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2. Unitized loads in the holds of surface ships.

3. Containerized loads on container ships, on the decks

and in the holds of cargo vessels.

4. Transport by lighters and barges; on open boats

through surf to sandy beaches.

5. Rollon, rolloff (Ro-Ro).

6. Containerized air shipments plus ground handling

and transportation.

7. Individual packages and unitized loads shipped by

air freighters and in the holds of passenger planes.

8. Miscellaneous combinations of modes of transporta-

tion, handling, and intermittent storage in foreign

places and unknown conditions.

Export packaging encounters at least two domestic

movements that involve the usual hazards of handling,

loading, dropping, compression, and moisture, plus the

hazards of transportation. One major exposure is during

the movement to the seaport or the gateway airport. The

second involves the customs procedures and storage of the

country of debarkation, multiple handling, unforeseen

conditions of transshipment, and eventual delivery. Pro-

ducts must be packed for the toughest part of the journey

and survive all of the exposures.

Only about 30% of the cargo losses can be classed as

fortuitous losses resulting from sinking, stranding, ﬁre,

collision, seawater, or heavy weather. One ship is lost,

somewhere in the world, almost every day. Approximately

70% of the losses may be preventable:

10% Water damage—freshwater, sweat, condensation,

and saltwater.

20% Theft, pilferage, and nondelivery

40% Handling and stowage—container damage, break-

age, leakage, crushing, contact with oil and other

cargo, contamination, and failure of refrigeration or

other equipment

During transit, cargo must withstand the conditions of

rough seas, turbulent air, and substandard roads.

At sea, a ship may move in six different directions at

the same time. A top-loaded container may travel 70 ft

(B20 m) in each direction with each 401 roll as often as

eight or nine times per minute. Figures 1 and 2 diagram

the movements involved. The center point is least subject

to these movements, but the shipper does not control the

location and stowage of the cargo or of individual

containers.

SPECIFIC CONDITIONS

With the above as a brief introduction to the hazards of

international cargo movements, the packaging engineer

should further study this subject and the conditions that

may exist at the ports of embarkation and debarkation.

Brief descriptions of port conditions and equipment are

provided in Ports of the World (1, 2) and similar publica-

tions. Information can be obtained from the cargo carriers

regarding their equipment, methods of handling, routes,

stops, trans shipment, and other details. They can also

provide some information about local conditions at the

port facilities. Some marine underwriters may have con-

structive suggestions. The consular staffs and the com-

mercial development departments of each country are well

informed. Foreign sales agents can provide details about

the customers and their facilities, needs, and preferences.

Too often the shipper is uninformed and relies on gossip

rather than acquiring a realistic understanding of this

complex subject.

PRODUCT ANALYSIS

Every facet of the products to be exported must be

analyzed, and the potential problems, even marginal

ones, must be identiﬁed. Export handling and shipping

expose the weaknesses if they are overlooked.

Corrosion and Mildew. All metals must be protected

against water and moisture for surface transportation and

even for air shipment from or to humid and tropical areas.

All natural materials (e.g., leather, cotton, wool, and

paper) must be protected against moisture, fungus, and

mildew.

Figure 1. The six directions of motion of a ship at sea.

Figure 2. The distance (B20 m) traversed by a top-loaded con-

tainer in a 401 roll.

428 EXPORT PACKAGING

Problems can begin because the products are not

thoroughly cleaned for ﬁngerprints, cutting or cooling

oils, dirt, and foreign matter, or because of unnoticed

rust, mildew, or local moisture in the packages. In transit

and in storage, the moisture and mildew, condensation, or

sweat may ﬁnd the weak point in the packaging and cause

irreversible damage. If the product surface is critical or if

the item is expensive or choice, the damage may be total.

When the problems are identiﬁed, corrosion inhibitors

(special oils and petrolatums) or properly used vapor-

phase inhibitors and waterproof wrapping are applied.

For electronic equipment and critical metal surfaces or

hidden areas, desiccants with sealed moisture-vapor bar-

riers are used. If possible, the critical sections or elements

are removed from the larger units and packaged sepa-

rately (inside the larger box or a sheathed crate). A small

vapor-phase inhibitor or moisture-vapor barrier package

provides better protection more economically than a thor-

ough package for an entire unit. Containerization does not

assure protection against water damage or moisture. Air

shipment does not offer full protection because of en-

trapped atmosphere, and the cargo is on the ground

more than in the air.

