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ME 8883 Physical Properties of Paper MeasurementLecture 10: Surface Roughness:

a)Parker Print Surf, b) Sheffield, c) Bendtsen, d) Emveco Stylus

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Adventures in Printability and Roughness

• Generally the air leak method for roughness is still the best predictor of printability although the correlation is poor

• it is an off-line and destructive test• several different paper topographies can

produce the same apparent air leak roughness although their printabilities will be different

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A dynamic paper compressibility test

• A modified Emveco caliper gauge was linked to a digitizing oscilloscope, load applied by a solenoid valve connecting a piston to an air reservoir

• A Kelvin-Voigt spring dashpot viscoelastic model fitted the resulting waveforms to determine the effective out of plane modulus and response to the impact

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The modulus here is the out of plane modulus measured by a dynamic caliper gauge whose output is measured by a digitizing oscilloscope

WRC –western red cedar

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Here we mention PPS – Parker Print Surf measurement of surface roughness

The suggestion here is that as paper has a higher modulus (through higher density) it will also be smoother – but will it print better ?

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Parker print Surf measuring head detail

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Slide from Testing Machines Incorporated

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“Head” geometries of various air leak instruments

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However, the gap g for paper is not a simple constant but is dependent of the surface pore number, shape and distribution

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Why contact air leak measurement ? Why not measure the roughness optically ?

This has been considered before, started with the reflection of radar waves off ocean waves in WWII, theory in Beckmann and Spizzichino “the Scattering of Electromagnetic Waves from Rough Surfaces”

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Warren and Peel (1973) did use long wavelength IR scatter to characterize paper roughness – not easy.

SCATTERING OF INFRARED RADIATION AS A METHOD OF MEASURING PAPER ROUGHNESS. Part 1Warren, C. A. Peel, J. D; Source: Paper Technology, v 14, n 2, 91-8 [T55-62], April, 1973

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For specular reflection, the incident and reflection angles are the same, moreover the reflected light is largely linearly polarized in the s direction, i.e., perpendicular to the plane of incidence.

Optical scattering from surfaces works well for ground metal surfaces, paper because it contains many different scales of roughness does not work.

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A built in visible laser diode emits a laser beam from the bottom of the gage illuminating the surface beneath it at a shallow incident angle. After striking the surface, the laser light is reflected and scattered back into the Lasercheck detection system. As surface roughness increases, the "specular" laser reflection decreases in relative intensity and the diffuse or "scattered light" reflection increases in relative intensity. This overall intensity and distribution of the reflected and scattered light is measured by an array of small closely packed detectors. This measurement is digitized by Lasercheck electronics, and then Ra roughness is calculated for the illuminated area

“Lasercheck” by Optical Dimensions CA - used on metal roll grinding

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Walter Bloechle, Hohner Corp.

Figure 2.An absolutely smooth surface reflects light from a point source at an angle qr equal to the angle of incidence qi.

Figure 3. Light from a point source is scattered by a rough surface into multiple reflections of equal intensity. The amount of light reflected at angle qr is equal to that reflected at an angle normal to the surface

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One attempt a optically measuring the roughness of paper was in the Domtar version of determining the amplitude of uneven light reflectance at high frequency as the paper moved past the sensor,

Most attempts at optical surface roughness detection to date ended up being too sensitive to paper gloss or brightness and cannot measure the small changes in surface roughness

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So back to contact air leak, but it is dependent on the number and shape of surface pores and paper compressibility.

The “gap” g measured in an integral of the topography over the measurement geometry

Fo here is the fraction of surface in flat contact with the top

Taken from P. Mangin (1989)

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Is all paper rough?

