Print Inspection systems
(Written by N.R. Jayaraman )
Multi color printing for high value brand products and any other printed material inbuilt with elements of designs with some security features call for defect free printing. Print material such as International magazines for Airlines, high quality advertising material, Brand promotional publicity material etc too require defect free printing. When it is stated defect free printing it is implied that the quality of print should match to international standards. The appearance of the printed matter and illustration should be uniform on entire stock, the printed images and texts should not show broken images, or show ink specks, spots, blind spots etc. Such defects damage the reputation of the agencies who distribute them as well the printers who printed them. Therefore all possible print defects which may appear glaring to the naked eyes are to be weeded out by thorough examination before being delivered. Hence such printed material is manually checked before supply. This does not apply to standard small order jobs like invitations of several varieties, greeting cards, bill books and other normal print material handled by small and medium presses. We are discussing only the high volume, special but regular printed material running to millions.
Excise Labels meant for liquor bottles, Printed materials such as Lottery Tickets, Raffle tickets, Bank Notes, Stamps , several of business forms, vouchers etc some of which contains some elements of security features falls under high quality defect free printing. Especially where numbering is involved it needs to be supplied error free. In all these documents barring the business forms and vouchers even the finest detail will be essential in the printed documents as they govern the economy of the nation. Even though the print volume of the said group of material is huge and running to millions, still they were mostly examined to sort out defective prints by manual labor . However of late some agencies have opted for machine examination gadgets and equipments for the said purpose replacing huge labor force.
Voluminous normal print material such as books, magazines, labels of low value consumer products like beedi/cigarette packets, match boxes, sweets, toffees, stationery, and many other eatables are manually examined but not critically as they are just meant for product identification. In contrast to these materials the critical examination will be necessary in the case of important printed documents such as Lottery Tickets, Raffle tickets, Bank Notes, Stamps printing, several of business forms, vouchers etc.
Take the case of printed leaflets of the life saving drugs. The print on them must be clear as even smallest defect in the printed image can lead to complaints. Security inbuilt design elements are incorporated on the labels used by many of the major consumer industries such as Pharmaceutical firms, labels for branded automobile spare parts, labels printed on BOPP films and used on components for many other internationally accredited machineries and equipments that are prone to counterfeiting. Though the print order of these products may not be high , they are examined both manually and with some gadgets because the overt or covert feature can not be easily examined manually alone and need some gadgets to authenticate them.
The manual checking has several limitations as they are subjective and varies from person to person. The defect which may appear to be minor to one person may be acceptable to the other person as no defect print. The tolerance limits can not be explained for visual inspection. Such issue lead to litigation and disputes. Therefore uniformity can not be maintained in manual examination where criticality is required. In light of the same, to streamline the production of high volume work with critical examination, the concept of automatic or semi automatic inspection systems were developed to take away the subjectivity factor from examination, and to hasten the examination at much faster pace.
Constraints in inspection systems:
There are certain constraints in the inspection systems some of which are:
1. High cost of investment of the machineries and equipments for machine inspection.
2. Need high quality maintenance.
3. Spare parts not easily available as updates keep continuing.
4. The equipments and machineries being electronics and computerized only high skilled person can effectively handle them.
5. If they fail to function and not rectified within few hours, the ongoing production operations will get affected.
6. Not suitable for small order runs.
7. Ideally suitable for presses which are engaged in continuous operation producing several millions of the same printed material and many such factors are involved.
Advantages of inspection systems:
However there are certain positive factors too which are:
1. Such machineries and equipments may reduce the labour force
2. The examination will be much faster compared to manual examination
3. Subjectivity in examination can be avoided
4. The defect analysis is possible for future planning
5. Help in standardization of printing.
6. Supply quality print material
Types of inspection systems
Let us see various groups of inspection/print quality checking machines which are available. Basically there are three types of systems in use. They are :
· Online print inspection machines
· Off line print inspection machines
· Semi automatic and Tabletop machineries and equipments for inspection
I. Online sheet inspection systems
The Online inspection machines fitted on the machines is the one that inspect the sheets online - immediately after printing but before reaching the delivery pile, where two sets of delivery piles are provided. One pile is meant for taking all good printed material and the second one for taking defective sheets. These are custom prepared to meet the demand of the purchaser. The sheets are fed in the usual manner and when the sheets comes out of the print center, they travel below the inspection chamber fitted with high resolution digital cameras where the sheets will be automatically examined for defects. They are identified as good and defective sheets similar to the manual examination process and directed to the good and suspect sheets delivery piles after authenticated by the print command center where the true images are stored. This is again customarised and given by the manufacturer as the size of equipments and inputs differ from machine to machines.
II. Online print inspection systems
The other type of online inspection machine is meant to only project the defects on the monitor screen to enable the operator to address the problem as quickly as the defect begin to appear. The inspection chamber will quickly display the defects on the monitor screen the moment the defect commences and before the print reaches the point of rejectable quality, the defects are corrected by the operator without stopping the machines thus saving cost. This system is mostly useful for color density control purposes on high speed Offset, Letterpress and Intaglio Rotary machines. The printing defects other than the ink feed, color density and shade can not be checked by this system. This speeds up the process of production activity. Even though the operators on the machine regularly check the printed sheets, the manual process commences only after the sheet gets delivered in the delivery pile and by the time the defects are detected by visual examination and corrective action taken by the operator by rushing to the proper points, much sheets would have been printed and delivered with the defects. The entire sheets will however be delivered in the normal delivery pile as usual. In order to enable faster detection of the defects for prompt corrective action, this system has been developed. This type of print inspection machines cost much less compared to the sheet inspection machines which separate the good and suspect sheets in two separate piles. This is also customarised and given by the manufacturer as the size of equipments and inputs differ from machine to machines.
