ISO 8791-4:2007
(Main)Paper and board — Determination of roughness/smoothness (air leak methods) — Part 4: Print-surf method
Paper and board — Determination of roughness/smoothness (air leak methods) — Part 4: Print-surf method
ISO 8791-4:2007 specifies a method for determining the roughness of paper and board using an apparatus which complies with the Print-surf method, as defined in ISO 8791-4:2007. It is applicable to all printing papers and boards with which it is possible to form a substantially airtight seal against the guard lands of the measuring head.
Papier et carton — Détermination de la rugosité/du lissé (méthodes du débit d'air) — Partie 4: Méthode Print-surf
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 8791-4
Second edition
2007-11-15
Paper and board — Determination of
roughness/smoothness (air leak
methods) —
Part 4:
Print-surf method
Papier et carton — Détermination de la rugosité/du lissé (méthodes du
débit d'air) —
Partie 4: Méthode Print-surf
Reference number
ISO 8791-4:2007(E)
©
ISO 2007
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ISO 8791-4:2007(E)
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ISO 8791-4:2007(E)
Contents Page
Foreword. iv
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Principle.2
5 Apparatus .2
6 Sampling.6
7 Conditioning.6
8 Preparation of test pieces.6
9 Procedure .7
10 Calculation.8
11 Precision.8
12 Test report .9
Annex A (normative) Calculation of roughness in micrometres.10
Annex B (normative) Maintenance of Print-surf roughness testers .12
Annex C (normative) Calibration of variable-area flowmeters .14
Annex D (normative) Calibration of Print-surf instruments against ISO reference standards .16
Annex E (normative) Calibration of an impedance instrument for the purpose of assigning values
to ISO reference standards.18
Bibliography .22
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ISO 8791-4:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 8791-4 was prepared by Technical Committee ISO/TC 6, Paper, board and pulps, Subcommittee SC 2,
Test methods and quality specifications for paper and board.
This second edition cancels and replaces the first edition (ISO 8791-4:1992), which has been technically
revised.
This version of ISO 8791-4 differs from the previous (1992) version as follows:
⎯ a definition of Print-surf compressibility has been added;
⎯ a description of a modified backing holder for testing high-stiffness papers and board has been added;
⎯ Annex D describing the calibration of Print-surf instruments has been revised and expanded;
⎯ some minor editorial changes have been made.
ISO 8791 consists of the following parts, under the general title Paper and board — Determination of
roughness/smoothness (air leak methods):
⎯ Part 1: General method
⎯ Part 2: Bendtsen method
⎯ Part 3: Sheffield method
⎯ Part 4: Print-surf method
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INTERNATIONAL STANDARD ISO 8791-4:2007(E)
Paper and board — Determination of roughness/smoothness
(air leak methods) —
Part 4:
Print-surf method
1 Scope
This part of ISO 8791 specifies a method for determining the roughness of paper and board using an
apparatus which complies with the Print-surf method, as defined in this part of ISO 8791. It is applicable to all
printing papers and boards with which it is possible to form a substantially airtight seal against the guard lands
of the measuring head.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 186, Paper and board — Sampling to determine average quality
ISO 187, Paper, board and pulps — Standard atmosphere for conditioning and testing and procedure for
monitoring the atmosphere and conditioning of samples
ISO 4094, Paper, board and pulps — International calibration of testing apparatus — Nomination and
acceptance of standardizing and authorized laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
Print-surf roughness
mean gap between a sheet of paper or board and a flat circular land pressed against it under specified
conditions
NOTE The mean gap is expressed as the cube root mean cube gap calculated as specified in Annex A. The Print-
surf roughness is expressed directly as the average value of roughness, in micrometres.
3.2
Print-surf compressibility
K
percentage decrease in surface roughness when measurements are made consecutively at the two standard
clamping pressures specified in this part of ISO 8791
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ISO 8791-4:2007(E)
4 Principle
The test piece is placed between a circular flat metal sensing surface and a resilient backing, and inner and
outer circular lands form a seal with the test piece. Under the influence of a pressure difference, air flows
across the measuring land between the measuring land and the test piece. The rate of air flow is measured on
a variable-area flowmeter, or the pressure difference across the measuring land is compared to the pressure
difference across a known impedance. In both cases, the result is expressed as the air gap, in micrometres.
