Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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REFERENCES

Cited Publications

1. A. A. Schoengood, ed., Plast. Eng. 32(3), 25 (1976).

2. M. McMurrer, ed., ‘‘Update: PVC Heat Stabilizers’’ in Plastic

Compounding, Resin Publications, Cleveland, OH, 1980, pp.

83–90.

3. L. R. Samuelson, Plast. Des. Process. 21(8), 14 (1981).

General References

P. Bredereck, J. of Vinyl Technology 1(4), 218–220 (1979). Guide to

Plastics, McGraw-Hill, New York, 1979, p. 27.

M. McMurrer, ed., Plastic Compounding 1984/85 Redbook, Vol. 7,

No. 6, Resin Publication, Cleveland, OH. 1985.

M. McMurrer, ed., ‘‘Update—U.V. Stabilizers,’’ Plastic Compound-

ing, Resin Publications, Cleveland, OH, 1985, pp. 40–57.

Thermal Stabilization of Vinyl Chloride Polymers, Technical

Report 3250, Rev. 7/68, Rohm & Haas, Philadelphia, 1968.

L. R. Samuelson, Plastics Design and Processing 21(8), 13–15

(1981).

V. Struber, Theory and Practice of Vinyl Compounding, Argus

Chemical Corp., New York, 1968.

FILM, SHRINK

GEORGE D. WOFFORD

Cryovac Division, W. R. Grace &

Co.-Conn., Duncan, South

Carolina

Shrink ﬁlm is a name given to a unique family of ﬁlms

that are distinguishable by their attributes, production

processes, and end uses. They are composed of two basic

categories: poly(vinyl chlorides) (PVCs) and polyoleﬁns.

Although most plastics exhibit some amount of free shrink

and shrink force at elevated temperatures, true shrink

ﬁlms (sometimes known as heat-shrinkable ﬁlms) provide

a high degree of free shrink with a controlled level of

shrink force over a broad temperature range.

Shrink force must be controlled to prevent crushing or

deforming the product being packaged. Providing the

proper level is important to the marketing of toys; games;

cards, calendars, and other paper products; hardware;

food; and a variety of merchandise where a tight, glossy

package appearance is essential.

Shrink ﬁlms are produced by uniaxially or biaxially

orienting a sheet or tube of ﬁlm by imposing a draw force

at a temperature where the ﬁlm is softened but kept below

its melting point, then quickly cooled to retain the physi-

cal properties generated during orientation. It is impor-

tant to note that the orientation temperature occurs

between the vicat softening point (ASTM D1525) and the

melting point, but is not directly related to the glass-

transition temperature (T

) as some literature would

indicate. This can be seen in Table 1, which compares

glass-transition temperatures to vicat softening points for

several polymers.

Prior to orientation, the molecules of the ﬁlm are

randomly intertwined, exhibiting no particular align-

ment. However, when a draw force is imposed, the amor-

phous regions are straightened and oriented to the

direction of force. By applying proper cooling, the mole-

cules will be frozen in this state until sufﬁcient heat

energy is applied to allow the chains to shrink back. One

can visualize this phenomenon by stretching a rubber

band and dipping it into liquid nitrogen. It will remain

stretched as long as it is kept at sufﬁciently cold tempera-

tures. However, when enough heat energy is applied, the

rubber band will shrink back to its original relaxed state.

Orientation on a commercial scale can be achieved

using either of two methods: a tenter-frame or a bubble

process. Tenter-frame technology produces a variety of

‘‘heat-set’’ products, of which biaxially oriented polypro-

pylene (BOPP) is the most common. Heat setting is a

process whereby a ﬁlm is reheated in a constrained state

such that the shrink properties are destroyed. Other

important characteristics derived from orientation (optics,

tensile strength, and modulus) remain intact, however.

Current tenter-frame technology does not allow the pro-

duction of materials with a high degree of free shrink and

shrink force due to the mechanics and thermodynamics of

the process.

In the tenter-frame process, a ﬂat sheet is produced and

cooled on a chill roll, which is usually immersed in a

waterbath. The sheet then proceeds through a machine-

direction orientation (MDO) unit, where it is heated and

stretched to the desired ratio. On exiting the MDO unit,

the sheet enters the transverse-direction orientation

(TDO) unit, where it is reheated and stretched. BOPP is

commonly stretched 700–800% in both the machine and

transverse directions. After exiting the TDO, the material

is wound into large mill rolls for aging (aging allows

secondary polymer crystallization, ensuring ﬁlm ﬂatness

and roll uniformity) and converting. Some tenter-frame

systems have additional downstream equipment for re-

tensilization and further heat setting or annealing. The

primary suppliers of tenter-frame systems are Bru

ckner

(Germany), Mitsubishi (Japan), and Marshall & Williams

(United States).

The second commercial method is the bubble process,

sometimes referred to as a tubular process. A primary tube

is produced by blowing the ﬁlm onto an external mandrel

or casting it onto an internal mandrel. Water is used to

help cool the tube at this point.

