Flexo Inspection

Published: April 01, 2001, By John D. Thome, BST Pro Mark

During the course of the last few years, we have seen remarkable advances in inspection technology — advances that have made 100% automated inspection of the entire printed web a reality.

Everywhere you go today in our industry you hear about “defect detection.” It seems that the converting community is fascinated with the possibility of being able to find every flaw on the web automatically. I believe that this new technology will have a profound impact on all forms of printing.

Expanding the Process Window

Flexography has undergone a multitude of process improvements in recent years — improvements that have made printing a 1% dot with 1,200-line anilox rolls on 0.5-mil poly substrate a reality. Such improvements mean flexo now competes with offset and even gravure in print quality.

Example (1) shows the fine work being done on flexible substrates today. The separations on this job were done using Samba screening technology, and they were done over the Internet! (Samba is a combination of stochastic and classic screening technology for flexo and silk screening applications.) This particular job was printed on an eight-color, central impression press, using up to 700-line anilox rolls.

Another example is photo (2), a flexible package that uses Samba screening technology. The colors are bold, and the finished product looks natural. The lower dot gain in the highlight areas and in the midtones produces a bag that truly sells.

Now let us turn to the subject at hand — 100% inspection. What is it? Does it work and on which kinds of presses and substrates? And is there any real benefit for a flexo printer to use 100% inspection of labels, cartons, envelopes, and flexible packages?

What Exactly is 100% Inspection?

One of the technological advances that played a significant role in allowing flexo to become competitive with offset and gravure was the widespread adaptation of video web inspection systems. This technology was instrumental in empowering press operators to manage the quality process rather than identify defects after the fact.

The impact on productivity probably has been greater than anyone expected, so great, in fact, that today converters are pushing the flexo envelope further, toward that elusive goal of 100% inspection. Is this a real improvement in our process management, or is 100% inspection just a passing fad?

Not only is 100% inspection coming — it is here, and in five years it will be the norm for all forms of printing.

The Current State of the Technology

There are two basic types of defect detection systems on the market today: 100% Sampling and 100% Comprehensive.

The 100% Sampling systems analyze one segment of the repeat at a time, monitoring 100% of the repeat over a period of time that may vary from a few seconds to many minutes.

The 100% Comprehensive systems monitor 100% of the material produced — virtually every square inch of product that runs through the machine.

These system are using two distinct camera technologies — line scan and area scan.

The 100% Comprehensive systems based on area scan technology are fairly new, but they have advantages over line scan technology for some packaging applications.

Let's explore some of the issues to consider when looking at 100% inspection of any kind.

Coverage and Sampling Ratio

A 100% Sampling system inspects the entire plate area over a period of time dependent upon the field-of-view, processing power, and speed (Hz) of the system. A 2-Hz system captures two pictures every second, while a 10-Hz system captures ten pictures every second. Given the same field of view and processing time, a 10-Hz system samples five times as much of the web as a 2-Hz system.

Example: Two systems, both with 7×9-in. fields-of-view (and for purposes of this discussion, the same processing speed) analyzing a 35-in. web running at 800 fpm. Such an application has 336,000 sq in. of web passing through the machine every minute.

The 2-Hz system will inspect 7,560 sq in. of material/min, which represents 0.02% of the web, a statistically invalid sample size. The 10-Hz system will inspect 37,800 sq in. of material/min, which represents 11% of the web, a statistically significant and valid sample.

Repeating vs Nonrepeating Defects

What about random defects like a bug on the web? Defect detection is king here, right? Wrong!

A 100% Sampling system will inspect the entire repeat and would find a bug on the web if, and only if, the camera just happened to be looking at that spot at that particular point in time. The fact is that 100% Sampling systems do not allow you to find every defect on the web — particularly nonrepeating defects. Their strength is in utilizing sampling techniques to monitor process quality attributes — color, register, streaks, and repeating defects — to allow effective monitoring of the process on a continual automatic basis and to alert the operator as trends develop.

