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

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General References

Modern Plastics Encyclopedia, McGraw Hill, New York, pub-

lished annually.

The Packaging Encyclopedia, Cahners Publishing, Denver, pub-

lished annually.

The PMMI Packaging Machinery Directory, The Package Machin-

ery Manufacturers Institute, Washington, DC, published

annually.

Food Master, equipment, supplies & services, Food Engineering

Network, Cahners, published annually.

J. M. Ramsbotton, ‘‘Packaging’’ in J. F. Price and B. S. Schweigert,

eds., The Science of Meat and Meat Products, 2nd edition,

Freeman, San Francisco, 1960, pp. 513–537.

THERMOFORMING

VIVEK CHOUGULE

MATT PIERCY

Printpack, Inc., Williamsburg,

Virginia

Thermoforming is a process of shaping thermoplastic

sheet into a product through the application of heat and

force. In most cases, the heat-softened plastic assumes the

shape by being forced against the mold until it cools and

sets up. Forming force may be developed by vacuum

(atmospheric pressure), positive air pressure, or mating

matched molds.

Thermoforming is an important thermoplastic fabrica-

tion process. Early developments involved thermoforming

of cellulose sheets. Industrial scale thermoforming began

in the early 1960s when containers and lids formed from

high-impact polystyrene (HIPS) were ﬁrst used by the

dairy industry to package cottage cheese, sour cream,

yogurt, and other dairy foods. The word sheet as it relates

to thermoforming is used to describe ﬂat extruded plastic

material that is generally relatively thick, in contrast to

comparatively thin plastic ﬁlms. It is ambiguous, however,

in two ways: Thermoforming is also used to shape rela-

tively thin ﬁlms, and the word sheet is also used to

distinguish between thermoforming machines that form

cut pieces (sheet-fed) rather than as extruded webs (web-

fed or roll-fed). The distinction between sheet-fed and

web-fed machines is similar to that between sheet-fed

and coil-fed equipment in the metal-can industry. The

exception is that in plastics forming, the wide sheet web

can come either from coiled stock or directly from the

extruder. When the thermoformer is fed directly by a sheet

coming from the extruder, it is called in-line thermoform-

ing. Sheet for thermoforming is commonly produced by

extrusion. Use of multilayered sheet structures is increas-

ing for a variety of applications. Multilayered sheet struc-

tures can be produced by coextrusion, coating, lamination,

or a combination of these processes. Roll-fed thermoform-

ing machinery is available that can produce beverage cups

at rates of 75,000–100,000 pieces/h while consuming plas-

tic sheet in excess of 1 ton/h (910 kg/h). Sheet consumption

for larger and heavier containers can exceed 2 tons/h (1.8

metric ton/h), but unit production rates may be lower

because fewer mold cavities can be mounted in the ma-

chines and more time is required to cool the thicker walls.

The ability to produce extruded HIPS foam sheet (see

Foam, extruded polystyrene) has provided additional

packaging markets for thermoforming. The ﬁrst of these

was meat and produce trays, followed by egg cartons.

Other applications include fast-food carryout cartons,

institutional dinnerware medical trays and blisters, and

inserts for rigid boxes. Use of oxygen and moisture barrier

thermoformed containers is on the rise for variety of shelf

stable and refrigerated food packaging. Thermoformed

plastic containers have rapidly replaced other packaging

materials (metal, glass) in the last 20 years. Common

applications include containers for fruit, apple sauce,

puddings and baby food. One of the growing new applica-

tions is the use of high-barrier thermormed containers for

retorted shelf-stable foods. Retort applications require

special considerations for material selection, structure,

and shape of the container.

PROCESS STEPS

The thermoforming process is used to make products from

thermoplastic sheet by a sequence of heating, shaping,

cooling, and trimming. Trimming is not necessarily an

integral part of the forming cycle. Only a few applications

(e.g., pharmaceticul blisters) can use the formed web

without some kind of trimming.

Heating

Thermoplastic sheet is heated to a temperature range

adequate for forming, depending on the thermoplastic

material used. Temperature control is critical because of

plastic’s poor thermoconductivity and because tempera-

ture affects the forming characteristics (i.e., ductility) of

the materials: Too much heat causes the sheet to ﬂow

without drawing; too little heat causes it to rupture early

in the forming process.

Heating time is governed by the heating process used,

materials used, sheet thickness, surface conditions of the

material, and the combination thereof. Any of the common

heat transfer processes such as convection, conduction, or

radiation can be used, but low thermoconductivity and

relatively high speciﬁc-heat capacity of thermoplastics

should be taken into consideration. Radiation heat could

be absorbed, transmitted, or reﬂected by thermoplastics,

depending on radiation wavelength and material proper-

ties. Most thermoplastics absorb infrared energy emitted

in the 3.0 to 3.5-mm wavelength range, which makes them

ideally suited to heating by radiation. Convection heating

would be ideal because the sheet could be soaked in hot air

at just the right temperature with the assurance that the

entire sheet would be uniformly heated. However, straight

convection systems are not practical because they are

relatively slow and they present materials-handling pro-

blems relative to transferring the sheet from the oven to

the forming station. Conduction heating requires contact

1228 THERMOFORMING

of a heated surface to the thermoplastic sheet. Although

conduction heating by contact is more efﬁcient than con-

vection heating, it has unique challenges of material

handling such as thermoplastic sticking to heated surface.