After it has been cooled, the tube is reheated and air is

used to inﬂate it into a bubble. On inﬂation, it is oriented

Table 1. Glass-Transition Temperatures and Vicat

Softening Points of Common Polymers

Polymer T

(1F) Vicat (1F)

Polystyrene 181–201 208

LLDPE 188 to 5 177–220

Polypropylene 7–41 307

PVC 158–176 183

498 FILM, SHRINK

in both directions simultaneously, typically 700–800% for

BOPP. Other ﬁlms are oriented 200–1000% in either

direction. After orientation, the bubble is cooled using an

air ring. The tube is slit and separated, and then each half

is wound onto mill rolls for aging and converting. Addi-

tional in-line processing, such as corona treatment for

materials that will be printed at a later time, is often

used with this technology. Commercial tubular orientation

equipment suppliers include Prandi (Italy) and Gloenco

(England). ICI, the developer of much of the early tech-

nology, exited this market in the mid-1980s.

Shrink ﬁlms were limited to monolayer constructions

until recently. Technological advances over the past dec-

ade have led to the development of multilayer coextru-

sions. These process improvements have enabled ﬁlms to

be designed with greater control over desired character-

istics to meet a much broader range of packaging require-

ments. Table 2 lists the major shrink ﬁlms offered in the

U.S. market.

The key attributes that are important to shrink ﬁlms

include shrink, sealability, optics, toughness, and slip.

Each of these attributes is composed of several facets.

For shrink properties, they are onset temperature, free

shrink, shrink force, shrink temperature range, memory,

and overall package appearance. For sealing properties,

one must also consider ease of trim sealing, trim seal

strength, trim seal appearance, static lap sealability, and

thermal lap sealability. For optics, it is important to

consider clarity, gloss, and haze.

There are three aspects to toughness: impact strength,

slow puncture resistance, and tear resistance. Impact

strength measures how well a material resists a sudden

force, such as when a box is dropped from a certain height.

Slow puncture resistance measures how well a material

resists a gradual increase in tension, such as when some-

one tries to poke a ﬁnger through a piece of ﬁlm. Tear

resistance is the measure of how well a ﬁlm resists tearing

once it has been nicked or abraded.

Finally, for slip, it is important to consider both hot slip

(such as a warm package being placed in a carton during

packoff) and cold slip (important in removing a package

from a carton). Many other attributes must be considered

for any given application, depending on the nature of

the product being packaged. Film properties along with

potential advantages and disadvantages are shown in

Tables 3, 4.

Packaging equipment for shrink ﬁlms are available in a

variety of models and price ranges, depending on the

features desired (see Wrapping machinery, shrink ﬁlm).

An important attribute for equipment is the sealing

method. Until recently, most sealing systems for typical

shrink-ﬁlm applications used a hot wire in order to seal

and cut the ﬁlm at the same time. More recently, hot-knife

systems have gained popularity on many models because

of their increased durability.

The wire or knife seals the ﬁlm against a pad that is

covered with Teﬂon tape. Impulse seals are also employed

for a number of applications. Time, temperature, and

pressure are the three variables that must be balanced

to optimize seal quality. If the time that the sealing head is

held on the ﬁlm is not long enough, the temperature is too

low, or the pressure is insufﬁcient or uneven, the seal

quality will be poor. Conversely, if the sealing temperature

or the pressure is too high, the result may be a weak

spot just behind the seal area, causing failure during

distribution.

To prevent seal failure problems, there must be an

appropriate amount of time for cooling of the seal prior to

any force being applied to the ﬁlm. This cooling time

Table 2. U.S. Shrink-Film Offerings

Polyoleﬁns

Polyethylene monolayer DuPont Clysar HP, LLP

Cryovac D601

Polyethylene multilayer Cryovac D955, D940, RD106,

D959

Polypropylene/monolayer

ethylene–propylene

copolymer

DuPont Clysar EHC, CHS, RSW

Okura Vanguard 100

Gunze PSS

PP/EPC multilayer Cryovac MPD2055, MPD2100,

J960, J961

Okura Vanguard 501

Intertape Exlﬁlm IP-33

DuPont Clysar EZ

Poly(Vinyl Chloride) (PVC)

Biaxially oriented Reynolds Reynolon 1044, 2044,

3044, 4044, 5044

Allied Krystaltite T111, T122,

T133, T144, T15, R11, R22,

R44

Gunze Fancywrap

Preferentially oriented Reynolds Reynolon 3023, 4061,

5032, 7052

Allied Krystaltite PT152

Specialty Films

Aroma and oxygen barrier Cryovac BDF2001, BDF2050

Moisture barrier Cryovac BDF1000

Moisture, gas, and aroma

barrier

Cryovac BDF3000

Table 3. Typical Shrink-Film Properties

Film Type Unrestrained Shrink (% at 260 1F) Clarity (%) Ball Burst (cm/kg) Coefﬁcient of Friction

Polyethylene monolayer 70–80 40–65 16–24 0.11–0.14

Polyethylene multilayer 70–80 80–90 21–28 0.22–0.32

Polypropylene or EPC monolayer 25–45 75–82 4–10 0.26–0.38

PP/EPC multilayer 50–60 75–85 10–20 0.30

PVC, biaxial 30–55 75–80 6–10 0.20—blocked

FILM, SHRINK 499

allows the molten seal to solidify and prevents it from

wrinkling or bunching. PVC shrink ﬁlms can exhibit

another sealing problem. They form a carbon buildup on

the seal head, which must be cleaned regularly; otherwise,

this will create a charred seal on the package. Some of the

more common equipment manufacturers include Doboy,

Great Lakes, Hanagata, Ilapack, Omori, Shanklin, and

Weldotron (see also Sealing, heat).

