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with hot water and also serves the function of cleaning

the jars or bottles. Cleaning is particularly important

with glass, as glass is often received in corrugated

shipping and, thus, may be contaminated with carton

dust. After cleaning and preheating, glass containers are

ﬁlled with hot product, ranging from approximately

170 to 2001F (77–931C). A small headspace is maintained.

The headspace is usually ﬂushed with steam in the capper.

Metal twist-on, lug-style caps are usually used. These caps

contain a gasket compound used to seal the jar or bottle.

The caps are preheated in the cap shoot to soften the

compound. A small initial vacuum is generated directly

following the capper by the condensation of the steam in

the headspace. This initial vacuum varies depending on

the size of the headspace and the amount of steam in

the headspace. An initial vacuum of at least 4–5 in. Hg

(10.2–12.7 cm of Hg) is necessary prior to cooling to ensure

that the seal between the glass jar or bottle and the cap

compound is secure prior to generating the larger vacuum

created by product contraction as it cools. Conveyors are

designed to allow for at least a 60-s hold after capping

prior to entering the cooler. This hold provides time for the

hot product to sterilize the jar or bottle. The ﬁrst zone of

the cooler uses a preheated water spray to prevent glass

breakage due to thermal shock. The 751F (241C) tempera-

ture differential, important in the preheating operation,

is also critical in cooling. The water-spray temperature

should be no more than 751F (241C) less than the product

temperature. Often the cooler is designed such that cold

water enters the discharge end of the cooler and is

transported to the infeed end. Thus, the water is warmed

as it cools the product throughout the cooler, which

prevents thermal shock glass breakage while minimizing

energy cost. The product temperature at the end of the

cooler is usually 90–1101F (32–431C). The ﬁnal vacuum

depends on the viscosity characteristics of the product

and on the size of the headspace, but usually it is 16–22 in.

Hg (41–56cm Hg).

As mentioned, barbecue sauce is not cooled after ﬁlling.

Products such as this are packed in corrugated shipping

containers, palletized, and cooled over several days in

normal warehouse storage. As containers in the center of

the pallet are insulated by the product stacked on the

outside, top, and bottom, they may cool at a much slower

rate than product on the outside. To reduce the variation

in cooling rates, pallet patterns with ventilation chimneys

are used to facilitate heat removal from the center of the

pallet.

As hot-ﬁlled glass jars and bottles contain a vacuum,

they must be handled carefully after cooling. Glass con-

tainers, particularly those ﬁlled with highly viscose pro-

ducts, break as a result of a phenomenon commonly

known as ‘‘water hammer’’ breakage, which occurs when

a glass jar or bottle containing a vacuum is dropped or

otherwise impacted. On impact, a vacuum bubble forms

and collapses quickly. This results in the creation of a

hydraulic action that concentrates forces on any defect in

the container causing breakage (5).

Care must be taken to properly adjust drop case pack-

ers to prevent water-hammer breakage. Corrugated ship-

ping containers are designed to cushion the bottom of

glass jars and bottles to minimize the impact of dropping.

Partitions are used to minimize glass-to-glass impact. See

also Glass container design.

PLASTIC PACKAGES

Historically, the hot-ﬁll process has required packages

that can withstand the hot-ﬁll temperatures and are

sufﬁciently rigid to withstand the vacuum developed after

cooling without paneling or otherwise distorting. Until

recently, these packaging material requirements have

limited the use of materials for hot-ﬁll packages to metal

and glass. During the 1980s, packaging material and hot-

ﬁll technology developments occurred that allowed the use

of, not only lightweight aluminum cans as already dis-

cussed, but also plastics (6).

Many plastic package technologies were developed in

the 1980s that resulted in the commercialization of sev-

eral hot-ﬁll plastic packages that replaced traditional

glass or tinplate packages. These technologies included

plastic squeeze bottles with dispensing closures for

ketchup and barbeque sauce, plastic table-ready bowls

with peelable membrane seals for fruit sauces, plastic cans

with double-seamed metal ends for a variety of products,

and clear plastic bottles for fruit juices, juice drinks, and

isotonic sport drinks (see also Blow molding).