5 Apparatus
5.1 Print-surf tester (two types)
5.1.1 Print-surf tester, which operates according to one of the following principles.
5.1.1.1 Variable-area flowmeter type, in which a standard pressure difference is created across the
measuring land and the air-flow rate is measured on a variable-area flowmeter. The air-flow rate varies with
roughness and the flow rate is converted to roughness, in micrometres. The flow diagram for this type of
instrument is shown in Figure 1.
Key
1 incoming air 300 kPa to 600 kPa
2 filter
3 pressure-regulator valve
4 6,2 kPa or 19,6 kPa
5 on/off valve
6 flow indicator tubes
7 to atmosphere
8 sensing head and clamping device
Figure 1 — Flow diagram for variable-area flowmeter type
5.1.1.2 Impedance type, in which the air from the controlled pressure source passes first through a
fluidic impedance and then through the sensing head, after which it discharges to atmosphere. The pressure
differences across the fluidic impedance and across the land are each measured by a transducer. These
pressure differences vary with roughness and the signals are converted to roughness, in micrometres. The
flow diagram for this type of instrument is shown in Figure 2.
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ISO 8791-4:2007(E)
Key
1 incoming air 300 kPa to 600 kPa
2 filter
3 pressure-regulator valve
4 19,6 kPa
5 fluidic impedance
6 pressure transducer
7 analog signal
8 to atmosphere
9 sensing head and clamping device
Figure 2 — Flow diagram for impedance instrument type
5.1.2 Procedures for maintaining these testers in good working order are given in Annex B.
5.2 Principal components of the system
5.2.1 Air supply, supplying clean air, free of oil and water droplets, at a steady pressure within the range
300 kPa to 600 kPa.
5.2.2 Sensing-head pressure regulator, allowing setting of the sensing-head differential pressure to
19,6 kPa ± 0,1 kPa or, on variable-area flowmeter instruments only, to either 6,2 kPa ± 0,1 kPa or
19,6 kPa ± 0,1 kPa.
5.2.3 Sensing head, (see Figures 3 and 4), consisting of three concentric, annular lands composed of
suitable material which have coplanar, polished surfaces. The centre or measuring land shall be
51,0 µm ± 1,5 µm wide and have an effective length of 98,0 mm ± 0,5 mm. The two guard lands shall each
be at least 1 000 µm wide at any point, and the radial distance between them at any point shall be
152 µm ± 10 µm. The measuring land shall be centred between them to within ± 10 µm.
The lands shall be mounted in an airtight mounting, constructed so that air can be passed into the gap
between one guard land and the measuring land, and exhausted from the gap between the measuring land
and the other guard land. The back of the mounting shall be flat and form a ground mating surface with the flat
surface of a manifold fitted with air inlet and outlet ports.
A spring-loaded protective collar may be fitted outside the guard lands. If such a protective collar is fitted, the
force exerted by the loading spring shall be taken into account when setting the clamping pressure.
NOTE In many instruments fitted with the protective collar, the force exerted by the loading spring is 9,8 N.
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ISO 8791-4:2007(E)
Key
1 measuring land
2 guard lands
3 passage connected to air supply
4 passage leading to flowmeters or atmosphere
5 recess vented to atmosphere
Figure 3 — Plan of the measuring and guard lands of the sensing head
Key
1 paper
2 resilient backing
3 measuring land
4 regulated low-pressure air
5 to flowmeters or atmosphere
Figure 4 — The sensing head sectioned on two radii
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ISO 8791-4:2007(E)
5.2.4 Backing holders, consisting of rigid metal discs of known mass, each recessed to accommodate a
resilient backing at least 10 mm greater in diameter than the outside diameter of the outer guard land. The
mass of both the resilient backing and the holder shall be allowed for in the initial adjustment of the clamping
pressure.
It has been observed that high-stiffness papers and boards can interact negatively with the flat metal backing
holder and cause erroneously high roughness results. This problem can be solved by using a modified
backing holder which relieves those areas of the backing holder not directly below the measuring land, as
shown in Figure 5.
Key
1 resilient backing
2 paper
3 regulated low-pressure air
4 measuring land
5 to flowmeters or atmosphere
6 new modified clamp platen showing machined cut-away
Figure 5 — The sensing head sectioned on two radii showing cut-away platen
5.2.5 Two resilient backings, of different types, which can be held in the recessed holders by means of
double-sided adhesive tape.
5.2.5.1 Soft backing, resilient, consisting of an offset printing blanket composed of a layer of synthetic
rubber, at least 600 µm thick, bonded to a fabric backing giving an overall thickness of 2 000 µm ± 200 µm.
