- July 01, 1995, Thome, John D.
This is the final installment of a three-part series on video web inspection. This month's article focuses on how to select the right system and manufacturer and how to apply video web inspection technology to specific applications. The author also takes a look ahead to what kinds of systems users can expect to see as the technology evolves.
In previous installments, we explained the various benefits of video inspection and presented detailed explanations of the technology involved.
In this issue, we will shift our focus to applying the technology to your operation and how to go about selecting the right system and manufacturer for your specific needs.
When considering the purchase of video inspection, a first step should be to analyze your operation's needs. This can easily be accomplished with a snapshot appraisal or a similar evaluation. The purpose of this appraisal is to quantify your operation's needs, to identify specific print problems that a video system could address and to determine the degree to which your operation will benefit from video inspection.
The process is fairly simple, lint it does require operator input, conscientiousness and cooperation. Take a two-week period and record daily the specific kinds of print problems experienced and how often each problem occurs.
At the same time, track the specific kinds of work produced. You can do this for one press or for a group of presses.
The final analysis should quantify the specific on-press problems you're experiencing and the degree to which each is a problem.
If the appraisal indicates that video inspection will be beneficial, you should assess the various technologies available and determine which is best for your application.
Passive vs. Active Inspection
Presently the passive and active inspection processes are the most common categories of on-press inspection used in the converting industry. The operator's role in the inspection process varies in each.
Passive inspection systems automate the entire inspection process or elements of it, and, in some cases, remove the operator from the loop. Such line-scan systems can inspect 100% of the web and 100% of the material printed. In the process, they can identify a wide variety of random and repetitive print defects such as streaks, hickeys and missing print. These systems are also capable of sorting good material from bad.
Aside from cost, passive inspection systems are limited in performing tasks such as monitoring color or determining if printed bar codes are readable, and they provide little or no useful means for the operator to view the moving web.
Line scan-technology has not been widely adopted as a process-control tool in the converting industry primarily because of the cost value. One application where they are used sporadically is roll-to-sheet gravure operations in which the $200,000 to $500,000 expenditure is justified.
Passive line-scan systems do require some advanced operator input and programming, which can be a drawback if your operation doesn't have well-trained operators. Generally, the systems are considered very effective by those who have installed them.
The second most common form of on-press inspection is active inspection, or inspection that requires operator involvement.
Some of the older equipment uses a rotating-mirror device that the operator uses to view the printed web. This is generically called a "scan-a-web," although that's actually a specific brand name.
Today, such systems are rapidly being replaced with video inspection systems which utilize electronic charge-coupled-device camera technology and computers to scan and digitally reproduce images on the web. Such equipment is the most common method used in the converting industry today.
The common denominator among active inspection systems is that they require operator involvement. At one end of the spectrum are the rotating-mirror and basic visual video inspection systems. At the other end are video systems which are far more active and which do some of the work for the operator, such as monitoring color, evaluating bar-code quality, monitoring and correcting registration as well as checking for other print defects.
The high-end active video inspection systems can also alert the operator when errors are found and can show them the location of the defect on the web. Closed-loop active inspection systems not only find the errors but also send signals to the press that initiate corrective action and monitor the corrections.
As a general rule, the greater the amount of process automation, the more costly the investment. Basic visual systems start at under $7,000, while the high-end active video inspection systems run from $20,000 to $150,000, or substantially more if you wish to close the loop.
There are a number of other important distinctions between active and passive inspection equipment, starting with basic philosophy. Line-scan systems seek to inspect every inch of product produced and do so with minimal operator involvement. They seek to dramatically reduce, if not eliminate, operator responsibility for print quality.
Charge-coupled-device systems, on the other hand, require operator involvement, thus supporting a quality and management philosophy that the operator is an integral part of a variable print production process. As such, operators can be empowered to control the quality of their work and provided with the necessary tools to maximize productivity.
Sampling vs. 100% Inspection
As web width increases, so do [TABULAR DATA FOR FIGURE 1 OMITTED] press speeds. Consequently the need for a systematic, well-defined approach to active inspection of the web typically increases with both of these factors. Proper sampling methodology and operator use of the equipment becomes increasingly important as web widths increase. The greater the amount of time spent sampling the web, the greater the sampling ratio of your inspection and the higher the likelihood that the operator has identified and corrected all defects and maintained quality to acceptable standards.
Failure to inspect to the proper level or sample size may lead to a false sense of security and may prevent maximization of the productive value of the equipment. This could complicate statistical process control in eliminating defects in a cost-effective manner.
Video web inspection then is an important tool of statistical process control, and it will play increasingly important process-control roles in the years ahead.
Simplicity is Essential
Generally, the simpler it is to learn and use a video inspection system, the more it will be used. Maximum productivity can be obtained when system functions are executable in one or two steps that are easy to remember. Also, fewer and color-coded controls may facilitate learning and remembering. Uncomplicated control panels and remote controls with larger touch pads seem to be preferred. Picture and color reproduction is also necessary.
Additional features may be more costly. A popular rule of thumb is to buy a system based upon your needs for the next three to four years. If those needs change, an inspection system that is expandable and adaptable will allow you to add new features and technology.
