LITHOGRAPHIC OFFSET PRESS ACCEPTANCE TESTING
And
CRITERIA FOR ACCEPTANCE
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Section 1: Testing and Acceptance Conditions
Introduction
This document is sub-divided in four sections:
Section 1: Testing and Acceptance Conditions
Section 2: Description of Acceptance Testing
Section 3: Suggested Aim points and Tolerances
Section 4: Forms and Testing Procedure
Press acceptance testing should start after the complete installation of all mechanical, electrical and control sub systems and after the successful completion of all commissioning tasks. Acceptance testing schedules should be agreed upon the Press Manufacturer and Printing site at the time of contract.
Statistical analysis is used for all evaluations. All values are reported as an average (mean) followed by the variation from the mean.
Example 1: SNAP recommends that density variation not exceed ± 0.05 units. In this document, we express the same information statistically by stating that the standard deviation should not exceed 0.025. Under these conditions 95% of all samples will meet the SNAP Specification. Please see the graph in the back of this document.
Example 2: SNAP recommends register be kept at ± 0.012 inches. In this document we express the same information statistically by stating that the standard deviation is 0.006. Under these conditions 95% of all samples will meet the SNAP Specification.
This approach simplifies analyses and can be accomplished using software applications such as Microsoft Excel that calculate the standard deviation
The SNAP committee recommends that the acceptance testing be performed through three separate press runs:
1. The first test run is performed without dampening solution to evaluate the uniformity and consistency of ink application
2. The second test run is performed with both ink and dampening solution to evaluate the uniformity and consistency of dampening solution
3. The third test run is performed with both ink and dampening solution to evaluate pressline system presets, density, register, dot gain, starvation/ghosting, and slur.
Two selected press units from the four or more units on the pressline shall be used for the print quality acceptance testing in accordance with the procedures and criteria described below. More units would be tested only in the case of unacceptable results.
All acceptance testing referred to in this document should adhere to the following conditions:
1. Testing procedure and qualitative criteria conform to the SNAP guidelines.
2. The quantitative criteria for acceptance are strictly press related. Attainment will therefore be dependent on the:
a. quality, weight, condition, and characteristics of the newsprint
b. accuracy and image placement on the printing plate
c. accuracy of the plate bends relative to the plate location punched slots in the plates
d. quality of the ink and dampening solution
e. environmental conditions such as humidity and temperature
3. The tolerance figures cited are strictly press-related and do not include variation resulting from any measuring device.
4. The density tolerances cited reflect variation associated with a typical inking system fitted with digital inking. Open fountain inking systems may allow more density variation. If the press being evaluated employs an open fountain inking system then these tolerances should be discussed and written in the contract.
5. All plates shall come from the same plate processor line and meet the strict geometric criteria defined by the equipment manufacturer. The accuracy of the lay-down of the printing image on the plate and the squareness accuracy of the leading edge plate bend relative to the plate-to-press location slots shall be within ±0.025 mm.
6. All newsprint rolls used for the acceptance testing should:
a. have a moisture content not less than 8%
b. be wound evenly and in a concentric manner
c. be free from mill splices and edge damage
7. The equipment satisfies the criteria for acceptance when it is shown to be capable of substantially (95% of the samples) meeting the SNAP recommendation.
8. The difference in the newsprint color between the non-image area of the plate and the blanket gap shall be 0.03 optical density units or less. . This criteria does not apply to gapless presses.
9. The press must reach a typical operating temperature before sampling. Though not specified, the temperature typically ranges between _____ and _____.
10. The temperature in the pressroom should be between 68 and 83 degrees Fahrenheit and relative humidity between 50 and 65%.
11. If possible, all test sample copies for a particular test should be collected from one roll of paper.
12. Densities shall be reported as absolute, which means that paper density shall be included in these density measurements.
13. Dry-back is defined as the differences in a printed density immediately after printing and three hours later. Dryback is caused by to ink penetration into the paper. Dryback is defined as the try experience has seen typical dry-back values of 0.05 for offset lithography. In accordance with the SNAP recommendations, dry-back density values must be added back to any measurements to understand the actual printed density at the time the paper was printed.
