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conceived the idea of having valve corners inserted into a

gusseted tube, or ‘‘in-fold’’ feature. These were ﬁrst made

in the mid-1920s, and their advantages were promptly

recognized. During the ﬁrst year of production, millions

were manufactured and sold, and by 1927 about 130

million were made annually.

While the cotton bags with their paper-ﬁlling sleeves

had solved the bag-ﬁlling problem, the other problem of

their return and reuse to reduce bag costs still existed.

Into this breach came the multiwall paper sacks (either

with or without ﬁlling sleeves), and thus the paper ship-

ping sack industry was born.

Portland cement and masonry cement plants, grain

products and ﬂour factories, and saltworks were the ear-

lier multiwall sack users, because these new bags solved

the dusty conditions that had been so dangerous, health-

wise. Now the workers were freed from breathing unde-

sirable dust and airborne mineral deposits, and working

conditions were greatly improved.

The terms multiwall bags and paper shipping sacks are

often used synonymously. Other industries followed suit as

soon as more producers of multiwall bags were established,

and the shipping sack industry prospered accordingly.

World War II caused severe shortages in burlap and manila

ﬁbers due to the extremely long distance from the primary

sources in Southeast Asia; concurrently, cotton ﬁber for

bags was in short supply, due to U.S. military and naval

needs (as well as for civilian clothing). These shortages

resulted in a large expansion of the use of paper shipping

sacks, and the cotton bag never recovered its earlier

dominance, just as cotton and burlap had replaced the

rigid containers such as wood boxes, crates, and barrels.

During World War II the multiwall bag really came into

common useage. W ith a wide range of special papers to

control moisture and insect infestation plus the ability

to tailor-make whatever package strength was required to

ﬁt the rough military handling, multiwalls came to be the

approved speciﬁcation packages for government purchasing.

From these early beginnings the sack industry has

grown into a multi-billion-dollar business with shipments

of over three billion sacks annually for over 2000 different

products in the United States. Over four billion sacks are

used annually in Europe (1). These end-uses fall into four

major categories: agricultural products and supplies, food

products, chemical products, and rock and mineral

products.

The primary advanges of paper shipping sacks are low

tare weight, ﬂexibility, ease of ﬁlling and handling, low

cost, minimum storage space, biodegradability, and good

graphics. Other advantages include the basic material,

which is a renewable resource; protection of contents from

moisture absorption; control of contents from moisture

loss; protection of contents from chemical action; control of

seepage or penetration of hot-packed products; provision

of loss of gas or vapor; prevention of product sifting or

contamination; good stacking and utilization of warehouse

space; FDA approval for human food products; excellent

graphics; and ease of use in merchandising displays.

Multiwall bags exclude single-ply bags and bags of

duplex construction, but do include those with three or

more plies of paper or other barrier protectors. They are

produced by combing several layers of paper (or other

substrate) over a metal former that nests the walls into a

long continuous tube (ﬂat or gusseted). These walls are

bonded together with adhesives so that each tube is

independent of the other tubes, and therein lies the

bag’s strength. The tubes may be cut straight (all at

once) or in stepped-end formation where each wall is ﬁrst

perforated, then simultaneously snapped apart.

Next, in order for the tubes to be ﬁlled and used, the

tubes are delivered to bottoming machines in the bag

plant where they are either sewn closed at one or both

ends. Or, the tubes may go to a pasting bottomer to form

either pasted open-mouth, stepped-end pasted valve, or

pinch-bottom open-mouth sacks (see Figure 1).

TYPES OF MULTIWALL SACKS

There are two basic multiwall types: the open-mouth type

and the valve type bags. Multiwalls are custom-made to

order according to the customer’s requirements and for

adequate strength to ensure the safe arrival of their

contents to the destination locations.

Whether the product is packaged with high-speed open-

mouth weighing and ﬁlling machines with automatic closing

and sealing machinery, or with even faster valve packers

and automatic bag placers, a proper and successfully de-

signed and printed multiwall bag may be readily developed.

Most multiwall bag suppliers have very well trained

sales personnel, as well as experienced factory technical

representatives with a wide range of user experience who

are well qualiﬁed to recommend the best bag for any

product requirement.

OPEN-MOUTH BAG TYPE

Sewn Open Mouth (SOM)

This style, with any number of walls, has a factory sewn

bottom and open top, and may be either ﬂat or with side

gussets. These bags are used primarily to package gran-

ular or large-particle products. The product is delivered

into the open top, and the top may be closed by sewing or

other means. If a polyethylene liner ply is speciﬁed, this

may also be heat-sealed. These types of packaging lines

may be fully automated.

Pasted Open Mouth (POM)

This style has a pasted bottom and open-mouth top, and it

sealed after ﬁlling

by folding and pasting, or, in some

cases, sewing the top. This latter style is not preferred for

packaging and shipping free-ﬂowing products because a

ﬁnlike (nonﬂat) package is obtained. But it is ideal for

containing a number of previously ﬁlled and sealed unit

bags–for example, 12.5-lb sugar or 5- to 10-lb bags of

potatoes. These are called master container or baler bags.

