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

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being LDPE-PET-alufoil-PET. For speciﬁc applications,

EVOH, OPA, or PP can be included in the structure.

Recent developments in barrier coatings for PET have

led to increasing use of PET bottles for fruit juices and

drinks, and this trend is likely to accelerate as production

ramps up and costs come down. Frozen concentrated

orange juice (421Brix) is usually held at 121C, at which

temperature it is still liquid; typical packaging materials

consist of a spiral wound paperboard tube with aluminum

ends, or an aluminum can.

Carbonated Soft Drinks. Soft drinks are prepared al-

most exclusively using the pre-mix system whereby the

blended syrup, after ﬂash pasteurization if necessary, is

mixed with carbonated, treated water prior to delivery to

the ﬁller. The degree of carbonation ranges from 1.5 to 5

volumes, each volume producing about one atmosphere of

internal pressure. The two major deteriorative reactions

in carbonated beverages are (a) loss of carbonation and (b)

oxidation and/or acid hydrolysis of the essential ﬂavor oils.

The ﬁrst is largely a function of the effectiveness of the

package in providing a barrier to gas permeation, while

the latter can be prevented to a large extent by the use of

high-quality ﬂavorings and antioxidants and by deaerat-

ing the mix prior to carbonation (1). Typical packaging for

soft drinks are aluminum cans and PET bottles.

Beer. Beer is an alcoholic beverage made by brewing

and fermentation from cereals (usually malted barley) and

ﬂavored with hops to give a bitter ﬂavor. Owing to its low

pH (about 4.0), microbial degradation is not usually a

problem, and the use of pasteurization and aseptic cold

ﬁltration excludes wild yeasts which could thrive. How-

ever, during storage, beer can undergo irreversible

changes leading to the appearance of haze, the develop-

ment of off-ﬂavors, and increased color. Flavor loss is

accelerated in the presence of light and certain metal

ions. Beer is particularly vulnerable to O

, which causes

signiﬁcant ﬂavor deterioration; ideally the O

level in beer

should be less than 50 ppb immediately after packaging.

The traditional packaging media for beer was the glass

bottle sealed with a crown closure; but today in many

countries, two-piece DWI aluminum cans with a stay-on

tab now outnumber bottles. A small but increasing quan-

tity of beer is packaged in PET bottles containing an O

barrier to provide acceptable shelf life.

BIBLIOGRAPHY

1. G. L. Robertson, Food Packaging Principles and Practice, 2nd

edition, CRC Press, Boca Raton, FL, 2006.

2. J. M. Krochta, in D. R. Heldman and D. B. Lund, eds., Hand-

book of Food Engineering, 2nd edition, CRC Press, Boca Raton,

FL, 2007, pp. 847–927.

3. K. Grob, M. Biedermann, E. Scherbaum, M. Roth, and K.

Rieger. Crit. Rev. Food Sci. Nut. 46 , 529–535 (2006).

4. M. Wormuth, M. Scheringer, M. Vollenweider, and K. Hunger-

hler, Risk Analysis 26, 803–824 (2006).

5. A. L. Baner in O.-G. Piringer and A. L. Baner, eds., Plastic

Packaging Materials for Food. Barrier Function, Mass

Transport, Quality Assurance and Legislation, Wiley-VCH,

New York, 2000, pp. 427–443.

6. M. Haldimann, A. Blanc, and V. Dudler, Food Additiv. Contam.

24, 860–868 (2007).

7. T. Rothenbacher, M. Baumann, and D. Fu

gel, Food Additiv.

Contam. 24, 438–444 (2007).

8. M. G. Sagilata, K. Savitha, R. S. Singhal, and V. R. Kanetkar.

Comp. Rev. Food Sci. Food Safety 6, 17–35 (2007).

9. M. Sivertsvik, in C. L. Wilson, ed., Intelligent and Active

Packaging for Fruits and Vegetables, CRC Press, Boca Raton,

FL, 2007, pp. 151–164.

PACKAGING OF FOOD FOR HIGH PRESSURE

TREATMENTS

RAFAEL GAVARA

RAMO

N CATALA

PILAR HERNA

NDEZ-MUN

Institute of Agrochemistry and

Food Technology, IATA-CSIC,

Packaging Laboratory,

Burjassot, Spain

High hydrostatic pressure processing (HPP) is a novel food

preservation technology that is gaining commercial im-

portance as a consequence of current consumption trends

toward high-quality products with natural ﬂavor and

taste, fresh-like characteristics, and no preservatives.

HPP presents unique advantages over conventional ther-

mal treatments due to low-temperature processes that

improve retention of food quality. Moreover, it is indepen-

dent of product size and geometry, and the pressure effect

is uniform and almost instantaneous; and although the

HPP has been reported to affect the texture of the food

product, other characteristics such as ﬂavor, color, and

nutritional value are largely maintained (1).