Pilferage and Nondelivery. It is essential to recognize

that the exported products may stay days or weeks in

conditions where pilferage is easy. Therefore, the

packages should not identify the contents as worth pilfer-

ing for private use or resale. Identiﬁcation of the product

on the outside of the package by brand name, manufac-

turer’s name, or the shape and size of the box is an open

invitation to steal. A damaged package exposes the con-

tents for pilferage.

Cigarettes, liquor, cameras, stereos, jewelry, furs, small

appliances, and many other items are ideal targets for

pilferage. Many consignees use code identiﬁcation because

their names on the packages would encourage pilferage.

Some even have the items delivered to a ‘‘front’’ to avoid

identiﬁcation.

With the potential problem of pilferage identiﬁed, steps

should be taken to assure special handling and account-

ability. Packages should be stored in separate locked areas

and on board in lockers or safes.

Nondelivery occurs when the packages are stolen,

destroyed, lost, unloaded and left at the wrong destina-

tion, or misdelivered. Adequate, permanent, and promi-

nent markings help to avoid some of these problems.

Unitizing or using master overpacks is also helpful.

Breakage. The internal weaknesses of electronic pro-

ducts equal the fragile construction, design, or materials

of many products; any part that extends; and any item

that is not in balance are all subject to breakage or

physical damage. It may be necessary to protect the

package from the product because of a dense weight load

or severe imbalance.

Any product that is hard to handle, tends to fall over, or

is especially awkward for any reason must be given special

consideration. Often, production equipment requires spe-

cial handling, and provision must be made in the package.

Skids and other devices may help to keep the equipment

on its base and handled properly.

Cushioning, blocking, and bracing are all essential (see

the Cushioning design article). Built-in lifting eyes or

other devices, special skids and pallets, and guide marks

for the stevedores are helpful. On large items, the balance

and the lifting points should be marked. Occasionally, an

experienced exporter includes special instructions and

photographs with the documentation and on the outside

of the packages or crates.

Contamination. Some products can be spoiled or da-

maged because they can absorb the odors or fumes from

the other items that may be in the same container or

nearby in the holds or warehouses. Often the people who

work with these items ignore or forget these problems.

Precaution can be taken to speciﬁcally request the

carriers not to expose the items to adverse conditions.

Sealed moisture-vapor barriers may give further assur-

ance. Packages should be marked to indicate that they

should not be stowed with potentially harmful cargo.

If photographic supplies and other sensitive items are

exposed to excess heat, light, or moisture, then damage

can result. This type of problem must be identiﬁed, the

product shielded in the packages, and full instructions

provided to the carriers.

Hazardous Materials. The shipper must identify any

hazardous materials. These should be separated and given

special documentation and packing under strict rules and

speciﬁcations. It is essential that each hazardous material

is classiﬁed and properly identiﬁed on the packages, with

the correct and legal labels.

It cannot be assumed that compliance with domestic

regulations will assure foreign acceptance of the ship-

ments. This requires special study. Improperly labeled

cargo may not be shipped and can cause great confusion

(see the Standards and practices article).

MARKS AND SYMBOLS

It is essential that proper markings and symbols be used.

Even a poor package, adequately closed and marked, has a

good chance of being delivered in reasonable order. An

excellent package that is not adequately marked may

never reach its destination.

Only those markings that are essential and appropri-

ate should be used. Any other markings, or too many, can

be confusing and serve no purpose. New, clean packages

with

no advertising or

other printing should be used. The

selected markings can be printed by the manufacturer of

the packages or stenciled permanently. Crayon, chalk,

marking pens, tags, and cards should not be depended on.

Code marks for the name of the consignee and of the

shipper (if used) are best for products that might be pilfered

(see the Code marking and imprinting article). These and

port marks should be large, clear, and applied on the side,

end, and top of each package (Figure 3). Required weight

and dimension information should be clear.