Pinch-Bottom Open-mouth (PBOM)

This style has the plies cut in a stepped fashion with the

bottom sealed by gluing or heat sealing at the bag factory.

88 BAGS, MULTIWALL

Figure 1. Common bag styles in current use: (a) sewn open-mouth; (b) sewn valve, (c, d) pasted open-mouth; (e) pasted-valve stepped-

end; (f) pinch-bottom open-mouth.

BAGS, MULTIWALL 89

After ﬁlling at the user’s plant, the open mouth top is

folded over and sealed by reactivating preapplied hot-melt

adhesive. This style of bag provides completely moisture-

proof and sift-proof packages. PBOM bags are the fastest-

growing segment of the total multiwall bag industry

because of the secure and strong closure ends, and full

control of sifting or infestation.

VALVE BAG TYPE

Sewn Valve (SV)

These are made with the plies at the top and bottom ends

sewn at the bag plant usually with sewn-through crepe

paper tape. The bags are ﬁlled with a valve packing

machine by forcing the product through a built-in valve

corner. For small particles and large powders, the contents

may be retained without any leakage, by hand-folding in

the paper valve extension.

This old style with sewn tape at both ends has fallen

out of favor in recent years, and the modern pasted valve

style has supplanted it.

Pasted Valve Stepped End (PVSE)

As in the pinch-bottom open-mouth bag, the plies are cut

off in a stepped conﬁguration; then the bag factory bot-

toming machine folds and, with adhesive, seals both top

and bottom ends of the tubes. An added valve sleeve is

built into a corner, permitting rapid bag placement on a

valve packer; and when the ﬁlled package falls to a take-

away conveyor, the valve automatically is closed by the

internal pressure of the product.

PVSE-type bags, introduced in 1956, are the present

standard package for most automatic or high-speed opera-

tions such as cements, powders, feeds, fertilizers, or other

chemical or building products. When stacked or palletized,

they provide stable and attractive three-dimensional

containers.

Valve bags may also be purchased with paper tuck-in

sleeves that, when folded in, will control all possible leakage.

SIZING MULTIWALL BAGS

First, the weight to be packaged and the product density

must be determined. This latter may be done by using a

box of known dimensions (1 ft 1ft1 ft) or 1 ft

to deter-

mine the weight per cubic foot. With the known weight

and volume requirements and with the bag type selected

(by product characteristics), an educated estimated set of

dimensions can be offered. A supplier can ﬁne-tune these

numbers by offering several handmade factory samples.

When test-ﬁlled, these may be adjusted to supply a larger

number of machine-made test packages to ﬁnalize all

speciﬁcations such as dimensions, ﬁlling, handling and

shipping, and palletizing conditions.

CONSTRUCTIONS

The paper most commonly used to fabricate multiwall

bags is natural-colored virgin pulp brown kraft. Made

primarily in the Southern U.S. states using a modiﬁed

sulfate pulping process, it affords longer ﬁbers that en-

hance the paper’s cross- and machine-direction strength.

Typically brown, it may be lightened through bleaching, or

semibleaching sulfate pulping.

Kraft is usually designated in basis weights ranging

from 40 to 80 lb per ream of 500 sheets of 24 in.  36 in. in

area. These grades are known as multiwall kraft and have

higher strength speciﬁcations than grocery-bag kraft (NK)

used for light-duty brown paper bags.

Other popular grades used in multiwall bags are

extensible (XTK) and free-dry (FDK) process papers.

These developments during the 1960s and 1980s resulted

in improved cross- and machine-direction stretch speciﬁ-

cations and greater overall bag strength. Other grades,

such as high-ﬁnish, rough, calendered, machine-ﬁnish,

and wet-strength, are available for speciﬁc end-use pur-

poses (see Paper).

Subsequent to the mid-1940s with the development of

tailor-made plastics, polyethylene (PE) ﬁlms have been

integrated into many bag constructions. Prior to this,

asphalt laminated kraft (ALK) provided moisture barrier

properties to multiwall bags, but it was difﬁcult to fabri-

cate on the tuber and bottoming equipment, and it is no

longer in use.

Multiwall paper bags may be lightly coated with high-

shear, low-stress adhesives to overcome bag slippage when

stacked on pallets or in storage. This inexpensive treat-

ment is invisible yet very effective.

Heavy-duty single-wall plastic ﬁlm bags are also avail-

able, however, when compared in cost with multiple-ply

kraft or a kraft paper ply plus a lightweight poly ply

or polyethylene-coated kraft, the single-wall plastic bags

are not cost-effective. They also are slippery, and palle-

tized ﬁlled bags cause shipping difﬁculties (see Bags,

plastic).

Depending on the physical characteristics of the pro-

duct being packaged in multiwall bags, they can be

custom-designed to provide all the protection that is

necessary. Low-or high-density polyethylene, poly(vinyl

chloride), poly(vinyl dichloride), nylon, saran, foil, and

polypropylene are some of the available substrates.