Poly(Ethylene Terephthalate). The late 1970s and most

of the 1980s was an era of rigid plastic food-packaging

development. Nearly all the major U.S. packaging compa-

nies were actively developing plastic packaging tech-

nologies. Several Japanese and European packaging

companies were also focusing their research and develop-

ment

activities on rigid

plastic package developments for

food and beverages. In the mid-to-late 1970s, polyethylene

terephthalate (PET) was commercialized for carbonated

soft-drink bottles. The initial application was the 2-L

carbonated soft-drink bottle. Carbonated soft drinks con-

tain preservatives and, thus, are cold-ﬁlled. They require

a high gas-barrier to retain CO

but do not require the

high heat resistance or physical strength necessary for

hot ﬁlling. PET, as used in soft-drink bottles, softens at

temperatures too low for hot ﬁlling or retorting. However,

the success in the market of PET soft-drink bottles proved

the consumer acceptance of plastic, and it conﬁrmed the

business potential that supported other major plastic food

and beverage R&D programs.

Acrylonitrile. At the same time as PET was being

developed for cold ﬁll, another plastic resin—acrylonitrile

(AN)—was under development for food and beverage

packaging. Although AN, like PET, was clear and provided

an adequate gas barrier for carbonated soft drinks and

oxygen-sensitive foods and beverages, it had a signiﬁ-

cantly higher melting point and, thus, could be hot-ﬁlled

and potentially even retorted. In the 1970s, AN was test-

marketed for carbonated soft drinks. In addition, several

hot-ﬁll containers were developed and test-marketed. A

squeeze bottle with a dispensing closure and an induction

aluminum-foil seal was test-marketed with barbeque

578 HOT-FILL TECHNOLOGY

sauce. At least two fruit-juice processors introduced into

test markets apple- and cranberry-juice products in

¼ fl ¼ oz (162.6-mL) AN cans with seamed-on metal

ends. Several other developments were under way. In

1977, the U.S. Food and Drug Administration FDA banned

the use of nitrile polymers in direct contact with beverages

because of the potential migration of AN monomer from

AN packages into beverages and the possibility that it

could be a carcinogen. AN was reapproved in 1984 with

limitations (7).

Polypropylene. Polypropylene containers were also

being developed for food and beverage applications in

this period. Polypropylene can be thermoformed or extru-

sion blow-molded and, thus, made into bowls, tubs, and

bottles. It has a high melting point and, thus, is well suited

for hot-ﬁll applications. Polypropylene is not crystal-clear,

like PET and AN, but it is semiclear (contact-clear). The

oxygen-barrier properties of the polypropylene are rela-

tively poor. This limited polypropylene’s application to

foods and beverages, until coextrusion technology was

developed in the 1980s to allow the addition of small

amounts of high-barrier materials to polypropylene

packages. Coextrusion technology resulted in the produc-

tion of multilayer extrusion, blow-molded bottles

and thermoformed containers containing 5 to 7 layers.

Figure 2 shows a typical 6-layer, hot-ﬁllable, high-oxygen-

barrier polypropylene container. The inner and outer

layers consist of virgin polypropylene. Ethylene—vinyl

alcohol (EVOH) is used in the middle layer to provide a

high-oxygen barrier. Polypropylene regrind is incorpo-

rated to use the scrape generated in the extrusion blow-

molding process. Two adhesive tie layers are required for

proper bonding of the EVOH to the polypropylene. Con-

tainers made of this material are hot-ﬁllable, are abuse-

resistant, and provide a high-oxygen barrier and contact

clarity. In the 1980s, several food processors modiﬁed

traditional hot-ﬁll processes to allow conversion to poly-

propylene/EVOH containers from glass and tinplate. Ex-

amples include 4-oz apple-sauce cups as well as 12–16-oz

cranberry-sauce containers, ketchup and barbecue sauce

bottles, and some fruit-juice containers. Most of these hot-

ﬁll packages were sealed with heat-sealed or induction-

sealed aluminum-foil laminates.

Heat-set PET. Polypropylene/EVOH co-extrusions pro-

vide an excellent hot-ﬁll food and beverage packaging

material with the possible shortcoming, for some products,

of a lack of glasslike clarity. Although PET provides the

clarity of glass, it was not hot-ﬁllable until advances in

injection blow-molding technology were made in the mid-

1980s. Heat-setting technologies were pioneered, primar-

ily by the Japanese, in the early-to-mid-1980s. More

advances were made shortly thereafter by U.S. PET bottle

manufacturers in conjunction with global injection and

blow-molding equipment suppliers. The ﬁrst commercial

application of hot-ﬁllable PET bottles, technologically

possible because of the heat-setting process, were for fruit

juice and juice drinks. In September 1985, 64-ﬂ-oz (1.89-L)

heat-set PET bottles were commercially introduced with a

line of cranberry-juice drink products using the Japanese

technology. Shortly thereafter, U.S. bottle manufacturers

commercialized alternative thermally stable technologies

for PET, and the commercial use spread beyond fruit

juices and juice drinks to isotonics. Ketchup also con-

verted from contact-clear polypropylene/EVOH bottles to

hot-ﬁllable PET/EVOH.