EXPORT PACKAGING 429

Handling precautionary instructions should be printed

or stenciled on the outside in the language of the destina-

tion country (Figure 4). Many times the cargo handlers

cannot read the language of the country in which they are

working. Pictorial precautionary markings may be the

most helpful (Figure 5). In general use, they are recognized

and have replaced a wide variety of symbols used in the

past. Regardless of the markings, it is essential to package

adequately and protect the products being shipped because

the precautionary marks are frequently ignored.

When a large number of units are being moved in a

single shipment, it helps to corner-code mark them with a

distinctive symbol or to color code the opposite top corners

with triangles or stripes. If a series of shipments is being

sent to the same destination to be collected and staged for

further inland shipment, the same method of coding should

be used. This helps to reduce the number of stray packages.

When a number of packages are being shipped to-

gether, they should be numbered on the packages with

the same numbers as the documentation. If there are 18

packages in a shipment, they should be numbered on the

packages as 1/18, 2/18, 3/18, y, 18/18. Thus, if a package

is missing, it can be checked on the documents for size,

weight, and contents.

PACKAGING FOR EXPORT

Some exporters use their domestic packages for export.

This is not good practice, but it may be economical.

Preparation of domestic packages for export includes the

following:

1. Waterprooﬁng by liners, overwrapping, or over-

bagging.

2. Master packing small to medium size packages.

3. Unitizing a quantity of packages with suitable

strapping, adequate pallets, and shrink or stretch

ﬁlms.

Unless conditions are thoroughly understood, this ap-

plies even for air shipments because of ground handling,

storage, and trans-shipment.

Packaging for Break Bulk. This is the traditional form of

shipment for small and large items stowed in holds or on

the decks of surface ships. The individual packages are

handled by the ship’s gear individually or on stevedore

pallets. They are pushed or moved manually, or some-

times by roller conveyor or lift truck, out of the square of

the hatch to wings. They are stacked with other cargo

and used to help brace the loads against movement as

the vessel rolls and pitches. There is a loading plan

specifying that the lightweight items should go on top

of the heavier and more rugged cargo and that special

classes of products should be given special treatment.

The plan must also consider late cargo and the sequence

of discharge at ports of call. Packages must withstand a

static load of similar material 20 ft (6.1 m) high without

distortion or rupture throughout the intended voyage.

This applies in the lateral as well as the vertical direc-

tion. Problems of shock and vibration may be 50 times

those normally experienced in domestic transit. Moisture

is usually encountered during a sea voyage at deckside,

in customs, and in lighters. Wooden boxes, sheathed

crates, and cleated ‘‘export’’ plywood are appropriate for

break bulk shipments when properly constructed,

packed, and secured.

Figure 3. Assignment of codemarks for shipping.

430 EXPORT PACKAGING

Unitized Loads. In addition to the details already cov-

ered, it is necessary to note that the outside boxes in unit

loads may be chaffed by other cargo during the ship’s

movement. The 20-ft (6.1-m) static load rule applies. It

may be necessary to provide a wood or heavy-duty cover to

distribute the superimposed weight and to protect the

individual packages.

Containerized Loads. Cargo containers provide physical

protection so the 20-ft (6.1-m) guideline does not apply.

The moisture problem can sometimes be intensiﬁed be-

cause of minimum air circulation. Containers should be

secured and loaded evenly from end to end. Containers

should be thoroughly cleaned and inspected before use.

There are special containers for unusual loads and condi-

tions. The packages may be removed at portside and then

face another domestic shipment to the consignees. Cus-

toms inspectors in some foreign ports do not restuff

containers the way they were shipped. They sometimes

repack with the larger and heavier items on top of the

smaller items, for example.

Lighters, Barges, and Open Boats. Some modern barges

may be similar to large containers. Lighters and open

boats may provide the most difﬁcult tests of the trips. The

handling is doubled and may be crude or rough, partly

because of greater exposure to sea water, fog, mist, and

spray. Sound packaging is required.

Rollon, Rolloff (Ro-Ro). In this method of shipment,

highway trailers and other units on wheels can be loaded

and secured in the vessel for relatively short voyages.

These modern ships offer good conditions, and the hand-

ling is minimized; however, the loading, unloading, cus-

toms, and storage are portside.