One other piece of equipment needed to provide a

sharp, tightly ﬁnished package is a shrink tunnel. In

order for adequate shrinkage to occur, the package must

be exposed to the correct temperature and airﬂow for the

proper amount of time. The ultimate goal is to have a

package that is tightly shrunk with no excess material or

‘‘ears’’ on the corners of the ﬁnished package. PVC ﬁlms do

not require the same level of temperature control as do

polyoleﬁn ﬁlms, since they shrink more easily when

exposed to heat. Shrink tunnels are generally purchased

in conjunction with packaging equipment to ensure cor-

rect sizing for speed and product application.

BIBLIOGRAPHY

General References

J. L. Throne, Plastics Process Engineering, Marcel Dekker, New

York, 1979.

R. J. Young and P. A. Lovell, Introduction to Polymers, 2nd edition,

Chapman and Hall, London, 1991.

E. C. Bernhardt, Processing of Thermoplastic Materials, Krieger,

Malabar, FL, 1974.

FILM, STRETCH

THOMAS B. BRIGHTON

Tenneco Packaging, Deerﬁeld,

Illinois

Unitization with elastic plastic ﬁlms (stretch ﬁlm) is a U.S.

innovation that began in the early 1970s and has spread

throughout the world to become the most widespread

means of containing unit loads. This article focuses on

the selection, use, and properties available for stretch

packaging unit loads and individual products. Little re-

ference is made to mechanical equipment systems avail-

able to accomplish stretch-ﬁlm unitization.

SAVING WITH STRETCH FILM

Stretch ﬁlm is a product that thousands of companies

throughout the United States and abroad used every day

in tremendous quantities. And, although acceptance and

audience of stretch ﬁlm continue to grow, the majority of

stretch-ﬁlm users know little of its true potential or how it

has developed over the years.

INTRODUCTION

Stretch ﬁlm is such an effective product that it is often

easy to take all that it does for granted. This section is

Table 4. Advantages and Disadvantages of Various Film Types

Film Type Advantages Disadvantages

Polyethylene monolayer Low cost Low modulus

High slip Narrow shrink temperature range

Strong trim seals

More dimensionally stable than PVC

Printable

Polyethylene multilayer Excellent optics Low modulus

Low cost

Strong trim seals

Broad shrink temperature range

High abuse resistance

Printable

PP or EPC monolayer High modulus Low abuse resistance

High gloss Easy tear

Low cost High shrink temperature required

High shrink force Low seal strength

PP/EPC multilayer High gloss Low abuse resistance

Low cost Easy tear

Strong trim seals High shrink temperature required

Broad shrink temperature range Low seal strength

High abuse resistance

High modulus

Printable

PVC, biaxial Low shrink temperature Weak seals

High modulus Noxious fumes created during sealing

Low shrink force Corrosion problems

Poor machinability

Poor slip

500 FILM, STRETCH

intended to serve as a ‘‘reminder’’ of (a) the beneﬁts of

unitizing and (b) the additional beneﬁts that you reap by

unitizing with stretch ﬁlm. As part of the discussion on the

beneﬁts of stretch ﬁlm, the testing of packages and unit

loads are addressed. Finally, in order to give you back-

ground knowledge on stretch ﬁlm, this section also re-

views the development and reﬁnement of stretch ﬁlm over

the years. All these issues are covered under the following

sections:

. Why Unitize?

. Why Unitize with Stretch Film?

. Testing Packages and Unit Loads

. The Evolution of Stretch Film

Why Unitize?

Unitization is deﬁned as a process in which smaller

individual items are gathered and made into larger unit

loads.

The beneﬁts of unitizing products are many. Although

unitization can be achieved with other packaging materi-

als, none does it as effectively and cost-efﬁciently as

stretch ﬁlm. But before discussing the merits of stretch

ﬁlm, it is important to ﬁrst understand why its necessary

to unitize. Here are the reasons.

Lower Handling Costs. In general, whenever a load of

packaged products increases in quantity, size, and

weight, the cost per unit handled becomes lower.

Unitizing products, therefore, signiﬁcantly reduces

handling costs.

Labor Savings. Without exception, the handling of

loose boxes is more time-intensive than handling a

unitized load. Thus, another beneﬁt of unitization is

the amount of labor hours saved in the movement

and shipment of packaged goods. The end result is a

quicker ﬂow of goods throughout factories, ware-

houses, and cross-dock operations while utilizing

fewer hands.

Transportation Savings. Since unitizing accelerates

the speed at which goods are moved, carrier vehicles

spend less time at unloading/loading docks. This

reduction in unloading/loading time provides valu-

able cost savings to the company that unitizes and

the customers receiving its prodcuts.

Protection. The protection that unit loads afford an

operator are threefold:

1. Unit loads reduce incidents of pilferage and theft

because packages from an enclosed load are

difﬁcult to remove. Moreover, unit loads are

much easier to track than individually packaged

products, making the ‘‘disappearance’’ of goods

more detectable.

2. Unit loads, which are placed on pallets and

moved by lift truck, are less likely to suffer

damage in transit than lose boxes carried by

hand, because lift trucks keep pallets closer to

the ground. For instance, whereas small boxes

(r20 lb) may often receive impacts from 30 in. or

higher when accidentally dropped, a unit load on

a pallet will seldom receive 12 in. in drop.

3. It is less likely in LTL (lift-truck load) shipments

that dense, heavy items will be parked on top of

palletized unit loads. Hence, your goods stand a

greater chance of not being damaged. This ben-

eﬁt, however, does not hold true for cartons

shipped individually.