Most multiwall bag suppliers have well-trained sales

personnel and specially trained factory technical repre-

sentatives who are well qualiﬁed to recommend the best

construction for any particular requirement.

SPECIFICATIONS

With the complete knowledge of a product’s physical and

ﬂow characteristics, a bag style may be selected. This is

only a minor portion of the information required to design

a new multiwall bag that will deliver the product in

acceptable condition. The product density and the desired

weight to be bagged must be known; from this, the

required capacity (in cubic inches) is calculated.

General rules are usually followed to select the total

kraft paper basis weight and the number of kraft plies. For

instance, 50 lb of product might require three walls of

50-lb natural kraft to total 150 lb, and 80 lb would need

90 BAGS, MULTIWALL

three or four walls of combined 40- and 50-lb basis weight

multiwall kraft, totaling ~180 lb.

The requirements for product protection must be care-

fully evaluated. This might include moisture protection

and control, gas or odor control, grease or oil control,

siftproofness, chemical resistance, mold protection, and

toxicity or hazardous chemical protection.

Each of these factors can be controlled by the use of one

or more of the following available sheets: low-density PE

(LDPE), high-density PE (HDPE), polyethylene-coated

kraft paper (PEK), saran, Tyvek, saran-coated polypropy-

lene (SCPP), polyfoil–polykraft laminate (PE-AL-PE-K)

biaxially oriented polypropylene (BOPP), or even poly-

coated crepe kraft tape, or heat-sealed waxed kraft tape.

About 25% of the multiwalls now being produced specify

some kind of special liner ply.

Special barrier plies are commonly used in multiwall

bags to protect the product from gaining or losing its

intended moisture content. This is stated in terms of its

water vapor transmission rate (WVTR). A wall of material

may be laboratory-tested, to determine its WVTR rating in

terms of g/m

per 1001F at 95% RH (relative humidity) in

24 h. The more effective barriers usually are the heavy

2.5–4.0-mil HDPE and LDPE, or 0.75-mil BOPP ﬁlm,

while those most frequently needed are the less expensive

0.5-mil HDPE and 0.8–1.0-mil. LDPE ﬁlms.

Some products have speciﬁc needs, such as controlled

porosity to permit proper air or moisture transfer, grease

or oil penetration, wet-strength protection with urea for-

maldehyde, or melamine formaldehyde, which enable the

paper to retain its original strength when saturated by

water. All these and more—toxicity, hazardous nature,

odor control, acidity, or alkalinity—can be controlled.

Or, for a really attractive consumer package, the man-

ufacturer might select a supercalendered high-ﬁnish full-

bleached white sheet with gravure-quality printing.

Multiwall prices are developed by applying cost factors

for bag style, bag size (sheet width and tube length), basis-

weight factors for kraft, and the plies of other substrates.

To these the supplier adds the cost for printing by area and

number of colors required. Bags are usually ordered by the

carload or trailerload to obtain the lowest unit price.

In cases where a product is highly aerated and moist-

ure protection is only nominal, one or more kraft plies may

require allover pinhole perforations (AOPHP) of different

diameter and spacing. These allow the internal pressure

to be expended during bag ﬁlling and they prevent bag

breakage.

Discussing the particular needs that are required for

your product with a multiwall bag supplier will direct you

to the correct and complete bag speciﬁcations from the

weighing and ﬁlling stages to the customer’s ultimate

place of use.

PACKAGING EQUIPMENT

For each style and size of bag used, there is a wide choice

of equipment available for weighing, ﬁlling, and closing

the packages. There are two main styles of bagging

equipment: open-mouth and valve.

Open-Mouth Packers

The older, original method of open-mouth bagging required

a scale, a vertical delivery spout, and a closing function.

With today’s modern high-speed open-mouth equipment,

there are automatic and accurate weighing scales, auto-

matic bag placers, and ﬁnally, automatic bag closures.

These systems usually run up to 20 or more preweighed

charges of up to 100 lb of material per minute with only

token supervisory requirements. There are two preferred

methods of closing open-mouth multiwall bags: sewn

closure or heat sealing.

Most common of the sewn types is the stitching of the

bag mouth with cotton or polypropylene sewing thread

with an industrial sewing head. With more secure or

special product requirements, either sewn-through tape,

adhesive, or heat-sealed tape over sewing may be provided.

Valve Bag Packer

With the invention of the valve packer in the mid-1920s,

the valve-style bag may be ﬁlled by forcing the product

through a spout (usually horizontal) into the valve corner

and then down into the bag. The product is either pre-

weighed and then forced into the bag, or it is weighed in

the package (gross-weighed).

Choices are available when weighing and ﬁlling valve

bags, and they depend on the product’s physical charac-

teristics, price, and production speed requirements.

Most valve packers use either the impeller-wheel

method or the belt-feed method. With the former, rotating

high-speed paddles force the material into the package.

This causes a great amount of aeration to occur, and the

PV or PVSE bags must be capable of withstanding great

pressure. One operator with a four-spout impeller valve

packer may produce up to thirty 94-lb (206-kg) ﬁlled bags

per minute.