Heat-set, or otherwise thermally stable, PET provides

an adequate oxygen barrier for many products, excellent

abuse resistance, good recyclability characteristics, hot

ﬁllability up to 180–1851F (82–851C), and the clarity of

glass. By the mid-1990s, hot-ﬁllable PET became a pre-

valent and popular packaging material for several cate-

gories of hot-ﬁlled foods and beverages (e.g., fruit juices,

juice drinks, isotonics, and ketchup) that traditionally

were packed in glass.

SUMMARY

In summary, hot-ﬁll p rocessing and packaging is a basic

traditional method of preserving high-acid foods and

beverages. It has been practiced since the early days

of commercial food processing and before that in home

canning. It is one of the food-processing technologies

that has allowed nu tritious foods to be available year-

round in all parts of the nation. H istorically, its appli-

cation has been limited to rigid metal and glass

container s.

However, technological development of new materials,

new package manufacturing processes, and hot-ﬁll meth-

ods in the 1980s have resulted in the commercialization of

a variety of new hot-ﬁll packages that provide the product

protection beneﬁts of metal and glass with additional

consumer beneﬁts.

BIBLIOGRAPHY

1. N. N. Potter, Food Science, 4th edition, AVI Publishing, West-

port, CT, 1986, pp. 297–298.

2. Ibid., pp. 172–189.

3. Thermal Processes for Low-Acid Foods in Metal Containers,

Bulletin 26-L, 12th edition, National Food Processors Assoc.,

Washington, DC, 1982, pp. 22–56.

4. Ref. 1, p. 197.

Figure 2. A typical hot-ﬁllable polypropylene/EVOH/polypropy-

lene bottle.

HOT-FILL TECHNOLOGY 579

5. J. F. Hanlon, Handbook of Package Engineering, 2nd edition,

Technomic Publishing, Lancaster, PA, 1992, pp. 9–11.

6. R. A. Bourque, ‘‘Hot Fill and Aseptic Packaging,’’ Food PLAS

VI-89 Conference Proceedings, Hoboken, NJ, Plastics Institute

of America, Stevens Institute of Technology, 1989.

7. R. Juran, Modern Plastics Encyclopedia, Vol. 64, No. 10A,

McGraw-Hill, New York, 1988, pp. 34.

General References

M. Bennion, The Science of Food, Harper and Row, New York,

1980.

O. R. Fennema, M. Karel, and D. B. Lund, Principles of Food

Science, Part II, Physical Principles of Food Preservation,

Marcel Dekker, New York, 1975.

580 HOT-FILL TECHNOLOGY

INDICATING DEVICES

DEE LYNN JOHNSON

3 M Company

One summer in the 1930s, Eastman Kodak Co. received a

telegram from the Kitt Observatory in Arizona stating

that a shipment of photographic plates had no light

sensitivity. Each of these plates was developed completely

and was uniformly black. A new set of plates was carefully

packed and shipped to the remote observatory. Again, all

of the plates were black when developed. A well-respected

Pinkerton guard accompanied the next shipment. He

never let the box of plates out of his sight, and the mystery

was solved at a small siding in Arizona. The rail clerk

received the box clearly marked ‘‘FRAGILE GLASS

PLATES,’’ and methodically opened the box, took out

each plate in the bright afternoon sun, removed the black

paper cover, and examined each one. ‘‘All in perfect shape,’’

he said to the guard as he rewrapped the plates and

returned them to the carton.

A shipper must know what happens to a package after

it leaves the shipping room, but it is impossible to send a

security guard with every carton. Instead, a variety of

indicating devices have become the security guards for the

packages.

Perishable products may speak for themselves by their

physical condition or their odor. Damage to other products

from inappropriate handling may not be evident until

actual use. As an example, whole blood stored below

501F (101C) is stable for months. If the temperature rises

above that point for 20 min, however, enzymatic changes

could occur that would make it life threatening if trans-

fused. The potency of vaccines is lost by exceeding critical

temperatures; an even greater hazard may result by

presuming that it is effective. Because microcircuits are

sensitive to mechanical shock and electrical potential,

individual components and wired circuits must be

monitored.

