Mark J. Kirwan Paper and Paperboard Packaging Technology

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overcome by inserting a belt feeder between the pre-weigher and the packer,

which regulates the delivery rate of the material.

Impeller packers

For packing a range of ground rock–type products, the impeller packer is recom-

mended (Fig. 8.9). A continuously running impeller feeds the product, through

a cut-off valve gate and a flexible connection, to the filling spout and into the sack.

A saddle on a weigh beam supports this, and, once the correct weight is obtained,

the cut-off gate is actuated and the sack discharged. Impeller packers are available

with weighing systems based on load cell controls.

They are also supplied with up to four spouts for high-speed packing of cement

and similar products. For low outputs, a simple impeller packer is available in which

the filling spout and chair assembly is mounted on a platform scale, the cut-off

gate being manually controlled.

Fluidising packers

Fine and very fine powders may be made to flow readily by passing air through

them. With valve packers based on this property, the fluidising is achieved by

admitting low-pressure air through a porous pad in the base of a chamber. The

fluidised product flows out by gravity, through a cut-off gate and the filling spout,

into the sack. In the ‘pressure flow’ version of this machine, applying air pressure

above the product in the chamber increases the packing rate. The control of weighing

is similar to that of an impeller packer.

Fluidising packers can handle a wider range of products than any other type but

are most suitable for material with a sufficiently high proportion of fine powder to

hold the fluidising air (Fig. 8.10). They are particularly recommended for fine

powders with difficult handling characteristics. By increasing the fluidising air

considerably, some granular products may also be packed. Since the product

delivered by a fluidising packer will be aerated, provision for the escape of

entrained air may be needed. A vent may be incorporated in the filling spout, and

if the sacks include a barrier ply, some perforations may also be necessary.

Weigh beam

Rotary valve

Cut-off gate

Filling spout

Sack chair

Figure 8.9 Impeller packers.

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Screw packers

For powder products which do not flow freely, a screw packer may be used, either

in connection with a pre-weigh scale or as a hand-controlled unit mounted on

a platform scale (Fig. 8.11). The material is delivered from a small hopper fitted

with an agitator, through a screw feed in the filling tube. Provision to assist settling

of the product can be made during filling with net weighing.

8.5.2.3 Rotary packing system

For industries which pack one unvarying product at a constant high rate, rotary

packing machines have been developed. These consist of a number of individual

packing units which are mounted on a slowly rotating structure. Empty sacks are

placed either manually or automatically on each of the filling spouts at one point.

Filling takes place as they travel round to the automatic discharge point. The

advantage of these machines is that the output is no longer governed by the time

taken to complete the weighing and filling of each sack. Rotary packers are available

for both valved and open mouth sacks. They are used mainly for filling cement,

Inlet valve

Fluidising chamber

Cut-off gate

Filling spout

Sack chair

Fluidising pad

Weigh beam

Figure 8.10 Fluidising packers.

Agitator

Packing screw

Filling spout

Pre-weigh scale

Figure 8.11 Screw packers.

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plaster or chemicals into valved sacks at rates up to 120 tonnes per hour. A fully

automated rotary system requires sacks to be presented to the spouts as flat as

possible and free from sticking together, using one of three types of magazine as

follows:

•

horizontal sack-holding magazine with capacity of 600 sacks

•

vertical magazine with storage capacity of up to 6000 sacks

•

roll system (sacks on reel) which serves as a magazine holding approximately

3000 sacks shingled together to form a reel with a maximum diameter of

160 cm held in place by two plastic tapes.

In machines for filling open mouth sacks, it is more usual to employ two or

three fixed pre-weighing scales, each serving several filling spouts. After filling,

the sacks are led off tangentially onto a conventional sewing line or for heat seal-

ing. If the operation is automatic, only one person is required to place empty

sacks into a cassette magazine. If the operation is manual, up to three operators

could be involved.

8.5.2.4 Output levels of valved sack systems

Table 8.4 summarises the output levels of available valved sack packing systems

for various products. The list of products is not intended to be comprehensive and

is provided for guidance only.

