when reviewing this type of file, it is important to consider:

• Image resolution
• Orientation of the image (right reading/wrong reading)
• Linescreen, angle, and dot shape
• Screen configuration (halftones and solids)
• Compensation (bump and printing curves)
• Dimensional accuracy (distortion)
• Registration marks or guides

All elements of the ripped file must be verified and certified that they correctly adapt to the standards defined by both: the basic workflow and those unique to the job. The use of an application, such as a bit-map viewer software (Image 2), for the visualization of this type of file is an excellent process control tool.

Image 2. Esko Grapholas visualization tool

Imaging (mask ablation)

The quality of the image ablation on the carbon mask of the plate is a function of the raw material and the adjustments of the imaging device and its condition. The plate manufacturer must ensure a uniform and consistent application of this layer to avoid problems during ablation and/or exposure. Before and after ablation, the mask should be checked for visible marks, scratches or abrasions. Remember, at this stage, the plate is more susceptible to damage because it has yet to be cured.

Measurements must be obtained during this stage by a transmission densitometer, the following apply:

1. Fidelity of the image (focus adjustment)
2. Density of the mask from 3.5 to 4.3
3. Stain level of the plate after the device removes carbon mask (D_min) <0.07
4. 50% Density on 0.30 +/- 0.02

It is important to maintain a consistent ablation on any digital imaging device with any digital plate. Ensure the laser is properly focused on the carbon mask to concentrate its energy in the smallest possible area and achieve a clean removal. Defects due to poor focus are seen as visual lines in areas that have been removed or malformed dots (Image 3).

Image 3. Out of focus ablated image 

A weekly ablation test should be carried out to monitor that both the device and the material do not vary from their original setting parameters. Using a transmission densitometer (Image 4), standardized targets are measured; these consist of 100% and 50% removed areas which are compared to the material with the mask removed manually. (NOTE: Calibrate the transmission densitometer with the control strip supplied by the manufacturer to confirm the correct amount of transmitted light is consistent with the amount transmitted when the unit was manufactured.) If the readings are below recommended targets, then there is too much energy being applied by the laser and the physical size of the dots will be larger than desired. On the contrary, if they are higher, then very little energy is acting on the plate to remove the carbon mask, resulting in lines present in solid areas and smaller than the desirable dot size affecting the applied bump curve. In either case, the energy (mJ/cm2) applied to the plate must be adjusted by modifying the RPM or power.

Image 4. Transmission densitometer

Finished plate

After the plate is processed, it is necessary to carry out measurements to ensure specification compliance. These results are dependent on the condition of the platemaking equipment and adequate monitoring of the processing conditions found during optimization. You should consider the following control points:

• Caliber
• Relief
• Dot formation
• Tackiness
• Cleaning

Caliber

Both the caliper and the relief can be measured with an analog or digital reading micrometer

Image 5: Micrometer

(Image 5). (NOTE: Calibrate a micrometer using a precision block placed between the surface to ensure the readings are accurate.) The test area must be at least 1" x 1", and multiple measurements throughout the plate are necessary to determine uniformity. The caliper of the plate is one of the most important factors when controlling the impression setting on the press machine; any low spot will not print or will cause overprinting for the remainder of the printing plate. All areas must be at full print height including solids and halftones. Flexographic Image Reproduction Specifications & Tolerances (FIRST) specifies variations for raw material thickness of digitally-imaged photopolymer plates and sleeves as follows:

        Within the same plate: +/- 0.0005" (0.0127 mm)

        Within a set of plates:  +/- 0.001" (0.0025 mm)

The flatness of the finished plate depends on the platemaking process; however, any variation will be accumulated with those of other variables, so the lower the better.

Relief

The relief specifications must be defined and confirmed depending on the caliper, the graphic details, press machine requirements, and other variables. Target relief specifications should be confirmed with the printer.

Dot Formation

The accuracy of the dot and the formation of the minimum dot is inspected with a microscope (100X) and a flexo plate analyzer. Specific characteristics are evaluated and verified to confirm a successful minimum dot; this method ensures that, although the minimum dot is small, it can be used as an effective control point. This may or may not be the dot percentage used in the production job (Image 6).

Image 6: MinDot test with two different screen configurations (stochastic and circular)

Once the smallest stable dot has been determined, the objective in the production of plates is to maintain this stable value by type of plate. Considerable changes in processing conditions can lead to considerable deviations (e.g. changes in UVA light emission intensity, etc.).

Additionally, a control strip must be recorded and processed with each plate for immediate verification of the consistency, not only of the platemaking but also of the compensation curves applied (Image 7). The use of a constant objective provides a single set of standards and tolerances to meet.

Image 7. Prepress scale where the values that must be measured on the plate for each tone are labeled. Source: FIRST

To verify the tonal accuracy of the finished plate, tools such as flexographic plate analyzers are used to measure the size of the halftones. Utilizing this tool improves process control, production efficiency and consistency of print quality. We recommend the BetaFlex Pro and the Plate to Print to create a record of the data collection verifying the specifications of the plate have been met. In any case, whatever the device used, calibration procedures and the process of taking measurements must be clearly documented along with the approval/rejection criteria.

Finally, it is important to consider the following frequency of operations to maintain stable conditions:

1. Measure and record the lot number, relief, caliper, minimum dot percentage and halftone percentage of at least one color of each job. 
2. Conduct a weekly "MinDot finder" test when there is a lot change of the photopolymer plate. This weekly test will help establish an appropriate main and back exposure.
3. Perform a daily "focus test" and a weekly "stain test" every time there is a lot change This testing will confirm adequate mask ablation. Maintenance for cleaning lens optics and changing parts must be programmed with the manufacturer.
4. Conduct a bi-monthly audit of the intensity of the UV exposure lamps. It is recommended to change lamps when the readings fall below 80% of their original power especially for jobs that incorporate special effects in the highlights or solids.
5. Weekly measure the Brix degrees or density of the processing solvent and adjust according to the manufacturer's recommendations.
6. Review the back exposure at each lot change and adjust accordingly.
7. Only back expose plates that will be used in less than half an hour.

There are many more initiatives that can be incorporated, but a quality control process must aim for continuous improvement based on the customer's needs. The process should detect any defects in the early stages of the manufacturing process where the cost is much lower than having a defective product in the machine. It should also be noted that a quality control program involves raw materials duly certified by the manufacturers. The goal is to reduce waste, decrease reprocessing, and increase consistency.

The return on investment in these types of programs is easily justifiable when packaging is produced consistently, downtime costs are reduced, margins are improved, and time-to-market is reduced. At MacDermid Graphics Solutions, we not only offer quality products under the tightest tolerances, but we also have a team of specialists willing to help you implement this type of initiative to increase your competitiveness.

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

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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.