Waste & Downtime Summary

Defect	% Random	% Repeating	Lineal Feet of Waste Per Month	% of Total	Avg. Down-time Per Month (Hr)	% of Total	Est. Cost1
Streaks	2	98	87,000	11.0	3.0	13.0	1,272
Color	0	100	246,000	31.3	9.0	39.1	3,726
Misregister	0	100	300,000	38.1	4.5	19.6	2,925
Splashes	70	30	44,000	5.5	0.5	2.1	389
Hickeys	5	95	28,800	3.7	1.3	5.7	499
Bar code quality	0	100	82,000	10.4	4.6	20.5	1,642
			787,800	100	22.9	100	10,453
1 Based on substrate cost of $0.10 MSF and press time valued at $250/hr.

The 100% Comprehensive technology would allow you to find random defects on the web, such as a bug, but these systems generally require three times the investment, so you better have a serious random defect problem to justify it. They also are used commonly as devices to separate good material from bad, such as on doctor machines. But consider the impact of a strategy that weeds out bad product versus a strategy of controlling and managing the process better.

In addition to understanding some of the basic differences in the two kinds of systems, it is important to understand the root cause of the waste stream. To do this, it is essential to have actual data on waste and downtime. It is necessary to identify the specific types of defects you are experiencing and quantify the impact each is having on press productivity.

An analysis of at least 3 months of data is recommended, although 6-12 months is preferred. A chart such as this is ideal (see Figure 1).

It identifies the print-related causes of waste and downtime and quantifies the impact of each in terms of the lineal feet of waste (or some other equivalent means) and the amount of downtime from each.

With this data in hand, it is possible to estimate a cost for each type of process problem and to set priorities for correctional actions, i.e., set clear objectives for the process control system.

If the majority of your print quality issues are trend defects, a 100% Sampling system will be the best solution. Those systems are designed to identify trend defects such as color or register, while giving operators useful information that will allow them to manage the process better.

If the root cause of your waste problem is random defects, a 100% Comprehensive system is likely to be the best solution.

Before you invest in an automated inspection system, invest the time and resources to develop this data first. Without it, you run the risk of installing a system that will not address your particular quality issues.

Prevention vs Detection

There are defect detection systems available today that will actually increase waste and downtime — the exact opposite of the desired result. An example is the traditional 100% Comprehensive line scan-based system. It is great at spotting defects but not effective in identifying trends. Effective process control requires effective trend analysis.

To be a truly useful pressroom tool, software and defect detection algorithms must be created that will perform defect prevention. With this additional dimension, these systems can be powerful process management tools in the pressroom and can be extremely effective in preventing waste and downtime caused by print quality problems.

A real advantage of 100% Sampling systems is that they detect defects, quantify them, and spot trends, a capability lacking in many 100% Comprehensive systems.

Also, open architecture allows peripheral devices and capabilities to be integrated effectively, such as wide-angle laser scanners for reading and ANSI grading bar codes, or spectrophotometer units for analyzing color.

The 100% Comprehensive systems based on line scan technology also have limited defect sensitivity control, no visual capability, cannot identify trends, and are merely defect detectors, not defect preventors. As such, they do little, if anything, to prevent waste and are used most effectively to sort good material from bad in post-processing operations.

The 100% Comprehensive systems based on area scan technology generally have more versatility, better defect sensitivity control, are very effective at identifying trends, and offer more opportunity for integration of software and peripheral hardware into their architecture.

Viewing Capability and The Human Element

The need for operators to visually inspect on-press print is an often overlooked element in the design of these high-end systems. The thinking is that because the defect detection system will do all the work, the operator will no longer need a visual inspection capability.

But what about a little thing called makeready? Just ask your operators how they would like to do an eight-color setup with no visual inspection capability, how long it will take, and how much material will be used.

Visual inspection of the web is a basic need that will never go away. There will always be traps, screens, vignettes, die-cuts, perforations, holographs, and many other factors that require the human element to manage on press properly, especially during makeready.

As a result, any web inspection system that does not have a full-featured viewing capability for the operator will be significantly less effective as a pressroom tool and will yield disappointing results as an investment.

Another important consideration is defect size. All systems have limitations on the size of the defects they identify, which is a function of the technology employed and the software in the system. Some systems go well beyond that and have the ability to learn operator defect acceptance patterns.

Areas that can be improved with a 100% inspection system include register, color, splash, spots, and streaks.

Such a system is quite effective for most packaging applications. For example:

• Where the need exists to monitor quality of key process variables and to improve the overall productivity of the machine;
• Where one or two specific trend defects are causing a substantial waste stream, i.e., significant enough to justify the capital investment;
• In high-speed, wide web applications where there is a desire to reduce dependency upon the operator as print inspector, or a desire to upgrade print process control to a higher level;
• Narrow web applications in which a high sampling rate is necessary to ensure the desired quality level.