Food preservation through HPP basically consists on

the application of hydrostatic pressures between 100 and

1000 MPa, with holding times typically between 0 and

3 min, at refrigeration and ambient temperature. Current

HPP research is exploring future applications at subzero

temperatures to more than 1001C and holding times up to

30 min. The simplest process consists of (a) heating (cool-

ing) to the treatment temperature (b) pressurization,

which produces an inherent adiabatic heating (c) pressure

holding time, which can include heat transfer between the

food, pressurizing ﬂuid, pressure equipment, and the

environment, and (d) depressurization, which results in

an adiabatic cooling and ﬁnal cooling (or heating) to

product storage temperature (2). However, to attain the

desired extent of the shelﬂife of foodstuffs by means of the

inactivation of microorganisms and enzymes in a short

treatment process, the HPP process can be very complex

with successive changes of pressure, as occurs in pulsed

high-pressure treatments (3).

Most HPP equipments work through batch processes,

although a few commercial semicontinuous units for the

898 PACKAGING OF FOOD FOR HIGH PRESSURE TREATMENTS

treatment of liquid foods and sauces that may contain

solids under 5 mm are under operation. In this latter case,

the food product ﬂows and ﬁlls the treatment vessel,

which is then closed and pressurized. The treated product

is then driven to an aseptic (or clean) packaging step. The

food is not sterilized during the HPP treatment; therefore,

HP treatment can be classiﬁed as a nonthermal pasteur-

ization and can be used commercially for refrigerated,

lowacid (pH o 4.5) or low-water-activity products (e.g.,

jams). Since the food is processed prior to packaging, no

special requirements need to be fulﬁlled. Therefore,

packaging solutions for the packaging of HPP-treated

food in a semicontinuous unit are similar to those of

aseptic packaging. Refer to the aseptic packaging section

for packaging solutions.

The HPP treatments by batch processing present sev-

eral differences with respect to the semicontinuous one,

the most relevant being that the product is packaged

before the treatment. Figure 1 presents a batch HPP

scheme. The packaged product is introduced in the

vessel, which is then ﬁlled with a pressure-transmitting

ﬂuid; this can be water or aqueous solutions of ethylene

glycol, propylene glycol, or other soluble oils. Then the

vessel is closed, and pressurization (100–1000 MPa) takes

place by pumping additional pressure-transmitting ﬂuid.

After the treatment, pressure is released in a few seconds,

the vessel is opened, the ﬂuid may be removed, and the

packages are removed, cleaned, and sent to the next

packaging step.

Except for the case of foods processed by semicontin-

uous units, packaging is a key factor to the feasibility of

high-pressure processing, because it must preserve the

intrinsic qualities of the food products without losing

integrity, barrier performance and mechanical character-

istics. This section highlights the requirements that a food

package has to fulﬁll to be used in high-pressure treat-

ments, lists the effects of such preservation technology

over packaging materials, and presents some of the most

conventional solutions.

PACKAGING REQUIREMENTS

During the HPP process, the food packages are immersed

in water or in aqueous solutions and exposed to high

pressures. Therefore, besides typical requirements of

food packaging such as airtightness, minimal food-packa-

ging interactions, and adequate optical and barrier prop-

erties, the package must transmit the hydrostatic

pressure to the packaged product, withstand the action

of the pressurizing liquid, and resist the action of pressure

without relevant changes. To transmit the hydrostatic

pressure to the packaged product, ﬂexible packaging is

more convenient than rigid packaging. Rigid packages

such as metal cans or glass bottles tend to fracture or

become distorted as a result of the high pressure applied

(4). The use of packaging structures designed with cellu-

losic or other hydrophilic materials at the outermost layer

should be reconsidered since it could present signs of

deterioration caused by the direct exposure to water.

It is then crucial to understand the effects of high

pressure on relevant properties of plastic materials to

ensure the safety of the foodstuffs along their shelﬂife.

Isostatic pressure causes the food product to compress

three-dimensionally up to approx 12% at 600 MPa.The

package should follow this volume change without altera-

tions. Rigid containers do not compress to this extent, and

therefore they usually get distorted. On the other end of the

spectrum, expanded polymeric materials such as expanded

polystyrene or expanded polypropylene can be compressed a

minimum of 80% due to the large percentage of air occluded

in the package walls, and their use should be avoided.

PACKAGING SOLUTIONS

Many ﬂexible packaging materials have been shown to

withstand HPP without visible signs of integrity loss. One

of the most important factors that have contributed to the

success of plastic ﬁlms and sheets is the availability of

Figure 1. Scheme of a batch high hydrostatic pressure process.