Field of View
Another important consideration in selecting a video inspection system is field of view. This term refers to the size of the image area that is seen on the monitor. It can range in size from a business card to the approximate size of a sheet of computer paper. Generally, as fields of view increase, so too does the cost of the equipment.
There are several reasons for this relationship. Lenses with greater magnification are required for larger fields of view and can add substantially to the cost. Large image areas also contain greater amounts of information for the electronics to process and the monitor to display. Greater spatial image resolution will then be required, which is attained by using three-chip cameras. As image areas increase, the camera must be mounted farther from the web. Because ambient light affects image quality, shrouding around the image is required. This may add to the system cost and can complicate mounting.
One last important issue related to field of view is magnification. As Fig. 1 shows, using the same lens but changing the diopter has two effects. It will increase field of view while it decreases the amount of magnification provided by the system. So to some degree, the cost of larger fields of view is reduced magnification. This emphasizes the importance of conducting the snapshot appraisal and defining your application needs. It will be necessary to balance the amount of magnification required for your application, the amount of field of view, the camera-mounting requirements, space available on your press and your budget.
Inspection System Requirements
For what kinds of defects are video inspection systems effective? Following is a list of some of the more common application uses of video inspection systems:
* Monitor and control registration: color-to-color, print-to-die, paper-to-paper;
* Monitor and control print defects: halo, plate squeeze, fill-in or spread, striation, roller marks, beading, ragged edges, chalking, pinholes, fisheyes;
* Monitor and control traps on the fly;
* Process-color control and registration control: visual through the use of split-screen features, automatic via color-monitoring software;
* Monitor and control line and screen color consistency: roll-to-roll, run-to-run;
* View the lay down of clear varnish and ultraviolet coatings: identify pin-holing and poor coverage areas;
* On-the-fly bar code inspection: are printed bar codes scannable and readable? generate ANSI grades;
* Monitor ink drying in the anilox roll: insufficient transfer of cell volume from anilox roll to plate;
* Monitor adhesive lay down;
* Monitor perforation and die-cut quality, print-to-die registration.
Using Video Inspection in Roll-to-Sheet Applications
Roll-to-roll applications account for an estimated 80-85% of the systems presently installed in the market. But, there is a distinct trend toward the use of video inspection in roll-to-sheet applications. Because an operator can readily pull a sheet and take it to a viewing station, many have felt video inspection is unnecessary.
However, at the higher speeds of today's presses, seconds are critical. Imagine pulling a sheet off the press, and losing in five minutes time 2,500 ft. of web without any regard for quality. With an on-the-press inspection system, those five minutes are far more effectively devoted to inspection and on-the-fly corrections.
This allows the operator to build quality into the printed product rather than inspecting it after the fact.
Selecting a Vendor
In the past three years, new manufacturers of video systems have entered the market at a furious pace.
Your choice of which vendor to buy from is almost as important as the system you buy.
Here are some questions to ask when evaluating vendors of video inspection systems and, in some cases, can be broadly applied to the purchase of other equipment as well:
* How long has the company been in business?
* What is their track record and are customer referrals available?
* What are their service capabilities including installation, troubleshooting, phone support and training?
* What are the manufacturer's engineering capabilities?
* What is their product offering?
Once you have familiarized yourself with each vendor, you'll want to evaluate different systems. Here is a list of questions one can use for this purpose:
* Is the system easy to learn and easy to use?
* How well do your press operators like the system? Will they use it?
* How many commands does it take to execute the most common features?
* How long does it take for new operators to learn to use the system?
* Does the system use a charge-coupled-device RGB camera with an RGB green-stripe filter?
* What is the camera resolution: horizontal, vertical and total?
* How much space is required to mount the camera?
* What is the field of view and range of magnification?
* What is the image size at maximum and minimum zoom?
* Does the system use Xenon strobes? How many? How much do they cost to replace?
* Are the strobes adjustable, and in how many ways?
* What types of materials are used in the traverse?
* Are manual and/or motorized traverses available?
* How heavy duty is the traverse motor and gear assembly?
* How long does it take to get service? Parts?
* At what speed does the traverse move across the web?
* Is the system's monitor interlaced or noninterlaced?
* What is the monitor refresh rate? Dot-pitch rate?
* What monitors are available?
* Does the system use a 16- or 24-bit frame grabber?
* What internal electrostatic-discharge protection is already built into the system?
* Does the manufacturer use off-the-shelf or custom electronics designed for the press room environment? How appropriately is the equipment designed for this environment?
* Is a wireless remote control available for use with the system?
Impact of Video Inspection
Studies of video inspection users have quantified that these systems can not only have dramatic impact on productivity and the bottom line but that they also contribute greatly to quality and morale.
We have only scratched the surface of potential. Voice-activated control systems, wireless network linking, integration with all other press-control systems, closed loop control...the possibilities are endless. The evolution of video inspection will continue.
By the turn of the century, video inspection devices will be standard operating equipment designed into every flexo and gravure press made. The technology will also have evolved from a basic tool for the operator into a process-control tool for management as well.
As you can see, the impact of video inspection can be substantial. You should demand that the system you buy has an impact on productivity and quality. That makes cost justifying the purchase of a video web inspection system fairly easy.