Print Quality Control System
Preventive quality control is before-the-fact action utilizing quality control procedures to prevent
problems before they occur and thus minimize downtime and waste. Quality control procedures are succesful when they are in sync with maintenance planning, continuous productivity improvement programs and business alignment strategies.
Total Production – and Prepress – Maintenance (TPM) concepts integrate with those of Preventive Maintenance (PM) and Total Quality Management (TQM), aiming at zero breakdowns. Apart from any formal certification program, print consistency can largely be obtained by adherence to standards as defined in the ISO 12647, “Graphic technology – Process control for the production of half-tone colour separations, proof and production prints”.
1. Viewing Conditions
The pressroom’s light levels must be high enough around the press to permit press crew to install
plates, blankets and packing easily and to make press adjustments without eye strain. Having a light shining between each pair of press units is recommended, not placed directly above the press but off to one side. Booths or tables where press sheets are viewed must conform to viewing standards. At one time, 7,500 K was recommended for assessing artwork, proofs and press sheets, but this is no longer the standard. Viewing at 5,000 K is the current standard approaching average white light as specified by ISO 3664.
ISO 3664 details the following spectral viewing conditions for prints and transparencies:
correlated colour temperature of lighting at 5,000 K;
spectral power distribution D 50;
colour rendering of natural daylight at least 90;
light intensity of print illumination 2,200 at + 4,770 lux;
uniformity of print illumination, i.e. viewing plane edges with at least 60% of the intensity in the center;
surround: neutral matt gray of 60% reflectance.
2. Densitometers
Reflection densitometers can measure density, dot gain, contrast, trap, hue error and grayness changes during a pressrun, and indicate differences between proofs and press sheets. Densitometers are to be conveniently located at each press inspection table and any other quality control area.
A formal density measurement program, determining the optical density tolerances of each job, will result in minimal colour fluctuations within a pressrun and keep the press operator aware of any changes in ink balance on the press.
Optical density tolerances for each pressrun are determined from density readings taken on an
approved press sheet before the pressrun starts. These readings are either manually recorded or stored in computerized press densitometers or in stand-alone electronic scanning densitometer databases.
Density aimpoints and tolerance ranges are established by a quality control department, and for each job one of the approved sheets is returned to the press. It is then the press operator’s responsibility to maintain colour balance both numerically (with a densitometer) and visually (by matching printed sheets with the ok-sheet). Printing densities are measured periodically throughout the pressrun, with a frequency depending on run length. Adjustments are made immediately if deviations from established optical density tolerances are detected.
Modern computerized presses permit density tolerances to be maintained within +0.04 density units.
Realistic tolerances for older mechanical presses may be between +0.05 and +0.06, but density
tolerances of +0.04 are often achieved when other press conditions are properly maintained.
In addition, spectrocolorimeters such as photospectrometers are to be used according to the
characterization tables as established by FOGRA and within the tolerances recommended by ISO
12647.
3. Plate-to-Blanket Squeeze
Plate-to-blanket squeeze is one of the most neglected conditions on printing presses. A mottled and broken printed image usually indicates inadequate squeeze. Excessive squeeze, which is more severe but also more difficult to detect, can result in dot gain and slur in the printed image, premature plate wear, and image blinding. These problems are often attributed to poor plates, inferior inks, or the mechanical conditions of the press, but the real cause may just as well be improper plate-to-blanket squeeze.
As part of a preventive quality control program, pressrooms should be equipped with at least one
blanket packing gauge for every four press units. The gauge can be used to quickly determine plate-to blanket squeeze after the plate and blanket cylinders are packed, and, if necessary, at convenient intervals during pressruns.
Plate-to-blanket squeeze tolerances vary depending on the type of blanket being used. Conventional blankets normally require a plate-to-blanket squeeze from 0.05 to 0.10 mm (0.002 to 0.004 in).
Compressible blankets usually require from 0.10 to 0.15 mm (0.004 to 0.006 in) squeeze, although
some produce optimum highlight and shadow detail with a squeeze as high as 0.20 mm (0.008 in), on both sheetfed and web presses.
