Performance and Appearance of packaging grades of paper Study on quality measurement methods

Author and Presenter:

Manoj SukumaranManager – Lorentzen and Wettre ProductsABB India Limited

Presented at IPPTA Workshop on 10th December 2015

World packaging material consumption

World paper and board packaging consumption by region

Performance of a Corrugated board BoxPractical considerations…• A corrugated-board box is intended to be filled with contents

which will to a greater or lesser extent exert forces on the insides of the box so that these tend to bend outwards.

• The load will act for a long period of time.

• During transport the box will be exposed to vibrations and shock loads.

• The corrugated-board box will in all probability be exposed to different atmospheric conditions.

• The load in a stack in a storeroom is probably not as uniform as in a compression tester.

Notations used in Corrugated board

Flute types and dimensions

Structure of Corrugated Board Box

Liner and Fluting Medium in use worldwide

* Natural kraft linerboard – Mainly unbleached kraft fibre, some recycled content allowed* White top linerboard – Bleached top layer on an unbleached base layer* Testliner – Top layer and base layer made from 100 % recycled fibres* Other recycled liner– Mainly kraft top layer, which is a natural kraft layer on a recycled base layer* Recycled medium – 100 % recycled fluting * Semi-chemical medium – Contains mostly NSSC hardwood and/or softwood (NSSC= Neutral Sulphite Semi-Chemical pulp. Produced by defibration in a disc refiner.)

Testing of components

McKee Formula for Box Compression strength (BCT)

BCT = k1 × (ECT b)0.75 × (SbB )0.25×Z0.5.

•ECT is the edgewise crush resistance of the corrugated board, the value in kN/m

•SbB is the geometric mean given by

SbB = √Sb

B MD x SbB CD

where SB,MD and SB,CD are the bending stiffness in the machine and cross-machine directions of the corrugated board, in Nm•The periphery of the box, Z in m.

McKee found the formula useful if the relationship of

depth/periphery is > 1/7 (0.143)

Simplified McKee formula

BCT = k2 × ECTb × T0.5 × Z0.5

•In the simplified equation, the bending stiffness is replaced by the thickness of the corrugated board T.

The theoretical background is that the bending stiffness of corrugated board to a very great extent is influenced by the distance from the neutral bending center line of the sheet to the centers of the surface liners, i.e. roughly the thickness of the board

Stress distribution along the edges of a corrugated board box

The corners take higher load than the sides. Failure of the vertical edge at corners lead to breakdown of the Box

Edgewise Compression Test (ECT)according to FPL method

The relationship between the ECT- value and the compression strengths of liner and fluting medium can in general be written as:

ECTb = k (σbC,L1 + σb

C, L2 + α σbC,F)

Where σbC is the compression strength, L1= liner1, L2= Liner 2…F= fluting

medium, α = flute take up factor and k is a constant

The value of k was determined by SCA depending on the method used for measuring compression strength of the liner and medium and published as:

•RCT 1.28 ± 0.08

•CCT 0.97 ± 0.04

•SCT 0.71± 0.03

Short-span Compression Test (SCT)

In the measurement of the compression strength of liner and fluting medium according to the “short (span) compression test” method (SCT), the paper sample is placed between two clamps with a 0.7 mm free clamping length. When the clamps are moved towards each other, the length is reduced and the stresses in the strip increase. Since the sample is short in relation to its thickness, the slenderness ratio is low and buckling is prevented. Consequently failure occurs due to compression

Ring Crush Test (RCT)

In the Ring Crush Test (RCT) method, the test strip is formed into a ring when it is inserted into an annular gap in a special holder. The width of the gap is adapted to the thickness of the test sample by using discs of different diameters. The great disadvantage of the RCT test is the buckling which occurs especially at the ends of the ring-formed test piece

Concora Crush Test (CCT) and Concora Linear Test (CLT)

In the Corrugated Crush Test (CCT), or Concora Corrugated Test method, the test strip is first corrugated in a laboratory corrugator, after which it is clamped vertically into a jig having the same profile as the rollers of the corrugatorIn the Crush Linear Test (CLT) or Concora Liner Test method, a straight test strip is held in a vertical position in a special holder.

CCT CLT

Comparison of different Compression methods

The relative ranking of the methods is clearly evident. The CLT- and RCT-methods in particular show strongly increasing values with increasing grammage, which must indicate that at low grammages failure has arisen through buckling and not through compression failure as was intended

Summary

• In Ring Crush Tests on lightweight materials failure occurs by buckling. On heavy weight materials failure occurs at the loaded edges, which are weakened by cutting as the specimens are prepared.