With the belt valve packer, the product is thrust

between a moving belt and a rotating grooved pulley and

then into the valve opening. This is gentler on the

material, but not as fast as the impeller method. Fertili-

zers and small-pelleted materials are usually packaged on

belt-style valve packers.

Three other less popular valve packer styles are avail-

able auger, gravity, and airﬂow. The latter is more modern

than the other two and has the advantages of handling

non-free-ﬂowing ﬁnely powdered materials, without degra-

dation with much better accuracy. The auger packer uti-

lizes a horizontal screw within a hollow tube. These work

well for non-free-ﬂowing products, and since the screw’s

rate of rotation can be adjusted, better weight accuracies

are obtained. The last and least popular valve packing

method is the vertical-gravity style. But, since it is used for

very free-ﬂowing or inexpensive materials, accurate bag

weighments are sometimes difﬁcult to obtain.

GRAPHICS

Previously, the fabrics of multiwall paper bags were

printed with simple yet effective designs to identify the

manufacturer, the product, the net weight, and some

methods for using the contents.

BAGS, MULTIWALL 91

Cotton and burlap bags had a second life, since they

must either be returned, cleaned, and ﬁlled for reship-

ment or, in the case of the cotton bags, be washed and then

sewn into towels or items of apparel at home.

With the conversion from fabric to paper bags came the

important advantage of superior printing capabilities.

With smooth surfaces and all-around six-sided printing

of up to four colors, a company’s marketing message could

be extolled, thereby differentiating itself from all its

competitors.

If brown kraft is not attractive enough, a supplier can

readily furnish semibleached (SBW) or full-bleached

(FBW) kraft. Smoother or brighter papers and coatings

in conjunction with modern printing capabilities provide

appealing and forceful advertising that make the bags

effective billboards for marketing their contents. Whether

they were small-unit consumer bags or large industrial-

size bags, they can utilize ﬂexographic printing to carry

the intended messages.

If more deﬁnition is required, prepress gravure print-

ing using up to six colors will result in photographic

quality printing, with the following advantages: excellent

product and protection; unlimited supplier capabilities;

biodegradable materials; and proven shipping and storage

environments

Pet-food and charcoal briquette bags are excellent

examples of effectively printed packages found in all

supermarket shelves today.

Multiwall bags are readily accepted and desirable as

shipping containers in all domestic and export markets,

including hundreds of uses for agricultural products, build-

ing materials, chemicals, food products, and minerals.

TRANSPORTATION

In the past 40 years the shipment of ﬁlled multiwall bags to

the market where they are used has swung almost com-

pletely from railroad boxcars to over-the-road trailers or

straight trucks. Also, pallets have replaced the individual

handling of ﬁlling bags. Coincidentally, plastic ﬁlm has in

many cases been incorporated into the bag’s construction.

These changes have resulted in lighter basis-weight re-

quirements and lower costs. Multiwall sacks are covered by

the Uniform Freight Classiﬁcation for Rail Rule 40; Na-

tional Motor Freight by Truck Item 200; Hazardous Mate-

rials Regulations title 49 CFR; and Federal Speciﬁcations

UUS-48, sacks for government shipment (2).

ENVIRONMENTAL CONSIDERATIONS

At present, the essential element in all paper sacks is virgin

ﬁber. Secondary or recycled ﬁber is not an option because

such material can result in an inconsistent pack that would

be weaker and unsuitable for automated ﬁlling and in

handling systems. The bags would be more costly to

produce and in many cases would be unacceptable for direct

contact with food. As a result of continuous research, there

has been substantial reductions in the average construction

weight of a multiwall paper sack. Recovered bags can be

recycled. It has been suggested that speciﬁcations should be

revised to maxmize the content of secondary fber. A levy on

virgin ﬁber has been proposed in Europe.

USES

Some examples of products that are packaged in weights

of under 20 lb are additives, animal food, baking products,

cat litter, charcoal, chemicals, coffee, cookies, dried foods,

grits, insecticides, microwave popcorn, plant food, theater

popcorn, salt, and sugar. Industrial products weighing

more than 20 lb and packed in multiwall bags are absor-

bents, abrasives, cement, lawn and garden supplies, and

animal feeds (3).

Helpful Hints. The list below is meant to be of use to a

packager in selecting an appropriate multiwall paper

sack.

1. In the initial planning stage of a new use for multi-

wall shipping sacks, obtain the guidance of bag

suppliers.

2. Develop accurate product physical and chemical

properties.

3. Finalize decision on desired package weights.

4. Determine annual production requirements.

5. Select proper bag style and features.

6. Consider styles of packaging equipment available.

7. Decide on the transportation and distribution

modes.

8. Determine probable bag sizes and constructions.

9. Plan bag transporting methods to provide minimum

individual handlings.

BIBLIOGRAPHY

Allan M. Baylis,‘‘Bags, Multiwal’’ in The Wiley Encyclopedia of

Packaging, 2nd edition, by John Wiley & Sons, New York,

1997, pp. 61–66.