Squeeze readings are made at the beginning of a pressrun to ensure that the recommended tolerances are met. Squeeze may change during a pressrun because of the compression set of a new blanket or because of blanket swelling caused by washup solvents that evaporate too slowly. When determining plate-to-blanket squeeze, the press operator should not assume that supplier specifications (for plate thickness, blabket thickness, packing sheet thickness and press cylinder undercuts) are accurate.
Variations often occur and they cannot be determined accurately without the use of a packing gauge.
4. Micrometer Measurements
Caliper of plates, blankets and packing sheets are often measured with machinist micrometers. Such micrometers are not recommended, because readings are sensitive to differences in feel and pressure, and different press operators will naturally obtain different readings. Ideally, the pressroom should be equipped with a table-model deadweight micrometer with a wide anvil of at least 14 mm (9/16 in) diameter. It has the advantage of allowing measurements both at the center and the edges, and because the device works on the principle of deadweight, human error is eliminated.
5. Fountain Solutions
Fountain solutions should provide clean startups of non-image areas, maintain a stable ink/water
balance, reduce emulsification and piling of ink and paper dust, keep maximum ink gloss, and improve
drying.
Recommendations for specific fountain (dampening) solution formulations are difficult to make because of the number of proprietary etches on the market. Fountain solution preparation is further complicated by variations in pH levels between different plants.
Fountain Solution pH
To achieve the optimum pH, it is recommended that fountain solutions be mixed to a pH between 4.5 and 5.5. This acidic tolerance seems to work best for most plants, and also provides the latitude to increase acidity to 4.0 if necessary. (Typically, pH is maintained at 4.5 for sheet offset, 5.0 for web offset, and 5.5 for newspaper printing.) Fountain solutions with a high acid concentration (below 4.0 pH) frequently cause a number of predictable problems, the most common of which are retarded ink drying and ink setoff, reducing print sharpness.
Once optimum pH is achieved, it is important to maintain pH consistency, which in turn is achieved by standardizing on one type of fountain solution throughout the pressroom. Fountain solution pH is measured at least once a shift, and even more often during longer high-speed runs where acid extracted from paper contaminates the fountain solution, increasing its acidity.
An alternative to conventional pH meters and pH strips is the monitoring of fountain solutions’
concentration with conductivity meters or, better, combined pH/conductivity meters.
Dampening Consistency
The ideal ink/water balance is achieved when the thickness of the fountain solution on the plate is half that of the ink – typically one micron of fountain solution to two or three of ink. To obtain and maintain a thin and uniform film of moisture on the plate, its surface tension has to be reduced. Mostly, alcohol is being used for this, although it may also increase the solution’s viscosity, resulting in film thickness variations. Alcohol is not supposed to affect pH, but impurities may change the fountain solution’s acidity and pH measurements should be made before and after adding alcohol.
An optional device with fountain solution recirculation systems is a refrigeration unit. The rate at which chemical reactions take place doubles for every 10°C (18°F) rise in temperature, and refrigeration minimizes viscosity and other chemical variations. Refrigerated recirculators are particularly valuable in dampening systems using alcohol substitutes.
Water Conditioning for Fountain Solutions
Water can be the principal contributor to problems encountered in the printing process. Erratic
dampening fluctuation, plate problems, and slow ink drying are just a few of the problems that can be solved by water treatment. Examples of water treatment systems and methods are filters, water softeners, de-ionization, and reverse osmosis.
Recycling
Filtration systems are designed to increase the life of fountain solutions and reduce wastewater
discharge. A filtration system engineered to fit a fountain solution recirculator can measure the rate of flow and incorporate filters to pick up particles as small as one micron. Filtration systems pick up contaminates that accumulate in the fountain solution, allowing for better pH and conductivity control for an extended time.
6. Ink Film Thickness
Inconsistent print quality is mostly the result of ink film thickness variations throughout pressruns. It may be the single most critical area for monitoring and improvements. Ink film thickness tolerance limits for sheetfed offset presses are normally between 0.2 and 0.4 mil (0.005-0.010 mm, 0.0002-0.0004 in), as measured on the oscillating roller above the form roller farthest from the dampening system. Webfed presses, which use less viscous inks, usually require a slightly higher ink film thickness, ranging from 0.3 to 0.6 mil (0.008-0.015 mm, 0.0003-0.0006 in).