The apparent hardness of the complete backing shall be 83 IRHD ± 6 IRHD (International Rubber Hardness
Degrees).
5.2.5.2 Hard backing, resilient, usually made from a polyester film bonded at its periphery to cork, offset
blanket or similar material. A small exhaust hole shall be provided to prevent air being trapped between the
film and the backing. The apparent hardness of the assembly shall be 95 IRHD ± 2 IRHD.
5.2.6 Clamping mechanism, allowing clamping of the resilient backing at pressures of either
980 kPa ± 30 kPa or 1 960 kPa ± 30 kPa, the pressure being calculated from the total area of the measuring
and guard lands.
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ISO 8791-4:2007(E)
2 2
NOTE 1 On some earlier instruments, these values may be displayed on the gauge as 10 kgf/cm and 20 kgf/cm .
Note that the spring loading in the protective collar (5.2.3) and the weight of the backing and its holder need to
be taken into account. The rate of clamping shall be such that the pressure reaches 90 % of its final value in
about 0,4 s, and 99 % of its final value in about 0,8 s.
2
NOTE 2 A third pressure of 490 kPa (5 kgf/cm ) is available on most instruments, but is not acceptable for use with this
part of ISO 8791 because of a tendency for air to leak under the guard lands.
Variable-area flowmeter measurement systems shall have a pressure gauge fitted to the instrument to
indicate the clamping pressure, which shall be adjustable. Impedance measurement systems shall have
integrated pneumatic and electronic circuitry which automatically controls the clamping pressure. In each
case, the actual pressure achieved shall be verified as specified in B.3.
5.3 Measuring system
5.3.1 The air-flow rate shall be measured with either a set of variable-area flowmeters or by measuring the
pressure drop across an impedance.
5.3.2 Variable-area flowmeter instruments shall be fitted with flowmeters which are graduated to show
the “cube root mean cube gap” between the paper and the measuring land surface, in micrometres (see
Annex A). The flowmeters shall be calibrated by the procedures outlined in either Annex C or D.
5.3.3 Impedance instruments measure air leakage by means of fluidic impedance, a pressure transducer
and a function generator. They display or print the roughness, in micrometres to the nearest 0,1 µm, based on
automatic measurement of pressure difference, over the range 0,6 µm to 6,0 µm. The value displayed shall be
the value calculated after 3 s to 5 s. This device shall be calibrated by the procedure described in Annex D.
6 Sampling
If the tests are being made to evaluate a lot, the sample should be selected in accordance with ISO 186. If the
tests are made on another type of sample, make sure that the test pieces taken are representative of the
sample received.
7 Conditioning
The sample shall be conditioned in accordance with ISO 187.
8 Preparation of test pieces
Prepare the test pieces in the same atmospheric conditions as those used to condition the sample. Cut at
least 10 test pieces for each side to be tested. The minimum size of each test piece shall be 100 mm × 100 mm,
and their surfaces shall be identified in some convenient way (for example, side one or side two).
The test area shall be free of all folds, wrinkles, holes or other defects, and should not include watermarks. Do
not handle that part of the test piece which will become part of the test area.
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ISO 8791-4:2007(E)
9 Procedure
9.1 Carry out the test in the same atmospheric conditions as those used to condition the sample (see
Clause 7).
9.2 Ensure that the instrument is on a rigid horizontal surface free from vibration and that it is level. Before
use on any particular day, check the system for leakage as specified in B.1.
9.3 Select and fit the backing disc appropriate for the material being tested. In general, the hard backing
should be used for papers that are to be printed by letterpress presses fitted with paper backings. Papers to
be printed by other processes, and boards however printed, should be tested with the soft backing.
9.4 Select and adjust the clamping pressure, using the following as a guide:
⎯ Hard backing letterpress 1 960 kPa ± 30 kPa
⎯ Soft backing letterpress 1 960 kPa ± 30 kPa
⎯ Soft backing offset 980 kPa ± 30 kPa
9.5 For a tester of the variable-area flowmeter type, select the lowest-range flowmeter which will give a
reading greater than 20 % of the scale range.
Always start with the highest-range flowmeter and turn the flow range selector switch successively to a
flowmeter of lower range, in order to avoid subjecting the low-range flowmeters to a high air flow.
9.6 Test the first test piece by the following procedure.
9.6.1 Variable-area flowmeter type
Set the sensing-head differential pressure to 6,2 kPa ± 0,1 kPa by adjusting the pressure from the low side.