Conduction heating is used in applications where thermo-

plastic requires relatively less heat to form or as prehea-

ters where contact surface temperatures are relatively

lower. Therefore, many thermoforming machines are

equipped with systems that take advantage of radiation,

convection, and conduction. Most of these ovens are

electrically heated, but there are also a large number of

cut-sheet machines using gas-ﬁred ovens. Most are

capable of heating the sheet from two sides, which is

especially advantageous if the sheet is X40 mil

(X1 mm). The ovens should also be appropriately zoned

and temperature-regulated by instrumentation enabling

the operator to maintain good control over the heating

process.

Today, any number of electrically powered heating

systems can be speciﬁed, including the conventional tub-

ular steel rods, glass or ceramic panels, quartz lamps,

emitter-strip panels, and small ceramic modules. The

most common continues to be the calrod type, but use of

rectangular ceramic elements has been steadily increas-

ing. Their relatively small size permits them to be incor-

porated into elaborate microprocessor-control systems to

provide preferential heating in localized areas. Emitter

strip panels have been introduced as an alternative to

provide temperature uniformity with a less-complex in-

stallation. They are gaining popularity because they

provide a full-area heat source and can be mounted closer

to the sheet and are operated at lower temperatures,

thereby conserving energy.

Conduction or contact heating, sometimes called

trapped-sheet heating, has been used successfully with

easy-to-form materials such as polystyrene (PS) and PVC.

Here, the plastic sheet is trapped between the mold and the

temperature-controlled hot plate, which is perforated with

extremely small holes [i.e., B0.020-in. (0.5-mm) diameter].

The holes are drilled in a grid pattern on p1-in. (2.5-cm)

centers. After the sheet has been in contact with the heated

plate for a predetermined time, compressed air is injected

through the plate, forcing plastic off the heated platen and

into the mold. This system is generally reserved for mate-

rials p0.015 in. (0.38 mm) thick.

Solid-Phase and Conventional Thermoforming

Solid-phase pressure forming (SPPF) means forming

below the crystalline melting point—that is, generally

5–8% lower than melt-phase forming, depending on the

material. For example, homopolypropylene (hPP) is

formed in the melt phase at 310–3151F (154–1571C) but

at 285–2951F (141–1461C) in SPPF. SPPF does not require

special thermoforming equipment but demands much

tighter process control. A thermoforming machine

forms products within the thermoforming window, and

product-conﬁguration requirements (i.e., stress, strength,

rigidity, ﬂexibility) determine the proper temperature

range.

Figure 1 indicates that SPPF is done in a temperature

range within the window, as in melt-phase forming.

Compared with products formed in the meltphase, SPPF

can produce stiffer parts with lower part weight. Solid-

phase forming improves the sidewall strength of a con-

tainer and increases the stress factor. When forming cups,

trimming in the mold is recommended. Plug assist and

high form air pressures [i.e., 100 psi (689 kPa)] are re-

quired, and the plug design is more critical than in

conventional melt-phase forming.

Forming

There are two basic forming methods from which all

others are derived: (1) drape forming over a positive

(male) mold and (2) forming into a cavity (female) mold.

Product conﬁguration, stress and strength requirements,

and material speciﬁcations all play a part in determining

the process technique. Generally, forming into a female

mold is used if the draw is relatively deep (e.g., cups).

Female molds generally provide better material distribu-

tion and faster cooling than do male molds. Malemold

forming is preferred for certain product conﬁgurations,

particularly if product tolerances on the inside of a part

are critical. Male-mold forming produces heavier bottom

strength; female-mold forming produces heavier lip or

perimeter strength. An advantage of straight forming

into a female mold is that parts with small sidewall angle

(near vertical) can be formed and extracted, stress-free,

from the molds because of the shrinkage that occurs as the

part cools.

In drape forming, when the hot plastic sheet touches

the mold as it is being drawn, it chills and start to set up.

To successfully drape-form, several variables must be

considered. One of the most signiﬁcant is shrinkage. Since

all plastic materials have a high coefﬁcient of thermal

expansion

and contraction (i.e.,

about 7–10 times that of

steel), care must be taken when designing the mold to

provide sufﬁcient draft on the sidewalls so the part can be

extracted from the mold. It is not unusual for parts to

rupture on cooling on an improperly designed drape mold.

Another potential problem is that the part may become so

highly stressed during forming and cooling that it loses

most of the physical properties that the sheet would

otherwise provide. High stress in the part may also cause

shape distortion also known as warpage.