Light forms, such as those composed of type only, usually require ink film thicknesses towards the lower end of this range, while heavy forms often require thicknesses approaching the higher tolerance limits. It should not be necessary to exceed the higher limits when using conventional lithographic inks.Fluorescent and metallic inks may require higher thicknesses to print properly, but even with such inks the high-end tolerance rarely exceeds 0.8 mil (0.02 mm, 0.0008 in).
At least one interchemical wet film thickness gauge, calibrated from 0 to 1 mil, should be available in the pressroom.The instrument need not be used often, but it should be used to establish ink film thickness tolerances during makeready and to determine how much ink is being run when problems occur thatmay be attributed to excessive ink, such as shadow area fill-in, slur, dot gain, slow ink drying, and setoffs. Inks are to comply with ink standard ISO 2846.
7. Roller Settings
Form rollers set too tightly against the plate are likely to result in plate wear and poor ink transfer. Form roller settings are to be checked at least once a week by dropping the ink form rollers onto the plate and by measuring the width of the resulting stripe. For most pvc (polyvinyl chloride) and urethane rollers, acceptable roller-to-plate stripes should measure between 3 and 5 mm (1/8 – 3/16 in, or about 1/16 in (1.6 mm) for each inch of roller diameter), but manufacturer’s specs should always be followed. See also ISO 12647-2.
Form rollers should be set to produce a slightly broader stripe on the oscillating rollers (5-6 mm or 3/16 to 1/4 in) than on the plate, so that they are driven by the oscillating rollers. If the setting between the form rollers and oscillating rollers is less than between the form rollers and the plate, the form rollers may be driven by the plate, which may result in plate wear, slur, improper inking of the plate, and a loss of control of the inking system. Insufficient pressure may also result in roller slip due to ink lubrication, causing printing problems and possible roller damage. Devices are available to help determine roller settings, such as roller setting gauges and (roller manufacturers’) ink stripe gauges.
8. Roller Hardness
Hard or glazed ink rollers result in poor ink distribution and premature plate wear. As part of preventive quality control, the roller hardness should be checked on a regular basis with a shoremeter or durometer. The term durometer also refers to the readings taken with the instrument. Form rollers should have a durometer between 25 and 30, and ink distributor rollers between 30 and 35. These tolerances apply to most rollers but may vary from manufacturer to manufacturer.
An ASTM Type-A Durometer measures the resistance of the roller compound with a spring-loaded
probe, giving a reading in units from 0 to 100. A reading of 100 indicates an inflexable surface such as cast iron. The hardness specs of a newly manufactured roller vary considerably depending on where the roller will be used and how it was made. For example, a form roller has a slightly lower durometer reading than an ink distributing roller. Upto 25% of the weight of a roller’s rubber covering is plasticizer.
Since plasticizer is leached out by washup solvents and solvents in the ink vehicle, some of the
plasticizer is inevitably lost. In addition, plasticizer is driven off by friction, further decreasing the roller diameter and hardening the roller.
Typically, a roller’s durometer increases 10 to 15 units during its useful lifetime. Beyond that, the roller becomes less and less effective and contributes increasingly to ghosting and roll-out problems, particularly if glaze is present. The roller’s durometer can often be reduced by applying a deglazing paste or, to a lesser extent, a regenerating solvent.
9. Spray Powder Control
Many advances have been made in developing rapid ink drying systems, but most printers still have to contend with spray powder to prevent setoff. The most common problems with spray powder, such as mechanical malfunctions, plate wear, hickeys and other spots, are caused by excessive use and/or by inadequate housekeeping. If excessive amounts of spray powder are applied to overcome ink setoff in the delivery pile, the press operator may be overlooking other causes of the problem, such as the acidity of the fountain solution, the thickness of the ink film, or the choice of ink and paper. A rule of thumb is that too much powder is being used when powder can actually be seen dropping onto the sheet in the press delivery.