NOTE 1 If the pressure gauge indicates differential pressure in metres of water gauge, 0,63 m is equivalent to
6,18 kPa.
NOTE 2 The pressure gauge on some instruments has been found to be sensitive to jolts and, if the adjustment is
made downwards from a higher pressure, the resulting pressure for a given scale reading will be higher than if the
adjustment is made upwards from a lower pressure.
Clamp the first test piece under the sensing head, with the side to be tested uppermost. This operation can
cause the reading on the sensing-head pressure gauge to change, but such a change may be ignored.
Record the reading on the flowmeter to the nearest 0,05 µm, 3 s to 5 s after application of clamping pressure.
Readings shall be taken level with the top of the flowmeter float. Select the lowest-range flowmeter which
gives results greater than 20 % of the scale range.
If the reading obtained is less than 20 % of the range of the lowest-range flowmeter, increase the sensing-
head pressure to 19,6 kPa ± 0,5 kPa (2,0 m water gauge). All readings taken at this pressure shall be
multiplied by 0,667 [(for the background of this factor, see Annex A, Equation (A.1)] to give the roughness, in
micrometres, unless the flowmeters are calibrated for this pressure.
9.6.2 Impedance type
Place a test piece under the head with the side to be tested uppermost. Clamp the test piece either
automatically or manually. Record the reading, 3 s to 5 s after application of clamping pressure.
9.7 Repeat step 9.6 for the other test pieces and calculate the arithmetic mean and standard deviation or
coefficient of variation for the side tested. For variable-area flowmeter-type instruments, do not repeat the
procedure for selection of the appropriate flowmeter and sensing-head pressure.
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ISO 8791-4:2007(E)
9.8 If a result is required for the roughness of the other side, take a second set of test pieces and repeat
steps 9.6 and 9.7.
9.9 If Print-surf compressibility is to be determined, the lower of the two clamping pressures shall be
selected and adjusted first. Follow step 9.6 and record the result and, without unclamping the test piece, next
select and adjust the higher clamping pressure and again record the result. Repeat this sequence for the other
test pieces. Calculate Print-surf compressibility using the equation in Clause 10.
10 Calculation
The Print-surf compressibility, K, can be defined mathematically by the equation:
100(GG− )
12
K=
G
1
where
G is the surface roughness value obtained at a nominal clamping pressure of 1 MPa;
1
G is the surface roughness value obtained at a nominal clamping pressure of 2 MPa.
2
11 Precision
The following estimates of repeatability and reproducibility, calculated according to TAPPI Test Method
T 1200 and published in TAPPI Test Method T 555, are based on data taken from the CTS-TAPPI
Interlaboratory Program for Paper and Paperboard and are reprinted and used by permission of TAPPI.
Testing is based on 10 determinations per test result and 1 result per laboratory, per material. The estimates
were determined prior to the availability and use of standard reference materials. The reproducibility is
expected to improve with the introduction of a reference standard system.
Table 1 — Roughness measurements in µm
Repeatability Reproducibility
Number of
Material description Grand mean Range
laboratories
r and %r R and %R
Coated cover, gloss 0,824 1,192 – 0,590 0,026 3,2 % 0,368 44,7 % 67
Coated face stock 1,125 1,390 – 0,887 0,030 2,6 % 0,320 28,4 % 71
Coated offset 1,193 1,420 – 0,984 0,043 3,6 % 0,291 24,4 % 67
Coated offset 1,255 1,500 – 1,049 0,045 3,6 % 0,281 22,4 % 67
Speciality paper 2,701 3,031 – 2,358 0,106 3,6 % 0,410 15,2 % 71
Laser bond, uncoated 3,511 3,965 – 3,031 0,172 4,9 % 0,580 16,5 % 33
Offset 4,602 5,141 – 4,000 0,145 3,2 % 0,772 16,8 % 30
Offset 5,415 6,117 – 4,680 0,167 3,1 % 1,049 19,4 % 30
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ISO 8791-4:2007(E)
12 Test report
The test report shall include the following information:
a) a reference to this part of ISO 8791;
b) date and place of testing;
c) all the information necessary for complete identification of the sample;
d) the type of instrument used;
e) the backing and type of backing holder used;
f) the number of test pieces tested;
g) the sensing-head differential pressure, in kilopascals;
h) the clamping pressure, in kilopascals;
i) the mean of the test results for each side tested;
j) the standard deviation or coefficient of variation for each side tested;
k) any deviation from this procedure which may have affected the results.