Natural-process evolution has combined the two sys-

tems to take advantage of the better parts of each method.

The plug-assist process, similar to matched-die forming,

involves a male mold (of plug) that ranges in size from

B60% to 90% of the volume of the cavity. By controlling

Figure 1. Thermoforming window.

THERMOFORMING 1229

the geometry and size of the plug and its rate and depth of

penetration, material distribution can be improved for a

broad range of products. The plug-assist technique is used

to manufacture cups, containers, and other deep-draw

products.

Many thermoforming techniques have been developed

to obtain better material distribution and broaden the

applicability of the process. Some of the more popular

methods are illustrated and described in Figures 2–10.

Most of these techniques can employ vacuum, pressure, or

a combination to apply the force necessary to shape the

heat-softened plastic sheet.

Cooling

The time required to cool the heat-softened plastic below

its heat-deﬂection temperature while it is in contact with

the mold is often the key to determine the overall forming

cycle time. Cooling is accomplished by conductive heat loss

to the mold and convective heat loss to the surrounding

air. Cooling rate depends on the tooling, because in all

methods except matched mold, the plastic is in contact

with the mold on one side only. Time duration for which

the mold is closed after forming signiﬁcantly affects cool-

ing. The opposite side of the thermoformed part is cooled

convectively by forced air or ambient air. Water sprays are

sometimes used but often pose as many problems (e.g.,

water spotting) as they solve. Pressure forming helps

minimize cooling time because the higher air pressure

keeps the sheet in more intimate contact with the mold

surface.

Trimming

A number of trimming methods are available, including

hand cutting, rough shearing of the web, punching parts

out from the web, or trimming around a periphery with a

steel rule or sharp-edged die used to penetrate the web.

The punch-and-die trim provides the greatest accuracy

and longest life, but at the highest cost. These dies are

usually designed with a hardened-steel punch and will

pass through a slightly softer steel die that can be peened

when it dulls. These trim dies are designed so the parts

are punched through the die successively and exit from

the trim station in a nested fashion. This simpliﬁes the

material handling required to pack or prepare the parts

Figure 2. Shallow-draw forming, female-mold bottom. (a) The plastic sheet is clamped and heated to the required forming temperature;

(b) a vacuum is applied through the mold, causing the plastic sheet to be pushed down by atmospheric pressure—contact with the mold

cools the newly formed plastic part; (c) areas of the sheet reaching the mold ﬁrst are the thickest. Applications: ice-chest lids, tub

enclosures, and so on.

Figure 3. Vacuum (drape) forming. (a) The plastic sheet is

clamped and heated to the required forming temperature; (b)

the sheet is sealed over the male mold; (c) vacuum from beneath

the mold is applied, forcing the sheet over the male mold and

forming the sheet. Material distribution is not uniform through-

out the part, because the portion of the sheet touching the mold

remains nearly the original thickness. The walls are formed from

the plastic sheet between the top edges of the mold and the

bottom-seal area at the base. Applications: trays, tubs, and so on.

1230 THERMOFORMING

for the next postforming operation (e.g., rim rolling or

printing).

Steel-rule dies are better suited for the lower-volume,

thin-sheet operations. Steel-rule trim dies are used pre-

dominantly in custom thermoforming operations, where

the run size does not justify the cost of matched shearing

dies. They are used almost exclusively in the manufacture

of blister packages and decorative packaging for the

cosmetics industry.

The trimming operation can be done inside or outside

the mold. The remote-trim conﬁguration utilizes a sepa-

rate trim press, which punches out one or more rows of

parts at a time. The parts are discharged in nested stacks

ready for subsequent operations or for packing. Trimming

within the mold provides the most accurate alignment of

the trim cut to the formed part, but the close proximity of

the trimming to the mold body is a complex arrangement

that may limit cooling capacity near the cutting surface;

and after trimming, precise control of the part ceases.

Complex mechanisms are required to sort and nest these

parts downstream. Trimming is done by sawing or routing

in the production of low-volume odd-geometry industrial

parts (e.g., boat hulls, recreational vehicle bodies, bath-

tubs) but not in packaging.

Forming Machines

There are basically two types of thermoforming machines:

sheet-fed and web-fed (roll-fed). Sheet-fed machines oper-

ate from sheet cut into deﬁnite lengths and widths for

speciﬁc applications. The sheet is generally heavy [i.e.,

0.060–0.5 in. (1.5–13 mm)] and the products are industrial.

Packaging-related applications include dunnage trays and

pallets, large produce bins, shipping-case dividers, box

liners, crates, and carrying cases formed to the shape of

the product.

Packaging applications generally employ web-fed ma-

chines, which use either coil stock or a web that comes

directly (i.e., in-line) from a sheet extruder. The need to

form a coil limits the thickness of the web to B0.125 in.