If sacks with tuck-in sleeves are used with valve packers having automatic

discharge, an arresting device may be provided to hold the sack temporarily at

a convenient angle.

8.5.3 Sack identification

It is often necessary to add information or a batch identification mark to sacks

during the packing operation. This may be done by simple printing devices or by

attaching pre-printed tickets.

On open mouth sacks, printing may be added by a rotating contact roller mounted

beside the sewing head, applying a code or batch number close to the sewing line.

Alternatively, for any type of sack, the printing may be done after filling, by an

ink-jet printer or a roller device as it passes along a conveyor. Special printing

devices are available for valved sacks which are either incorporated into the clamp

which secures them to the packing machine spout, or are an addition to certain

sack applicators.

If printed tickets are required, these may be manually or automatically inserted

into the sewing line or open mouth sacks. Automatic ticket dispensers either take

the tickets from a reel or will feed them from a magazine. Self-adhesive tickets

may also be applied to either valved or open mouth sacks after they have been

filled.

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Table 8.4 Valve sack output levels for various products and packing systems

Type of packer Products kg

Output in sacks per minute

with one operator weight for

given number of spouts

General

description Typical examples 1 2 3 4

Gravity with

platform scale

(semi-auto weigh)

Granular,

free-flowing

Plastic pellets

Dry sand

Gravity free fall

with pre-weigh

scale

Granular,

free-flowing

Plastic pellets

Granular fertiliser

5–6

12–14

12–16 — —

Belt, grooved

wheel with

pre-weigh scale

Granular,

free-flowing

Granular fertiliser

Granular sugar

Plastic pellets

Provender pellets

Grain

D.V. Salt

6–7

4–5

6–7

14–15

8–10

14–15

—

Belt, grooved

wheel with

pre-weigh scale

and belt feeder

Large cubes

Light fine flake

Beet pulp nuts

Cattle cubes

Provender meals

Soya meal

Shredded beet

pulp

4–5

2½

8–10

12–15

7½

16–20

Impeller on

platform scale

Pure ground rock Portland cement

Gypsum

Hydrated lime

China clay

Fly ash

2–3

1–2

Impeller with

automatic

weighing and

sack discharge

Pure ground rock Portland cement

Gypsum

Hydrated lime

China clay

Fly ash

5–6

3–4

4–5

10–12

8–10

15–18

12–15

20–24

16–20

Fluidising

pressure flow

Powder

Powder/

fine-granule

mixture

PVC resin

Carbon black

Starch

Refractory

powders

Flour

5–6

3–5

4–5

5–6

4–5

8–10

Screw with screw

feeder, mounted

on platform scale,

hand control

Fine and very fine,

non-free-flowing,

powder

Resin powder

Starch

Screw with

pre-weigh scale

Fine and very fine,

non-free-flowing,

powder

Resin powder

Starch

Flour

Provender meal

1½

1–2

2–3

4–5

4–6

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8.5.4 Sack flattening and shaping

To gain the maximum advantage from paper sacks as a means of storing and trans-

porting materials, it is often desirable to flatten or shape them after filling. This

provides firm, uniform packs, offering substantial savings in storage and transport

costs, together with safer stacking or palletising.

The choice of equipment depends on the nature of the product and the type of

sack. For light and medium density free-flowing products, a twin-roller sack-

shaping machine, which serves to distribute the product evenly along the length of

the sack, is adequate. For denser products which pack more solidly, the compression

flattener, in which the whole length of the sack is subjected to uniform pressure as

it passes between belts, is recommended. If the product is sensitive to excessive

pressure or is of a coarse lumpy nature, a vibratory flattener should be used.

There is generally a greater benefit to be obtained from flattening sewn open

mouth sacks than pasted valved sacks. Open mouth sacks should always be fed

bottom first into roller or belt flatteners.

The flattening of sacks with barrier piles containing highly aerated products

must be done very slowly, to avoid burstage, even if they are specially perfor-

ated. It is preferable to reduce aeration as much as possible by mechanically

settling or ‘possing’ during filling and to allow time for further de-aeration

before sealing the sack.