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ISO 8791-4:2007(E)
Annex A
(normative)
Calculation of roughness in micrometres
For the purposes of this part of ISO 8791, the cube root mean cube gap, G , in metres, in the direction of the
3
air flow between the measuring land and the test piece is calculated from the equation:
1
⎛⎞12××η bq× 3
v
G = (A.1)
⎜⎟
3
lp×∆
⎝⎠
where
η is the viscosity, in pascal seconds, of air at room temperature;
b is the width, in metres, of the measuring land;
q is the volume of air flowing in unit time, in cubic metres per second;
v
l is the median length, in metres, of the measuring land;
∆p is the pressure difference, in pascals, across the measuring land.
6
The roughness, in micrometres, is then equal to G × 10 .
3
If the differential pressure exceeds 1 % of the absolute pressure then ∆p should be calculated as in Equation
(A.2) to correct for the compressibility of air:
22
p − p
ud
∆=p (A.2)
2 p
m
where
p is the absolute upstream pressure;
u
p is the absolute downstream pressure;
d
p is the pressure at which the flow q is measured.
m v
Equation (A.2) is derived on the assumption that the gap between the measuring land and the test piece is
uniform across the width of the land, but that it varies along its length.
Equation (A.1) is subject to the assumptions that the flow is laminar, that the temperature is constant
throughout, and that the kinetic energy changes per unit volume of air are negligible compared with ∆p. The
flow conditions are normally well within the laminar range, but the kinetic energy can be important when rough
papers are measured, unless the differential pressure is restricted. To estimate the extent of the error, the full
equation for flow over the measuring land may be used:
2
12××ηρbq× C× ×q
vv
∆=p + (A.3)
322
lG××2 l×G
33
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ISO 8791-4:2007(E)
where
ρ is the density of air measured at pressure p ;
m
C is a coefficient found by experiment for a number of papers, and is approximately equal to 2,5.
Additional information about the background of Equations (A.1), (A.2) and (A.3) may be found in a paper
[2]
published in Paper Technology .
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ISO 8791-4:2007(E)
Annex B
(normative)
Maintenance of Print-surf roughness testers
B.1 Leakage
The apparatus shall be maintained free of leakage, visible surface irregularities of the backings and pressure
gauge error, as detailed in B.1.1, B.1.2 and B.3. Check for leakage at the lowest clamping pressure available
and a sensing-head differential pressure of 19,6 kPa.
B.1.1 Leakage between the back of the sensing head and its supporting manifold is indicated by a
measurable air flow when the soft backing is clamped directly against the head. Such leakage can be
corrected by a thin smear of petroleum jelly on the mating surfaces.
B.1.2 Damage to the sensing head is detected as follows:
a) Carefully wipe the face of the sensing head with a lint- and oil-free, soft clean material.
b) Clamp a smooth scratch-free piece of 125 µm thick film, such as cellulose acetate, between the sensing
head and the hard backing. Measure the air flow.
This test is very sensitive to dust, due to static charges and even to fingerprints. If a measurable flow is
found, carefully wipe the surface of the film and repeat the test.
It is recommended that a suitable film be obtained from the instrument manufacturer/supplier.
c) If it is impossible to obtain a zero reading on the lowest range flowmeter, confirm damage by inspecting
the measuring surface at a magnification of about × 50 with a stereoscopic microscope. On impedance
instruments, a reading greater than 0,8 µm indicates the possibility of damage.
d) If pits or scratches are apparent, replace the sensing head.
B.2 Sensing head
At frequent intervals, inspect the head, preferably with a stereoscopic microscope, to ensure that the gaps
between the measuring land and guard lands are free from debris. If necessary, clean as advised by the
instrument manufacturer.
B.3 Pressure gauges
Whenever the instrument is used, check that both gauges register zero when the air supply is disconnected.
At least once a year, check the accuracy of pressure gauges and transducers by connecting in parallel a
manometer or transducer, the latter having been calibrated against dead weights. Operate the instrument
normally and record the actual static pressures achieved.
Convert the clamping pressure reading to force per unit area of guard plus measuring land surface. Correct for
the weight of the resilient backing plus holder and for the force exerted by the spring-loaded protective collar.
Compare the corrected clamping pressure and the measured head pressure to the gauge readings and
pressure settings specified in 5.2.2 and 5.2.6.
Replace defective gauges or repair faulty control systems.
12 © ISO 2007 – All rights reserved
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