PACKAGING OF FOOD FOR HIGH PRESSURE TREATMENTS 899

multilayer structures. A suitable multilayer structure

used to pre-package foods must have sufﬁcient ﬂexibility,

resilience, and resistance to delamination during the

compression process, because a loss of packaging integrity

would result in major food safety and quality deterioration

(5). Initially, the innermost layer of the structure should

be selected to limit food–package interactions (unless the

substance exchange is desired and beneﬁcial; see Active

Packaging) and to provide hermetic a seal. Heat-sealable

materials such as polyoleﬁns could be a good option,

although other sealing process can widen the materials

spectra (see the sealing section). The outermost layer of

the structure should resist the action of the pressurizing

ﬂuid. With the exception of highly hydrophilic materials

such as polyvinyl alcohol, most polymeric materials could

be suitable. Reversed printed oriented polypropylene,

polyester, or polyamide ﬁlms can be good choices.

Finally, the barrier properties to oxygen can be tailored

to the product requirement by varying the materials and

the number of layers in the structure (see the barrier

section). Many structures have been tested using PA,

EVOH, PVOH, aluminum foil, or polymer ﬁlms coated

with aluminum, silicon oxide (SiOx), or aluminum oxide

(AlOx). Most of these packaging structures have been

tested with no relevant changes caused by the HPP

treatment. The packages showed sufﬁcient degree of ﬂex-

ibility and resilience to compensate for the reduction in

volume, no signiﬁcant changes in tensile strength being

observed between controls and HPP treated samples.

Heat seal strength is a critical point in packages,

because if any void is present or generated by the treat-

ment along the seal of the package, the safety of the

packaged food will be seriously compromised. In general,

thermoplastic materials have been proven to withstand

HPP without signiﬁcant losses in sealability. In other

technologies, such as in retortable food packages, the

seal is exposed to high temperatures and pressure

changes in both directions, which can affect and even

break the sealing. However, the sealing is not exposed to

such critical conditions in HPP treatment, since the

package always receives the pressure from the outside

and the temperature rarely reaches 701C.

High-pressure-induced delamination has been ob-

served in ﬂexible packages made of composite materials

and having relatively large headspace volumes. Gases

reduce their volume during the HPP treatment much

more than food and packaging materials, promoting ten-

sions and failures. Headspace volumes should be mini-

mized to reduce the chances of having seal damages,

delaminations, and large deformations. The presence of

large headspace volumes increases largely the time to

obtain the required pressure since more pressurizing ﬂuid

has to be pumped in the vessel. Also the packaging design

with materials of diverse nature can produce changes

in packaging properties. Thus, aluminum and metal oxi-

des present lower compressibilities than do polymeric

materials. This difference can be responsible for

the delamination and blistering observed in barrier

structures containing aluminum layer or coatings which

increase in delamination with exposure time and tem-

perature (5, 6).

BARRIER PROPERTIES

The barrier properties are often the most relevant char-

acteristic to be considered in the design of functional

polymeric packages for food. Therefore, HPP-induced

barrier changes can have a very negative impact concern-

ing packaged food quality. In general, the permeability to

oxygen, carbon dioxide, and water vapor do not change

relevantly after the HPP treatment. Nevertheless, a large

decrease of barrier has been observed in metalized ﬁlms

apparently due to the damage caused in the aluminum

layer (4). It is also relevant to mention that HPP treat-

ments produce much less damage than do conventional

thermal treatments in the barrier properties of hydrophi-

lic materials such as EVOH. The concurrence of low

temperatures of treatment, shorter treatment period,

and the presence of high pressure produces a much lower

water sorption in those polymers, which results in the

reduction of package plasticization and of the subsequent

loss of barrier. The volume reduction of the voids present

in the polymer (free volume) which takes place during the

high-pressure treatment could be responsible for a de-

crease in solubility and a much slower molecular diffusion

though the polymer matrix. These two parameters con-

tribute to reduce the extent of mass transport through

packages and food–package interactions. In agreement

with this statement, migration studies showed that HPP

treatment do not alter the migration values of ﬂexible

packaging structures (7).

Tests should be carried out to conﬁrm the validity of a

package for HPP treatments keeping in mind that,

although most polymeric materials are suitable since their

main properties are not modiﬁed, good sealing properties,

design of the package structure with materials of similar

compressibility, and the reduction of the package head-

space volume will lead to a successful packaging solution.

BIBLIOGRAPHY

1. E. Palou, A. Lo

pez-Malo, G. V. Barbosa-Ca

novas, and B. G.

Swanson, ‘‘High-Pressure Treatment in Food Preservation’’ in

M. S. Rahman, ed., Handbook of Food Preservation, Marcel

Dekker, New York, 1999. pp. 533–577.

2. J. A. Torres and G. Velazquez, ‘‘Hydrostatic Pressure Proces-

sing of Foods’’ in S. Jun and J. Irudayaraj, eds., Food Proces-

sing Operations Modeling: Design and Analysis, CRC Press,

Boca Raton, FL, 2008, Chapter 7.