Inventory Control. Unitizing products facilitates im-

provements in control of inventory since large uni-

tized loads (as opposed to parcels of smaller ones)

can be identiﬁed, counted, and managed more easily.

Customer Service. Customers appreciate the effects of

unitizing because it allows them to unload their

trailers and move goods through their warehousing

systems more efﬁciently.

Why Unitize with Stretch Film?

From the advantages listed, the importance of unitizing is

clearly evident. What is equally important to realize is

that stretch ﬁlm accomplishes each of these unitizing

functions better than any other packaging material.

Also, stretch ﬁlm performs additional functions that other

unitizing materials simply cannot:

. Low Supply Cost. Compared to other packaging and

bundling materials, stretch ﬁlm is the lowest in cost

to use because it offers the most yield for the dollar.

. Protection from Moisture, Dirt, and Abrasion.

Stretch ﬁlm protects against these elements, safe-

guarding the goods it wraps.

. Reliable Performance. With stretch ﬁlm, you can

quickly see the holding performance and protection

it offers. The holding power, however, of other

packaging

materials such as

adhesives is not always

immediately noticeable since they can unravel,

loosen, or come undone without being noticeable a

few minutes or hours after observation. Stretch ﬁlm,

manufactured by a company committed to quality,

will produce consistent results every time it is used.

. Automation. Stretch-ﬁlm machines are either par-

tially or fully automated, which improves the pro-

ductivity of the packaging process, reduces labor,

and alleviates back-breaking and time-consuming

work.

. Scan-through Optics. The clarity of the stretch ﬁlm

enables UPC codes to be quickly read and scanned

while protecting labels and lot codes printed under-

neath the ﬁlm.

. Ease of Removal. Stretch ﬁlm only clings to itself

and not surfaces, making its removal fast, clean, and

easy.

. Elimination or Reduction of Other Packaging

Materials. The superior load-holding capability

of stretch ﬁlm either eliminates the need for or helps

scale down the bulk of other packaging materials

(e.g., in some cases, stretch ﬁlm may enable thin-

ner cardboard cartons to used for packaging

goods).

FILM, STRETCH 501

. Recycling. Stretch ﬁlm is recyclable. In fact, several

stretch-ﬁlm manufacturers will pick up and recycle

your stretch ﬁlm if you qualify for their recycling

programs.

Testing Packages and Unit Loads

Stretch ﬁlm saves you money. By offering superior unitiz-

ing and protective capabilities, it drastically reduces

product damage, which contributes to your bottom line.

But you do not have to take anyone’s word for it. There are

two in-plant tests you can perform to get an initial feel for

the durability of the unit loads you wrap in stretch ﬁlm

before sending them off to your customers.

The ﬁrst preshipment test is ‘‘test course handling.’’ In

this test a lift truck equipped with forks or slip sheet

attachment approaches the unitized load, picks it up,

accelerates, corners, decelerates, and sets the load down.

The lift truck then repeats this same test once again,

except this time moves in reverse. A typical test sequence

includes two to seven repetitions, with an average of four

repetitions for the assurance required by most users.

The second preshipment test is the ‘‘free-fall drop.’’ This

test does exactly what its name implies. In this test, one

end of a unit load is raised to a speciﬁed height of 3–12 in.

and released to fall ﬂat on the ﬂoor. The drop is then

repeated on the opposite end of the unit load. Drop height

will depend on the weight of the unit load and the

assurance level that is desired.

Further shake-table or vibration-table test procedures

will provide you with stretch-ﬁlm load–force data for your

products. It is recommended that you work closely with

your local stretch-ﬁlm packaging sales professional when

packaging new products or considering stretch-ﬁlm

changes.

The Evolution of Stretch Film

Although many people enjoy the beneﬁts of stretch ﬁlm

today, stretch ﬁlm was not developed overnight. The crea-

tion and development of stretch ﬁlm has been a gradual

process, one that continues to undergo reﬁnements to this

day. For a better idea of where stretch ﬁlm has been and

where it’s headed, let’s take a look at this timeline:

NEW PACKAGING SOLUTIONS

When you mention stretch ﬁlm, most people conjure up

the image of a load of boxes or goods wrapped on a pallet.

For good reason. This application is the most popular one

for stretch ﬁlm. But an ‘‘on pallet’’ application is just one of

hundreds that stretch ﬁlm is capable of performing. All it

takes is a little imagination and ingenuity to customize

stretch ﬁlm for your own uses. An increasing number of

companies are looking to stretch ﬁlm to bundle and

package goods once wrapped in other materials such as

tape, twine, strapping, and corrugated boxes. In this

section, some of these innovative applications and the

beneﬁts derived from them are shared with you under

the following headings:

. Stretch-ﬁlm advantages

. Versatility

. Protection

. Productivity

. Cost beneﬁts

Year Machine Evolution Film Development Volume and Cost per

Average Unit Load

Wrapped

Early 1960s Shrink-wrap use begins to grow in Europe

1965 Shrink wrap adopted in United States 30 oz/$1.50–2.00

1973 Lantech develops ﬁrst U.S. stretch-ﬁlm

wrapping equipment

Mobil develops the ﬁrst stretch ﬁlm

Introduction of Mobilrap ‘‘C’’ (LDPE) 30–50% stretch 18 oz/$1.10

1974 PVC stretch ﬁlms introduced

1976 EVA stretch ﬁlms introduced

1978 First LLPDE ﬁlm introduced with much higher

stretch levels

10–12 oz/50–80b

1980 Prestretch introduced Hand-wrap market begins to grow

Powered prestretch equipment

introduced 150–250%

Stretch ﬁlm cling at high stretch a challenge

1981 Convenience hand wraps introduced 8 oz/50b

Machinery and ﬁlm advances allow

prestretch up to 300%

Cling consistency improves with better control by

ﬁlm manufacturing

1983 Nonmigratory cling introduced

Higher-load–force cast ﬁlms introduced 4–6 oz/25–40b

1987

Thinner gauge (50, 60) become available

Bi-stretch (trademark) introduced Mobile introduces stretch-ﬁlm recycling program 4 oz/25b

1991

Postconsumer recycled content (PCM) hand wrap

introduced

1992

502 FILM, STRETCH

Stretch-Film Advantages

Think about all the areas in your operation where you

currently use tape, twine, strapping, and so on, and

chances are that stretch ﬁlm can do a better job. Stretch

ﬁlm surpasses other bundling materials for the following

reasons:

VERSATILITY

Stretch ﬁlm is able to ﬁt the contours of any product.

Door and window manufacturers have switched over in

increasing numbers to stretch ﬁlm. Stretch ﬁlm, because

of its stretchability, has largely replaced the custom-size

boxes that were once needed to package the different-size

doors and windows being produced.

Stretch-ﬁlm clarity makes products easier to iden-

tify. Labels and lot and UPC codes can be read as well as

scanned because of the transparency of stretch ﬁlm. Food

wholesalers and distributors have taken advantage of

this feature in implementing and expediting their cross-

docking practices.

PROTECTION

Stretch ﬁlm provides better holding force than do

other bundling and packaging materials. Because

stretch ﬁlm spreads across the entire surface area of a

load, its holding force is dispersed, not concentrated at

only a few points like strapping. This attribute enables

stretch ﬁlm to maintain exceptional load integrity. The

furniture industry has come to recognize the ability of

stretch ﬁlm to hold even the most unstable of loads

together, which is why an increasing number of compo-

nent manufacturers have turned to stretch ﬁlm to bundle

loose furniture parts, such as table and chair legs and

stair banisters.

Because stretch ﬁlm clings only to itself, it does

not leave any sticky or messy residue on product

surfaces. This feature has made stretch ﬁlm a favorite

of the furniture industry. With stretch ﬁlm, there are no

glues or adhesives that can damage fragile ﬁnished wood.

As a result, furniture manufacturers use stretch ﬁlm for a

wide variety of tasks, some of which are to keep furniture

drawers shut so that they do not open in transit, attaching

accessories to furniture pieces and to wrapping-desk and

table tops.

Unlike other bundling and packaging materials,

stretch ﬁlm protects the products it wraps from dirt,

moisture, and abrasion. Carpet and textile manufac-

turers depend on the protective attributes of stretch ﬁlm

to safeguard their rolls of carpet and fabric.

PRODUCTIVITY

Stretch ﬁlm increases productivity. Thanks to auto-

mated stretch-ﬁlm machines, newspaper companies have

been able to bind stacks of newspapers together more

time-efﬁciently and cost-effectively by reducing much of

the manual handling involved.

Cost Beneﬁts

Stretch ﬁlm costs less than most conventional bundling

and packaging materials. But the savings you earn with

stretch ﬁlm goes way beyond just a lower unit cost. Stretch

ﬁlm also saves you money in the following ways.

Stretch ﬁlm offers more yield than do other bund-

ling materials. Because stretch ﬁlm has the ability to

stretch, you get more product per square inch than with

any other bundling material.

Stretch ﬁlm can eliminate or reduce the need for

secondary packaging materials. One example of a

stretch packaging cost savings is that of a national T-shirt

manufacturer who used to package its T-shirts for ship-

ment in expensive corrugated boxes. However, once it was

seen that stretch ﬁlm could do the same job at a fraction of

the cost, the company switched to stretch ﬁlm. Today, the

company is saving $5000 a day in packaging and shipping

costs.

Stretch ﬁlm eliminates or reduces the need for

tertiary packaging. Because of superior load-holding

force of stretch ﬁlm, money once spent on tertiary materi-

als such as strapping, twine, and kraft paper can now be

saved and better spent.

Stretch ﬁlm saves on shipping costs. Since products

bundled in stretch ﬁlm are usually more compact—this

reduction in size is achieved by eliminating the need for

extrabulky packaging materials like boxes—they take up

less space in freight trucks. More space in the back of a

truck means additional space for more of your products—

and that saves you money!

SELECTING THE RIGHT FILM FOR THE RIGHT JOB

There is a wide array of stretch ﬁlms available on the

market, each designed to meet different needs. Choosing

the right stretch ﬁlm(s) for your operation can be a bit

confusing. In this section, the variables to choosing the

proper stretch ﬁlm for your particular needs are ex-

plained. Finally, this section discusses the characteristics

of the different types of stretch ﬁlm and explain how they

are made. All these issues are covered under the following

headings:

. Proper

ﬁlm selection

. How stretch ﬁlm

is made

. Extrusion

. Coextrusion

. Cast fabrication process

. Blown fabrication process

. Process inﬂuence on stretch-ﬁlm properties

. Opaque and color tint ﬁlms

Proper Film Selection

Whenever selecting ﬁlm, the following variables should be

taken into account.