(Note: The Tappi methods T818 and T822 for RCT are intended for paperboard between 0.28 mm (0.011 in.) and 0.51 mm (0.020 in.) thick. )

• The SCT-method measures the pure compressive strength i.e. what it is intended to measure. The SCT-method is quick, and easy to use. The SCT results are more indicative of the fluting medium contribution to ECT than the RCT results

• To predict the print results on unprinted paper/board, for different printing techniques.

• It is important for the print results when using Flexo Gravure Offset

Why characterize the surface?

By characterizing unprinted substrates it is possible to predict print quality in order to reach the specifications.

• Air-leak – PPS, Bendtsen, Sheffield– Small surface measured– ”wrong” resolution to be relevant for printing

• Stylus roughness– Reference head, laser triangulation,

chromatic aberration etc.– Small surface measured– ”wrong” resolution to be relevant for printing

• Optical topography– Photometric stereo technique– Large area measured– ”right” resolution to be relevant for printing

Height mapGradient

Methods to characterize the paper surface

Phenomenon: topography and low angle light = shadows

Measurements: shadows and known light angle = topography

Optical image analysis measurements

Picture from rightPicture from left

Gradient map

Height map

Stereographic picture

Gradient map

Height map

Stereographic picture

CRATERSOSD

OptiTopo Surface DeviationO S D

Small scale surface variations

– relevant for printing

It is very important to divide the surface variation in fine, mid,and large scale variation because some intervals are important for the print results and some are not.

This is not possible to do with air-leak instruments (e.g. PPS, Bendtsen).

Paper and board surface characterization

Printability according to OSD vs. PPS

Band passing into different wavelength ranges creates a spectrum from original profile

These ranges may play different roles for the printability

Paper surface

Different wavelength ranges of the original profile are sometimes given specific names:

Form

Waviness

Roughness

Sta

ndar

d de

viat

ion

Wavelength

Mathematical characterization of surface

LARGE scaleFINE scale MID scale

L&W OptiTopo extracts information from stereoscopic picture

Total image– a combination of all surface variations

Roughness variations

Surface roughness

Sm

oo

thR

oug

h

Standard Deviation is calculated for each wavelength band

LARGE scaleFINE scale MID scale

Roughness variations

Surface roughness

Sm

oo

thR

oug

hOSD

Important for printing

BendtsenSheffieldEmveco

PPS

LARGE scaleFINE scale MID scale

Roughness variations

LARGE scaleroughness variations

FINE scaleroughness variations

MID scaleroughness variations

L&W OptiTopo extracts information from stereoscopic picture

Total image– a combination of all surface variations

OptiTopo measures the surface variation in a large number of wave-lengths and group these into 10 different bands

These ranges may play different roles for the printability

Large scale roughness variations,Bendtsen, Sheffield, Emveco(wavelength band 5-6)

Mid scale roughness variations,PPS (wavelength band 4-5)

Small scale roughness variations,OSD (wavelength band 1-4)

Paper surface total variation = small scale + mid scale + large scale roughness variations,+ very large scale variations (waviness, cockling etc.)

Very large scale variations, form, waviness, cockling(wavelength band 7-10)

Bendtsen: ”land”, D= 150µm, circumference 104 mm

Measured area of paper = 15,5 mm2

D

Actual paper surface (wavelength band 1-10)

D

Paper surface ”seen” by Bendtsen = large scale roughness variations (wavelength band 5-6)

A closer look at Bendtsen

Fine scale, mid scale roughness and craters are invisible to Bendtsen

PPS: ”land”, D=51µm, circumference 100 mm

Measured area = 5,1mm2

D

D

Paper surface ”seen” by PPS = mid scale roughness variations (wavelength band 4-5)

Fine scale scale roughness and craters are invisible to PPS

A closer look at PPS

Actual paper surface (wavelength band 1-10)

OptiTopo resolution: 15,6 µm

Measured area = 1000 mm2

Variations that OptiTopo actually ”see”= all variations from small scale roughness variations to large scale roughness variations, all the way up to 32 mm wide variations, divided into 10 wavelength bands.