Cited Publications

1. The Environmental and Technical Association for the Paper

Sack, ‘‘Multiwall Paper Sacks’’ in M. J Kirwan, ed., Paper and

Paperboard Packaging Technology, Blackwell Publishers, Lon-

don, 2005.

2. Transportation Regulations, Paper Shipping Sack Manufac-

turer’s Association, Inc., Coopersburg, PA, www.pssma.com,

accessed June 2008.

3. End Uses, Paper Shipping Sack Manufacturers Association,

Inc., Coopersburg, PA, www.pssma.com, accessed June 2008.

BAGS, PLASTIC

Plastic bags, available in virtually all shapes, sizes, colors,

and conﬁgurations, have replaced paper in most light-

duty packaging applications. Paper has been more difﬁcult

92 BAGS, PLASTIC

to replace in heavy-duty applications (see Bags, heavy-

duty plastic). Light-duty plastic bags are generally de-

scribed in one of two ways: by the sealing method or by

application. This article explains the methods used to

produce plastic bags and deﬁnes the various types of

plastic bags in terms of their intended use. A plastic bag

is deﬁned here as a bag manufactured from extensible ﬁlm

(see Films, plastic) by heat-sealing one or more edges and

produced in quantity for use in some type of packaging

application.

METHODS OF MANUFACTURE

By deﬁnition, all plastic bags are produced by sealing one

or more edges of the extensible ﬁlm together. The proce-

dure by which this heat-sealing occurs (see Sealing, heat)

is typically used to identify and categorize types of plastic

bags. Three basic sealing methods are used today that

include the following: sideweld, bottom seal, and twin seal.

Sideweld Seal. A sideweld seal is made with a heated

round-edged sealing knife or blade that cuts, severs, and

seals two layers of ﬁlm when the knife is depressed

through the ﬁlm material and into a soft rubber backup

roller. The materials are fused by a combination of pres-

sure and heat (see Figure 1).

The sideweld seal is the most common bag-sealing

method. Typical high usage bags, e.g., bread bags and

sandwich bags, are produced using the sideweld techni-

que. The term ‘‘sideweld’’ is derived from the fact that

many of the bags produced in this fashion pass through

the bag machine with the length (or depth) of the ﬁnished

bag perpendicular to the machine ﬂoor. The ﬁlm fed into

the machine is either prefolded, i.e., J stock, or folded

during the in-feed process (see Figure 2).

Bottom Seal. The bottom-seal technique seals the bag

at the bottom only. Tube stock is fed into a bag machine,

the single seal is produced at the bottom of the bag,

and the bag is cut off with a knife action that is separate

from the sealing action. A bottom seal is generally made

by a ﬂat, heated sealing bar, which presses the layers of

ﬁlm to be sealed against a Teﬂon (Du Pont)-covered rubber

pad, i.e., seal pad (see Figure 3), or another hot-seal bar

(see Figure 4). A separate cut-off knife is used to separate

the bag from the feedstock while the seal is made or

immediately thereafter. Both bottom-seal mechanism de-

signs produce a bag with only one seal, unless the tube has

been manufactured by slit-sealing (see below). The small

amount of unusable, wasted ﬁlm between the edge of the

seal and the cutoff point, called the ‘‘skirt’’ of the bag, is an

important factor in the total cost of the bag. Any disad-

vantages caused by the presence of the unwanted skirt are

usually offset by greater control of the sealing process. The

sideweld method actually melts through the plastic ﬁlm,

and overheating of the ﬁlm resins can change the physical

structure of the plastic molecules when the seal cools. In

contrast, the bottom-seal method controls the amount of

heat and the dwell time, i.e., the time that the heat is

applied to the ﬁlm, to produce a seal that does not destroy

the ﬁlm or change its physical properties. In addition, the

total amount of ﬁlm sealed together is usually larger,

because the seal bar has a ﬁxed width and none of the

ﬁlm material is melted or burned away.

The bottom-seal method is commonly used to produce

(HDPE) merchandise bags and (LDPE) industrial liners,

trash bags, vegetable and fruit bags, and many other types

of bag supplied on a roll. In contrast to the sideweld

method, designed primarily for high-speed production of

bags made from relatively light-gauge ﬁlms — i.e., 0.5–2.0

mil (13–51 mm)—bottom-seal methods are often used to

produce bags from ﬁlm from 0.5 to Z6 mil (13–Z152 mm)

at slower production speeds. Bags manufactured by

bottom-seal methods are delivered through and out of

the bag machine with the length (or depth) of the bag

parallel to the machine direction. Because all of the bags

Figure 1. Sideweld-seal mechanism.

Figure 2. Sideweld process (top view).

Figure 3. Bottom-seal mechanism (top heat only).

BAGS, PLASTIC 93

are produced from tube sock, multiple-lane production of

bags is limited only by the widths of the machine and the

bags and the ﬁlm-handling capability of the bag machine

(see Figure 5).