TIME AND TEMPERATURE INDICATORS

The fully integrating monitoring device can indicate the

temperature gradient to which it has been exposed, as

well as the amount of time that it has been at that

temperature. Many inventors and their companies have

focused on developing such devices, which operate on the

following principles: physical change, chemical change,

electrochemical indication, electromechanical indication,

electronic readout, and others. The devices monitor per-

ishable foods such as ﬁsh, fruit, and vegetables, as well as

pharmaceuticals and vaccines. Table 1 lists the U.S. Food

and Drug Administration’s (FDA) temperature limits for

various biological preparations.

Indicators that show time only are also available in

many styles. These are used primarily on shipping con-

tainers, not on consumer packages. Life-dated products,

such as photographic ﬁlm and some foods, can have

greatly extended salable time by refrigerated storage.

A time indicator would beneﬁt the manufacturer and

retailer as well as the consumer.

The Andover Laboratories manufactures a time–tem-

perature integrator called the Tempchron, which is for-

merly known as the Ambitemp (see Figure 1), which

functions with a ﬂuid that has a speciﬁc melt temperature

for the product to be monitored. This device can be

described as an integrator because it provides information

Table 1. Biologics, Shipping Temperatures

Product

Maximum

Temperature, 1F(1C)

Cryoprecipitated antihemo-philic factor

o0(18)

Measles, mumps, and rubella virus

vaccine, live

o50 (10)

Measles and rubella virus vaccine, live o50 (10)

Measles-smallpox vaccine live o50 (10)

Measles virus vaccine, live, attenuated o50 (10)

Mumps virus vaccine, live o50 (10)

Poliovirus vaccine, live, oral, type 1 o32 (0)

Poliovirus vaccine, live, oral, type 2 o32 (0)

Poliovirus vaccine, live, oral, type 3 o32 (0)

Poliovirus vaccine, live, oral, trivalent o32 (0)

Red blood cells,

frozen o(85) (65)

Red Blood cells,

liquid 34–50 (1–10)

Rubella and mumps virus, live o50 (10)

Rubella virus vaccine, live o50 (10)

Single-donor plasma,

frozen o(18)

Smallpox vaccine, liquid o32 (0)

Source plasma

23 (5)

Whole blood

34–50 (1–10)

Yellow fever vaccine o32 (0)

Ref. (1).

Human.

Figure 1. Integrated time–temperature indicator. The Temp-

chron (Andover Laboratories) uses the melt temperature of the

product to be measured. To convert in. to cm, multiply by 2.54.

581

on the multiple of the time and temperature, giving a

readout in degree minutes that can be interpreted from a

chart. A wide range of temperatures can be monitored by

selection of the liquid that is to be frozen in the tube.

Figure 2 shows an interesting concept of a time–tem-

perature integrator (TTI) that was at one time manufac-

tured by Honeywell Corp. This device, comprised an

electrolytic battery that was activated by breaking an

ampul of electrolyte between copper and cadmium strips.

An electrochemical color reaction that was accelerated at

higher temperatures ensued.

Kokum Chemical in Malmo, Sweden, manufacturers a

time–temperature monitor (TTM) that uses an enzyme to

bring about a color change. This product (see Figure 3),

the I-Point TTM, is intended to match the enzyme reac-

tions of the product it is monitoring. A progression of color

changes occurs.

Another time–temperature integrator functions by the

use of a selective gas-absorption plastic membrane and

leuko (colorless) dyes that are sensitive to oxygen (2). The

leuko dye is converted to the colored form inside the pouch

in proportion to the rate of oxygen penetration through

the membrane and the time. The rate of penetration of the

oxygen through the membrane is also related to the

temperature. The storage life of foods for emergency

survival has been studied as a function of the tempera-

tures at which they were stored. With the accurately

calibrated barrier ﬁlms (see the Barrier polymers article)

now available, this may prove a valuable area of

investigation.

Allied Chemical Co. has a patent (3) on work in which

combinations of conjugated acetylene compounds

irreversibly change color when heated. After ﬁnalizing

the above design, the Program for Appropriate Technology

in Health (PATH) in Seattle, WA., has marketed a product

called the PATHmarker, which indicates the change in

color.

There are two patents (4, 5) for time–temperature

integrators under the name Monitor Mark [see Figure

4(a)]. In this product, a dyed meltable solid travels down a

porous wick when heated above its melting point. The

time interval is a function of the travel of the color down

the wick. The device is activated by removal of a barrier

ﬁlm that is positioned between the wick and the meltable

solid [see Figure 4(b)]. The product is available in several

response temperature ranges.