Powder tends to accumulate in and around presses, and should be removed regularly. A sheet cleaner can be used to remove excess from prints that will be run through the press again, and will be even more effective by the use of a static eliminator. The removal of excess spray powder from the delivery and from the pressroom air may be facilitated by air purifying systems.
10. Paper Conditioning
Optimum paper storage conditions are at -rH of +/- 50% at 20 °C. If that can not be realised then
wrappers are to be left on until printing and again after printing.
Paper temperature should always be the same as the room temperature. Paper should be brought into the pressroom for climatisation but with wrappers on.
Paper prepared for printing usually contains some 6% moisture and is hygroscopic, i.e., takes on or gives off moisture until a balance is reached with the relative humidity of the room. A sword hygroscope allows press operators to determine the moisture content in a stack of paper and, therefore, predict potential printing or folding problems. Excessive moisture content (above 7%) causes register problems because of the increased dimensional instability of the paper. Inadequate moisture (below 5%) often causes folding problems.
Paper will pick up moisture from the fountain solution on the blanket. If paper moisture is below 6% and the pressroom’s relative humidity is more than 50%, the moisture content of the paper will become particularly excessive as the paper passes through the first printing unit of the press, because upto 70% of the moisture that dampens a plate may be absorbed by the paper.
Reducing or increasing the moisture content in a given stack of paper is difficult, and the only solution to chronic problems in this area would be the regulation of the pressroom’s relative humidity and temperature. If infrared dryers are used, the pile temperature of sheets should not exceed 35°C (100°F), in order to avoid blistering and folding problems.
Diffusion of moisture to or from a pile of paper is so slow that only the paper’s edges become
acclimatized to the surrounding air. This uneven acclimatization inevitably leads to wavy or tight edges and subsequent problems in printing. Loads should be printed without delay once they are unwrapped.
Printed loads should be rewrapped or covered with reusable plastic moisture-proof covers immediately after each pass through the press to minimize dimensional change and edge distortion.
11. Paper Waste
Waste levels can only be reduced when at different stages (makeready, printrun, finishing) paper
consumption can be measured. Direct machine interfaces provide the data needed to focus on specific improvement efforts. Other technology succesful in reducing both makeready and paper waste are electronic ink key presets and semi-automatic plate changers. However, for paper waste controls to provide optimum benefits, management will have to provide incentives and budgets for continuous staff training, while creating a culture of waste reduction consciousness and well-defined objectives.
Often, paper is wasted because stacks get ruined by forklift operators, or are handled inefficiently during transportation to the press. Forklift prongs and clamp truck jaws can be lined with soft material to protect paper stacks, and proper paper handling needs to be included in staff’s instruction.
12. Overall Pressroom Conditions
Unnecessary accumulation of grease and waste on floors around presses and excessive dust and spray powder buildup on overhead light fixtures, rafters, presses and postpress equipment indicates that staff is paying little attention to the working environment and reflects the attitude that attention to detail and consistency would be unimportant.
Housekeeping can be included in a preventive maintenance program, and will thus benefit both quality and productivity.
Using the appropriate washup solvents makes the cleaning job easier, more efficient, and less
hazardous to health. Most blanket and roller manufacturers provide specifications as to washup solvents to be used. Automatic blanket washing devices can be retrofitted to most presses.
Regardless of whether a formal quality control department exists or not, every printing company needs a clearly defined preventive quality control program with which the controls, inspections and standards can be enforced, monitored and, where necessary, remediated that are necessary to achieve the company’s quality goals. When quality control is made everyone’s job and staff is knowledgeable about it, preventive inspections become normal production routines and will also benefit productivity improvement efforts.
13. Some Prepress Maintenance Issues
Prepress production rooms need to be clean and dust-free, and must be properly ventilated. Surfaces and floors should be damp-cleaned regularly.
Rapid or drastic changes in temperature and humidity may cause condensation, which can in turn cause electronic or mechanical failure. Room temperature should be maintained at 20-24°C (68-75°F), relative humidity at 45-55%. Air pressure should be kept positive, preferably using filtered air.
The use of anti-static devices and static eliminating equipment is recommended.
