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canmaking. Today most cans are identiﬁed by their di-

mensions and not by name. Some of those names and can

sizes listed are no longer applicable.

International standards for can sizes have been devel-

oped under auspices of the International Standards Orga-

nization (ISO), and cans are named in many countries on

the basis of ISO standards. The speciﬁcations, measure-

ments, and nomenclatures in ISO standards are used to

describe cans in several countries. As can sizes listed in

the ISO standards are the least likely to be restricted in

international trade, it would be advisable when develop-

ing a new product line or changing package sizes to

consider can sizes from the ISO standards. To comply

with the ISO standards, the Can Manufacturers Institute

has now recommended that U.S. can sizes be identiﬁed in

metric measurements according to their body plug dia-

meter rounded to the nearest whole number of milli-

meters, and their height must be rounded to the nearest

whole number of millimeters as shipped from the can

factory. For example, the 307 113 is identiﬁed as

83 46 mm.

Normally, during the selection of a container size for a

speciﬁc item the concerns are not with providing a can

with minimum surface area, but rather to minimize

package metal weight (i.e., cost) within practical limita-

tions. As an example, the ﬁlling, processing, and end

attachment operations demand that the circular ends be

made of thicker metal than the cylindrical body. The cost

impact tends to reduce end diameter while increasing

body height compared with the theoretical dimensions

for minimum surface area. Furthermore the ends are

attached to the body with mechanically overlapped

metal formed into the body and ends. Again, the require-

ment for thicker metal for the ends would favor smaller

diameter.

Similarly, the body beads (for strength against can

collapse) and end beads (for rigidity) increase the effective

surface area. This again favors a smaller diameter to

minimize metal weight. In addition, the ends are punched

out of a rectangular sheet of metal in the manufacturing

process. Not only must the weight of metal used in the

ends be minimized, but also the scrap produced from

manufacturing round ends from square sheets has to be

held to a minimal level.

Other limitations must also be considered. For exam-

ple, the can manufacturer provides to the customer a can

with one end attached (factory-ﬁnished can). The custo-

mer, in turn, ﬁlls the empty can and conveys it to another

machine, where the top end (customer/packers end) is

attached. To avoid spilling product until the top end can be

attached, the can is not fully ﬁlled but a small ‘‘headspace’’

remains. Besides preventing product loss, this void pro-

vides for product expansion during processing and allows

the consumer to open the can without splashing. This need

for a headspace requires the can to be slightly taller than

the theoretical height. In conveying, the laws of physics

(inertia) state that the liquid juice will try to remain in

place while the can begins moving. Without adequate

headspace, juice would ﬂow out of the open can. At a

given conveying speed, less headspace is required for a

smaller-diameter can than for a larger one. Therefore, it

would be required to either increase can height for a

Table 1. Continued

Style Dimensions in. (cm) Capacity Some Uses Convenience Features

Spice can, oblong (13) Wide range 1–16 oz (28.4–454 g) Seasonings Dredge top, various

dispenser openings

Square, oval, and round-

breasted containers

(14)

Powders Perforations for

dispensing, reclosure

feature

Two-piece drawn

redrawn sanitary can

208 207/108 (6.4 6.2/

3.8)

35 1/2 oz (85–156 g) Food Improved can integrity,

stackability

307 111 (8.7 4.3) 6 3/4 oz (191 g)

211 214 (6.8 7.3) 7 1/2 oz (213 g)

404 307 (10.8 8.7) 11/2 lb (680 g)

Table 2. Popular Can Sizes

Dimensions Popular name Inches (mm) Capacity

(oz) (cm

)

202 214 2.13 2.88 (54 73) 4.60 (137)

211 413 12 oz 2.69 4.81 (68 122) 12.85 (380)

300 407 #300 3.00 4.44 (76 113) 14.60 (432)

303 407 #307 3.19 4.38 (81 111) 16.20 (479)

307 409 #2 3.44 4.56 (87 116) 19.70 (583)

401 411 #2 1/2 4.06 4.69 (103 119) 28.60 (846)

404 700 40 oz 4.25 7.00 (108 178) 49.55 (1465)

603 700 #10 6.19 7.00 (157 178) 105.10 (3108)

Completely ﬁlled.

208 CANS, STEEL

larger diameter can, or customers would have to slow their

lines to avoid spilling product. Increasing the can height

would make air removal more difﬁcult. Increased air

(actually the oxygen in the air) content would have a

somewhat negative effect on product quality.