The Therma-Gard recorder (Impact-O-Graph Corp.)

monitors temperatures on a cassette and functions for

30 days. The TSI International Corp. has introduced a

recorder that operates for up to 90 days in the indicating

range of  20 to 1001F(29 to 301C).

Two workers (6), following the lead of another (7), have

fabricated two styles of electronic recorders with built-in

memories (see Figure 5); however, they are not commer-

cially available. Workers at 3 M Co. have invented a

Thermal History Indicating Device (THID)*. Another

product that interfaces with a computer for its readout

is the Tattletale thermograph (Onset Computer Co.) (see

Figure 6). This thermograph reports temperature transi-

ents to a memory and can be programmed to record at 5-s

intervals for 5.5–6-h intervals for 32 mo. The functional

temperature range is 41 to 1851F(41 to 851C).

TEMPERATURE INDICATORS

Temperature-measuring devices are numerous. In the

simplest form, a sphere of ice is frozen with one half clear

and the other red (see Figure 7). If the product thaws, the

Figure 3. The I-Point TTM (Kokum Chemical) is an integrating

time–temperature monitor utilizing enzyme-produced color

change. To convert in. to cm, multiply by 2.54.

Figure 4. (a) The Monitor Mark Time Temperature Tag (3 M

Co.), an integrator available in many temperature ranges. To

convert in. to cm, multiply by 2.54, (b) Cross section.

Figure 2. Integrated time–temperature indicator. The TTI (Hon-

eywell Corp.) used an ampul of electrolyte between metal plates.

To convert in. to cm, multiply by 2.54.

* The author and F. R. Parham, 3 M Co., Saint Paul, MN, have

been active in microprocessor monitors.

582 INDICATING DEVICES

sphere becomes pink. High-technology temperature indi-

cators include thermistors, circuits, and liquid-crystal

displays (LCD).

Most temperature indicators depend on a chemical

color change. These compact units provide direct, reliable

information on the temperature to which the product was

exposed. They are widely used for materials with critical

temperatures such as pharmaceutical and biological pre-

parations (Table 1) and fresh and frozen ﬁsh. Table 2 lists

some of these indicators; Figure 8 is representative of

these devices.

There are two other types of indicators not listed in

Table 2: the Criti-Temp (Schobl Enterprise, Inc.), which is

a bimetallic spring-loaded monitor, and the Precision

Digital Thermometer (TSI International Corp.), which

functions from a thermistor to give a temperature display.

FREEZE AND THAW INDICATORS

The main purpose of the indicators listed in Table 2 is to

show a response to a rising temperature well above room

temperature. Devices to indicate temperatures near the

freezing point of water require special design because they

must be unbroken before use and function at low

temperatures.

Akzo N.V. of the Netherlands has exerted a strong

inﬂuence in the monitor area through its subsidiary,

Organon, heir to the BMS disposable-thermometer tech-

nology, and through its Info-Chem Protective Products

Division. Info-Chem manufactures the Thaw-Watch and

the Freeze-Watch. These products utilize an ampul ﬁlled

with a colored ﬂuid. Freezing breaks the ampul, which

spills the contents onto a paper indicator (see Figure 9).

Biosynergy, Inc. manufacturers a liquid crystal Hemo-

Temp II for use on blood-collection bags. This device, after

being frozen, indicates the temperature of the blood (see

Figure 10).

The Check-Spot (Check-Spot, Inc.) (8) functions at

3–321F(16 to 01C) and is based on a solid emulsion

(see Figure 11).

The Monitor-Mark Button (3 M Co.) [see Figure 12(a)]

operates by means of a meltable, dyed compound con-

tained in a porous reservoir. In the inactivated form, a

domed indicator paper is separated from the reservoir by a

small distance. When the dome is pressed, the two mate-

rials come in contact, allowing wicking to occur when the

melt temperature is reached [see Figure 12(b)]. This

product has many different applications, including irre-

versible monitoring of bags of whole blood. The speciﬁca-

tions for blood banking have been outlined (9).

Other styles of freeze indicators are manufactured by

3 M Co. (10–12), including the Monitor-Mark Cold Side

Indicator, model 32F (see Figure 13). When temperatures

fall below 321F(01C), there is an irreversible color change.

In the past, the irreversible warmup indicator (IWI), was

manufactured by Artech Corp. (13).

HUMIDITY INDICATORS

Many items are sensitive to moisture in ways that are

irreversible. Simply drying out or humidifying does not

restore the original function, and the damage that may

have occurred during shipping or storage may not be

evident to the recipient. To protect the manufacturer

and the customer, humidity changes must be monitored.