Type of Product. Stretch-ﬁlm type is dependent on

what you are wrapping. Lighter, more fragile goods

require a lower-gauge ﬁlm to avoid being crushed

FILM, STRETCH 503

whereas heavier, more solid goods can withstand the

pressure of higher gauges.

Type of Load. There are two factors that determine

load type: weight of the load and uniformity of its

outside surface. In stretch ﬁlm, load types are dis-

tinguished by the ﬁrst three letters of the alphabet. A

loads have no irregularities, B loads have protrusions

of r3 in., and C loads have protrusions of W3 in.

Type of Equipment. Machine stretch levels will help

determine what type of ﬁlm is necessary to wrap a

load. Whereas higher gauges were once required for

machines with high prestretch levels, Mobil is now

manufacturing thinner, tougher ﬁlms that are able

to endure greater levels of stretch while maintaining

their strength.

Film Thickness. Commonly available gauges for

stretch ﬁlm range from 0.5 to 3.0 mil. Load type,

weight, and height are important determinants in

selecting ﬁlm thickness. As prestretch increases, it

is extremely important that the stretch ﬁlm not lose

its strength.

Light gauges are excellent dust covers for already

secure and strapped loads. Heavy gauges or new-

technology metallocene-based ﬁlms are excellent for

loads that require a high degree of containment.

Film Width. Most stretch ﬁlms range from 2 to 70 in.,

but can be made up to 100 in. by request. Rotary

spiral equipment usually uses 20-in.-width ﬁlm, but

can also use 25- and 30-in. ﬁlm. Full-web equipment

uses a wide variety of widths depending on load

height.

To determine which ﬁlm widths are best for your

individual applications, contact a packaging sales profes-

sional for assistance.

How Stretch Film Is Made

Understanding how stretch ﬁlm is made, and the result-

ing properties that each type of ﬁlm possesses, can help

you when it comes time to purchase the proper ﬁlm for

your operation. The following industry terms, accompa-

nied by diagrams, describe the various manufacturing

processes of stretch ﬁlm and the types of ﬁlm that are

produced as a result:

Extrusion. This is the ﬁrst step in the production of

stretch ﬁlm in which plastic (polyethylene) resin

pellets are converted into melted plastic. For this

melting to occur, the resin pellets are loaded into the

hopper funnel of the extruder and dropped by grav-

ity into the barrel where the mixture is heated to its

melting point. This liqueﬁed plastic is then pushed

forward by a continuously turning screw. In the ﬁnal

phase of extrusion, the molten material (or extru-

date) is pushed through an opening called a die,

which shapes the plastic for ﬁlm formation. (See

Figure 1.)

Coextrusion. Coextrusion is a form of extrusion using

multiple extrudates and dies to create stretch ﬁlm

with multiple layers. Coextrusion involves the si-

multaneous extrusion of two or more layers of

plastic that are brought together while still melted

and then cooled to form a multilayer ﬁlm. The layers

of this ﬁlm may or may not be different materials

and may or may not exhibit different properties.

(See Figure 2.)

Cast-Film Fabrication. After the extrusion process

melts the plastic, this plastic is pushed through a

slit-shaped die. The plastic is then formed into a

sheet as it falls into a roller and is pulled vertically

downward, orienting the polymer in that direction.

This sheet is cooled by passing it through chill rolls,

which are kept cool by circulating water inside

them. Finally, the sheet is cut into the proper

combination of widths and wound into ﬁnished

stretch ﬁlm rolls. (See Figure 3.)

Blown Fabrication Process. In the blown fabrication

process, the extrudate is pushed through a round-

shaped die. This action forms a continuous hollow

tube from the extrudate, which is blown vertically

upward. During this phase, the ﬁlm is pulled or

oriented in both the pulling (machine) and sideways

(transverse) direction. High above the die, the tube

is cooled and squeezed shut by a collapsing frame-

work and a set of nip rolls. The ﬂattened tube is then

slit and wound into rolls. (See Figure 4.)

Figure 1. Extruder.

Figure 2. Coextrusion.

504 FILM, STRETCH

Process Inﬂuence on Stretch-Film Properties

Cast ﬁlms are typically different from blown ﬁlm because of

the difference in the way the plastic is oriented and cooled.

In the cast process, ﬁlm is cooled more rapidly than in

the blown process. This faster cooling, which is induced by

the chill rollers, causes single-direction orientation in the

ﬁlm. In other words, the polyethylene molecules are cooled

so quickly that the molecular structure is formed long-

itudinally in long, bidirectional branches.

By contrast, blown ﬁlm is cooled slowly by air while the

tube of plastic is drawn upward. This slower cooling allows

for the polyethylene molecules to become oriented in

all directions, producing a ﬁlm with tougher puncture

resistance.

The differences in these two manufacturing processes

produce the following differences in cast and blown stretch

ﬁlms:

Cast Film Blown Film

Clear Hazy

High gloss Dull

Lower modulus—stretches

easily

Tough

Quiet unwind Noisy unwind

Stiffer ﬁlm—high load

containment

Tear resistance (cross

direction)

Extensibility

Nonmigratory cling Migratory cling

Good-to-excellent gauge

control

Poorer gauge control

Good puncture resistance Excellent puncture resistance

Gauge band potential Gauge spread across roll (rotating

die)

Although these are textbook examples of the differ-

ences between cast and blown ﬁlms, it is important to note

that through use of various polymers, additives, or process

alterations, these differences can be reduced and even

reversed. This ability to manipulate the manufacturing

process has enabled the stretch-ﬁlm manufacturer to

produce ﬁlms that combine the best properties of blown

and cast ﬁlms into one ﬁlm.