++

+

Band 1-4

Band 4-5

Band 5-6

Band 7-10

Actual paper surface (wavelength band 1-10 which is 0.0315 – 32 mm variations)

OptiTopo use band 1-4 to calculate OSD value

L&W OptiTopo Crater Value

0 µm

-3 µm(in this example)

Identify craters of different depths…

InkPaper surface

Shallow craters are not “dangerous” Deep craters cause

missing dot or un-covered area (UCA)

Flexo printed board

According to PPS these surfaces have the same roughness value. The PPS value will not say anything about the craters, which may cause print defects in flexo and gravure.

PPS 1.5 µm

PPS 1.5 µm

Craters deeper than 1.5 µm

Crater depth in µm

The crater value– a value of the surface deviation as regards to surface pores

Crater 1.5 – smooth Crater 3 – medium Crater 5 – rough

Double/triple coated surfaces, hard calendared qualities like SC, MG

Liner, liquid board, newsprint, copy paper, fine paper

Sack paper, uncoated board, uncoated paper, tissue

Double coated board Liquid board Sack

Craters deeper than -1.5 µm = 3.14%

Craters deeper than -3 µm = 1.78%

Craters deeper than -5 µm = 3.48%

The crater value– depths and paper grades

• The amount of craters with a specified depth• Different crater depths are relevant for different paper grades

To give a relevant value for printing the amount of craters in measured area should be in the range of 0.2–10%

Craters below 1.5 µm

11.4%

Craters below 5 µm

0.04%Amountof craters (%)

Craters below 3 µm

2.27%

Best choice for this paper grade

The crater value– a value of the surface deviation as regards to surface pores

Mottle

Uncovered area (UCA)

Missing dots

Case studies - Typical print defects

Case study 1 – rotogravure on double coated board

Missing dots

Case study – rotogravure on double coated board

Cyan 25% Full-scale gravure print Source: Innventia

Missing dots

19 samples with different print result

Missing dots were counted manually with microscope

Amount of missing dots [%]

Case study – rotogravure on double coated board

Source: Innventia

Missing dots

PPSvs

missing dots

Low correlation

Case study – rotogravure on double coated board

Source: Innventia

Missing dots

L&W OptiTopo

vsmissing dots

Amount of missing dots [%]

High correlation

Conclusion Case Study 1

OptiTopo gave a better ranking of missing dots compared to PPS

Correlation to the print results:

L&W OptiTopo R² = 0.94

PPS R² = 0.57

Case study 2 – Improved top ply printability after rebuilding of white-top liner machine

Uncovered area (UCA)

Importance of relevant topographical properties for print result

Before After

Rebuilt white-top liner PM ”reversed forming concept”

Source: Innventia

• Print mottle reduced• UCA reduced• Wet-in-wet printing

improved

Rebuilt white-top liner PM– improved flexo printability

Ste

reos

copi

cgr

adie

nt im

age

Before After

Fle

xo p

rint

res

ult

Source: Innventia

Before After

Conclusion Case Study 2

OptiTopo gave a better ranking of print prediction compared to PPS

Case study 3 – Liquid board, flexo printed

Predict mottling

Source: Innventia

Liquid board– flexo printed

• A set of 6 different samples gave different amount of mottle in a full scale test print.

• The mottle in the cyan 100% areas were measured using the Mottling-Expert software from Innventia

• The same 6 samples were measured using the L&W OptiTopo in the unprinted parts of the printed sheets and the amount of craters were measured

• Also PPS and Bendtsen was tested as reference

Cyan 100%Full-scale flexo print

Source: Innventia

Mottle value

OptiTopo– print prediction correlation

High correlation

Source: Innventia

Mottle value

Bendtsen– print prediction correlation

Medium correlation

Source: Innventia

Mottle value

Quite low correlation

PPS– print prediction correlation

Conclusion Case Study 3

OptiTopo gave a better ranking of mottling compared to PPS and Bendtsen

Correlation to the print results:

L&W OptiTopo R² = 0.91

Bendtsen R² = 0.54

PPS R² = 0.36

• What? A proven technique to predict printability, faster and more accurate compared to air-leak instruments, stylus instruments and visual assessments.

• Why? To minimize waste, reduce customer complaints and improve quality.

• For whom? Useful for producers of paper, board and sack products that will be printed on.

Summary

Surface topography– a key factor for print quality

Source: Innventia

(OSD)

Benefits

•Saves resources in terms of materials, energy and time

•Reduces costly customer complaints

•Reduces waste and environmental impact

Measure printability withL&W OptiTopo

Unprinted paper / board

Benefits with early print prediction

Questions?

The Measure of Quality