A humidity monitor based on a color change is available

from Herrmann Chemie and Packmittel (see Figure 14).

A U.S. patent has been granted for a time/history

humidity indicator (14). This device comprises a salt

Figure 5. Electronic recorder (6, 7). To convert in. to cm, multi-

ply by 2.54.

Figure 6. Tattletale thermograph electronic recorder (Onset

Computer Corp.). To convert in. to cm, multiply by 2.54.

Figure 7. Thaw indicator. Half-colored sphere of ice becomes

homogeneous when thawed. To convert in. to cm, multiply by 2.54.

INDICATING DEVICES 583

that absorbs moisture from the air, a water-soluble dye,

and an absorbent material. After activation by removal of

a barrir ﬁlm, the dissolved dye migrates into the absor-

bent material as the humidity increases. This product

resembles the product shown in Figure 4.

GRAVITATIONAL-FORCE INDICATORS

Gravity is a useful force in packaging because it keeps

containers in place. Many times the normal force of

gravity may be applied to a product in shipping and

Figure 10. HemoTemp II (Biosynergy, Inc.). To convert in. to cm,

multiply by 2.54.

Figure 9. Freeze-Watch (Info-Chem Protective Products).

Figure 11. Check-Spot (Check-Spot, Inc.). To convert in. to cm,

multiply by 2.54.

Figure 8. Temperature indicator representative of devices listed

in Table 2. To convert in. to cm, multiply by 2.54.

Table 2. Color-Change Temperature Indicators

Product Name Company and Location

Temperature Range,

1F(1C)

Celcistrip Solder Absorbing Technology (Agawan, MA) 104–465 (40–240)

105–500 (40–260)

Thermomarkers W. H. Brady Co. (Milwaukee, WI) 100–500 (40–260)

Tesa-Temperatur Indikatoren,

irreversible

Biersdorf AG (Hamburg, Germany) 175–360 (80–180)

Thermolabel Paper Thermometer Co. (Greenﬁeld, NH) 140–180 (60–81)

Tattleherm Everest Interscience (Tustin, CA) 100–500 (40–260)

Reatec MRC Corp. (Wayne, PA) 100–420 (40–213)

Template Wahl Corp. (Los Angeles) 100–1000 (40–590)

Telatemp Telatemp Corp. (Fullerton, CA) 100–350 (40–155)

Thermo Strip Archie Soloman and Associates (Roswell, CA) 219–435 (103–220)

Thermindex Thermindex Chemicals and Coatings, Ltd. (Missisauga,

Ontario, Canada)

100–500 (38–260)

OmegaLabels Omega Engineering (Los Angeles) 100–400 (40–202)

Hermet Markal Co. (Chicago) 100–500 (40–260)

Temp Tabs Jardine Engineering (Hong Kong) 100–900 (40–500)

Celsipoint Signalarm, Inc. (Springﬁeld, MA) 100–500 (40–260)

Templilabel

Temp-Alarm

Templistik Big Three Industries, Inc. (South Plainﬁeld, NJ) 125–750 (50–395)

Thermax Thermagraphics Measurements, Ltd. (Chicago, IL) 150–400 (65–200)

T-Dot Westemp Instruments Co. (Cardiff, CA) 100–400 (40–200)

584 INDICATING DEVICES

handling. This abusive treatment can cause hidden da-

mage. The Hump-Gard (Impacto-Graph Corp.) is a 30-day

monitor for the high G forces encountered in railroad

shipping.

The Shockwatch is a compact device for measuring

high gravitational forces (15). The active element is a

capillary tube with surface-energy differences in each end

(see Figure 15). A colored ﬂuid is placed in the -philic end,

and when forces exceed a predetermined value, the liquid

is forced into the -phobic end where it is visible. Once

distributed by 3 M Co., it is now available from Media

Recovery Co.

This article has discussed only a few of the hundreds

of indicating devices available

. A low cost indicator

is still needed, preferably one that could be imprinted

as part of the label of consumer products such as frozen

foods.

BIBLIOGRAPHY

D. L. Johnson, ‘‘Indicating Devices’’ in The Wiley Encyclopedia of

Packaging Technology, 1st ed., 3 M Company, pp. 400–406.

Cited Publications

1. Fed. Reg. 39(219) (Nov. 12, 1974).

2. K. H. Hu, Food Technol. 26, 56 (Aug. 1972).

3. U.S. Pat. 3,999,946 (Dec. 28, 1976), G. N. Patel (to Allied

Chemical Co.).