Twin Seal. The twin-seal method employs a dual bot-

tom-seal mechanism with a heated or unheated cutoff

knife located between the two seal heads (see Figure 6).

The unique feature of the twin-seal mechanism is that

it supplies heat to both the top and bottom of the ﬁlm

material and makes two completely separate and inde-

pendent seals each time the seal head cycles. Like the

bottom-seal method, the twin-seal technique can supply a

large amount of controlled heat for a given duration. This

ability makes the twin seal useful in sealing heavier-

gauge ﬁlms as well as coextrusions and laminates. Be-

cause two seals are made with each machine cycle, the

twin-seal method can be used to make bags with the seals

on the sides of the ﬁnished bag, i.e., like sideweld bags, or

on the bottom of the bag in some special applications, such

as retail bags with handles. Many special applications call

for the use of a twin-seal-type sealing method, but it most

often is used in production of the plastic ‘‘T-shirt’’ grocery

sack.

Slit Seal. Another type of sealing method, the slit seal,

involves sealing two or more layers (usually only two) of

ﬁlm together in the machine direction through the use of a

heated knife, hot air, laser beam, or a combination of

methods. The slit-seal technique is usually used to convert

a single large tube of ﬁlm into smaller tubes. In the

production of grocery sacks, for example, a single extruded

60-in. (152-cm) lay-ﬂat tube of ﬁlm (see Extrusion) can be

run through two slit sealers in line with the bag machine.

This process results in three tubes of 20-in. (51-cm) lay-

ﬂat material being fed to the bag-making system (see

Figure 7).

APPLICATIONS

Most plastic bags are characterized in terms of their

intended use, e.g., sandwich bags, primal-meat bags,

grocery sacks, handle bags, or bread bags. To provide

some order to this user-based classiﬁcation and deﬁnition

system, it is convenient to separate the bags into commer-

cial bags and consumer bags.

Commercial Bags. A commercial plastic bag is used

as a packaging medium for another product, e.g., bread.

Typical commercial bags and seal methods are listed in

Table 1.

Consumer Bags. Consumer bags are purchased and

used by the consumer, e.g., sandwich bags, trash bags.

The plastic bag is the product. Typical consumer bags and

seal methods are listed in Table 2.

Figure 5. Bottom-deal bag machine, three-level.

Figure 6. Twin-seal mechanism.

Figure 4. Bottom-seal mechanism (top and bottom heat).

Figure 7. Slit sealer (hot-knife type). (a) side view; (b) top view.

Table 1. Commercial Plastic Bags

Sealing Method Types of Bag

Sideweld Bread bags, shirt and millinery bags, ice

bags, potato and apple bags, hardware

bags

Bottom seal Vegetable bags on a roll, dry-cleaning

bags, coleslaw bags, merchandise bags

Twin seal Primal-meat bags, grocery sacks

With dual bottom seal.

94 BAGS, PLASTIC

In addition to the conventional bags, an enormous

variety exists of special bags that have been created by

bag producers to meet commercial and consumer needs.

These bags are usually described as specialty bags, but

they are all variations of the standard bags described

above. Specialty bags include rigid-handle shopping bags,

sine-wave handle bags, pull-string bags, patch-handle

bags, double-rolled bags, square-bottom bags, round-

bottom bags, deli bags, etc. Examples of both common

and specialty bags are shown in Figure 8.

BIBLIOGRAPHY

‘‘Bags, Plastic,’’ in L. L. Claton, ed. The Wiley Encyclopedia of

Packaging Technology, 1st ed., Wiley, New York, by FMC

Corporation, 1986, pp. 39–42.

General References

‘‘Bag Making: Inline or Off-Line,’’ Plast. Technol. 30(2), 55 (1984).

L. R. Whittington, Whittington’s Directory of Plastics, 2nd ed.,

Technomic Publishing, Lancaster, PA, 1978.

Polyethylene Film Extrusion, U.S. Industrial Chemicals Co., New

York, 1960.

BANDS, SHRINK

DAVID R. KOPSILK

ED FISHER

CMS Gilbreth Packaging

Systems, Trevose, Pennsylvania

Updated by Staff

INTRODUCTION

Shrink labels, bands, and sleeves provide today’s packa-

ging companies with a unique and versatile method for

labeling their products. Developed in Japan during the

1960s, shrink bands became popular in the United States

in the early 1980s, mainly as unprinted or one-color

tamper-evident neckbands. Today, the shrink band is

used as a primary label, as a decoration, for multipack

applications, and as a traditional tamper-evident feature.

The high-quality printing and gloss inherent in shrink

labels ensure packagers that their product will generate

tremendous visual shelf appeal.

MANUFACTURING

Shrink bands are generally produced using special poly

(vinyl chloride) (PVC) ﬁlms, although PETG ﬁlms,

oriented polystyrene ﬁlms, and poly(lactic acid) are also

used (1). Film thicknesses generally run from 1.25 to 3.0

mils, with 1.50 and 2.0 mils the most prevalent. Material

is available in two varieties: seamed ﬁlm, and tubing.