Containerized Air Shipments. If the containers are prop-

erly loaded and precautions are taken against moisture in

the packages and on the ground, this method of shipment

may be the safest. Domestic packaging can often be used if

the delivery is known to be normal.

Individual Units by Air. Unit loads by air freighters and

individual packages unitized by the airlines for shipment

on air freighters may be moved automatically and have

little or no handling or moisture problems. Individual

packages in the holds of passenger planes, however, may

experience many rough handlings. In the cargo space, the

Figure 4. Precautionary handling instructions.

EXPORT PACKAGING 431

Figure 5. Pictorial representation of precautionary markings.

432 EXPORT PACKAGING

handler usually cannot stand up straight and often resorts

to crude methods of moving packages. The small items

may be rolled or thrown and will not be thoroughly

secured.

Miscellaneous Modes. Packages shipped to inland

points may experience a good deal of rough handling,

storage under unexpected conditions, and exposure to

pilferage and corrosion.

GUIDELINES

If shipments can be fully controlled by the shipper and the

consignee by containerization, documentation, represen-

tation at both ends, and complete understanding by all

parties, there can still be serious trouble unless adequate

packaging is used and properly implemented. It is almost

impossible to design and develop packaging that is too

good for export shipment.

BIBLIOGRAPHY

F. W. Green, ‘‘Export Packaging’’ in The Wiley Encyclopedia of

Packaging Technology 1st ed., Point O’View, pp. 276–282.

1. Ports of the World, Benn Publications, Ltd., London, annual.

2. Ports of the World, A Guide to Cargo Loss Control, Insurance

Company of North America, Philadelphia, Pa, 13th ed 1984.

General References

Preservation, Packaging and Packing of Military Supplies and

Equipment, 2 volumes, Defense Supply Agency, Department of

the Army, the Navy, the Air Force, Washington, DC.

EXTRUSION

J. A. GIBBONS

Egan Machinery Company

INTRODUCTION

Extruders are the most common machines in the plastics

processing industry. They are used not only in extrusion,

but in most molding operations Molding and extrusion are

the basic techniques of forming polymers into useful

shapes. The molding process, which is normally intermit-

tent, can ﬁx three dimensions (height, width, length) of

an object. The continuous extrusion process through a die

can ﬁx only two (height, width). These two processes are

usually complementary rather than competitive and pro-

duce a wide variety of products as diverse as pigmented

pellets, threaded closures, and refrigerator liners (1).

The extrusion process in which an Archimedean screw

rotates within a cylindrical barrel is probably the most

important polymer processing technique used today. It

is used to manufacture continuous proﬁles such as ﬁbers,

tubing, hose, and pipe; to apply insulation to wire; and to

coat or laminate paper or other webs (see Extrusion coat-

ing). This article, however, deals primarily with single-

screw extruders for compounding polymers and producing

pellets, for producing rigid or foam sheet, and for making

blown or cast ﬁlms.

The extrusion principle was ﬁrst employed about 1795

for the continuous production of lead pipe. The ﬁrst

patents on an Archimedean screw extrusion machine

were granted to Gray in England (2) and Royle in the

United States (3). During the nineteenth century the

machinery became reﬁned for manufacturing rubber,

gutta-percha, cellulose nitrate, and casein products. Mod-

ern extrusion technology as applied to synthetic thermo-

plastic polymers began in about 1925 with work on PVC

(see Poly(vinyl chloride)). The ﬁrst screw extruder de-

signed speciﬁcally for thermoplastic materials appears to

have been made by Paul Troester in Germany in 1935 (4).

SINGLE-SCREW EXTRUDERS

Modern single-screw extruders designed to process ther-

moplastic resins normally are o1 in. to 12 in. in diameter,

although larger extruders have been built. The most

common diameters for production-sized machines are

2–8 in. (51–203 mm). Figure 1 shows an 8-in. vented

extruder with a barrel length of 32 diameter, and a 600-

hp (447.4 kW) drive. The main features of an extruder are

shown cross-sectionally in Figure 2.