Opaque and Color-Tint Films

Opaque and color-tint ﬁlms are available from ﬁlm sup-

pliers as special production runs. Because these requests

are customized, opaque, and color-tint ﬁlms have longer

lead times and higher costs than do standard stretch ﬁlms

and must meet sizable minimum-order requirements (see

Colorants).

MEASURING STRETCH-FILM PERFORMANCE

To most people, if a unitized load maintains its integrity as

it moves from point A to point B, it has performed well.

However, numerous tests can be completed to thoroughly

analyze and measure ﬁlm performance. Many of these

tests can be conducted and evaluated only in a laboratory,

but there are a few that any operator can complete with

some very simple tools.

This section answers the following stretch-ﬁlm perfor-

mance questions:

. What properties are important?

. How can the properties be measured?

. In the factory or warehouse

. In the laboratory

. What effect does my machine have on ﬁlm

performance?

What Properties Are Important?

Because there are hundreds of different stretch ﬁlms now

available, knowing how to evaluate ﬁlm is an important

key to success. Equally important is how to use this

information to determine load-wrapping cost.

The following is a list of the top six attributes most

commonly evaluated:

1. Tensile Property. How much even force can be used

before the ﬁlm breaks?

2. Puncturing. How much pointed force can the ﬁlm

take before puncturing?

3. Tearing. How much force, after a cut has been made,

can the ﬁlm take before tearing?

4. Optical Scanning. How much can be seen and read

through the ﬁlm?

5. Cling on Slip. How much does the ﬁlm stick to itself

or other surfaces?

Figure 3. Cast-ﬁlm fabrication.

Figure 4. Film-blowing equipment.

FILM, STRETCH 505

6. Load Force. How much actual force does the ﬁlm

place on the load?

It is important to note that all of these ﬁlm attributes are

related. Depending on the speciﬁc ﬁlm application, the

importance of each attribute will vary. Also, be aware that

each ﬁlm has a balance of attributes and that producing a

ﬁlm that meets maximum performance levels with each

attribute would yield an overall weaker ﬁlm because

you cannot affect just one attribute without affecting all

of them.

HOW CAN THE PROPERTIES BE MEASURED?

In the Factory or Warehouse

A knowledgeable stretch-ﬁlm sales professional will em-

ploy effective methods to measure ﬁlm performance on-

site. One such method tests the actual stretch level of ﬁlm

after being placed around a load. Using a stretch wheel

and a tape measure, the percentage of stretch your

machine delivers is calculated in order to ensure that

you are not overstretching or underutilizing your stretch

ﬁlm.

Force-to-load is another test that should be performed.

The point of this exercise is to measure the pressure the

stretch ﬁlm places against a load. By applying a force-to-

load gauge to the stretch ﬁlm, the apparatus reveals

whether the ﬁlm is administering the appropriate amount

of force to contain a load of goods. Snapback is yet another

demonstration test used to measure ﬁlm performance. In

the snapback test, you ascertain ﬁlm recovery; that is, how

much has the stretch ﬁlm returned to its original shape

after its been stretched? Film with poor recovery has been

stretched beyond its limits and will cause the load to lose

integrity over time.

Finally, the ‘‘cut and weigh’’ is a test used to demon-

strate the actual cost savings of a particular ﬁlm in

comparison to other brands. In the cut and weigh, a load

wrapped in stretch ﬁlm is cut, removed entirely from the

load, and measured on a scale. The same load is then

wrapped in another manufacturer’s brand of stretch ﬁlm

and is also cut, removed, and weighed. The two weights

are then compared.

To see these ﬁlm performance measurements done

ﬁrsthand, contact your local packaging sales professional.

In The Laboratory

The following properties are routinely tested in the lab:

. Tensile properties are determined by measuring ﬁlm

performance as it is elongated at a constant rate. By

measuring the force required to elongate the ﬁlm, the

following properties can be determined: yield

strength—the point where a ﬁlm permanently de-

forms as a result of force applied during elongation;

ultimate tensile strength—the maximum force a ﬁlm

can withstand in a tension test measured in psi

(pounds per square inch); and elongation—the per-

cent increase in ﬁlm length at its break point.

. Puncture and tear properties are easy to visually

identify while the ﬁlm is stretched around a pallet

load. A ﬁlm that has poor tear and puncture char-

acteristics will usually fail, forcing an operator to

compensate by increasing the number of pallet wraps

or loosen the tension. This is an expensive way to

compensate for having the wrong ﬁlm or a ﬁlm with

poor quality. In the lab, puncture resistance is mea-

sured by the force needed to puncture the ﬁlm. This

is accomplished by pushing a pear-shaped probe

through a tightly secured piece of ﬁlm. It is measured

in inch-pounds or in pounds per mil if ﬁlm-gauge

correction is required. Tear tests or an ‘‘Elmendorf

test’’ is simply the force required to propagate a

precut slit in a ﬁlm sample. Another common name

for ﬁlm tearing is ‘‘zippering.’’ The Elmendorf test is

measured in grams or grams per mil if gauge correc-

tion is required. Tears can occur in both vertical

(machine) and horizontal (transverse) directions.