3. G. D. Alema

n, M. Walker, D. F. Farkas, J. A. Torres, E. Y. Ting,

S. C. Mordre, and A. C. O. Hawes, ‘‘Pulsed Ultra High Pressure

Treatments for Pasteurization of Pineapple Juice,’’ J. Food Sci.

61, 388–390 (1996).

4. C. Caner, R. J. Hernandez, and M. A. Pascall. ‘‘Effect of High-

Pressure Processing on the Permeance of Selected High-

Barrier Laminated Films,’’ Packag. Technol. Sci. 13, 183–195

(2000).

5. C. Caner, R. J. Hernandez, M. A. Pascall, and J. Riemer, ‘‘The

Use of Mechanical Analyses, Scanning Electro

n Microscopy

and Ultrasonic Imaging to Study the Effects of High-Pressure

Processing on Multilayer Films,’’ J. Sci. Food Agric. 83, 1095–

1103 (2003).

900 PACKAGING OF FOOD FOR HIGH PRESSURE TREATMENTS

6. A. Schauwecker, V. M. Balasubramaniam, G. Sadler, M. A.

Pascall, and C. Adhikari, ‘‘Inﬂuence of High-pressure Proces-

sing on Selected Polymeric Materials and on the Migration of

a Pressure-Transmitting Fluid,’’ Packag. Technol. Sci. 15,

255–262 (2002).

7. J. Dobia

s, M. Voldrich, M. Marek, and K. Chuda

ckova

‘‘Changes of Properties of Polymer Packaging Films During

High Pressure Treatment,’’ J. Food Eng. 61, 545–549 (2004).

PALLETIZING

A pallet is a low platform used to stack or accumulate

several smaller units of product so that they may be

conveyed by mechanical means. Pallets may be made of

wood (see Pallets, wood), plastic (see Pallets, plastic), or

corrugated kraft board (see Pallets, expendable corru-

gated), and they are usually rectangular rather than

square. The arrangement of product on the pallet (pallet

pattern) is critical to an orderly, compact loading, and

distribution system. Patterns correspond to the pallet

shape and are designed to afford the maximum load in

the minimum space without forfeiting structural strength.

Pattern design may be restricted by the primary and

secondary product packages, the shipping method and

size of conveyance, as well as warehouse space.

The quantities of pallets and tiers also are determined

by the shipping container, shipping method, and

distributor space allotment. The second layer, or tier,

may be exactly like the ﬁrst, or rotated 1801, or it may

be an entirely different pattern. Unless the cases are

heavy and/or large, the pattern of the second layer is

changed to form a load that will not break apart. Rotation

by 1801 is a common solution. The stacking strength of the

load must be judged against the space limitations of the

pattern. A column stack (each layer identical) with cor-

ners one above the other is strongest, but it is most likely

to topple because the tiers are not interlocked. Many

different pallet patterns are shown in Figure 1.

PALLETIZING

Once the pattern and number of tiers have been designed,

the method of product-to-pallet transfer is determined.

Hand palletizing is the most versatile method and is

effective when loading is slow. Single-product, single-

pattern, high-level palletization developed from hand

palletization. Recent technological advances permit auto-

mated palletization of almost every product, but this

process may not always be economical. The selection of a

palletizer demands consideration of performance require-

ments and space limitations. The palletizer is an element

in the total packaging/distribution system, and it should

never be a limiting factor. The palletizer should be capable

of speeds exceeding normal line speed by 5–10%, and

change over time from product to product should be

minimal. A fully automatic palletizer does not require an

operator in attendance full time.

Figure 1. Pallet patterns.

PALLETIZING 901

High-Level Palletizers. High-level palletizers (also

called moving-pallet palletizers) pick up the product at a

level above the height of a full pallet and form the tier

pattern on a bed. The pallet is raised to the bed level, and

the product is transferred (by a sweep or rake off system)

to the pallet. Then, the pallet is lowered one layer, and the

next tier is formed in the same manner and placed atop

the ﬁrst. This process continues until the pallet reaches

the prescribed height. It is then transferred out to be

replaced by an empty pallet that is raised to the bed for a

repetition of the process. Numerically controlled tape

programs provide simple multipattern capability and are

adaptable for a variety of users.

Low-Level Palletizers. Low-level palletizers (also called

‘‘ﬁxed-pallet’’ palletizers) were developed more slowly

than high-level palletizers, but with the introduction of

programmable logic controllers, they have become highly

competitive with high-level units in cost and speed. The

low-level palletizer operates at ﬂoor level (see Figure 2).