III. Off line inspection systems (machines)
In this system the entire lot of printed sheets are loaded onto the separate stand alone machine exactly similar to a printing machine that has feeder and delivery units. The only difference is that this machine has no printing, inking and other such auxiliary units. The printed sheets are fed in the usual manner from the feeder unit. When the sheets travel below the inspection chamber fitted with high resolution digital cameras the sheets will be automatically examined for defects. They are identified as good and defective sheets similar to the manual examination process and pushed into two separate piles which are meant for good and defective sheets. The machine has only inspection systems fitted on them to do the job similar to manual examination.
IV. Off line Table top inspection systems
The semi automatic table top inspection machines are mainly used by the Label printers to replace the defective labels especially when supplied in continuous reel or Zig Zag folded sheets format which are used on automatic labeling machines . Such inspection is also carried out on sheets for supply as all good labels which are individually pasted on several products. Again one important point in the use of these table top inspection equipments needs to be told. They are useful only for any volume of work with regular run and where critical examination is insisted by the customer.
Basic principles of the inspection system :
1. The principle of inspection or checking involved in all the four varieties as mentioned above is same.
2. The inspection machines work with the highest grade of complex software algorithm.
3. The system work as per pre programmed data in respective of quality.
4. Out of the four types, two versions - machines for online inspection and off line inspection- will be provided with two delivery piles- one to accept all good sheets and the other to accept defective prints. The interfaces fitted with the machine controls will allow the said sheets get separated.
5. In all versions- online, offline and semi automatic- perfect defect free print images have o be first fed into memory of the machine. Such images are called Golden images or Absolute prints. If need be tolerance limits to quality can also be set.
6. The tolerance limits for the images are also fed along with the Absolute print images. Let us for example say an image with ‘X’ color density is fed into the memory of the console as Absolute image. Simultaneously images which may have ‘+/- 2%’ variation in color density are also fed into the memory of the console as tolerance images. Otherwise all the sheets which are not 100% true with the Absolute image will get rejected.
7. The high precision and high resolution digital camera inbuilt in a chamber will capture or scan the images when the sheets pass under them and transmit the images to examination center.
8. The transmitted images will get compared with the acceptable range of images which are already fed into the console .
9. If the inspection machine is fitted with some marker to indicate the defective areas, they may mark those areas or if no marker is provided on the machine, they will go into defect sheets delivery pile.
10. Similarly the sequential numbers if any to be inspected for logical conditions i.e. numbers in a given note should match, numbers should adhere to a certain sequence logic, registration should be proper etc will also be inspected provided the machine memory console is programmed accordingly. But the inspection systems for the numbered sheets are totally different and are to be customarised. The programme is set accordingly.
All the above features form the part of the inspection system in all the versions.
How does the inspection system work ? :
1. In On line inspection system, as the sheets comes out from the print area and travel towards the delivery pile, sets of cameras inbuilt in the inspection chamber quickly capture / scan the images and sends to the print command unit.
2. The printed images are quickly compared with the Absolute print in memory and direct where the sheet will get delivered –whether to the defect sheet pile or all good sheets pile.
3. The comparison is done in split of seconds when the process of comparison gets completed before the sheet reaches the command center point that directs the sheets to particular pile. The print errors include mis register, geometrical mismatches, color variations, slur, sledges etc.
4. During inspection, once the printed images are scanned and transmitted they automatically sit face to face in perfect register with the True image with tolerance limit already in memory. The beauty of all these inspection systems are that they act as fast as the machine prints (speed). The Absolute image is converted into some signals and kept stored in memory console. When the printing is through, the printed images scanned by the inspection chamber too convert the print images into the same signal format as the Absolute image and sends them to the image command center. Once the signals match the signals of the Absolute print, the status of the sheets – all good or sheets with defect- gets determined.
5. In the case of On line print inspection system the inspection chamber quickly capture the image and transmit them to the print command center where the print is compared with the Absolute image (in this case tolerance limits are not set) and display the images on the screen monitor indicating the errors in small circle. The operator quickly address the defects with suitable remedial action such as feeding more ink, lessen the ink or push the ink in duct or stop the press and clean the plates etc. However all the sheets will only go in the single pile which will have both good and suspect sheets. This system only work as additional operator who is constantly examining the sheets and takes corrective action. The difference between normal checking and checking with this system is the speed with which each and every sheet is shown the error spots instantly before sheets reach delivery.
In the case of Table top or semi automatic machines as used by the Label printing printers, the label reel or multi up sheets are slowly moved under the inspection camera which acts similar to that of automatic inspection machines for comparing the images with the Absolute print stored with required tolerance in the command center. Unless the next label is moved under the inspecting camera, no further action will take place. However by the flash of Red light or a beep sound the defective labels will be identified. When the label is positioned below the inspection camera it scans the image and transmits to the inspection interface cum monitor where the Absolute Print of the actual print is stored with required tolerance. The inspection interface inspects the image and shows the defective ones on the monitor.
Again there are two varieties of table top inspection equipments. In one case the operator pause the inspection which still show the defective label and manually move the label below marker. Based on the key board operation the marker puts an X on the rejected label . ( See the illustration) The operator uses the wheel again to bring the next set of labels below camera and switch on the inspection. He continues to only mark the defective labels and roll it on a spool. The said roll will be re wound in another spool by another operator after replacing the defective labels with good ones. In effect this will require two sets of operators, but work will go on faster.
In the next type of equipment as the normal manual operation as stated above continues, the defective labels are also simultaneously replaced with good labels by the same operator who examines the labels. However in such case, the marking will not be done with ink jet printer. Since the Roll is manually wound forward, there will be enough time to replace the defective labels.
These equipments are preferred over manual examination because the finest and minutest lines, and letters will be read perfectly and they either passed or rejected without subjectivity in examination.