(3.2 mm) due to sheet winding and reheating issues. Web

thickness can be increased by operating directly in line

with an extruder.

TOOLING

Among the plastic-fabricating processes, thermoforming

permits the use of the broadest range of mold-tooling

materials. Mold materials such as wood, epoxy, polyesters,

or combinations thereof can be used if the volume is

not sufﬁcient to warrant the cost of advanced, tempera-

ture-controlled metal molds or for prototyping. For high-

volume applications (e.g., food packaging) molds are

normally manufactured from cast or machined aluminum,

sometimes with hard coats applied to the wearing surfaces

for longer service life. Molds are occasionally made from

beryllium copper or brass that is chrome-plated for ex-

tended service life.

Figure 4. Matched-mold forming. The plastic sheet is clamped

and heated to the required forming temperature. The heated

sheet can be positioned over the female die, or draped over the

male mold. The male and female halves of the mold are closed,

forming the sheet, and trapped air is evacuated by vents located

inside the mold. Material distribution varies with mold shape.

Detailed reproduction and dimensional accuracy are excellent.

Applications: foam products, egg cartons, meat trays, and so on.

Figure 5. Pressure-bubble/plug-assist vacuum forming. The plastic sheet is clamped and heated to the required forming temperature. (a)

The heated sheet is positioned over the female mold and sealed. (b) Prestretching is accomplished by applying controlled pressure through

the female mold, creating a bubble. (c) When the sheet is prestretched to the desired degree, the male plug is forced into the sheet. (d)

When the male plug is fully engaged, a vacuum is applied through the female mold to form the sheet. Pressure may be applied through the

plug to assist forming, depending on the material and forming requirements. Applications: refrigerator liners, bathtubs, and so on.

THERMOFORMING 1231

Vacuum and compressed air are the primary means

used to form the heated, thermoplastic sheet to the mold

conﬁguration with rigid-type materials. For foam materi-

als, matched metal molds are frequently used to compress

the material to the desired thickness without compressed

air. Vacuum is transmitted through small holes that are

located to allow the sheet to come in intimate contact with

the mold surface. The diameter of these holes usually

ranges from B0.013 to 0.030 in. (B0.33 to 0.76 mm),

depending on the type of thermoplastic to be formed.

Other means of applying vacuum in the mold include

the incorporation of narrow grooves or slots, which often

are easier to incorporate than vacuum holes and allow

greater evacuation of the air in the tool. Polyoleﬁn materi-

als require small vacuum holes and slots; PS materials,

which cool faster, can use larger holes. Final product

design and material selection determine the exact size of

the holes required.

In addition to vacuum holes and slots, molds can be

sandblasted to minimize the possibility of trapped air

between the sheet and the mold. Sandblasting molds

also results in mat surface ﬁnish and better cooling due

Figure 7. Vacuum snapback. ( a) The plastic sheet is heated and sealed over the female vacuum box; (b) when vacuum is applied to the

bottom of the vacuum box, the plastic sheet is prestretched into a concave shape; (c) after the plastic sheet is prestretched to the desired

degree, the male plug enters the sheet. A vacuum is applied through the male plug while the vacuum box is vented, creating the snapback.

Light air pressure may be optionally applied to the vacuum box, depending on the materials used and forming requirements. Applications:

deep-draw parts (e.g., luggage, auto parts), gun cases, cooler liners, and so on.

Figure 6. Plug-assist forming sequence: (a) The plastic sheet is clamped and heated to the required forming temperature—the sheet is

then sealed across the female-mold cavity; (b, c) the male plug is forced into the sheet—the depth and speed of penetration, as well as plug

size, are are the primary factors in material distribution; (d) when the plug is fully engaged, vacuum, pressure, or a combination are

applied to form the sheet. Applications: cups, food containers, and so on.

1232 THERMOFORMING

to higher surface area. Eliminating trapped air is impor-

tant because it prevents imperfections on the part surface

and aids in rapid transfer of heat out of the material by

providing close contact between the plastic and the cavity.

Molds for PS are normally not sandblasted because this

material is relatively rigid and sandblasting would make

part removal difﬁcult. Vacuum is usually pulled through

the forming mold, male or female, and the compressed air

is introduced from the opposite side of the sheet material.

In foam-forming with matched metal molds, vacuum is

normally applied only through the female cavity.

Tooling for high-production applications is most often

aluminum because of its thermal conductivity, light

weight, and cost. Most crucial is thermal conductivity,

because in any thermoforming process the residual heat of

the plastic must be removed as rapidly as possible for

rapid cycling. The temperature control of the mold is

accomplished by water channels in the mold and design-

ing them to provide maximum heat removal from the

sheet. Ideally, all molds should provide a temperature

differential between inlet and exit water temperature of

no more than 51F(31C).