The pallet is neither raised nor lowered, and the tier is

formed at low level. The transfer bed or apron is raised or

lowered to the appropriate level, and the tier is trans-

ferred to the pallet. The bed returns to the tier-forming

position, and the next layer is formed. This pattern is

repeated until the pallet is full. After the full pallet is

discharged, an empty one is transferred into the load

station, and the process is repeated.

Robotic Palletizers. In the past, ‘‘pick-and place’’ sys-

tems picked up a product at one point and placed it at a

second point repeatedly. At present, robotic palletizers

may be thought of as intelligent, discriminating systems

capable of picking up several different products and

placing them at several different points. These units are

usually capable of movement in two or three planes, often

on rotational coordinates that operate from a ﬁxed point at

the center of a circle. They are capable of picking up a

variety of products from one point on the circumference of

the circle and placing it on one of several pallets located at

other points around the circumference. Robotic palletizers

are most applicable when a variety of products require

different pallet patterns and relatively slow speeds are

acceptable.

Bulk Palletizers. The shift to bulk handling has reduced

the demand for case depalletization of empty containers.

Numerous applications exist for palletizing without ben-

eﬁt of a case, for example, the transfer of empty cans from

the can manufacturer to the ﬁller. Cans are bulk palle-

tized successfully because they are strong enough to

withstand relatively rough treatment. Improved handling

methods permit bulk palletization of glass containers, and

many plastic containers now are being bulk palletized as

they gain a greater place in the market.

Nested container-to-container bulk pallets can save up

to 15% space for the same number of containers compared

with column-stacked cases. That means 15% more con-

tainers can be shipped in the same truckload to the user,

which cuts time and expense. The compact loads also

reduce warehouse requirements. In addition, bulk

palletizing eliminates the labor and expense associated

with uncasing equipment and additional conveyors for

empty cases.

Bulk palletization is essentially an adaptation of high-

or low-level palletization. The product is accumulated on a

table or bed, then transferred by lifting a tier or clamping

the perimeter and sweeping the tier onto a pallet. A

corrugated sheet the size of the pallet, called a tier sheet,

is generally used between layers to provide stability to the

load. Increased stability can be gained by use of an

inverted tray instead of a tier sheet.

Figure 2. Operation of a low-level palletizer. (a) Operation 1.

Sealed shipping cases feed in and are oriented to the prepro-

grammed pattern. When one row is formed, the cases move

forward. The next row forms and moves forward, which continues

until the layer is complete and the loading plate is ﬁlled.

(b) Operation 2. The layer is lifted to the height of the existing

pallet stack. The ﬁlled loading plate moves into position just

above the pallet stack. (c) Operation 3. The loading plate retracts

allowing the cases to settle, row by row onto the top of the pallet

stack. The pallet is squared by a squaring bar, which also assures

complete unloading. The loading plate returns to starting position

where another accumulated load is ready.

902 PALLETIZING

Miscellaneous Palletizers. Other types of palletizers in-

clude drum palletizers, keg palletizers, and bag palleti-

zers. In most cases, these machines address a particular

end-use and are concerned only with a narrow portion of

the market place.

DEPALLETIZING

The removal of product from pallet depends on the con-

formation of the product. Bulk depalletizers remove tiers

of product from the pallet in much the same manner as

bulk palletizers in reverse. In one approach, the tops of the

containers are gripped mechanically, pneumatically, or

with a vacuum, and the tier is lifted onto a discharge

table. In another, the tier is swept onto the discharge

table. Removal of the tier sheet or inverted tray is as

critical here as in bulk palletization. Product stability is a

key factor in all bulk handling operations and the primary

determinant of method.

Depalletization of plastic cases or crates may require

modiﬁed bulk depalletizers or specialized robotic depalle-

tizers. Plastic crates usually have an interlocking feature

that requires a tier to be lifted clear of the one below before

transfer to the discharge table, thus precluding a sweep

system. Most pail depalletizers must handle the products

individually in addition to lifting clear of the pail below.

Depalletization of corrugated cases is more difﬁcult

than palletization. The ﬂaps on the cases get caught on

one another, which prevents consistent sweepoff. Corru-

gated cases do not interlock, so clamping the perimeter

causes the center cases in the tier to slip down. The most

reliable way to remove a whole tier of corrugated cases is

to use tier sheets, but the additional cost discourages wide

acceptance. The next-best method is a combination clamp-

ing and vacuum system. Some automated warehousing

systems remove cases from pallets one at a time in a type

of ‘‘order-picking’’ operation. Little effort is being ex-

pended today on ﬁnding better case-depalletization meth-

ods because the shift to bulk handling has shifted research

and development work in that direction as well.

BIBLIOGRAPHY

‘‘Palletizing’’ in S. D. Alley, ed., The Wiley Encyclopedia of Packa-

ging, 1st edition, ABC-KCM Technical Industries, Inc.

pp. 486–488.