CAN PERFORMANCE

No other container has all the attributes of the can:

economy, strength, durability, absence of ﬂavor or odor,

ease in mass handling without breakage, compactness,

lightness, and no light damage to product; the can also

provides a hermetic seal and can be produced at excep-

tionally high speeds and be lithographed.

Food cans are expected to have tightly drawn-in ends.

A can with swelled ends or with metal feeling springy

or loose is not merchantable. Consumers associate such

appearances with spoiled contents. Gases produced by

microbial action can, indeed, cause such appearances. It

is also important to remove air from food cans to retard

adverse internal chemical reactions. Therefore, end units

must be properly engineered to cope with their many

environments. They must act as diaphragms that expand

during thermal processing and return to their tight

drawn-in appearance when a vacuum develops during

cooling. The necessary strength is built into the end units

by the use of adequate plate materials and design of end

proﬁle (see Figure 1).

Food ends are either proﬁled structures requiring a

mechanical means for opening or designed with scores for

easy opening. Since the introduction of the easy-opening/

convenience end in the mid-1960s, a broad variety of

products have been packaged in cans with this feature.

These easily removed ends are available in various

diameters, metals, coatings, and conﬁgurations to meet

product needs. Units can be removed as a whole with

separation occurring around the perimeter of the end or

only a portion of the top can be removed. For those

products where consumer protection is desired, easy open-

ing ends can be provided with safety features.

Ends used for carbonated beverages and aerosol pro-

ducts and those used for noncarbonated beverages packed

using liquid-nitrogen pressurization, need little or no

vacuum accommodation but rather resistance to high

internal pressure. An end unit will permanently distort

or buckle when the internal pressure exceeds its capabil-

ity. This also renders the can unmerchantable because the

distortion could affect the double-seam integrity.

Cans with inadequate body strength will panel because

of internal vacuum or collapse under axial load conditions.

A panel is the inward collapse of the body wall because

of pressure differential. The condition may appear as a

single segment, many ﬂat segments, or panels that de-

velop around the circumference of the cylindrical can body.

Axial load is the vertical pressure that a packed container

will be subject to during warehousing and shipping. The

axial load capacity of a can is greatest when the cylinder

wall is in no way deformed. A casual dent or designed

structure that breaks the integrity of the straight cylinder

will, in most cases, reduce the axial load capability of the

container. Panel resistance and axial load capability are

direct functions of the metal speciﬁcation.

Panel resistance can be enhanced by the fabrication of

beads in the body wall. This, in effect, produces shorter

can segments that are more resistant to paneling.

However, such beads predispose the container to early

axial load failure. The deeper the bead is, the greater the

paneling resistance but the greater the reduction in axial

load capability. Many shallow beads can provide addi-

tional paneling resistance with less reduction in axial

load capability, but labeling problems are often associated

with cans with such bead conﬁgurations. Consequently,

there are many bead designs and arrangements, all

of which are attempts to meet certain performance

criteria (7).

The steel container with proper material and struc-

tural speciﬁcations possesses, within limits, good abuse

resistance. Excess abuse causes obvious damage, and

severely dented cans are unacceptable in the marketing

of canned products. Insidious events run parallel with

excessive abuse. The double seams may ﬂex momentarily,

permitting an equally short-term interruption of the

hermetic seal. Under some unsatisfactory conditions,

this lapse can permit the entrance of micro-organisms

that cause spoilage. This leakage can also admit air that

accelerates adverse chemical reactions within the con-

tainer. Many cans that show some level of damage are

in the food distribution chain, and the safety of using

such products is frequently questioned. To prohibit the

sale of all containers that have insigniﬁcant amounts of

damage would be a waste of large amounts of acceptable

food (8).

Figure 1. End proﬁles (A—curl, B—seaming panel, C—steam-

ing panel radius, D—chuck wall, E—chuck wall radius, F—chuck

panel, G—chuck panel radius, H—bead, I—bead-edge radius, J—

ﬁrst expansion panel, K—ﬁrst expansion panel radius, L—ﬁrst

expansion panel step, M—ﬁrst expansion panel step radius, N—

second expansion panel, O—second expansion panel radius, P—

second expansion panel step, Q—second expansion panel step

radius, R—center expansion panel).