4. U.S. Pat. 3,954,011 (May 4, 1976), W. Manske (to 3 M Co.).

5. U.S. Pat 3,962,920 (June 15, 1976), W. Manske (to 3 M Co.).

6. A. J. C. Carselidine and R. R. Weste, CSIRO Food Res. Q. 36,

41 (1973).

Figure 15. Capillary tube of Shockwatch.

Figure 12. (a) Monitor-Mark Button (3 M Co.). To convert in. to

cm, multiply by 2.54. (b) Cross section.

Figure 14. Humidity indicator (Herrmann Chemie and Pack-

mittel of Sod-Chemie AG). To convert in. to cm, multiply by 2.54.

Figure 13. Monitor-Mark Freeze Indicator 32F (3 M Co.). To

convert in. to cm, multiply by 2.54.

W. Manske, 3 M Co., Saint Paul, Minn., has a working ﬁle of over

255 U.S. patents on Indicating Devices.

INDICATING DEVICES 585

7. M. J. M. Van’t Root, Proceedings of the 13th Congress on

Refrigeration, Washington, DC, Vol. 4, pp. 445–451.

8. U.S. Pat. 2,971,852 (Feb. 14, 1961), J. Schulein (to Check-Spot,

Inc.).

9. B. A. Myhre, Quality Control in Blood Banking, Wiley, New

York, 1976, pp. 168, 169.

10. U.S. Pat. 4,132,186 (Jan. 7, 1979), W. Manske (to 3 M Co.).

11. U.S. Pat. 4,457,252 (July 3, 1984), W. Manske (to 3 M Co.).

12. U.S. Pat. 4,457,253 (July 3, 1984), W. Manske (to 3 M Co.).

13. Product literature, Artech Corp., Falls Church, VA.

14. U.S. Pat. 4,098,120 (July 4, 1978), W. Mansek (to 3 M Co.).

15. U.S. Pat. 4,068,713 (Jan. 17, 1978), U.R. Rubey (to Detectors,

Inc.).

INJECTION MOLDING FOR PACKAGING

APPLICATIONS

LARRY PASCUCCI

Negri Bossi Inc., Mississauga,

Ontario, Canada

The art of making a form out of a crude mold dates back

centuries in the civilization of mankind. In the 20th

century the technology of mold-making and materials

science in the production of plastic forms evolved together

with the invention of the injection molding machine (see

Figure 1). Manually operated at ﬁrst, the injection mold-

ing machine evolved into a fully automatic machine cap-

able of faster and faster ‘‘cycle times’’ in the production of

plastic parts. This article will take you into the 21st

century and explain the art, science, and technology of

today’s injection molding technology.

This article consists of the following sections:

The Process of Thermoplastic Injection Molding

The Thermoplastic Injection Mold

The Thermoplastic Injection Molding Machine

Automation for Injection Molding Machines

Monitoring and Servicing the Machinery

THE PROCESS OF THERMOPLASTIC INJECTION

MOLDING

There are two types of injection molding processes. The

most predominant is thermoplastic molding, which con-

sists of injecting melted/heated plastic resin that must be

‘‘cooled’’ by the mold to solidify. The second process

involves a thermoset material that requires a ‘‘heated’’

mold to solidfy. The machinery is essentially the same for

both processes except for the control of the heating or

cooling phases of the material processed. This article will

concentrate on the thermoplastic molding process that is

most common for packaging applications.

Thermoplastic material in the form of small ‘‘pellets’’

(resin) each about the size of a small pea are placed into a

conical steel ‘‘hopper’’ ready to be fed by gravity into the

‘‘injection unit’’ of an injection molding machine. The

pellets are then drawn through a ‘‘feed throat’’ into a

horizontally mounted ‘‘screw ‘‘which revolves at a certain

speed, driven by an electric or hydraulic motor. The

screw resides in a ‘‘barrel’’ that is heated on its outer

perimeter by electric ‘‘heater bands.’’ As the screw rotates

it mixes and induces work energy into the resin and

together with the heated barrel melts the plastic. The

plastic resin is conveyed to the front of the screw down its

shaft. At this point, ‘‘injection’’ into the mold is ready to

take place.

The mold is clamped in the vice-like grip of the machine

ready to keep the mold closed under extreme injection

pressures that can climb to 30,000 pounds-per-square-

inch (psi) or more. The screw, on commands given by a

central computer that directs all machine functions, then

bolts forward to inject the melted resin into the mold. A

device on the end of the screw called a ‘‘check valve’’ stops

the plastic from backing up into the screw during

injection.