(See Figure 5 for clariﬁcation.)

. Optical properties describe the ﬁlm’s ability to trans-

mit light (haze) or reﬂect light (gloss). Both of these

properties are signiﬁcant in determining ﬁlm clarity.

. Haze is a quantitative measure of ﬁlm transparency,

which is very important for scanning labels and

identifying pallet-load contents.

. Gloss is a measure of surface ﬁnish and the ﬁlms

ability to reﬂect light shine. The higher the percent

gloss, the better the shine. Cast-manufactured ﬁlms

tend to exhibit better clarity than do blown ﬁlms and

have higher gloss.

. Load-force properties describe the amount of force a

ﬁlm can provide after it is stretched wrapped to a

pallet of goods. As mentioned earlier, this test is done

in the ﬁeld as well as in the lab. The load-force

property

is very important

because it is a key to

keeping pallets loads tightly unitized. The best way

to measure load force is to use a force-to-load gauge.

This gauge will show the actual force (pressure)

being placed on the load by the ﬁlm.

. Cling and slip properties are measured with an

Instron apparatus. Both cling and slip is measured

in grams. Cling is simply the ability of a ﬁlm to stick

to itself or another smooth surface. Cling is affected

by the amount of tackiﬁer additives, such as poly-

isobutylene (PIB) in blown ﬁlms, which are added

or by the smooth outer surface of cast ﬁlms. To

Figure 5. Full-dimensional orientation.

506 FILM, STRETCH

determine slip, a ﬁlm is measured for its coefﬁcient of

friction (COF), which is a measure of the amount of

force required to move a ﬁlm sample with no cling out

from underneath a standard unit of weight. (See

Figure 6.)

What Effect Does My Machine Have on Film Performance?

Film performance is critical when determining which ﬁlm

can offer the best load protection at the lowest cost. Often,

the equipment used to apply ﬁlm has a dramatic impact on

the ﬁlm. A ﬁlm that performs well on one machine may not

offer the same results on another system; for instance,

stretch levels may differ by machine. Even equipment

operators can affect ﬁlm performance by mishandling the

product, not keeping equipment clean and running efﬁ-

ciently, or not stretching the ﬁlm to its maximum

potential.

The challenge is to look beyond a ‘‘no complaints’’ or

‘‘it’s not broken, don’t ﬁx it’ mentality to evaluate true ﬁlm

perfomance. In other words, how can a user methodically

evaluate ﬁlm performance levels to discover the best ﬁlm

for the operation under consideration? By employing the

ﬁlm performance measurements outlined in this section,

you can make sure that your stretch ﬁlm is performing up

to its full potential.

INSPECTING AND HANDLING STRETCH FILM

Because stretch ﬁlm exhibits toughness when wrapped,

most people would think that stretch ﬁlm rolls are not

fragile. But they are. Caution must be exercised when

handling stretch-ﬁlm rolls and loading them onto the

wrapping machine to avoid edge damage, which will cause

tearing during prestretch. This section will teach you

proper roll-handling techniques and enable you to spot

roll defects by covering the following topics:

. Stretch ﬁlm handling and storage

. Manufacturing defects

Stretch-Film Handling and Storage

Stretch-ﬁlm rolls, especially their edges, are sensitive to

damage. Bumps, nicks, and cuts on the outer edges will

easily render the roll unusable. As a result, the rolls must

be handled with care. The following are some recom-

mended tips on handling stretch ﬁlm rolls:

. Step 1—Minimize Roll Handling. The packaging

that holds a stretch ﬁlm pallet together has been

designed to be opened as close as possible to the

wrapping machine. For exactly that reason, it is

recommended that pallets of stretch ﬁlm be stored

as close as possible to the wrapping machine to

minimize roll handling and prevent damage that

could be caused by dropping the rolls.

. Step 2—Inspect Rolls. After opening your pallet of

stretch ﬁlm, the rolls should be inspected for ship-

ping damage that can occur if they have shifted in

transit. This damage can take the form of dents,

product rubs, and gouges, nicks, and tears. Should

these exist, try to remove them by unraveling the

ﬁrst few layers of the roll before loading it onto the

machine.

. Step 3—Keep Film Away from Surfaces. Stretch-ﬁlm

rolls are designed with a 1/4-in core protrusion on

each end. When either placing stretch ﬁlm on the

ﬂoor for storage purposes or on a machine roll

carriage, always make sure that the core, and not

the ﬁlm, is touching the ground or machine surface.

The less the ﬁlm comes in direct contact with any

surface, the less likely that your roll edges will

become damaged.

. Step 4—Keep Film Away from Excessive Heat. To

prevent ﬁlm damage, keep rolls away from heating

vents or machines that give off an inordinate

amount of heat.

Manufacturing Defects

Several defects can occur in the manufacture of stretch

ﬁlm. Users of stretch ﬁlm should be aware of these defects

(highlighted in italics in legends to Figures 7–13) which

may adversely affect ﬁlm performance.

The best way to avoid manufacturer defects is to pick a

manufacturer who places extremely high emphasis on

quality, consistency, and performance.

Figure 6. Measurement of coefﬁcient of motion.

Figure 7. Gels—round, hard, clear spots that are high molecular

weight or unmelted polymer. Caused by abnormalities in resin or

poor mixing. Large gels can cause holes to occur during the

wrapping process, which may result in ﬁlm breaks.

FILM, STRETCH 507