The solid polymer fed to the extruder may be in the

shape of powder, beads, ﬂakes, pellets, or combinations of

these forms. The extruder conveys, melts, mixes, and

pumps the polymer at high temperature and pressure

through a specially shaped die. The die’s conﬁguration and

the solidifying or cooling process determine the shape of

the product.

All extruders consist of a barrel, a screw, a drive

mechanism, and controls. The heart of the extrusion

process is the screw. It is fashioned with a helical thread

or threads, along with varying channel depth. The func-

tion of the screw is to convey material and generate

pressure in order to produce pellets or other shapes. In

the case of a solids-fed screw the function is expanded to

include solids conveying, compression, and melting. Rota-

tion of the screw accomplishes all these functions.

Successful operation of an extruder depends on the

design of the screw. The depth and length of each zone of

the screw is determined by the product to be run. Barrier

ﬂights and/or mixing sections are sometimes built into the

screw to improve its efﬁciency in melting and delivering a

homogeneous polymer to the die at the proper tempera-

ture and pressure. The proﬁle of the melting process in a

mixing screw is shown in Figure 3.

An extruder interior schematic is shown in Figure 4.

Here the solid resin is introduced into the hopper and,

through the action of the rotating screw, is conveyed into

EXTRUSION 433

the heated barrel. The screw in this section is feeding or

conveying the solids and hence is quite deep. The geome-

try of the screw is such that the depth decreases in the

transition zone, and the solids start to melt. Melting

results from the shearing action of the screw as motor

horsepower is converted into frictional heat. Barrel hea-

ters are used for start-up and to supplement the melting

process. The melt continues to be pumped toward the

discharge, or die end, of the extruder through the meter-

ing section of the screw. (Metering is pumping at a given

rate in a uniform manner, within close temperature and

pressure tolerances.) The die then forms the polymer in

the desired shape. Downstream cooling equipment solidi-

ﬁes and maintains that shape.

Screws are cut from alloy steel. The tops of the convey-

ing ﬂights are hardened or surfaced with special alloys for

extended life. The clearance between screw and barrel is

close. In operation, the screw ﬂoats in the barrel on a layer

of melted polymer.

The barrel, or hollow cylinder in which the screw

rotates, is manufactured from machined steel and

built to withstand pressures of 7500–10,000 psi (51.7–

68.9 MPa). Barrels are also lined with special alloys or

hard-surfaced to extend life. The length of a barrel is

deﬁned as a multiple of its diameter (i.e., L:D ratio of

32:1 = a barrel:screw 32 diameter long). The length of an

extruder barrel is determined by the polymer and process

involved.

Figure 1. An 8-in. (203-mm) 32:1 L/D vented extruder.

Figure 2. A cross section of an extruder.

434 EXTRUSION

Extruders must be heated and cooled. Electrical heat-

ing or ﬂuid heating can be used on the barrel. In electrical

heating, resistance heating elements in various forms

surround the barrel. Tubes for cooling ﬂuid, cast in

aluminum, also contain heating elements. Some barrels

are built with jackets through which heating and cooling

ﬂuid can be circulated. Extruder barrels are usually

divided into zones of speciﬁc lengths, each of which can

Figure 4. Schematic of the interior of an extruder.

Figure 3. Melt proﬁle in a mixing screw.

EXTRUSION 435

be set at a desired temperature. The zones are controlled

by instruments or by microprocessors. Thermocouples are

normally used to sense temperature and to signal the

action of the controller (see Instrumentation).

The drive mechanism consists of a motor and a gear

reducer. The motor is usually a variable-speed dc drive

system with the ability to run a speed range of slow to fast

in a ratio of approximately 1:20. The gear reducer is used

to lower the speed of the motor output shaft (e.g., 1750–

2000 rpm top speed) to the desired screw rotation speed.

Most extruders operate in a variable screw speed range of

up to 200 rpm, but speeds considerably lower are used

with certain polymers, especially on very large machines.

The screw rotation speed depends on the diameter of the

extruder, the polymer to be extruded, and the production

rate desired. The limiting factor in a given-size extruder is

quality of the product, which is dependent on the melt

quality. As screw speed increases and more rate is

achieved, the melt quality deteriorates because of nonuni-

form mixing or excessive temperatures, or because of

degradation of the polymer from excessive heat generated

by high shear.