CANS, STEEL 209

CAN CORROSION

The steel container is not chemically inert and, therefore,

can react with its environment and its contents. Steel’s

major ingredient, iron, is a chemically active metal that

readily takes a part in reactions that involve water,

oxygen, acids, and a host of other elements and com-

pounds that can participate in oxidation reduction reac-

tions (see the Cans, corrosion article).

The application of tin to the surface of sheet signiﬁ-

cantly increases the corrosion resistance of steel. Never-

theless, the potential for corrosion attack persists.

Although numerous modifying factors produce varied

patterns of attack, the chemical fundamentals for oxida-

tive corrosion are the same. The basic requirements are

always (a) differences in potential between adjacent areas

on an exposed metallic surface to provide anodes and

cathodes, (b) moisture to provide an electrolyte, (c) a

corroding agent to be reduced at the cathode, and (d) an

electrical path in the metal for electron ﬂow from anodes

to cathodes.

Under normal conditions, tin forms the anode of the

couple, going into solution at an extremely slow rate and

thus providing protection to the canned food. Under some

conditions, iron forms the anode with resultant failure

because of perforations or the development of hydrogen

and subsequent swelled cans (hydrogen springers). Under

other conditions, as when depolarizing or oxidizing agents

(i.e., nitrates) are present, the removal of tin will be

greatly accelerated with a consequent signiﬁcant reduc-

tion in shelf life. Hydrogen is formed by the following two

distinct processes: (a) at exposed steel areas that are

protected cathodically by the tin–steel couple current

and (b) as the steel corrodes, either because it is not

completely protected by the tin or after the tin has been

consumed. When perforations occur, they are usually the

result of the same process in which hydrogen is developed

except the steel is consumed in a localized area (9–11).

PRODUCT COMPATIBILITY

Products can be loosely categorized in terms of their

susceptibility to chemical reaction with the can. Oils and

fatty products seldom react with the metal surface of can

interiors. However, when small quantities of moisture are

present, either by design or accident, adverse reactions

can develop. Highly alkaline products, usually nonfoods,

will rapidly strip off tin or organic coatings but will not

corrode the base steel. Acid products are corrosive, and

highly colored acid foods and beverages are also suscep-

tible to color reduction or bleaching, because of the reac-

tion of the product with tin.

In this regard, when tin is exposed to some food

products, a bleaching action occurs. Although this is

objectionable with many products, such as the red fruits,

and is avoided by the use of a suitable can lining, this

bleaching action is desirable in certain instances. This is

particularly true with the lighter-colored products such as

grapefruit juice and grapefruit segments and sauerkraut.

A slight bleaching action keeps the color light and

compensates for the normal darkening effect, which may

result from the processing or sterilization. Peaches and

pears packed in cans completely enameled inside will be

somewhat darker in color and slightly different in ﬂavor

than if packed in nonenameled plain tin cans. Although

some individuals may prefer peaches packed in all enam-

eled cans, it is doubtful that such cans will be produced

under current conditions because the presence of an

appreciable area of plain tin greatly increases the shelf

life of this canned product.

Foods with pHW4.6 often have sulfur-bearing consti-

tuents (e.g., protein) and react with both tin and iron. This

has been recognized as a problem for as long as such

products have been canned, and the cause has been sought

by many investigators. These low-acid foods form a dark

staining of the tin surface and react with the iron to form a

black deposit that adheres to both the can interior and the

food product or can cause a general graying of the food

product and liquor. This is often referred to as black

sulﬁde discoloration or sulﬁde black. It can be unsightly

but is harmless. Although sometimes exclusively a can

headspace phenomenon, any interior container surface

can be affected. Also, the black sulﬁde discoloration con-

dition generally occurs during or immediately following

heat processing, but occasionally it develops during sto-

rage. It is believed to be an interaction of the volatile

constituents from the food product and/or oxidation–

reduction agents in the food product with an oxidized

form of iron from the tinplate. The staining of the tinplate

is not part of this reaction (13, 14).

Although tin sulﬁde staining and black sulﬁde disco-

loration is harmless from a product standpoint and has

no detrimental effect on container or product shelf life,

it has been found to be objectionable from an aesthetic

standpoint.

In addition to container and product appearances, some

food products and beverages are highly sensitive to off-

ﬂavors caused by exposure to can metals (12).

CAN METAL

The term ‘‘tin can’’ is somewhat of a misnomer, because tin

cans are made of steel sheets that have either no tin or

coated with a thin ﬁlm of tin. The steel products commonly

produced for container components have a theoretical

thickness ranging from 0.0050 to 0.0149 in. (0.127 to

0.378 mm) expressed as weights per base box* of 45–

135 lb. Depending on end use, the plate (blackplate) can

be processed by the electrolytic deposition of metallic

chromium and chromium oxides and coated with a lubri-

cating ﬁlm (tin-free steel) or coated with tin by electro-

deposition.

