Mark J. Kirwan Paper and Paperboard Packaging Technology

Подождите немного. Документ загружается.

CORRUGATED FIBREBOARD PACKAGING

347

represents the respective positioning of the major families of industrial papers

by classes of creep rates. It is thus possible to define paper classes which are

homogeneous as to burst strength and fatigue resistance, where strong bursting

indices go together with a strong fatigue resistance of the paper and therefore of

the corrugated board case.

Theory of creep behaviour

When carrying out the creep test, a constant stress (σ

) is applied to a material, and

the change in its deformation (∈) in the course of time is analysed.

The creep function derived there from is of the type:

∈(t)

The material is characterized by its creep rate, which takes the form:

in Pa

−1

as t → 0

If the phenomenon is examined from a mechanical theory approach, it will be

shown that for the stress σ

, the energy released by the material, as it becomes

deformed through creep, can be written at every instant as a function of the creep

rate:

in Pa

where E is the Young’s modulus of the paper.

For standard experimentation, the designers commonly choose the safety factor

coefficients shown in Table 11.9.

There is a large amount of box performance safety data within the corrugated

board industry. The safety factor coefficients indicated in Table 11.9 are taken into

account at the design stage to ensure that the boxes used will have sufficient

strength to protect the contents during the expected storage life.

ft()=

-----

v =

δft()

δt

-----------

Wt()=

-------

+ σ

∫

t()dt

Table 11.9 Safety factor coefficients related to external conditions

Environment Specificity Safety factor

Stacking mode Column 1.00–1.33

Interlocked 1.67–2.00

Moisture (RH) 65% 1.07–1.25

75% 1.25–1.67

90% 1.82–2.50

Storage at 90% RH 1 hour 1.11–1.40

10 days 1.33–2.00

360 days 2.00–2.55

348

PAPER AND PAPERBOARD PACKAGING TECHNOLOGY

The basis of the calculation, which is the prediction of the required BCT,

is based on the BCT of a box which is tested after a short storage life

and which has not been stacked. This is known as BCT

. The predicted

required BCT is calculated from this value multiplied by a cumulative safety

coefficient, C

For design centre calculations, BCT

is measured in the laboratory at 23 °C

and 50% RH. It is multiplied by the cumulative safety coefficient, C

. This is

calculated by multiplying the various safety factor coefficients which correspond

to the factors which have an influence on box, or case, performance:

= C

· C

where C

is the coefficient which represents the packaging operation; C

for pallet stacking (interlocking or in columns); C

for fatigue during the storage

time; C

for the climatic conditions during transport and storage and C

is for

the vibration during transportation.

is generally evaluated at between 2 and 7, and often between 3 and 4. As an

example, a box stored in an interlocked arrangement in a pallet for 10 days at 90%

RH will require a BCT four times higher in compressive strength than a normal

individual box non-stacked and stored for a short time. (Note: C

× C

is 2× 2 = 4,

from Table 11.9.)

Hence a result of a BCT

of 200 kg in the laboratory will not be strong enough

to support more than 200/4 = 50 kg in real conditions.

Moreover, a factor of 25% must also be taken into account if tests are made

with hand-made boxes to take account of the mechanical stress which can be

experienced during erection filling and closing at speed on a packing line.

11.3.2 Containability and protection

The basic function of corrugated board packaging is the same as for any packaging,

namely to protect products during distribution until the product is removed from

the package. It may also protect the environment from the product – e.g. in the

distribution of dangerous goods.

The corrugated board package is mainly designed to contain products during

distribution. Containability together with adaptation of the box strength to logistics

systems (packing lines) are becoming important issues in corrugated board transit

packaging design.

11.3.2.1 Cushion performance

Corrugated board is a structural material which has few energy adsorption charac-

teristics compared, for example with expanded polystyrene or other cushioning

materials (Table 11.10).

The performance required from cushioning is to eliminate or minimize damage

to packaged product arising from impact and vibration.

CORRUGATED FIBREBOARD PACKAGING

349

The effect of a damaging shock or impact can be reduced by three principles:

•

Spreading the forces on impact, so that the force per unit area or the force on

any part in contact with the cushion is reduced.

•

Localizing the force, so that the forces at impact are directed to the stronger

parts of the package or the product.

•

Absorbing the energy of the packaged product.

This third principle is called cushioning, i.e. absorbing energy – the material is

compressed during impact and so absorbs much of the impact energy and,

subsequently, the force. It reduces the stress on the outer face of the product as

well as the shock, which causes the damage to the contents.

Corrugated board quality is essential to provide product protection by cushioning,

i.e. cushioning in this context means the amelioration of damage likely to be

caused by shocks such as dropping.