Seamed ﬁlm is converted by bonding a ﬂat sheet of printed

Table 2. Consumer Plastic Bags

Typical Sealing Method Types of Bag

Sideweld Sandwich bags, storage bags

Sideweld or bottom seal Trash bags, freezer bags,

can liners

Bottom seal Industrial liners

Figure 8. Bags, plastic: (a) conventional wicket bag; (b) conven-

tional bag; (c) square-bottom bag; (d) round-bottom bag; (e)

sandwich bag; (f) trash bag; (g) handle bag; (h) patch-handle

bag; and (i) sine-wave bag.

BANDS, SHRINK 95

or unprinted material into a sleeve conﬁguration. This is

achieved through a process of solvent seaming into the

desired dimension or ﬂat width. Tubing is extruded into

the desired ﬂat width before printing. Seamed ﬁlm can be

reverse printed with 3601 graphics and generally provides

more consistent dimensions in both ﬂat width and gauge

proﬁle. Tubing is generally less expensive, and it is used

for unprinted or one-color work.

Important dimensional measurements associated with

shrink bands are (a) ﬂat width, which represents the

dimension relating to the diameter or circumference of

the container, and (b) cut height or impression height,

which represents the length of the container or the length

of the section of the container to which the shrink band

will be applied. These dimensions are usually expressed in

millimeters.

The inherent shrink is imparted into the material in a

heated stretching process. With tubing, this is achieved

during the initial extrusion process. With seamed ﬁlm, a

process known as tentering is used. A tenter frame is a

modiﬁed oven that uses a combination of airﬂow and

temperature zones to orient the material. For example, a

30-in.-wide roll of 5.0-mil material will be converted into a

60-in. roll of 2.0- or 2.5-mil ﬁlm. Shrink ratios of 50–70%

can be achieved by adjusting the process. Various ratios

are used, depending on the contour of the container being

labeled.

Shrink bands are generally processed using the gra-

vure printing method. Reverse rotogravure allows the

image or copy to be printed on the back side of the ﬁlm,

which results in a glossy look to the package as well as

protection of the image from scratching or scufﬁng that

may occur in ﬁnal packaging or distribution. Gravure

printing provides excellent color reproduction consistency,

high speed, and productivity, and it is an excellent method

for printing smooth ﬁlm surfaces. Flexo printing has made

inroads into this market as the technology improves, but

gravure is still dominant.

After printing, several other features can be added to

the shrink band. Vertical and horizontal perforations, as

well as tear tape strips, can be added to make the band

removable for the consumer. Some packagers use a hor-

izontal perforation to make part of the label removable,

the tamper-evident feature is removed, and the primary

label stays on the container. Some examples of this are

toothpaste pumps, lip balms, syrups, and salad dressings.

Bands can be provided in continuous roll-fed form, or in

individual cut pieces for manual application.

APPLICATION

Bands can be applied manually or by automatic machin-

ery. Machinery on the market today is capable of applying

bands at speeds of 500 per minute or more, depending on

container size, container contour, and label dimensions.

For example, a lip-balm label of approximately 27-mm ﬂat

width and 60-mm length can be applied faster than a

190-mm-ﬂat width, 40-mm length band for a large dairy

container, due to both the label size and the container size.

Machinery is dominant in the dairy, packaged-food, and

pharmaceutical markets. Most machinery applies shrink

bands in a horizontal method, but recent developments

have allowed for vertical application of pen barrels, lip

balms, and other small cylindrical objects. Most machin-

ery requires material of 2.0 mils or more for process-

ing, and the use of a wedge or other device to open the

material before application is common. After application,

the container is sent through a heat tunnel, which uses a

combination of airﬂow and heat to shrink the band

securely onto the container. A dwell time of 3 s at roughly

3001F is used for most applications, although some pro-

ducts require an elaborate system of varying tempera-

tures and product rotation to achieve the desired effect.

Machinery is available to pack two containers together

with one band. This process is used in lieu of an overall

bundling method, which then may require the application

of another printed label for bar coding or to convey the

promotional message. Manual applications are used when

product volume is small or in market trial introductions.

Since shrink bands adhere using shrink energy, no glue

or glue applicating systems is required to apply shrink

bands.

USES

Shrink bands are generally used in three ways: as tamper-

evident neck bands, as primary labels and decorations, or

for promotional multipacking (see Figure 1 for examples).

Food and pharmaceutical packagers have been using

neckbands as a tamper-evident feature ever since the

Tylenol incident in Chicago in 1982. They are often used

in conjunction with vacuum packaging, inner seals, and

breakaway closures for product integrity. One advantage

for consumers is that neckbands provide evidence of

tampering before the consumer brings the product home,

as any attempt at removal is evident on the store shelf. In

the food-packaging market, yogurt, sour cream, salad

dressing, mayonnaise, syrup, mustard, and jelly are just

some examples of products that use neckbands (2). In the

pharmaceutical and health–beauty arena, use of neck-

bands is even more prevalent. Eye care, mouthwash,

cough syrup, pain relief, and vitamins as well as many

ethical medicines are among the products that utilize this

tamper-evident feature (see Tamper-Evident Packaging).