PALLETS, PLASTIC

L. T. LUFT

Menasha Corporation

Neenah, Wisconsin

Plastic pallet construction began during the late 1960s

when the low cost of commodity resins such as polystyrene

(PS) and polyethylene encouraged scores of molders to

enter this promising market. In the early 1970s, high-

density polyethylene (HDPE) was priced at about $0.16/lb

($0.35/kg). Still in its infancy, the plastic pallet market

was severely curtailed when prices of commodity resins

more than doubled at the time of the 1973 oil embargo. A

2:1 price differential between wood and plastic pallets

quickly became a 4:1 price disadvantage.

Several changes that took place in the 1980s have

encouraged and use of plastic pallets as follows:

Packaging is now often considered part of direct pro-

duction cost instead of ﬁxed overhead. This highlights the

savings generated by reusable pallets (1).

Adoption of the just-in-time inventory concept (2) in-

cluding reusable packaging, inventory reduction, and

higher quality.

Greater use of robots and automated palletizers that

require uniform size/weight pallets.

Increased awareness and regulation of plant

sanitation.

Typical single- and double-faced pallets are shown in

Figures 1 and 2.

Materials. Most plastic pallets are manufactured from

HDPE (see the Polyethylene, high-density article). Mate-

rials such as polystyrene, ﬁberglass-reinforced plastics

(FRP), and polypropylene are used occasionally. Heavy

pallet loads and unsupported pallet racking may dictate

the use of stiffer polystyrene (see the Polystyrene

article).

FRP (see the Thermosets article) are used for low-

volume custom pallet requirements or prototype pallets.

In this situation, low-cost wooden tooling is used with

the hand-layup ﬁberglass technique. Polypropylene (see

the Polypropylene article) has been used to construct

structural foam plastic pallets by companies with excess

virgin or regrind polypropylene. Polypropylene is not

normally used in pallet construction because it requires

relatively expensive impact modiﬁers for cold-weather

performance.

Polyethylene is favored for a many reasons, such as

commodity status (i.e., low cost, uniform performance,

readily available, and wide acceptance); excellent resis-

tance to impact; good performance under a wide range

of operating conditions (i.e., temperatures of –30 to 1501F

(34 to 661C), indoor or outdoor applications; light- to

heavy-weight loading; outstanding chemical resistance

to most acids and bases; U.S. Department of Agricul-

ture (USDA) and U.S. Food and Drug Administration

Figure 1. Typical single-faced plastic pallet.

PALLETS, PLASTIC 903

(FDA) clearance for use in food and pharmaceutical

plants; easy cleaning; and outstanding molding and de-

sign ﬂexibility.

Polyethylene’s one glaring weakness is its inability to

resist deﬂection (bending) under load. This deﬂection

problem is especially serious in pallet-racking applica-

tions. Unsupported racks do not have center supports or

decking. In these racks, the pallet must span an open

space while maintaining the load. With loads of over

2000 lb (907 kg), plastic pallets are prone to bending

(deﬂection). In addition to the initial deﬂection, the plastic

pallet will continue to bend or creep for up to 2 weeks.

Over time, it may become difﬁcult to reenter the pallet

with the forks of a lift. Most standard pallet rack, drive-

through rack, and gravity-ﬂow rack are ‘‘unsupported.’’

In situations where heavyweight racking is a must,

steel-reinforced plastic or stiffer PS are frequently used.

Steel reinforcements add expense, and compared to

HDPE, PS costs more and offers less chemical and impact

resistance. One solution to the racking problem is in

the design of rackable pallets. Two-way-entry pallets can

rack over 3000 lb (1360 kg) in an unsupported rack (see

Figure 3). Experimental plastic resins are also being tried

in an attempt to solve the racking dilemma.

Design and Construction. Pallet design and the method

of construction greatly inﬂuence pallet performance, price,

and acceptance. Today plastic pallets are designed and

built using several different processing techniques (see

Table 1).

Structural Foam Molding. Most plastic pallets made

today are made by structural foam molding (3, 4). This

low pressure injection molding process produces parts

with a solid skin surrounding a foamed core. Compared

to high-pressure injection molding, structural foam mold-

ing allows the economic production of heavy wall sections

and helps reduce stress points throughout the pallet. The

structural foam process provides outstanding design ﬂex-

ibility. Wall thickness of

in. can be molded to produce

pallets ranging from lightweight single-faced units to

super-duty racking pallets. Another beneﬁt is high-speed

production, with cycle times as low as 2–3 min. Good

resistance to impact, high strength per pound (kilogram),

and good deﬂection strength are all positive characteris-

tics that make structural foam a good choice for large-

scale production of both custom and proprietary pallets.