8.5.5 Baling systems

Baler packing equipment ranges from simple devices, which aid manual loading

of baler sacks, to fully automatic power-operated machines. The method of func-

tioning will depend on whether the goods to be packed are fully compressible,

such as woollen articles, semi-compressible, such as small bags of flour, or rigid,

such as boxes or cans.

A typical hand baler consists of a short chute, shaped to receive and clamp the

mouth of the sack and support it in an inclined position. Articles are simply pushed

through the chute into the sack. In another version, suitable for rigid articles only,

the goods are loaded into a horizontal open trough, over which the baler sack is

drawn. On releasing a catch, the trough tilts forward so that the goods and sack

slide clear.

Semi-automatic baling machines include a receiving hopper into which the

items are manually loaded. The sack is manually applied to the ‘duck bill’-type

jaws which grip it tightly. On closing the hopper, the charge is first subjected to

alignment or pressure, by inward movement of one side of the hopper, and then

pushed through a throat into the sack by a longitudinal ram. At the end of its

travel, the ram pushes the loaded sack off the jaws. If the items are compressible,

the side plate applies sufficient pressure to reduce the volume of the charge

slightly; if they are rigid, it merely serves to line them up firmly.

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In fully automatic baling machines, automatic collation of the articles forming

the complete pack may take place directly in the hopper from above, or by forming

the load alongside the machine and transferring it sideways into the hopper. The

placement of the baler sacks on the sack jaws may be by hand or by mechanical

means using suction cups.

Baling machines can be incorporated into continuous production lines, the

completed packs being discharged through a sealing station to a conveyor.

8.6 Standards and manufacturing tolerances

The paper sack industry participates actively on national and international

standards committees, ISO, etc. These committees are concerned with the elaboration

of standards dealing with terminology, dimensions, capacities, marking and test

methods in the fields of packaging and unit loads. (ETAPS participates in this

work.)

8.6.1 Standards

In some instances, there are separate standards for the three Standards groups

mentioned in Section 8.6 but there is an increasing tendency towards common

standards either internationally or across Europe.

Although many standards are not mandatory, they may become so. A standard

on manufacturing tolerances for paper sacks has not yet been ratified, but ETAPS

has discussed the subject with a number of user bodies and is able to set down

guidelines for the area of the UK and Ireland.

Table 8.5 includes relevant standards for plastic film which is now a common

constituent of the paper sack. Also detailed are those standards of a more general

nature which may be of interest to both manufacturers and users of paper sacks.

Table 8.6 indicates current manufacturing tolerances being observed by

ETAPS members in the absence of a specific agreement on tolerances with a sack

user or another trade association.

In recent years, much attention has been paid to the suitability of paper sacks

and other containers for the packaging and distribution of food products for

human consumption. The question of possible migration of hazardous sub-

stances from paper, board and plastic film is under active consideration at

national and international levels. Food manufacturers are now concerned about

the condition of the manufacturing plants and the distribution methods of their

packaging suppliers.

United Nations approved specifications are becoming common in relation to

the packaging of hazardous chemicals. In the United Kingdom, the Food Safety

Act 1990 is relevant as well as the following statutory instruments: SI 1523:1987

and SI 3145:1992.

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Table 8.5 List of Standards (ISO, EN and BS)

International or

European Standard British Standard Title

ISO 1924/2 1986 BS 4415/2 Determination of tensile properties of

paper/board

ISO 2206 1987 BS 4826/1 1988 Complete filled transport packages

ISO 2233 1986 BS 4826/2 1986 Complete filled transport packages

ISO 3676 1983 BS 6884 1987 Plan dimension of unit loads

ISO 6780 BS 2629 Pallets for materials handling through transit

specifications for principal dimensions and

tolerances (NB: ISO and BS are not similar but

are related.)