Documentation on the use and maintenance of all devices should be readily available, including a
maintenance program with specifications on deadlines and responsibilities.
problems before they occur and thus minimize downtime and waste. Quality control procedures are succesful when they are in sync with maintenance planning, continuous productivity improvement programs and business alignment strategies.
Total Production – and Prepress – Maintenance (TPM) concepts integrate with those of Preventive Maintenance (PM) and Total Quality Management (TQM), aiming at zero breakdowns. Apart from any formal certification program, print consistency can largely be obtained by adherence to standards as defined in the ISO 12647, “Graphic technology – Process control for the production of half-tone colour separations, proof and production prints”.
1. Viewing Conditions
The pressroom’s light levels must be high enough around the press to permit press crew to install
plates, blankets and packing easily and to make press adjustments without eye strain. Having a light shining between each pair of press units is recommended, not placed directly above the press but off to one side. Booths or tables where press sheets are viewed must conform to viewing standards. At one time, 7,500 K was recommended for assessing artwork, proofs and press sheets, but this is no longer the standard. Viewing at 5,000 K is the current standard approaching average white light as specified by ISO 3664.
ISO 3664 details the following spectral viewing conditions for prints and transparencies:
correlated colour temperature of lighting at 5,000 K;
spectral power distribution D 50;
colour rendering of natural daylight at least 90;
light intensity of print illumination 2,200 at + 4,770 lux;
uniformity of print illumination, i.e. viewing plane edges with at least 60% of the intensity in the center;
surround: neutral matt gray of 60% reflectance.
2. Densitometers
Reflection densitometers can measure density, dot gain, contrast, trap, hue error and grayness changes during a pressrun, and indicate differences between proofs and press sheets. Densitometers are to be conveniently located at each press inspection table and any other quality control area.
A formal density measurement program, determining the optical density tolerances of each job, will result in minimal colour fluctuations within a pressrun and keep the press operator aware of any changes in ink balance on the press.
Optical density tolerances for each pressrun are determined from density readings taken on an
approved press sheet before the pressrun starts. These readings are either manually recorded or stored in computerized press densitometers or in stand-alone electronic scanning densitometer databases.
Density aimpoints and tolerance ranges are established by a quality control department, and for each job one of the approved sheets is returned to the press. It is then the press operator’s responsibility to maintain colour balance both numerically (with a densitometer) and visually (by matching printed sheets with the ok-sheet). Printing densities are measured periodically throughout the pressrun, with a frequency depending on run length. Adjustments are made immediately if deviations from established optical density tolerances are detected.
Modern computerized presses permit density tolerances to be maintained within +0.04 density units.
Realistic tolerances for older mechanical presses may be between +0.05 and +0.06, but density
tolerances of +0.04 are often achieved when other press conditions are properly maintained.
In addition, spectrocolorimeters such as photospectrometers are to be used according to the
characterization tables as established by FOGRA and within the tolerances recommended by ISO
12647.
3. Plate-to-Blanket Squeeze
Plate-to-blanket squeeze is one of the most neglected conditions on printing presses. A mottled and broken printed image usually indicates inadequate squeeze. Excessive squeeze, which is more severe but also more difficult to detect, can result in dot gain and slur in the printed image, premature plate wear, and image blinding. These problems are often attributed to poor plates, inferior inks, or the mechanical conditions of the press, but the real cause may just as well be improper plate-to-blanket squeeze.
As part of a preventive quality control program, pressrooms should be equipped with at least one
blanket packing gauge for every four press units. The gauge can be used to quickly determine plate-to blanket squeeze after the plate and blanket cylinders are packed, and, if necessary, at convenient intervals during pressruns.
Plate-to-blanket squeeze tolerances vary depending on the type of blanket being used. Conventional blankets normally require a plate-to-blanket squeeze from 0.05 to 0.10 mm (0.002 to 0.004 in).
Compressible blankets usually require from 0.10 to 0.15 mm (0.004 to 0.006 in) squeeze, although
some produce optimum highlight and shadow detail with a squeeze as high as 0.20 mm (0.008 in), on both sheetfed and web presses.