Many techniques are used to incorporate cooling in the

molds for improved cycle times. The most common meth-

ods are contact cooling and direct water cooling. Contact

cooling consists of coring a mold base and carefully

machining cavity inserts so that contact occurs when the

cavity insert expands from the heat of the plastic sheet

while the mold base contracts from the coolant circulating

through it. The most common method today for rigid HIPS

is to circulate water through hollow-shelled cavities. This

is more expensive but is the most effective way to cool the

plastic sheet rapidly.

Tooling for low-volume work (e.g., box inserts, blisters,

and other nonfood packaging) and prototyping can often

be simple and effective without any of the special cooling

features described above. This is because it normally takes

longer to heat the sheet than to cool it, and the work is

generally done on slow, low-production equipment that

does not require the rapid cycle rates. With these molds,

cooling is very often augmented by the use of fans to

increase the air circulation over the material after it is

formed. In such applications, low-cost, simple molds of

plastic, plaster, wood, or other materials are used

Figure 8. Pressure-bubble vacuum snapback. (a) The plastic sheet is clamped and sealed across the pressure box; (b) prestretching is

accomplished by applying controlled air pressure under the sheet, through the pressure box, creating a bubble; (c) when the sheet

is prestretched to the desired degree (i.e., 35–40%), the male plug is forced into the sheet while the air pressure beneath the sheet remains

constant; (d) after the male plug is fully engaged, the lower air pressure is increased. A vacuum is simultaneously applied through the

male plug, creating the snapback. Applications: intricate parts where material distribution is critical.

Figure 9. Trapped-sheet, contact-heat, pressure forming; the plastic sheet is clamped between the female-mold cavity and a hot blow

plate. (a) The hot, porous blow plate allows air or vacuum to be applied through its face; (b) the mold cavity seals the sheet against the hot

plate; (c) controlled air pressure, applied from the mold, blows the plastic sheet in contact with the hot plate to ensure complete contact

with the heating surface; (d) when the desired heating has been accomplished, air pressure is applied through the hot plate, forcing the

plastic sheet into the mold. Venting of the mold can be simultaneous, depending on materials and forming requirements. Steel knives may

be inserted in the mold for sealing and in-place trimming if additional closing pressure is available. Applications: OPS containers, covers,

and so on.

THERMOFORMING 1233

effectively. Mold dimensions must allow for the different

shrinkage rates of different types of thermoplastics. In the

manufacture of a family of products, it is sometimes

possible to utilize the same cooling base and interchange

only the molding inserts. Depending on the trim dimen-

sions of the products, the same cutting tool might be

suitable for more than one product; if not, they can be

designed to be easily interchanged from the trim die shoe.

CUT-SHEET THERMOFORMING

Cut sheet is processed on two types of thermoforming

machines: the shuttle style and the rotary, or carousel,

style. In single-station shuttle machines, the plastic sheet

is placed in a clamping frame that shuttles into the oven

for heating and back again for forming. Another very

popular shuttle machine is the double-ender, which has

a common oven with a form station at each end. This

arrangement provides 100% utilization of the oven be-

cause heating and forming can be done simultaneously.

Rotary or carousel thermoformers are most frequently

used for heavier-gauge materials—that is, starting at

B0.050 in. (1.3 mm). Most rotary machines have three

working stations: load/unload, heat, and form. In a typical

machine, the carousel frame rotates 1201 on a time-con-

trolled basis. Generally, the time cycle is dictated by the

period required for the plastic sheet to cool below its heat-

deﬂection temperature. When cooling time is faster than

the heating time, four-station rotary machines are often

used; these have two heating stations. Another type of

four-station machine has a separate load and unload for

automation and parts handling.

Most cut-sheet thermoformers are designed to operate

as vacuum formers, but rotary and single-station (shuttle-

type) machines are available with pressure-forming cap-

abilities and sustain forming pressures up to 50 psi

(345 kPa), depending on mold area. Most sheet-fed ma-

chines can be adapted for any of the standard thermo-

forming techniques. Cut-sheet machines have been built

to accept sheet 10 ft (3.05 m) wide and 20 ft (6.1 m) long

with the capability of making parts up to 4 ft (1.22 m) deep.

TWIN-SHEET THERMOFORMING

Twin-sheet thermoforming can produce hollow parts (e.g.,

pallets) from cut sheet. These parts can be produced from

both cut-sheet and roll-fed machinery. In sheet-fed machin-

ery, the material to be formed can be 0.050–0.500 in. (1.3–

13 mm) thick. Roll-fed machines are limited to materials

0.005–0.125 in. (1.3–3.2 mm) thick. With a typical web-fed,

twin-sheet system, two rolls of plastic materials are simul-

taneously fed, one above the other. The webs are trans-

ported through the oven on separate sheet-conveyor chains

and heated to a formable temperature. At the forming

station, a specially designed blow pin enters the space

between the two sheets before the mold closes. Air pressure

is introduced between the sheets through the blow pin,

and, at the same time, vaccum is applied to each mold half.