Micrograph of lightweight coated paper

Fibres

Coating particles Pits

Several scales of roughness exist simultaneously

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Calendering is the process where dried paper on a paper machine is passed through several pressurized nips comprised of mating rolls to reduce the paper thickness by about 50%, this “squeezes” out the pore volume

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Relationship between paper bulk, roughness and gloss:

As paper is densified in calendering, it becomes smoother and glossier

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PPS Tappi method excerpts

The most common use of the PPS is the S-10 method, soft rubber backing and a clamping pressure of 10 kgf/cm^2

For very smooth papers, e.g coated magazine, the H-20 is also used: Hard backing with a higher clamping pressure of 20 kgf/cm^2

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Correlations between various air leak instruments is known to be grade dependent, here are some graphs taken from the 1996 L&W catalogue with their disclaimer

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Reference study on air-leak roughness interrelationships for newsprint

Inter-Relationship Among Air-Leak Roughness/Smoothness Methods: Canadian Newsprint StudyBichard, W. Source: Pulp and Paper Canada, v 93, n 6, 43-48, June, 1992ISSN: 0316-4004

Abstract: Canada, the world's largest exporter of newsprint, exported 8,600,000 mt to more than 55 countries in 1988. International trade of newsprint subjects it to tariffs and regulations. The Customs Co-operation Council (CCC), an international organization, has responsibilities that include the proposition of customs tariff nomenclatures. The CCC developed the Harmonized Commodity Description and Coding System (HS) that has been implemented by 52 countries to date. HS became effective January 1, 1988. Its definition of newsprint includes smoothness/roughness requirements measured with Bekk instruments, as does that of the European Community Tariff Schedule (ECTS). The Bekk method is not commonly used in Canada, and it is suggested thatits precision is questionable for newsprint. Smoothness/roughness testers used in Canadian laboratories include Parker Print-Surf, Sheffield, and Bendtsen. A study using 40 commercial newsprint samples from across Canada was conducted to determine how these test methods are related to the Bekk method. All four methods use the air-leak measurement principle, which states that the greater the roughness, the greater the flow or leakage of air across it. While the Bekk method measures smoothness, the other three measure roughness. Each method is described. The Bekk test had the highest in-sample error, followed by the Bendtsentest. The Parker Print-Surf method had the least error. The Bekk method was better suited to discerning differences in very smooth papers. The majority of newsprint samples tested met the HS classification for newsprint (Bekk smoothness under 200 sec), but a significant number had difficulty meeting the ECTS subclassification of 130 sec Bekk. The Parker Print-Surf measurement gave a better prediction of Bekk results. (3 fig., 12 ref., 5 tab.)

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Drawings of the Bendtsenmeasuring head illustrating the principles of measurement – from an 1996 L&W catalogue

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Developed by Bendtsen in Denmark in1940

laboratory air is fed from a small compressor to a mechanical pressure regulator which regulates the air supply to the measuring head at 1.47 kPa, equivalent to 150mm water gauge

The low pressure air is fed to the centre of a metal annulus, 0.15mm wide, which rests on the test sample. The sample is supported by a glass plate. The rate of flow of air between the annulus and the paper is measured by variable area flowmeters

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compressor pressureregulator

variablearea

flowmetersmeasuring

head

the Bendtsen roughness tester system

slide material from TMI ,Inc Michael Moore

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Deficiencies of the Bendtsen design

• the simple mechanical air pressure regulator often cannot maintain the measuring air pressure within the limits specified by most standards

• the variable area flowmeters are subject to wear of the floats and the bores of the tubes

• the length and internal diameter of the flexible tube connecting the measuring head to the instrument reduces the actual pressure at the measuring head by an unknown amount

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The design of the measuring head makes it act like a hovercraft and its effective mass decreases with air flow

This limits the effective range of the instrument to 50 - 1200 ml/min

the measuring land is easily damaged if droppedpaper

low pressure air

flow due toroughness

flow due topermeability

glass plate

steel annulus,0.15mm wide

cross section of Bendtsen measuring head

During the test a large area of the test piece is exposed to airunder pressure. Air flow due to porosity will be counted as flow due to roughness

Glass plate is in back of the air flowmeters Manostate

weights regulate pressure, typically use the 150 mm water column one

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The combined effect of these deficiencies results in an instrument with very poor reproducibility and high maintenance requirementBendsten tests are highly operator dependent because the measuring head is manually positioned and the time between placing the head and reading the flowmeter is not specified

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Sheffield air leak instrument: manometers, glass plate backing, popular for newsprint for many decades

Calibration air leaks for manometer tubes 1,2,3 are located here

Before measurement check:

Remove the quick-connect hose, place in a selected calibrated air leak and adjust the high and zero value with the toggle valve up or down accordingly .