Squeeze readings are made at the beginning of a pressrun to ensure that the recommended tolerances are met. Squeeze may change during a pressrun because of the compression set of a new blanket or because of blanket swelling caused by washup solvents that evaporate too slowly. When determining plate-to-blanket squeeze, the press operator should not assume that supplier specifications (for plate thickness, blabket thickness, packing sheet thickness and press cylinder undercuts) are accurate.
Variations often occur and they cannot be determined accurately without the use of a packing gauge.
4. Micrometer Measurements
Caliper of plates, blankets and packing sheets are often measured with machinist micrometers. Such micrometers are not recommended, because readings are sensitive to differences in feel and pressure, and different press operators will naturally obtain different readings. Ideally, the pressroom should be equipped with a table-model deadweight micrometer with a wide anvil of at least 14 mm (9/16 in) diameter. It has the advantage of allowing measurements both at the center and the edges, and because the device works on the principle of deadweight, human error is eliminated.
5. Fountain Solutions
Fountain solutions should provide clean startups of non-image areas, maintain a stable ink/water
balance, reduce emulsification and piling of ink and paper dust, keep maximum ink gloss, and improve
drying.
Recommendations for specific fountain (dampening) solution formulations are difficult to make because of the number of proprietary etches on the market. Fountain solution preparation is further complicated by variations in pH levels between different plants.
Fountain Solution pH
To achieve the optimum pH, it is recommended that fountain solutions be mixed to a pH between 4.5 and 5.5. This acidic tolerance seems to work best for most plants, and also provides the latitude to increase acidity to 4.0 if necessary. (Typically, pH is maintained at 4.5 for sheet offset, 5.0 for web offset, and 5.5 for newspaper printing.) Fountain solutions with a high acid concentration (below 4.0 pH) frequently cause a number of predictable problems, the most common of which are retarded ink drying and ink setoff, reducing print sharpness.
Once optimum pH is achieved, it is important to maintain pH consistency, which in turn is achieved by standardizing on one type of fountain solution throughout the pressroom. Fountain solution pH is measured at least once a shift, and even more often during longer high-speed runs where acid extracted from paper contaminates the fountain solution, increasing its acidity.
An alternative to conventional pH meters and pH strips is the monitoring of fountain solutions’
concentration with conductivity meters or, better, combined pH/conductivity meters.
Dampening Consistency
The ideal ink/water balance is achieved when the thickness of the fountain solution on the plate is half that of the ink – typically one micron of fountain solution to two or three of ink. To obtain and maintain a thin and uniform film of moisture on the plate, its surface tension has to be reduced. Mostly, alcohol is being used for this, although it may also increase the solution’s viscosity, resulting in film thickness variations. Alcohol is not supposed to affect pH, but impurities may change the fountain solution’s acidity and pH measurements should be made before and after adding alcohol.
An optional device with fountain solution recirculation systems is a refrigeration unit. The rate at which chemical reactions take place doubles for every 10°C (18°F) rise in temperature, and refrigeration minimizes viscosity and other chemical variations. Refrigerated recirculators are particularly valuable in dampening systems using alcohol substitutes.
Water Conditioning for Fountain Solutions
Water can be the principal contributor to problems encountered in the printing process. Erratic
dampening fluctuation, plate problems, and slow ink drying are just a few of the problems that can be solved by water treatment. Examples of water treatment systems and methods are filters, water softeners, de-ionization, and reverse osmosis.
Recycling
Filtration systems are designed to increase the life of fountain solutions and reduce wastewater
discharge. A filtration system engineered to fit a fountain solution recirculator can measure the rate of flow and incorporate filters to pick up particles as small as one micron. Filtration systems pick up contaminates that accumulate in the fountain solution, allowing for better pH and conductivity control for an extended time.
6. Ink Film Thickness
Inconsistent print quality is mostly the result of ink film thickness variations throughout pressruns. It may be the single most critical area for monitoring and improvements. Ink film thickness tolerance limits for sheetfed offset presses are normally between 0.2 and 0.4 mil (0.005-0.010 mm, 0.0002-0.0004 in), as measured on the oscillating roller above the form roller farthest from the dampening system. Webfed presses, which use less viscous inks, usually require a slightly higher ink film thickness, ranging from 0.3 to 0.6 mil (0.008-0.015 mm, 0.0003-0.0006 in).