14. Testing shall be performed for each press acceptance test at make-ready speed, mid-production speed, and production speed. These three speeds should be agreed-upon by the press manufacturer and the printer.
15. Clean Blankets verified for thickness, correctly torqued, and run in.
16. Press should be set to manufacturer specifications
17. Budgeting of testing time and material will have to be factored in at the beginning of the project.
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Section 2: Description of Acceptance Testing
First Round of Testing, Inking system Diagnostic test:
Test
To visually evaluate inking uniformity within the same unit and between units
Plates:
Dummy plates of the same thickness or clean used plates from any run.
Speed:
This test is performed at make-ready speed, mid-production speed, and production speed.
Condition:
The test shall be conducted with two full color webs running collect. It is advisable to avoid angle bars to simplify testing. The test is performed with one color at a time.
1. Set all ink keys to maximum coverage (one hundred percent).
2. Turn the water supply off but keep the dampening train engaged.
3. Clutch in the cyan unit and insure that magenta yellow and black units are not clutched
4. Print at make-ready speed. Wait for five thousand impressions if press is cold (one thousand if press is warm) then collect samples.
5. Density should be +/- .1 units from the SNAP aim points.
6. Change the press speed to mid-production speed.
7. Print without making any other changes. Wait for one thousand impressions and then collect samples.
8. Repeat step 7 production speed.
9. Repeat the above steps 1 to 8 for magenta yellow and black. Consult the press manufacturer in case of abnormalities.
Sampling:
Collect twenty consecutive impressions to produce sampling groups representing inking uniformity:
a. at make-ready speed after printing five thousand impressions if press is cold (one thousand impressions if press is warm)
b. at mid-production speeds after printing one thousand impressions
c. at full production speeds after printing one thousand impressions.
Label each sample impression collected for each of the three sample groups with the press speed and the sample number plus any other needed identifying information.
Samples measurement:
None. Although objective measurements can be taken, we recommend only a visual evaluation for samplings (a), (b) and (c) at this stage.
Measurement positions
None.
Evaluation:
Visually evaluate the 20 samples collected for each of the three sample groups for uniformity within one press unit. Compare the uniformity of the same color from different units. Consult with the press manufacturer in case of abnormalities such as uneven densities caused by factors such as streaks either across the web or in the direction of the web travel.
Acceptance Criteria:
All prints at all three speeds should be visually similar with no streaks across or along the web travel. Blanket gap streak should be the only exception if blankets are gaped. Consult the press manufacturer in case of abnormalities.
Add pictures to show what is acceptable or not
SECOND Round of Testing, dampening System Dianogist test:
Objective:
To evaluate registration accuracy and dampening uniformity visually and quantitatively within the same unit and between units
Plates:
“Twenty % Test Block” plates, which have a uniform solid coverage on 20% of the plate’s total image area (see attached test form), plus hairline registration marks at all four corners. The plate’s registration must be verified to be accurate prior to the plates being put on press.
Speed:
This test is performed at make-ready speed, mid and production speed.
Condition:
The test is conducted with all four colors at the same time on both sides of the web with two full color leads running collect. It is advisable to avoid angle bars to simplify testing.
1. Set all register gauges to zero.
2. Set the ink keys to twenty percent and ink keys next to margins will be needed to set slightly lower. The goal is even ink density across the web.
3. Print at make-ready speed with the lowest possible dampener setting while maintaining clean background. Wait for one thousand copies (five thousand copies for cold press) then collect samples.
4. Change the press speed to mid production speed. Print without making any changes. Wait for at least one thousand copies then collect samples.
5. Repeat the above step at production speed.
Sampling:
Collect twenty consecutive impressions to produce sampling groups representing inking uniformity:
a. at make-ready speed after printing five thousand impressions if press is cold (one thousand impressions if press is warm)
b. at mid-production speeds after printing one thousand impressions
c. at full production speeds after printing one thousand impressions.