Figure 10. Twin-sheet forming. This technique utilizes a rotary-type machine with two heating stations and bottom-load clamp frames.

The ﬁrst sheet (a) is heated and formed into the lower half of the mold (b, c) while the second sheet is being heated. After forming the ﬁrst

sheet, the second sheet is indexed into the forming station (d) and vacuum is applied to the upper mold half to form the sheet (e). A bond is

simultaneously made between the parts by pressure applied to the mating surfaces (f). After cooling, the molds are removed to give the

product (g).

1234 THERMOFORMING

Twin-sheet forming is done by a slightly different method

on specially designed rotary thermoformers.

EQUIPMENT IMPROVEMENTS

Rapidly expanding markets for thermoformed products

are attributable to technological advancements in equip-

ment design. Most of the development is focused on

continuous roll-fed machines for packaging. New produc-

tivity enhancements such as bigger machines, faster

cycles, quicker changeover, and greater automation are

gaining in importance. Better process control has enabled

converters to increase productivity and improve product

quality. Use of microprocessor-control systems allows the

operator to enter data, examine existing values in the

program, or troubleshoot problems by viewing a monitor.

The conventional control panel includes a cathode-ray

tube (CRT) and keyboard, form-on and emergency stop

push buttons, and indicator lights to signal form or

automatic modes. Machine functions may be displayed

on the CRT upon request, and function control is obtained

by means of the simpliﬁed keyboard. The newer systems

use touch screen LCD panels.

The microprocessor-control systems feature control of

oven-temperature orientation in multiple zoned ovens,

interface speed between former and trim press, sheet-

index length, machinery functions and timing, and control

and graphic display of start end points of forming func-

tions, storage of production information, and diagnostic

capabilities. Use of the microprocessor results in tremen-

dous savings in labor, energy, and downtime. Many of

these systems are available as a retroﬁt to existing equip-

ment. Equipment has also been designed to monitor

several lines of similarly equipped machines from one

central location. Such data-management systems provide

comparative data, setup parameters, production rates,

maintenance schedules, alarms and safety signals, and

storage of reference information, resulting in a true on-

line management-information reporting system.

Design improvements that have contributed to im-

proved productivity include redesigned forming presses

that provide front and rear access to the forming station to

facilitate tooling installation and periodic maintenance;

access doors to oven heaters for quick cleaning; ovens that

eject automatically if the sheet overheats; and automatic

lubrication systems that increase component life substan-

tially and allow faster mechanical operation due to re-

duced friction.

Microprocessor heating control allows the operator to

control the temperature proﬁle of the sheet from edge to

edge or to provide localized heating. Solid-state instru-

ments used in conjunction with mercury or semiconduc-

tor-controlled rectiﬁer (SCR) relays have made it possible

to maintain extremely close heater temperature control.

Forming presses are now available with precision

guidance systems on the platens to permit critical opera-

tions to take place in the forming molds that formerly had

to be done on auxiliary equipment. An example is the

method of incorporating the latching slots that are cut in

the sidewalls of egg cartons, fast-food cartons, and trays.

Previously, these slots were punched in on a special press

located downstream from the forming station. Registra-

tion problems relative to the slot location were often

encountered because of variations in the index length by

the thermoformer web-conveyor system. These problems

were overcome by expensive drive systems. On machines

with precision-guidance systems, mating punches can be

incorporated in the forming molds that engage during the

last 5–10% of press closure and form a slot in the heat-

softened plastic that has reinforced edges similar to

button holes.

High-speed trim presses are available with supported

moving platens that carry up to 85% of the gross-platen

and trim-tool weight. The advantage of this system is that

it permits the press tie rods to function primarily as

guidance members rather than as both platen transpor-

ters and guides, in addition to serving as tie rods. The

system reduces trim-tool maintenance by up to 50%.

Signiﬁcant improvements have been made in product-

handling systems, which can now automatically count,

stack, and package products at the trim-press discharge.

The design of these automated systems varies from rela-

tively simple systems using guide chutes or magazines

and transport conveyors to those employing robotics and

advanced packaging machinery.

Automation has been extended to the materials hand-

ling of large plastic sheets into sheet-fed machines. The

development of automatic sheet loaders provides auto-

matic loading systems that enable use of the entire

machine area. This results in a 20–30% increase in

productivity, achieved largely from savings in loading

time, since a manually loaded machine normally used

only one-quarter of the machine effectively.

IN-MOLD LABELING

Unlike injection molding for which in-mold labeling (IML)

is a very well established process, thermoforming process

has challenged IML to address several important limita-

tions in machinery and labeling technology that. Use of

IML for thermoformed products is on the rise in recent

years with development of automation and labeling tech-

nology. Even with the current improvements in output,

there still remains a gap between IML thermoforming

versus the productivity of conventional thermoforming.