The calculation of the required thickness of the cushion may be carried out by

an empirical formula as:

H = C ×

where H is the drop height; T is the board thickness; C, the cushioning factor

(for corrugated board), C is between 1.8 and 3.6; and G is expressing the level of

fragility of the packed product.

Charts can illustrate this formulae variation, Figure 11.38.

The cushioning possible with corrugated board packaging is the subject of

ongoing research.

11.3.2.2 Drop protection

The structure and quality of a corrugated board are essential to ensure the protection

of the product when the case is dropped.

Drop testing is carried out according to ISO 2248 standards. The height of the

drop is increased incrementally during the test and the percentage of boxes

damaged at each height is evaluated.

The evaluation of corrugated board packaging is performed with 8 and 15 kg of

packed sand and there are different modes of drop. The case can either be dropped

horizontally or on an edge.

For the same weight of contents, the drop-height failure depends on the

grammage of the board as shown in the Figure 11.39.

Table 11.10 Cushion properties for corrugated boards and other materials

Source: Swedish Packaging Research Institute (1978).

Material Energy absorption (kJ/m

)

PE bubbles 70–95

PS sheets – density 15 kg/m

140–270

Corrugated fibreboard 60–300

----

350

PAPER AND PAPERBOARD PACKAGING TECHNOLOGY

The potential energy of the box in drop testing is the product of the weight of

contents and the drop height.

The following histogram for B flute boxes, Figure 11.40, which represents this

potential based on the height at which 50% of the boxes fail the drop test

shows that the potential energy depends on the weight of the corrugated board

which protects the contents.

Figure 11.38 Relationship between the thickness of the board and the drop height.

700

600

500

400

300

200

100

123456

Board thickness (mm)

Drop height (cm)

C =2; G = 0.002

C =2; G = 0.005

C =3; G = 0.005

300

250

200

150

100

350 380 430

Board Grammage (g/m

)

Drop height (cm)

Figure 11.39 Drop height failure of B flute RSC boxes with 8 kg of packed sand.

CORRUGATED FIBREBOARD PACKAGING

351

The main material parameters which influence the protection of the case in

drop testing have been analysed in detail.

From drop test experiments conducted with B flute RSC boxes of

40cm × 30 cm × 30 cm and two content weights (8 and 15kg), correlations have

been made. The potential energy is directly related to the burst strength of the

corrugated board as presented in the Figure 11.41.

200

180

160

140

120

100

350

380

430

Board grammage (g/m

)

Potential energy

at 50% damaged boxes

Figure 11.40 Potential energy versus corrugated board grammage (B flute – RSC boxes

40 cm × 30 cm × 30 cm, 8 kg packed).

150

250

350

Potential energy at which

50% of boxes will fail

Corrugated board burst (kPa)

200 400 600 800 1000 1200 1400

Figure 11.41 Potential energy at which 50% boxes failed by drop versus corrugated board burst

(B flute – RSC boxes 40 cm × 30 cm × 30 cm, 15 kg packed).

352

PAPER AND PAPERBOARD PACKAGING TECHNOLOGY

There are large differences in corrugated board burst strength resulting from the

use of papers of different quality (see Figure 11.42). Figure 11.42 also shows the

importance of corrugated burst values in relation to the drop height, in particular

the difference resulting from the use of test liners with burst index 2.0–2.4 and

kraft liners with burst index >3.8. This shows that the maximum drop height for

the test liner boxes is 75 cm, approximately, whereas with the kraft liner, the drop

height starts at 120cm and the maximum is 230cm, approximately.

The conclusion from this drop test research, investigating drop test protection,

is that burst strength, measured in kPa, is the key parameter.

Table 11.11 indicates the protection requirements in terms of classification and

drop height for dangerous goods.

This classification is the usual recommendation for the transport of dangerous

goods indicated in all manuals of practice, for example FEFCO.

11.3.2.3 Puncture protection

Corrugated cases can be damaged by puncturing during distribution both

internally by movement of the contents and externally by impact with sharp objects.

Puncture resistance of the corrugated board is a requirement specified by some

national specifications (France, Germany and Spain).

300

250

200

150

100

0 200 400 600 800 1000 1200 1400

Board burst (kPa)

Drop height (cm)

Test liner

Kraft liner

Figure 11.42 Relationship between drop height and corrugated board burst strength.

CORRUGATED FIBREBOARD PACKAGING

353

As shown in Figure 11.43, the protection against puncture is provided by the

quality of the liners (kraft and test liners), the basis weight of the corrugated board

and the structure of the fluting (B or C flute).