COMPOUNDING

Extrusion compounding as it relates to packaging consists

of preparing polymers for use in speciﬁc product applica-

tions. Mixtures of polymers, ﬁlled polymers, pigmented

polymers, and a host of other polymer additives constitute

a huge market. Compounding with a single-screw extru-

der consists of mixing and dispersing one or more minor

constituents (e.g., pigments, stabilizers) into a major con-

stituent, a polymer. The product of a compounding line is

pellets. These are used by the converting industry. Con-

verting is the process of melting the pellets and producing

extruded sheet, ﬁlm, injection-molded parts, and so on.

Compounding can be separated by functions: resin-

plant extruding, blending, reclaim, and devolatization.

Resin-plant extruders take the products from a poly-

merization operation and make pellets. The feed to these

machines can be powder, granules, other irregular shapes,

or even a melt. A melt extruder has a molten feedstock and

only generates pressure, whereas a plasticating extruder

has the job of turning solids into a molten mass and then

generating pressure. During pelletizing, stabilizers and

processing aids are combined with the polymer (see Ad-

ditives, plastic).

A blending extruder is used to mix feedstocks of

compatible polymers, or different viscosities of the same

polymer. The blending operation tailors the physical prop-

erties to meet a speciﬁc end use.

Reclamation

Reclamation of polymers is an important segment of the

plastics industry. Film producers are satisfying sustain-

ability calls and reducing costs by replacing virgin plastic

with reclaim. Edge trimming is necessary when it comes

to cast ﬁlms. The trims are recycled by means of a special

extruder, fed back in-line, and used for the encapsulation

of the ﬁlm edges. For blown ﬁlm, recycled material can be

used in the center layer of the ﬁlm structure, and virgin

layers prevent recycled material from contacting pack-

aged food. Edge encapsulation prevents barrier material

from making it to the edge of the ﬁlm, which would

preclude that scap from reintoduction (5).

The problems of reclamatiom are numerous because

the feedstock for the extruder comes in many sizes and

shapes. Some examples are polypropylene battery cases,

polyester X-ray ﬁlm, soft-drink bottles, bread wrappers,

unusable foam products, ﬁbers, ﬁlaments, and off-grade

ﬁlm of all kinds. Specially designed extruders must be

used for processing these materials.

Some polymers require extraction of moisture or gases

from the melt before a satisfactory product can be pro-

duced from the die. To accomplish this, a vent hole with

vacuum pump is introduced along the extruder barrel, and

the screw is specially designed. Devolatizing extruders are

used where residual monomer, water, or other unwanted

materials must be removed from the extrudate.

The machinery required for the aforementioned pro-

cesses must be speciﬁcally designed for the application. A

wide variety of design features are utilized.

Specially designed extruders are used for the addition

of short ﬁbers to thermoplastic polymers (6). This is done

to improve the physical, mechanical, and structural prop-

erties of the virgin plastic. Cost-per-unit volume can also

be improved. The extruder uses a three-stage screw. The

polymer is melted in the ﬁrst stage. The short ﬁber,

usually chopped ﬁberglass, is screw conveyed into the

side of the extruder barrel. The glass and polymer are

mixed in the second stage. The third stage allows venting

and pressure generation for the die at the exit. A cross-

sectional view of a side-fed extrusion system is shown in

Figure 5.

Another unique design is used for reclaiming. Because

the bulk density of most of the scrap-plastic items is low,

use of a dual-diameter extruder has become prevalent.

The feed end of the screw has a larger diameter, so there is

a greater volume for the entering light ﬂuffy feedstock.

This facilitates high production rates.

Vertical extruders meet special needs such as limited

ﬂoor space, or other plant layout requirements.

BLOWN AND CAST FILM

Film is a relatively thin [usually p10 mil (p254 mm)]

ﬂexible web made from one or more polymers, either

blended or coextruded (see Coextrusion, ﬂat; Coextrusion,

tubular) but not to be confused with a fabricated extru-

sion-coated

or laminated web

(see Laminating; Multilayer

ﬂexible packaging).