The chief limitation of structural foam is that relatively

high volume (3000 total units minimum) is required to

amortize the relatively high tooling cost. When compared

with high-pressure injection molding, the tooling for

structural foam may be less costly. Most low-pressure

foam tools may be built from machined aluminum or

Kirksite, which reduces tooling costs by up to 50%.

When structural foam tools are built from steel, the cost

savings are negligible.

Injection Molding. High-pressure injection molding (see

the Injection molding article) also offers design ﬂexibility.

It is used for the production of pallets that range from

light weight disposables to heavy-duty 60-lb (27-kg)

reusables. Injection molded parts generally have nar-

rower wall sections than structural foam, r0.300 in.

(r7.62 mm), and rely on their rib design for structural

integrity (5). Injection molding excels in light weight,

large-volume production. Cycle times for

-in. (3.2 mm)

injection-molded pallets can be under one minute. Heavy-

duty parts with wall section

in (3.2 mm) offer high

strength and excellent durability. Because high molding

pressures require expensive equipment and hardened-

steel tooling, high-volume production runs Z10,000) are

generally required to amortize tooling and press costs.

Figure 2. Typical double-faced plastic pallet.

Figure 3. Two-way entry racking pallet.

904 PALLETS, PLASTIC

Table 1. Plastic Pallets, Production Methods

Molding Process

Plastic Pellets

Advantages

Plastic Pallets

Disadvantages

Ideal Pallet

Application Secondary Applications

Tooling Options Average Cycle

Wall

Thickness

Structural-foam

molding

Economic production of

heavy-wall sections

High-cost tooling

and processing

equipment

Large-volume

custom or

proprietary

pallets with

runs of 1000

pallets or more

Manufacture of heavy-

duty racking pallets is

possible by using ﬁlled

polyethylene pallets or

polystyrene

Kirksite

aluminum steel

2–4 min

21:0 in (4.8–

25.4 mm)

Short cycle times

Minimum custom

order quantity

3000 units

Wall thicknesses of up to

1 in. (2.54 cm) can be

used when necessary

Good impact resistance

Good deﬂection strength

High strength per pound

(kilogram)

Good weight and

dimensional tolerance

allows complex shapes

Injection molding Flexible process allows

production of light-

weight disposable as

well as heavy-duty

returnable pallets;

allows complex

geometry

Highest tooling

cost Highest

equipment costs

High energy

costs

Largest-volume

custom and

proprietary

pallets

Lightweight disposable

pallets can be

inexpensively produced

by keeping wall

sections narrow and

cycle times short;

heavy-duty racking

pallets can be

manufactured by using

heavier wall sections

and a well integrated

rib design

Hardened steel 30 s–3 min

in. (0.8–

9.5 mm)

Rotational

molding

Low equipment cost Relatively long

cycle times

Low-volume

production of

large pallets

Manufacture of heavy-

duty racking pallet is

possible by

ecapsulating steel

reinforcements into the

pallets

Cast- aluminum

fabricated

metal-plated

nickel

3–6 min

in. (3.2–

6.4 mm)

Low tooling cost

Limited weight

and

dimensional

stability

Custom pallet

projects of 1000

units or more

are feasible

Production of double

walled parts

Limited to simpler

design

(geometry)

(Continued

)

905

Table 1. Continued

Molding Process

Plastic Pellets

Advantages

Plastic Pallets

Disadvantages

Ideal Pallet

Application Secondary Applications

Tooling Options Average Cycle

Wall

Thickness

Vacuum forming Low equipment cost

Relatively long

cycle times

Lower-volume,

low cost,

lightweight

pallets

Heavier loads up to 3000

lb (1361 kg) can be

accomodated by using

twin-sheet vacuum

forming

Metal Plaster

Epoxy Wood

3–6 min

in. (3.2–

6.4 mm)

Low tooling cost

Limited wall

thickness

Pallets projects of

500 units and

above are

feasible

Vacuum-formed pallets

are not generally used

for heavy-duty racking

applications

Limited depth of

draw

Limited design

complexity

Reaction-injection

molding

Lighter weight tooling

and equipment costs

less than injection

molding processes

Limited deﬂection

strength

Lighter duty

custom and

proprietary

pallets

Fiberglass-reinforced

reaction injection

molding is used to

increase deﬂection

strength for heavier

applications

Lightweight steel

Aluminum

Kirksite

Sprayed metal

2–4 min

22:0 in. (3.2–

51 mm)

Allows complex designs Slightly longer

cycle times than

injection-

molding

processes

Pallet projects of

1000 units and

above should be

justiﬁable

Steel reinforcements can

be encapsulated for

additional strength

Lower pressures and

temperatures afford

signiﬁcant savings

(70%) over injection-

molding processes Limited

dimensional

stability

906

Rotational Molding. Rotational molding (3) (see the

Rotational molding article) uses a heated tool into which

solid or liquid polymer is placed. This process offers the

most economical tooling costs. Myriad sizes and designs of

a relatively low quantity (1000–2000 units) can be eco-

nomically justiﬁed. Design innovation, including the

molding of steel-encapsulated, smooth-skinned pallets is

a feature of rotational molding. Rotationally molded parts

offer good resistance to blunt impact and the repair of

small puncture damage is possible. Its drawbacks include

relatively long cycle times (as high as ﬁve min) and

relatively narrow

-in. (4.8-mm) wall thickness (6), which

limits rotationally molded pallets to medium-duty appli-

cations. Some rotationally molded designs can accommo-

date the addition of steel reinforcement for heavier loads

and pallet racking operations.