11.3.2.4 Preservation of the hardness

Hardness, or softness, is a quantified value. To evaluate the hardness preservation

and its relation to the performance of the box, the impact tests on the inclined

plane have to be carried out as illustrated below (Fig. 11.44) on the four sides of

boxes with a 5 cm × 5 cm transverse hazard. Hardness is evaluated by measuring

the BCT before and after impacts – this evaluates the loss of performance resulting

from the impacts.

As expected, cushioning properties and box and board performance correlate

with board hardness. This can be seen in Figure 11.45 which shows that a significant

and continuous drop in BCT is apparent as the board hardness is reduced.

In short, the BCT loss from transportation and handling impacts will not be

severe if board of higher hardness is used.

Table 11.11 Classification of dangerous goods packaging

Group Danger level Drop height (cm)

III Low 80

II Medium 120

I High 180

5.0

4.5

4.0

3.5

3.0

2.5

250 300 350 400 450 500

Single wall C flute

Kraft

Test liner

5.0

4.5

4.0

3.5

3.0

2.5

250 300 350 400 450 500

Single wall B flute

Kraft

Test liner

Puncture (J)

Corrugated board basis weight (g/m

)

Corrugated board basis weight (g/m

)

Puncture (J)

Figure 11.43 Puncture resistance in relation to liner (kraft or Test) and fluting (B and C).

354

PAPER AND PAPERBOARD PACKAGING TECHNOLOGY

On several occasions during manufacturing, corrugated board and boxes are

prone to be damaged by flat compressive forces exerted by various machines. To

understand and control such damage and to implement the experimentation

described in the literature, we made similar measurements in one corrugated plant.

During these trials, the board was crushed incrementally to create progressive

Figure 11.44 Inclined plane test.

–10

–20

–30

–40

50 100 150 200 250

BCT loss (%)

RSC boxes 40 cm × 30 cm × 30 cm

Hardness (kpa)

Figure 11.45 Box performance losses versus the hardness of the board (RSC boxes

40 cm × 30 cm × 30 cm).

CORRUGATED FIBREBOARD PACKAGING

355

damage and to evaluate the performance of board and boxes made from that

board. The extent of damage is expressed as the percentage of initial thickness

(calliper).

Decreased thickness (calliper) caused by crushing the board induced increased

softness in the board, as can be seen in Figures 11.46 and 11.47. The variation

Flexural stiffness

01020

–50

–40

–30

–20

–10

30 40 50 60 70 80

Extent of crushing

Loss (%)

Figure 11.46 Effect of crushing on flexural stiffness.

BCT

Extent of crushing (%)

Loss (%)

–10

–20

–30

–40

–50

0 1020304050607080

Figure 11.47 Effect of crushing on BCT.

356

PAPER AND PAPERBOARD PACKAGING TECHNOLOGY

and trends presented in these figures confirm previous findings as found in the

bibliography.

11.3.3 Boxboard packing line considerations

Corrugated board boxes (RSCs and die-cut blanks) are packed manually, semi-

automatically or fully automatically. The selection of loading method will depend on

the product, the package, the line speed and the capacity needs. The packages can be

either erected and then filled and closed, or formed around the product and closed.

In general, the filling line speed increases from 5–10 packages per minute to

30 packages per minute in a semi-automatic line and on an automatic line at around

one pack per second. To ensure packing line efficiency, the corrugated board

blanks and cases have to meet certain requirements:

•

flatness and structural stability

•

suitability for closure.

11.3.3.1 Flatness of corrugated fibreboard

Corrugated board sheets often exhibit curvature, also referred to as warp or curl

which can cause great difficulty in subsequent converting operations and in box

set-up in the customer’s facility. Hence, flatness or the avoidance of warp is

a major consideration in the corrugating industry.

Since warp varies inversely with board thickness, thin boards like F (1.2 mm),

E (1.7mm) and B (2.8 mm) flute are much more prone to warp than C (4.0 mm)

flute boards. As production of these thinner flutes and corrugator speed have both

increased dramatically, warp has become a much more important issue.

There are different forms of warp:

•

normal warp or curl in both MD and CD

•

twist warp.

Normal warp across or along the sheet is caused mostly by two factors. These

are the differences in both moisture content and hygroexpansivity within or between

liners. Figure 1.24 illustrates various types of curl and twist which can occur with

paper and paperboard.

Hygroexpansivity, induced by the papermaking process, is often ignored in

considerations of warp, but can differ by 50% or more between liners. Control

requires adjusting moisture content to compensate for both differences since

hygroexpansivity cannot be changed on the corrugator.

A well-tuned corrugator can provide adequate warp control for C flutes and

some B flutes, but may fall short of what is needed for thinner flutes.

Curvature along the diagonal of corrugated sheets, usually called twist

warp, can be avoided only by specifying liners with polar angles, as it cannot be