Controlled screw speed, injection velocity, and melt

pressure are crucial to making a quality ﬁnished part.

The mold is cooled via water channels to solidify the

thermoplastic resin. When the plastic part cools and

‘‘sets,’’ the mold is opened by the injection molding ma-

chine and the part is ‘‘ejected’’ by the action of both mold

and/or machine. At this point the part is left to free-fall to

a conveyor that guides the part out of the machine or

an automated robot enters and extracts the part for

Figure 1. This modern 700-ton clamp

force injection molding machine illustrates

a HYBRID machine design. Electric screw

drive with Hydro-Toggle clamp. PC-based

control and DIGITAL Canbus architecture.

(Photo courtesy of Negri Bossi.)

586 INJECTION MOLDING FOR PACKAGING APPLICATIONS

packaging or any downstream handling requirements.

This point in the process now deﬁnes the ‘‘cycle’’

completed.

Let’s review the sequences in a more detailed way:

The ﬁrst molding sequence is to ‘‘plasticize’’ the solid

thermoplastic pellet into a hot molten condition. The

second is ‘‘mold-close,’’ where the machine clamps the

mold under high forces to withstand the opening forces of

high injection pressure. The third is ‘‘injection forward,’’

where the screw bolts forward to inject the molten

thermoplastic resin into the mold and ‘‘hold’’ the injection

forward while the part is cooling. The fourth is ‘‘part

cooling,’’ where the mold via water-cooled channels cools

the thermoplastic part to solidify it. The ﬁfth is ‘‘mold

open’’ as the mold opens to ready the part for extraction.

The sixth is ‘‘part ejection,’’ where the machine and/or

mold ejects the part either by free-fall or into the clutch

of a robot’s grip. The ﬁnal stage is the to repeat the

‘‘cycle’’ to produce another thermoplastic part(s). To sum-

marize: the seven stages in the cycle of thermoplastic

injection molding are:

1. Plasticize or melt the thermoplastic into a molten

state.

2. Close the mold under high force to withstand high

injection pressures.

3. Inject the thermoplastic into the mold under high

speed and pressure and ‘‘hold’’ the injection forward

while cooling.

4. Cool the part by extracting heat by a water cooled

mold

5. Open the mold to ready part extraction.

6. Eject the part into free-fall or automated robot

removal.

7. Repeat the ‘‘cycle’’ to produce another part(s).

THE THERMOPLASTIC INJECTION MOLD

The mold design has a tremendous impact on system

productivity and product quality, and therefore on the

overall economics of the injection-molding operation. The

design of a mold is heavily inﬂuenced by characteristics of

the part. A thorough understanding of the various mold

classiﬁcations is also critical in the speciﬁcation of the

right type of mold best suited to your application. The

mold classiﬁcations section of this article identiﬁes each

mold type and the criteria for each.

Most injection-molded parts for packaging have rela-

tively large length-to-thickness (L:T) ratios. The length is

deﬁned as the maximum ﬂow length in the cavity and is

measured from the point where plastic enters the mold to

the furthest point it travels. The thickness is the average

wall thickness of the part. Early molds had L:T ratios of

up to 200:1, but today they can be as high as 500:1. For

such thin-wall parts, design emphasis must be placed on

the hot runner system, part ejection, mold cooling, align-

ment, and mold material selection. A typical mold for

thin-walled containers is shown in Figure 2 (also see

Figure 3).

Mold Classiﬁcations

Molds are classiﬁed into different SPI (Society of Plastic

Industry) mold categories based on expected mold lifetime

measured in cycles. The different classiﬁcations dictate

signiﬁcant differences in mold design and material which

will impact tooling costs. It is important to review and

identify the production requirements when specifying the

type of mold to be constructed. See Table 1.

Operation. As the mold closes, the cores and cavities

are aligned by some form of tapered mating interface.

Plastic melt (i.e., melted resin), is injected by the injection

unit of the molding machine into the sprue bushing and

forced through a network of ﬂow channels called the

runner system. In Figure 2, the runner system is heated

by electric heaters, hence the term ‘‘hot runner.’’ The hot

runner system maintains and controls the temperature of

Figure 2. An eight cavity thin-walled container mold with hot

runner technology. The part is a one-gallon paint can. (Photo

courtesy of Top Grade Molds.)

Figure 3. A four cavity thin-walled mold with hot runner tech-

nology. The part is a tamper-evident container. (Photo courtesy of

Top Grade Molds.)

INJECTION MOLDING FOR PACKAGING APPLICATIONS 587