Films are used in ﬂexible packaging (for overwraps,

bags) as industrial wraps (stretch and shrink ﬁlms), in

medical and health care products (disposable diapers,

backings, hospital bed liners), in agriculture (mulch ﬁlms),

for sacks (drum liners, garbage bags), and as laminates

(aseptic container stock). (Some packaging overwrap ﬁlms

are produced by casting a solvent solution of PVC resin on a

stainless steel belt and evaporating the solvent as the belt

436 EXTRUSION

travels through a heated chamber. This process normally

uses a pump to distribute the solution through the die.

Solvent casting is beyond the scope of this discussion.)

The same basic extrusion processes are used for produ-

cing both blown and cast ﬁlm. The ﬁrst two steps, melting

and metering, are part of the extrusion process described

above.

Forming

In the forming process, the polymer is squeezed through a

die as it leaves the extruder, to form a thin uniform web.

The cast-ﬁlm process produces a ﬂat web. In the blown-

ﬁlm process the die shapes the polymer into a tube. This

latter process is more versatile because it can produce not

only tubular products (bags), but ﬂat ﬁlm as well, simply

by slitting open the tube. Key to the success of both

processes is the die, which must distribute the polymer

uniformly.

Orientation

In the orienting stage of the blown-ﬁlm process, the tube is

blown up into a bubble that thins out, or ‘‘draws down’’ the

relatively thick tube to the required product gauge (thick-

ness). In certain blown-ﬁlm processes, the polymer is

blown downward to produce ﬁlms with special properties.

The ratio of the diameter of the blown bubble to the

diameter of the die is called the blowup ratio. Most

LDPE (see Polyethylene, low density)-blown ﬁlms used

in packaging are made using blowup ratios of 2.0:1–2.5:1.

The blowup ratio is changed depending on the character-

istics of the resin being extruded and the properties

desired in the ﬁlm. In cast ﬁlm, the molten polymer is

also drawn down to the desired ﬁnished gauge. Drawdown

ratios between 20:1 and 40:1 are typical. (If a polymer

exits a die at 40 mils (1 mm) thick and ﬁnishes up 1 mil

(25 mm) thick, the overall drawdown ratio is 40:1.) In the

orientation process the long molecules of the polymer line

up in the stretching direction, which improves the ﬁlm’s

strength in that direction. A key difference between the

two ﬁlmmaking methods lies in the manner of orientation.

Because both edges are free in cast ﬁlm, it is drawn

down only in the direction the material exits the die

(machine direction). Because cast ﬁlm is drawn in one

direction only, it usually exhibits excellent physical prop-

erties in the machine direction, and poor properties in the

cross-machine direction. The cast-ﬁlm process is shown in

Figure 6. In the blown-ﬁlm process, the extruded tube is

stretched in two directions: as it is blown into a bubble and

as it is drawn from the die in the machine direction by the

adjustable speed drive system. This results in strength

properties that are more uniform and can be balanced

depending on the blowup ratio and the takeoff speed.

Figure 7 illustrates the blown-ﬁlm process.

Certain ﬁlms can be biaxially oriented to enhance

properties for speciﬁc packaging uses such as shrink ﬁlms

(see Films, shrink) or overwrap. The blown process pro-

duces a thick tube that is then reheated and blown out

while increasing the take-away speed to maximize orien-

tation in both directions. The cast process extrudes a

thick, ﬂat sheet that is chilled, then reheated and

stretched in the machine direction and then reheated

and stretched across its width by means of a tenter frame.

The most common plastic materials to be biaxially or-

iented are polystyrene (see Polystyrene), homopolymers

and copolymers of polypropylene, usually coextruded (see

Film, oriented polypropylene); and polyester (see Film,

oriented polyester). These ﬁlms are then coated to en-

hance heat-seal or barrier properties.

Quenching

After the polymer has been extruded, it must be solidiﬁed

into ﬁnished ﬁlm. In blown ﬁlm the quenching (or cooling)

process is achieved by convection, by blowing air on the

outside and sometimes on the inside of the bubble. Air

rings at the die exit direct and distribute air uniformly to

the bubble. In cast ﬁlm the web leaves the die and is

Figure 5. Cross section of a side-fed extrusion system.

EXTRUSION 437