Inspection and Productivity

Visual systems have had an enormous impact on press productivity. The benefits of automatic 100% defect detection are just as significant but not of the same nature. Make-ready productivity improvements will depend on what kind of inspection process is being used currently. Remember, if the system lacks a full-featured visual capability, you can actually expect a negative impact on makeready.

Benefits of 100% defect detection include the following:

• Maximum impact on running productivity and reduced running waste;
• Allowing focus on elimination of specific high-impact defects or waste-stream generators, often sufficient to pay for the system;
• Faster identification of potential high-impact defects, resulting in more immediate corrective action as well as improved overall quality off the press and improvements in productivity;
• Reduced downtime;
• Management of the process.

Some Practical Applications

Let's look at examples of different print jobs and highlight some of the critical print factors of each job that any 100% inspection system would have to provide. This should offer an understanding of the complexities of 100% inspection.

This pharmaceutical label (3) is a prime candidate for 100% inspection. The package has three different kinds of bar codes, which presents an interesting challenge for any automatic inspection system. Some systems today actually can perform OCR (optical character recognition) on the text, a critical print factor in this type of work.

Critical elements of this lottery ticket job (4) make effective 100% inspection a real challenge:

• Color trapping and register are essential to proper appearance of the ticket.
• All the black ink must be in place before the scratch-off material is applied. Any missing ink must be identified in the process.
• When the scratch-off material is applied, it must provide full coverage in the proper areas.
• All the security features must be present.
• Front-to-back register must be within tolerance.
• Bar codes on the reverse must scan and meet ANSI standards.
• Color cannot vary more than ±1.0 ³E.

The proper 100% inspection system can do all of this automatically.

This prime label (5) has many critical features with very little or no margin of error, all of which must be accounted for by a 100% inspection system:

• The gold must be fully applied. Gold is particularly difficult for area scan systems to see effectively.
• Color-to-color register has no margin of error.
• Trapping of the gold in the red “London Distilled” must be in register or the label is waste.
• All the elements of the very ornamental background must be in place and print cleanly for the label to work.
• The embossing must be present and crisp.

Flex-Pack and 100% inspection

Flexible packaging presents its own unique set of challenges for 100% inspection systems. The system must be able to manage web stretch and snapback, twisting, and skewing, and it is critical the installation properly manage web shadows. If not, the system will constantly give false alarms, and the operator will eventually just turn it off.

The ingredient statement, proof of purchase, and bar code are all critical elements of inspecting the package seen in (6). Bar codes must meet ANSI standards, and the sampling ratio must be high enough to guarantee a 97% first scan rate in the store.

Color traps, seemingly simple vignettes, absolutely critical color, and a logo are all elements of this package (7) that have to be right every time. There are 100% systems that have proven themselves very effective at helping the press operator manage all these variables.

Conclusions

There are significant differences in the kinds of systems on the market today and the features they provide. Effective implementation of any 100% inspection system requires a proper match of technology to need. The right system will provide corrective or preventive action directed at the specific waste issues you are experiencing.

The potential return on investment and impact on a converting operation from a 100% inspection system can be significant. But, you must do your homework. You need to identify and to quantify the kinds of defects you are experiencing. Ideally, you will create Pareto charts of the errors — color, register, bar codes, splashing, pinholing, streaks, random defects, etc. Failure to do so can result in the purchase of a system that is not ideal for the application or that may not address the root cause of your waste stream.

It is likely that over the next three — five years, there will be a dramatic change in the kinds of inspection devices being installed on flexo presses of all web widths. Approximately 90% of all the units being installed today are visual systems. But by 2005 at least 50% of all the systems being installed probably will be 100% inspection systems.

And, because printing is a global business today, this will be a global phenomenon. You must be able to compete not only with the shop down the street but also with the shop half way around the world. These new inspection technologies will be essential tools for competing in a global market.

The investment required for 100% inspection is substantial, but so is the impact on productivity and on the bottom line.

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John D. Thome, VP of marketing for BST Pro Mark, Elmhurst, IL, has 17 years of experience in the printing industry. He can be reached at 630/833-9900.