Packaging – sacks – vocabulary and types

• Part 1 – Paper sacks

• Part 2 – Sacks made from thermoplastic film

ISO 6591/1 1984 BS EN 26591/1 & 2 1993 Packaging – sacks – description and method of

measurement

• Part 1 – Paper sacks

• Part 2 – Sacks made from thermoplastic

film

ISO 7023 1983 BS EN 27023 1993 Packaging – sacks – method of sampling empty

sacks for testing

ISO 7965 1984 BS EN 27965/11993 Packaging – sacks – drop test

ISO 8351 1994 BS ISO 8351-1 1994 Packaging – method of specification for sacks

• Part 1 – Paper sacks

ISO 8367/1&2 1993 BS ISO 8367/1&2 1993 Packaging – dimensional tolerances for general

purpose sacks

• Part 1 – Paper sacks

• Part 2 – Sacks made from thermoplastic

film

EN 277 1989 BS 7303/1 1990 Sacks for transport of food aid (polypropylene

fabric)

pr EN 1086 1993 Sacks for transport of food aid – selection

recommendations for type of sack and liner

(provisional recommendations)

BS EN 770 1994 Sacks for transport of food aid (paper sacks)

ISO 535 BS 2644 1985 Sizing properties of paper – methods of

detecting the degree of water resistance (Cobb

method)

ISO 3689

ISO 3781

BS 2922 Part 1 1985

BS 2922 Part 2 1985

BS 2987 1958

Methods of testing paper and board after

immersion in water

Notes on application of statistics to paper

testing

ISO 2758/9 BS 3137 1972 Methods of determining bursting strength of

paper and board

ISO 187 1993 BS EN 20187 1990 Methods for the conditioning of papers and

boards for testing

ISO 536 1976 BS 3432 Method of determining grammage

(basis weight) of paper and board

ISO 1974 1990 BS 4468 Method of determining internal tearing

resistance of paper and board

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Table 8.5 (Continued)

International or

European Standard British Standard Title

ISO 5636/5 1987 BS 6538/3 Determination of air permeability (Gurley) of

paper and board

ISO 6599 BS EN 26599/1 1993 Method of conditioning paper sacks

BS 6563 1985 (1990) Surface roughness determination

(Bendtsen)

BS 3725 1984 Non-returnable paper sacks for loose

potatoes

BS 3755 1964 (71) Methods of test for assessment of

odour from packaging materials used

for foodstuffs

BS EN 645 1994 Foodstuffs contact – cold-water extract

preparation

BS EN 646 1994 Foodstuffs contact – colour fastness

determination

BS EN 647 1994 Foodstuffs contact – hot-water extract

preparation

BS EN 648 1994 Foodstuffs contact – fastness of fluorescence

determination

BS EN 643 1994 List of European Standard qualities of waste

paper

ISO 2875 BS 3110 1959 (85) Method of measuring rub resistance

of print

ISO 9001/1/2/3 1987 Water spray test

ISO R 181, 174,

307, 8618

BS 2782 Methods of testing plastics

ISO 291, 3205 BS 2782:Part 0:1982 Introduction

BS 2782:Part 2 Antistatic behaviour of film

Method 250A 1976(83) Charge decay method

Method 250C 1976(83) Field window method

ISO 1184 BS 2782:Part 3

Methods 326A to

326C:1977

Determination of tensile strength and

elongation of plastic film

Stiffness of plastic film BS 2782:Part 3

Method 332A:1976(83)

BS 2782:Part 3

Method352D:1979

Determination of falling weight impact

resistance of thin flexible sheet (film)

BS 2782:Part 3

Method 353A:1991

Determination of multi-axial impact behaviour

by the falling dart method

BS 2782:Part3

Method 360A:1991

Determination of tear resistance of plastic film

by Elmendorf method

BS 2782:Part 6

Method 630A:1982

Determination of thickness by mechanical

scanning of flexible sheet

ISO 1183 1987 BS 2782:Part 6

Method 620D:1991

Determination of density and relative humidity

of non-cellular plastic

ISO 1133 BS2782:Part 7 Determination of melt flow rate of thermoplastics

Method 720A:1979

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8.6.2 Manufacturing tolerances