In the primary label and decorating market, shrink

bands provide a versatile and unique way to package

various products. The reverse printing capability allows

graphics

to stand out

on the store shelf as well as provide

durability. Batteries have been labeled using shrink ﬁlm

for years, because the ﬁne copy and metallic look required

on these products are achievable with shrink labels. Lip-

balm labeling combines a primary and tamper-evident

label using a horizontal perforation for easy removal of

the tamper-evident feature. Since this product is often

sold loose at drug and convenience-store checkout coun-

ters, the lip-balm package can be easily displayed without

any further packaging. In recent years, writing instru-

ments decorated with multicolor graphics have become

popular, especially those with designs targeted to the

teenage market. One advantage of shrink labels on this

product is that the reverse printing protects the image

from the oils and dirt that occur naturally on people’s

96 BANDS, SHRINK

hands, which can erode an image directly printed on a

writing instrument. Other products using shrink bands as

primary labels or decorations are Christmas ornaments,

Easter egg decorating kits, children’s shampoos and soaps,

deodorant sticks, plastic baseball bats, golf clubs, yarn and

thread spools, craft paints, and tobacco containers. Seal-it

Inc offers holographic labels (3).

Full-body sleeves cover 3601 of a product from top to

bottom., and they provide the maximum promotional area.

Perforations are available that are easily removed, but the

label remains on the containers. Up to 10 colors are

available (2).

A third use of shrink labels is for multipacking pur-

poses. Two or more products are bundled together using a

shrink band. Printing the bands eliminates the need to

apply a paper label later in the distribution process. Two

products are placed side by side, and the label is applied

vertically over the top. It is not uncommon to put two

different products together. Examples of this would be

16 oz of shampoo and a trial size of conditioner, or

mouthwash and a toothbrush. When three containers

are banded, placing the products in a triangle conﬁgura-

tion provides for a more secure package, because this

prevents the third product from falling out of the middle.

Just about any product can be packaged this way; some

common uses are for packaging of hair-care products,

vitamins, car-care products, lubricants, peanut butter,

caulking compounds, and cooking sauces. Three tubs of

baby wipes are stacked on top of each other and vertically

sleeved using a shrink band. Most multipacks are done

manually, and almost any product or container allows for

this banding method. Basic uses for multipacks are trial

sizes, values, twin packs, buy-one-get-one-free, and seaso-

nal promotions (4).

Shrink bands will continue to provide packaging com-

panies with a unique method of labeling their products.

The double beneﬁt of increased tamper evidence and

integrity along with attractive graphic impact is beneﬁcial

for almost any product. Shrink bands will continue to

grow as a labeling method in the beverage, industrial, and

toy markets as well as maintaining an important position

in the traditional food, health and beauty, and pharma-

ceutical markets.

BIBLIOGRAPHY

D. R. Kopsilk and E. Fisher, ‘‘Bands, Shrink’’ in A. J. Brody and

K. S. Marsh, eds., The Wiley Encyclopedia of Packaging

Technology, 2nd edition, Wiley, New York, 1997, pp. 69–70.

Cited Publications

1. ‘‘Shrink Bands and Sleeves,’’ www.servipak.com, September

2008.

2. J. C. Brennan and B. P. F. Day, in J. G. Brennan, ‘‘Packaging’’

ed., Food Processing Handbook, Wiley-VCH, Weinheim,

Germany, 2006.

3. ‘‘Shrink Bands,’’ www.sealit.com, September 2008.

4. ‘‘Multipack Bands,’’ www.mpilabels.com, September 2008.

BARRELS

FRANK J. SWEENEY

Sweeney Cooperage Ltd.

A barrel, or cask, is a cylindrical vessel of wood that is ﬂat

at the bottom and top, with a slightly bulging middle. The

three primary parts of a barrel are heads (bottom and top),

staves (sides), and hoops (rings that bind the heads

and staves together) (see Figure 1). Speciﬁcations are

contained in the Department of Transportation regula-

tions (1).

In architecture and physics, the arch is probably the

stongest possible structure. The more pressure or weight

exerted on the top (keystone) of the arch, the stronger the

arch becomes.

The wooden barrel is designed according to the double-

arch principle of strength. Like an egg shell, it is doubly

arched, both in length and girth. The bend in the stave’s

length is the ﬁrst arch, and the bilge circumference of the

stave’s width is the second arch. These arches impart

great strength.

There are three basic cooperage operations: logging the

timber, milling the logs, and assembling the barrel staves

and heading material. Saws reduce the logs to length,

produce edge-grained pieces of cylindrically shaped and

jointed wood for the staves, and produce ﬂat pieces of wood

for the heading. In recent times, staves have been quarter-

sawn as opposed to earlier cylindrical-sawn staves. The

Figure 1. Examples of commercial uses of shrink bands.

BARRELS 97