Light forms, such as those composed of type only, usually require ink film thicknesses towards the lower end of this range, while heavy forms often require thicknesses approaching the higher tolerance limits. It should not be necessary to exceed the higher limits when using conventional lithographic inks.Fluorescent and metallic inks may require higher thicknesses to print properly, but even with such inks the high-end tolerance rarely exceeds 0.8 mil (0.02 mm, 0.0008 in).
At least one interchemical wet film thickness gauge, calibrated from 0 to 1 mil, should be available in the pressroom.The instrument need not be used often, but it should be used to establish ink film thickness tolerances during makeready and to determine how much ink is being run when problems occur thatmay be attributed to excessive ink, such as shadow area fill-in, slur, dot gain, slow ink drying, and setoffs. Inks are to comply with ink standard ISO 2846.
7. Roller Settings
Form rollers set too tightly against the plate are likely to result in plate wear and poor ink transfer. Form roller settings are to be checked at least once a week by dropping the ink form rollers onto the plate and by measuring the width of the resulting stripe. For most pvc (polyvinyl chloride) and urethane rollers, acceptable roller-to-plate stripes should measure between 3 and 5 mm (1/8 – 3/16 in, or about 1/16 in (1.6 mm) for each inch of roller diameter), but manufacturer’s specs should always be followed. See also ISO 12647-2.
Form rollers should be set to produce a slightly broader stripe on the oscillating rollers (5-6 mm or 3/16 to 1/4 in) than on the plate, so that they are driven by the oscillating rollers. If the setting between the form rollers and oscillating rollers is less than between the form rollers and the plate, the form rollers may be driven by the plate, which may result in plate wear, slur, improper inking of the plate, and a loss of control of the inking system. Insufficient pressure may also result in roller slip due to ink lubrication, causing printing problems and possible roller damage. Devices are available to help determine roller settings, such as roller setting gauges and (roller manufacturers’) ink stripe gauges.
8. Roller Hardness
Hard or glazed ink rollers result in poor ink distribution and premature plate wear. As part of preventive quality control, the roller hardness should be checked on a regular basis with a shoremeter or durometer. The term durometer also refers to the readings taken with the instrument. Form rollers should have a durometer between 25 and 30, and ink distributor rollers between 30 and 35. These tolerances apply to most rollers but may vary from manufacturer to manufacturer.
An ASTM Type-A Durometer measures the resistance of the roller compound with a spring-loaded
probe, giving a reading in units from 0 to 100. A reading of 100 indicates an inflexable surface such as cast iron. The hardness specs of a newly manufactured roller vary considerably depending on where the roller will be used and how it was made. For example, a form roller has a slightly lower durometer reading than an ink distributing roller. Upto 25% of the weight of a roller’s rubber covering is plasticizer.
Since plasticizer is leached out by washup solvents and solvents in the ink vehicle, some of the
plasticizer is inevitably lost. In addition, plasticizer is driven off by friction, further decreasing the roller diameter and hardening the roller.
Typically, a roller’s durometer increases 10 to 15 units during its useful lifetime. Beyond that, the roller becomes less and less effective and contributes increasingly to ghosting and roll-out problems, particularly if glaze is present. The roller’s durometer can often be reduced by applying a deglazing paste or, to a lesser extent, a regenerating solvent.
9. Spray Powder Control
Many advances have been made in developing rapid ink drying systems, but most printers still have to contend with spray powder to prevent setoff. The most common problems with spray powder, such as mechanical malfunctions, plate wear, hickeys and other spots, are caused by excessive use and/or by inadequate housekeeping. If excessive amounts of spray powder are applied to overcome ink setoff in the delivery pile, the press operator may be overlooking other causes of the problem, such as the acidity of the fountain solution, the thickness of the ink film, or the choice of ink and paper. A rule of thumb is that too much powder is being used when powder can actually be seen dropping onto the sheet in the press delivery.