A paper sack is a pre-made form of packaging manufactured to the user’s

requirements of size and construction. During the manufacture, a small degree

of variation can occur, which good manufacturing practice will keep to an absolute

minimum. Table 8.6 summarises the tolerances which apply in sack manufacture

Table 8.5 (Continued)

International or

European Standard British Standard Title

ISO 255 BS 2782 Part 8

Method 821A:1979

Determination of gas transmission rate of

plastic film

BS 2782:Part 8

Method 822A:1992

Determination of water vapour transmission

rate of plastic film

BS 2782:Part 8

Method 824A:1984(90)

Determination of coefficients of friction of

plastic film

BNS 3130 Part 8 1987 Packaging terms – plastic sacks

BS 7344 1990 Specification for reeled low density

polyethylene for general purposes

BS 3177 1959 Permeability of water vapour into flexible sheet

material

BS EN 9001/1/2/3 Quality systems (formerly BS 5750)

BS EN 7750 1994 Environmental management systems

BS 7850 1992 Total quality management

BS 6143 Part 2 1990 Guide to the economics of quality prevention,

appraisal and failure model

• Part 1 – Process Cost Model not yet published

ISO 10011/1/2/3 1993 BS EN 30011–1/2/3 1993 Guide to quality systems auditing

BS 8750 1993 Environmental health

Proposed new work items

ISO TC122/SC2/N292 Friction of filled sacks

ISO TC122/SC/N293 Method for testing the strength of seams. Determination of

the peel resistance of glued seams in a pasted sack bottom

ISO 3852-1 ISO/TC122/SC2/N162

ISO 8352-2 ISO/TC122/SC2/N163

ISO 8352-3 ISO/TC122/SC2/N164

Packaging paper sacks – method of testing the strength of a

single glued seam

Part 1 – Strength of a single glued seam under normal

climatic conditions

Part 2 – Strength after immersion in water

Part 3 – Determination of the water resistance by immersion

in water

Food contact

The Food Safety Act 1990

SI 1523:1987 – Food The materials and articles in contact with Food

Regulations 1957

SI 3145:1992 – Food The plastic materials and articles in contact with Food

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and which are generally accepted throughout the industry, although ETAPS has

agreed more stringent tolerances for some of the measurements with the Chemical

Industries Association (CIA).

8.7 Environmental position

The paper sack is a product invented and developed in the twentieth century. It

continues to be a significant form of packaging. From its humble origin as a sheet

of paper, rolled and tied at one end, filled with product and tied again at the other

end, it has developed into one of today’s leading packaging products. It is ideal for

handling and containing a wide range of products in a packed weight ranging from

10 to 50kg.

The essential element in all paper sacks has to be virgin fibre. Their

strength, handling and efficient manufacture depend mainly upon raw material

derived from sustainable temperate forests in North America and Europe, including

Scandinavia and the Russian Federation. Secondary or recycled fibre at the moment

is not an option as such material can result in an inconsistent pack which is

weaker and is unsuitable for automated filling and in handling systems. The sacks

would be more costly to produce and in many cases would be unacceptable for direct

contact with food products.

As a result of continuing research and development there have been substantial

reductions in the average construction weight of a paper sack. In conjunction with

Table 8.6 Generally accepted multiwall sack manufacturing tolerances

Note: This table conforms largely to ISO 8367/1 but the ISO standard has a face width tolerance of

plus 5 mm and minus 5 mm.

Tolerances

Item Plus Minus Notes

Sack kraft grammage 4% 4%

Sack length 10 mm 10 mm

Face width 3 mm 3 mm

Circumference (girth) 10 mm The upper limit controlled

by upper limits on the face

width and gussets

Gusset width 3 mm 3 mm

Bottom widths 5 mm 5 mm

Valve width 3 mm 0

Valve sleeve position 5 mm 5 mm

Seam overlap of

20 mm

3mm

Stitchline position Not less than 12 mm from sack transverse edge