For example, IML slows down the process reducing output

to 20 cycles/min versus 40 cycles/min for standard thermo-

forming of PP cups. Most of the current machines for IML

incorporate tilting-mold technology, which provides

greater accessibility and easier label placement. Another

new technology for IML uses a dual-mold conﬁguration

instead of a tilting mold. The dual-mold system allows

removal of the ﬁnished part and placement of the label in

one mold while a part is being formed in the other mold.

In-mold labeling (IML) is a technique that typically

uses a heat-activated adhesive applied to the paper or ﬁlm

label backing. The in-mold label is placed in a mold and

held in place by vacuum, electrostatic attraction, or other

means

. The mold

is then closed and the part is produced as

normal. The adhesive’s activation takes place during the

THERMOFORMING 1235

manufacture of the packaging itself via thermoforming.

The label becomes molded to the container wall. The

material used for in-mold labels is generally paper or

ﬁlm. It must have sufﬁcient temperature resistance and

stiffness to resist the rigors of the molding operation. The

adhesive that is utilized is generally heat-seal coating

adhesives. Depending on the packaging and labeling

materials, the in-mold label may not even require an

adhesive. The container resin itself will, in certain cases,

provide the adhesive material necessary to ﬁrmly and

permanently attach the label.

One of the advantages of IML is that the label appears

as part of the container compared to a label that is applied

by conventional (e.g., pressure sensitive adhesives) tech-

niques and appears above the surface of the container.

Thus, the in-mold label has greater consumer shelf appeal.

IML also eliminates the need, equipment, and labor

required of a separate labeling process. Other advantages

of IML include reduction in container weight (the IML

provides structural as well as decorative value, allowing

as much as 10–15% reduction in package weight) to

provide improved container sidewall strength and better

squeeze resistance.

High-density polyethylene (HDPE), polypropylene

(PP), polyester, and polyvinyl chloride are the most com-

mon container resins that are used with in-mold labeling.

Among the wide variety of consumer products packaged

with in-mold labels are laundry products, household clea-

ners, personal care products, automotive products, agri-

cultural and garden products, food, and beverages.

BIBLIOGRAPHY

V. A. Chougule and M. N. Piercy, ‘‘Thermoforming’’ in K. L. Yam,

ed., The Wiley Encyclopedia of Packaging Technology, 3rd

edition, John Wiley & Sons, Hoboken, NJ, 2009, pp. XYZ–XYZ.

L. McKinney, W. Kent, and R. Roe, ‘‘Thermoforming’’ in The Wiley

Encyclopedia of Packaging Technology, 1st edition, John Wiley

& Sons, New York, 19XX, pp. 668–675.

General Reference

‘‘Thermoforming Techniques’’ in J. Agranoff, ed., Modern Plastics

Encyclopedia, McGraw-Hill, New York, 1990.

TIME TEMPERATURE INDICATORS

R. PAUL SINGH

Department of Biological and

Agricultural Engineering,

University of California, Davis,

California

Temperature is one of the key environmental factors

inﬂuencing the storage stability of a food product. For

most foods, an increase in storage temperature causes a

reduction in their storage life. To determine the deleter-

ious effects of temperature, it is often necessary to know

for how long a food is exposed to that temperature. The

cumulative effects of temperature and time cause irrever-

sible changes in foods. In the commercial food-distribution

chain, speciﬁc guidelines for temperature management

have been developed. However, when these recommended

temperature guidelines are not met, food quality is ser-

iously impaired. Time-temperature indicators (TTIs) are

devices that provide useful information regarding the

temperature history experienced by a food during storage

and distribution.

A food material may undergo physical, chemical, or

microbiological change during storage. When these

changes accumulate to such an extent that they exceed

some predetermined criterion of acceptable quality, the

food is considered to have reached the end of its shelf life.

A variety of changes such as color, ﬂavor, and texture may

lead to the end of a food’s shelf life. Similarly, the micro-

biological changes may render a product unsafe for human

consumption. In such cases, a monitoring system that

allows an early warning of the approaching undersirable

increase in the microbial level would be necessary. The

common practice in the modern food-distribution system

is to use some type of open dating that indicates the

expiration of the shelf life of a product. The open-dating

policy assumes that a product has been kept at the

recommended temperature. However, if the temperature

during storage and distribution exceeds the recommended

conditions, the open-dating policy fails. In these situa-

tions, TTIs are more suitable predictors of product’s shelf

life (see also Shelf life).

A TTI afﬁxed to a food package undergoes a tempera-

ture history similar to that of food. The indicator response

is measured through some physical means, or a certain

property of an indicator may change, causing a visually

discernible response. The indicator response may be a

change in its color such as green to yellow or a change in

light reﬂectance measured with special instruments, or

there may be an advance of a boundary between two

different colors that is observed visually. When the change

in the property of an indicator is irreversible and cumu-

lative with time and temperature exposure, the indicators

are referred to as time-temperature integrators, full-his-

tory TTIs, or all-temperature TTIs. For certain applica-

tions, the indicators may be designed to respond only

when the food is exposed to temperatures above some

preselected threshold level, thus giving a partial history of

the storage period. These types of indicators are called

partial history TTIs or threshold-temperature TTIs. The

TTIs respond at a rapid rate when the temperature goes

up, and their response slows down as the temperature

decreases.