Label each sample impression collected for each of the three sample groups with the press speed and the sample number plus any other needed identifying information.
Measured Samples:
Measure one impression of samplings (a), (b), and (c) for both sides of the web.
Measurement Positions:
Measure solid ink density at eight points in the horizontal direction inside each circle per page, side, and color. Measure register using a magnifier/loupe at the hairline cross hairs.
Evaluation:
Compare the lowest dampener settings achieved during Step 3 to the dampener presets. The two settings should be close.
Visually compare the printed density of one color lead to the other. If colors appear similar quantify the accuracy of density profile of each individual color relative to SNAP targets. The mean and standard deviation define the density profiles and is calculated from the average of one impression. Also evaluate register values and variation from the mean
Acceptance criteria
Determine the mean density of each of the four colors for each sample group collected at the three different speeds.
The difference between the three sample group means shall not exceed ± 0.05 density units from the SNAP target value. SNAP target solid ink density values (absolute density values measured prior to any dryback) are:
Cyan 0.90
Magenta 0.90
Yellow 0.85
Black 1.05
The standard deviation of density for each sample group across any web shall not exceed 0.03 density units for each color.
Without any manual or automatic registration adjustments being made or used, the cumulative, total mean register value for all samples shall not exceed ± 0.012” from black. The standard deviation of the “mean register” from black shall not exceed ± 0.009” (which is equal to a range of .036”) in either the sidelay or circumferential directions.
Example:
Third Round of Testing, Print with SNAP TEST Form:
Proceed with the following tests only upon the successful completion of the previous tests. All the following tests are conducted with one full color leads printing on one side of the web running collect. It is advisable to avoid angle bars to simplify testing.
Pre-setting
Test (1): Ink and Register Pre-setting
Repeatability
Test (2) Density Repeatability
Test (3) Register Repeatability
Stability
Test (4) Density Stability on Press Deceleration
Test (5) Register Stability on Press Deceleration
Test (6) Density Stability and Accuracy at production Speed
Test (7) Register Stability and Accuracy at production Speed
Test (8) Density Stability on Press Acceleration
Test (9) Register Stability on Press Acceleration
Dot Gain
Test (10) Dot Gain
Starvation
Test (11) Starvation
Ghosting
Test (12) Ghosting
Slur and Doubling
Test (13) Slur and doubling
Tinting and toning
Test (14) Tinting and toning
Pre-setting
Test 1: Ink and Register Pre-setting (When specified)
Objective:
To assess the accuracy of ink and register pre-setting during make-ready speed and quantify make-ready waste.
Plates:
SNAP Press Acceptance Test Form in all plate position, only one side of the sheet. The plates’ registration must be verified to insure accuracy prior to press. Dot area coverage (to within ± 1) and slur targets must also be verified.
Speed:
This test shall be performed at make-ready speed.
Condition:
Transmit the Raster Image Processor (RIP) data to the press from the pre-setting system, if available. RIP digital data must be calibrated prior to this test. RIP data accuracy shall be within 1% of the actual pre-press digital files.
Sampling:
Collect the first two thousand impressions off the folder, after white-waste, to quantify make-ready waste and then collect twenty consecutive samples after five thousand impressions (or after one thousand impressions if press is warm).
Measured Samples:
Measure four consecutive samples in each plate position.
Measurement Positions:
Density is measured in six points in the horizontal direction per page per color, as indicated on the attached map.
Evaluation:
Count the number of impressions before a visual saleable copy. Evaluate the accuracy of density profile of each individual color relative to SNAP targets. The mean and standard deviation define the density profile and are calculated from the average of four consecutive samples, forty eight readings per sample.
Quantify the average sidelay and circumferential register error in the crosshairs and vernier scale readings for four consecutive samples for all pages
Acceptance Criteria:
The number of impressions before the first saleable copy should be established based on a mutual agreement between the printer and the press manufacturer.