Thermoforming. Thermoformed plastic pallets (see the

Thermoforming article) are offered in dozens of low-cost

lightweight designs. Inexpensive tooling allows faster

amortization of low-volume custom pallets. For example,

custom reusable dunnage trays are often thermoformed.

Thermoforming, too, has its disadvantages. With cycle

times averaging 5-min (5), high-volume projects are

sometimes impractical. In addition, relatively narrow

wall thicknesses limit these pallets to lighter loads,

usually under 3000 lb (1360 kg). They are not often found

in heavy-duty racking applications. Twinsheet vacuum

forming allows heavier loads with reduced deﬂection,

but it lengthens cycle times and adds cost.

Reaction Injection Molding (RIM). RIM polyurethane

pallets have entered the market. RIM uses two or more

liquid components (polyol and isocyanate) that are mixed,

then injected into a closed mold. These two components

react to form a ﬁnished polymer, which takes on the shape

of the tool. The chief advantages of RIM are lower cost

equipment and tooling, especially in building large parts

such as pallets. The chief disadvantage of RIM pallets is

the lower resistance to deﬂection. For this reason, many

large RIM parts are steel reinforced or manufactured with

ﬁberglass or mineral ﬁllers. These stiffening techniques

add cost.

Advantages. Plastic pallets are used primarily in the

food, pharmaceutical, textile, high-technology, and auto-

motive industries. Because of the higher cost of plastic

pallets, most purchasers use their pallets in-plant, or in

closed-loop shipping system. Plastic pallets are almost

always found in applications where the user can retrieve

most of the pallets after each trip.

Plastic pallets of all types offer certain generic advan-

tages which make them attractive alternatives to other

pallet materials. Listed below are some of the plastic

pallet’s chief beneﬁts.

Long Pallet Life. The relatively expensive plastic pallet

must offer a long service life. Many customers

experience plastic pallet life of 5–9 years and

more (7).

Reduced Load Damage. Smooth molded plastic helps

eliminate product damage. There are no broken

boards or protruding nails to damage-sensitive

loads (7).

Easy Cleanup. Plastic pallets are easy to clean and

keep clean (8).

USDA and FDA Clearance. Both polyethylene and

polystyrene are acceptable in food and pharmaceu-

tical plants. Pallets made from these materials can

be approved on a case by case basis by the on-site

inspectors.

Reduced Worker Injury. Smooth construction and con-

sistent weights help to eliminate minor cuts and

back strain (7).

Chemically Inert. Polyethylene plastic pallets are

highly resistant to acids and bases, and at ambient

temperatures, hydrocarbon solvents.

Moistureproof. Plastic pallets will not absorb moisture

and soak loads. Plastic pallets will not rust or break

down in wet conditions (8).

No Harbor for Pests. Plastic pallets will not harbor

or support the growth of worms, eggs, molds, or

mildew.

The design advantages of plastic pallets can include the

following:

Nestability (single-faced pallets can nest with each

other when unloaded). This feature can save over

50% of valuable truck or dock space.

Interstacking (the ability to positively locate one loaded

pallet on top of another loaded pallet).

BIBLIOGRAPHY

‘‘Pallets, Plastic’’ in L. T. Luft, The Wiley Encyclopedia of Packa-

ging Technology, 1st ed., Neenah, WI: Menasha Corporation,

pp. 488–492.

1. U.S. Industrial Outlook, United States Department of Com-

merce, Jan. 1984.

2. J. M. Callahan, ‘‘Just-in-Time a Winner,’’ Automotive Ind. Mag.

65(3), 78 (March 1985).

3. Modern Plastics Encyclopedia, 1983–1984 ed., McGraw-Hill

Publications, New York.

4. ‘‘What’s Available in Plastics?’’, Warehouse Supervisor’s Bulle-

tin, National Foreman’s Institute, Waterford, CT, June 25,

1984.

5. ‘‘Pallets Take off in All Directions,’’ Modern Plast. Mag., 64–66

(March 1971).

6. ‘‘Fitting Plastic Pallets to the Job,’’ Plast. Design Forum 9(3),

57 (May/June 1984).

PALLETS, PLASTIC 907