For a TTI to correctly predict the end of the shelf life of

a food, it must be able to accurately mimic the kinetics of

the product quality. Therefore, the ﬁrst step in the selec-

tion of a TTI is to identify one or more quality attributes

of a food that are the key determinants of the shelf life

of a food. The next step is to quantitatively determine

their kinetics. For example, if a food material undergoes

an undersirable change in color due to storage at

1236 TIME TEMPERATURE INDICATORS

temperatures higher than ideal, and the off-color makes

the food unacceptable, then the kinetics of color change in

the food, as inﬂuenced by temperature, must be known (1).

The kinetics of quality change in a food material is

usually expressed by the rate constants. For example, if

the change in the attribute is described by a ﬁrst-order

reaction, then the magnitude of the ﬁrst-order rate con-

stant will indicate how rapidly the attribute may be

undergoing change at that temperature. The rate con-

stants are usually determined for at least three different

temperatures and then combined into the Arrhenius

equation to relate rate constants to temperature. Another

common way to express shelf-life kinetics is to use the

Arrhenius expression as follows:

¼ Ae



where t

is the shelf life (days or months), A is a constant

with same units as t

, T is temperature (K or 1C + 273), R is

a constant (0.001987 kcal/(mole degrees), and E

is the

activation energy (kcal/mol). If the shelf life of one tem-

perature (T

) is known, the above equation may be used to

estimate the shelf life of the same product at a different

temperature (T

) as follows:

¼ t





From the Arrhenius equation, one obtains the activa-

tion energy of a reaction or shelf life. The Arrhenius

equation is also used to predict the shelf life at any

unknown temperature. The shelf-life data and the activa-

tion energy are the key design parameters for a TTI. It is

absolutely essential that the activation energy of a TTI

match the activation energy of the quality attribute of

concern. This equality in kinetic parameters allows the

TTI to reach the same level of change as experienced by a

food material when exposed to identical storage time and

temperature.

The commercially available TTIs use either a biochem-

ical reaction between an enzyme and a substrate (Figure 1),

a polymerization reaction in some material resulting in a

highly colored polymer (Figure 2), or diffusion of a dye

along a wick (Figure 3). In each case the reactions are

temperature-dependent and the rates of reactions are care-

fully controlled by controlling the concentration of the

indicator’s constituents. Some of the indicators are acti-

vated at the time of manufacture, requiring their storage at

ultralow temperature until they are afﬁxed to a product

package. Other indicators are activable at any desired time,

such as when the TTI is ﬁxed on a package at the start of

the distribution chain.

TTIs differ in how they display their response to time

and temperature. A TTI based on biochemical reactions

involves a pouch containing an enzyme and a substrate that

are initially kept separate by a thin seal. At the time of

activation, the seal separating the pouch contents is broken

and the contents are mixed. The pouch is made of a

transparent ﬁlm and is placed inside a plastic sleeve with

a window. As the enzymatic reaction in the pouch proceeds

as a result of the combined effect of temperature and time,

the pH of the mix changes, resulting in a change in color.

The user observes the color of the pouch’s contents to

determine the time–temperature exposure. In another

type of TTI, the movement of a dye along a wick is

indicative of time and temperature exposure because the

diffusion process is temperature-dependent. A rectangular

wick is typically sandwiched inside an outer sleeve that

contains viewing holes and an appropriate scale. The

advance of the dye along the wick is visually observed

and related to the extent of the reaction. Another method

used to display the indicator response is the use of a bull’s-

eye concept. In this case an indicator is located at the inner

circle, while a surrounding ring is painted with a color that

is eventually reached by the indicator when the shelf life of

the product is deemed to have expired. In this type of a TTI,

a user compares the color of the indicator seen in the center

with the color of outside ring(s) as shown in Figures 1 and 2.

TTIs are useful in implementation of inventory man-

agement policies other than the commonly used ﬁrst-in-

CheckPoint

III

Temperature

Label

www.vitsab.com

INACTIVE

2.00

< The word ACTIVE must appear here

DO Not Use

If Circle Is

PINK

1.00

ACTIVE

.75

"Activator Label"

Figure 1. Check point III

temperature label (dimensions in

inches) (http://www.vitsab.com/Products2.htm).

Sensor

If color of "ACTIVE" large circle

matches color of small circle,

TT Sensor

is expired

= Expired

TT Sensor

is a time temperature indicator

35-F

ACTIVE

Figure 2. Time–temperature indicator label (http://www.global-

spec.com/FeaturedProducts/Detail/TIPTemperatureProducts/

TimeTemperature_Indicator_Labels/32742/0).

TIME TEMPERATURE INDICATORS 1237