Manometer adjustments for calibration check

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•The apparatus consists of an air supply, a pressure controller, a pressure measuring device, an air flow measuring device, and a test head assembly which houses a flat plate, the measuring head, and a mechanical device that lowers the test head onto the specimen that is inserted between the measuring head and flat plate.

• The measuring head has concentric annular lands of a total area of 97+/- 3 mm2

• The total mass of the measuring head is 1.640+/- 0.005kg.• Range is 0 to 3400mL/min (0-400 SU)

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Parker Print Surf instrument operational detail

1965• John Parker was a scientist with the Bowater Corporation at its newsprint mills in Kent, UK

• He wanted a roughness test for newsprint whichgave results in a form which was meaningful to printers

Why design another roughness tester?

letterpress printing plate

press packing

paper

ink

half-tone dot, 0.050mm diameter

letterpress printing plate

Roughness is an important contributor to printability

Existing roughness testers did not attempt to simulate the nip pressures and packing hardness of the letterpress and litho printing processes

BUT

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measuring landguard land guard land

air in air out

centre lineof head

central airvent

cross section of Print-surf measuring head

The Print-surf reference plane is a steel strip 100mm long and 0.051mm wide, formed into an annulus

steel guard rings spaced at

each side of the annulus prevent the escape of air along the paper surface. The small overall dimensions of the measuring head minimize the area of paper exposed to possible leakage through air permeance

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Parker-print Surf components

sensing head

test piece

resilient backing assembly

pneumaticclamp

glass flowmeter tubes

vent

analoguepressure

gauge

clamppressureregulator

air in

pressureregulator

analoguepressure

gauge

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Printing on rough paper

Printing plate

Ink film

paper

Effect of the measurement being proportional to the cube root of the mean gap, different topographies can get the same air leak roughness however…

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Select “cP 1000” and “S” for the S-10 measurement, using the soft backing

For very smooth papers select cP2000 and “H” for PPS H-20

“R” is for roughness

Parker Print Surf - front panel options

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Print-surf Calibration

• The measuring system and electronics may be checked by measuring an external fluidic impedance (“dummy head”)

• The dummy head simulates a piece of paper of known roughness, placed in the measuring nip.

• The instrument reading should agree with the assigned roughness value of the dummy head to within ±0.05µm

• The dummy head does not check the most critical part of the instrument – the measuring nip.

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Test and Calibration Disc Set

Three sets of 6 discs, in 3 ranges

Spare hard backings

Zero check film

Cotton gloves for handling

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Print-surf MaintenanceRoutine Check #1

Clamp a piece of soft rubber sheet in the nip and take reading

Result should be <0.3µm

If not, check for leaks in the air tubing or at the head manifold.

Fix leaks or reapply grease seal on manifold

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Print-surf MaintenanceRoutine Check #2

Measure a piece of clean zero check film at 1000kPa

Result should be <0.3µm

If not, clean film with lens tissue and remeasure

If still high, inspect measuring land for scratches

Look for air leaks as in #1

Damaged measuring heads must be returned for refurbishment

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The Print-Surf and Printability

letterpress Excellent Original design brief

Litho Excellent Original design brief

Gravure Good Clamp pressure just low enough

Flexo Not good Range too lowClamp pressures too high

Ink jet Not good Clamp pressures too high

xerography Not good Clamp pressures too high

This has been shown to correlate well with linerboard flexographic printability, a stylus rides along the surface as a strip is pulled through, the measured surface undulations calculated as a “microdeviation”

Output and graph are on the attached PC

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Microdeviation are the differences squared, microaverage is equation (1)

Load on “STANDBY” sample strip in CD or MD orientation with strip under the pulley wheel, set to “RUN”,start measurement form software

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Assignment #2

• Using the sample set from assignment #1: linerboard, medium, magazine paper, newsprint, synthetic paper, Mylar, measure the surface roughness using

1. Bendtsen2. Sheffield3. Parker-print Surf S-10 and PPS H-204. Emveco stylus, MD and CD Describe your procedure in detail, instrument checks made, plot

the results and comment/explain on the similarities/differences, try to find correlations between the different measurements. Remember there are 2 sides and 2 directions to a paper specimen