Powder tends to accumulate in and around presses, and should be removed regularly. A sheet cleaner can be used to remove excess from prints that will be run through the press again, and will be even more effective by the use of a static eliminator. The removal of excess spray powder from the delivery and from the pressroom air may be facilitated by air purifying systems.
10. Paper Conditioning
Optimum paper storage conditions are at -rH of +/- 50% at 20 °C. If that can not be realised then
wrappers are to be left on until printing and again after printing.
Paper temperature should always be the same as the room temperature. Paper should be brought into the pressroom for climatisation but with wrappers on.
Paper prepared for printing usually contains some 6% moisture and is hygroscopic, i.e., takes on or gives off moisture until a balance is reached with the relative humidity of the room. A sword hygroscope allows press operators to determine the moisture content in a stack of paper and, therefore, predict potential printing or folding problems. Excessive moisture content (above 7%) causes register problems because of the increased dimensional instability of the paper. Inadequate moisture (below 5%) often causes folding problems.
Paper will pick up moisture from the fountain solution on the blanket. If paper moisture is below 6% and the pressroom’s relative humidity is more than 50%, the moisture content of the paper will become particularly excessive as the paper passes through the first printing unit of the press, because upto 70% of the moisture that dampens a plate may be absorbed by the paper.
Reducing or increasing the moisture content in a given stack of paper is difficult, and the only solution to chronic problems in this area would be the regulation of the pressroom’s relative humidity and temperature. If infrared dryers are used, the pile temperature of sheets should not exceed 35°C (100°F), in order to avoid blistering and folding problems.
Diffusion of moisture to or from a pile of paper is so slow that only the paper’s edges become
acclimatized to the surrounding air. This uneven acclimatization inevitably leads to wavy or tight edges and subsequent problems in printing. Loads should be printed without delay once they are unwrapped.
Printed loads should be rewrapped or covered with reusable plastic moisture-proof covers immediately after each pass through the press to minimize dimensional change and edge distortion.
11. Paper Waste
Waste levels can only be reduced when at different stages (makeready, printrun, finishing) paper
consumption can be measured. Direct machine interfaces provide the data needed to focus on specific improvement efforts. Other technology succesful in reducing both makeready and paper waste are electronic ink key presets and semi-automatic plate changers. However, for paper waste controls to provide optimum benefits, management will have to provide incentives and budgets for continuous staff training, while creating a culture of waste reduction consciousness and well-defined objectives.
Often, paper is wasted because stacks get ruined by forklift operators, or are handled inefficiently during transportation to the press. Forklift prongs and clamp truck jaws can be lined with soft material to protect paper stacks, and proper paper handling needs to be included in staff’s instruction.
12. Overall Pressroom Conditions
Unnecessary accumulation of grease and waste on floors around presses and excessive dust and spray powder buildup on overhead light fixtures, rafters, presses and postpress equipment indicates that staff is paying little attention to the working environment and reflects the attitude that attention to detail and consistency would be unimportant.
Housekeeping can be included in a preventive maintenance program, and will thus benefit both quality and productivity.
Using the appropriate washup solvents makes the cleaning job easier, more efficient, and less
hazardous to health. Most blanket and roller manufacturers provide specifications as to washup solvents to be used. Automatic blanket washing devices can be retrofitted to most presses.
Regardless of whether a formal quality control department exists or not, every printing company needs a clearly defined preventive quality control program with which the controls, inspections and standards can be enforced, monitored and, where necessary, remediated that are necessary to achieve the company’s quality goals. When quality control is made everyone’s job and staff is knowledgeable about it, preventive inspections become normal production routines and will also benefit productivity improvement efforts.
13. Some Prepress Maintenance Issues
Prepress production rooms need to be clean and dust-free, and must be properly ventilated. Surfaces and floors should be damp-cleaned regularly.
Rapid or drastic changes in temperature and humidity may cause condensation, which can in turn cause electronic or mechanical failure. Room temperature should be maintained at 20-24°C (68-75°F), relative humidity at 45-55%. Air pressure should be kept positive, preferably using filtered air.
The use of anti-static devices and static eliminating equipment is recommended.
Documentation on the use and maintenance of all devices should be readily available, including a
maintenance program with specifications on deadlines and responsibilities.
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