Improvements in electrolytic

tinplate, since its

commercial introduction in 1937, have increased both its

* Base weight—approximate thickness in pounds per base box:

(base weight 0.00011 = theoretical thickness). Base box—unit of

area: 112 sheets, 14 in. 20 in. = 31,360 in.

(217.78 ft

); dimen-

sions in increments of 1/16 in.

Package: 112 sheets of any dimensions. Number of base boxes in a

package: use ratio tables (ASTM A623) or L(in.) W(in.) 112 =

31,360. Number of base boxes in a coil: L(in.) W(in.) = 31,360.

210 CANS, STEEL

versatility and uniformity of performance. Electrolytic

plate has entirely replaced hot-dipped plate in present-

day cans. The result of these changes/improvements has

allowed for reductions in tin coating weights. Tin plate

carries a coating of tin that may vary from 0.000015 to

0.000100 in. in thickness, depending on the grade. Differ-

ential coated electrolytic tinplate, with different coating

weights of tin on each side of the steel baseplate, has been

used commercially since 1951. Containers made of differ-

ential coated tinplate have an inside tin coating of

sufﬁcient thickness to withstand the corrosive attack of

processed foods and an outside coating adequate to with-

stand the rigors of processing and atmospheric conditions.

The thickness of tin coating is designated by the total

amount of tin used on coating one base box of plate. For

example, #25 plate is electrolytic tinplate on which 0.25 lb

of tin was electroplated on one base box, covering both

sides of the sheet. Thus, the coating on one side of the

sheet is one half that amount of tin, or 0.125 lb.

For structural integrity, base weight or gauge thickness

is a prime consideration, but any increment of base weight

is an increment in container cost. Often, the temper or

design of can components can provide added strength

without the need to add base weight. The chemistry and

general metallurgy of the steel plate has considerable

inﬂuence on the performance of steel containers. Before

the advent of continuously annealed and cold-rolled dou-

ble-reduced steel plate, which inﬂuence plate stiffness or

temper, it was necessary to add ingredients to the steel to

produce high tempers. Temper as applied to tin-mill

products is the summation of inter-related mechanical

properties such as elasticity, stiffness, springiness, and

ﬂuting tendency. During the development of the use of

cold-reduced steel for mill products, the fabricators deter-

mined that various steel compositions and degrees of

hardness in the base metal would permit the production

of containers for a wider variety of products. Although

there are certain limitations to the Rockwell hardness

test,

it has been adopted as an industry standard because

of its simplicity and overall good correlation with fabrica-

tion requirements. As a result, ranges of Rockwell hard-

ness were developed to guide the steel manufacturers, and

these Rockwell hardness ranges have become accepted as

temper ranges.

Rephosphorized steel was necessary to fabricate beer

can ends with sufﬁcient buckling resistance. When the

market opened for canned carbonated soft drinks, repho-

sphorized steel was not acceptable because of the corrosive

nature of soft drinks and the high susceptibility of this

plate to corrosion. Test pack experience indicated that

some could perforate the endplate in 4–6 weeks. The

availability of continuously annealed plate without the

corrosion sensitivity properties of rephosphorized steel

permitted the canning of soft drinks. It was still necessary

to use heavier base weight material for the high-carbona-

tion drinks. The added stiffness afforded by cold-rolled

double reduced plate permitted the canning of all soft

drinks with carbonation ranging from 1.0 to 4.5 volumes

in cans with the same basis-weight ends. These same

economies have been applied to other products where

container fabrication could be adapted to the degree of

stiffness characteristic of this plate (14).

CAN FABRICATION

One who is not associated with the container industry

seldom realizes that the manufacture of cans is classed as

one of the most mechanized industries, and in addition to

the large amount of automatic equipment used, the speed

at which cans are made no doubt rates among the faster

automatic operations (see the Metals Cans, Fabrication

article).

The most common type of steel can produced today are

three-piece cans that consist of a body with a welded or

soldered side seam in conjunction with two end compo-

nents (see Figure 2). Another popular structure is the

two-piece can, which is produced by either a single-draw,

draw-redraw (DRD), or drawn-and-iron (D & I) process.

Some limited quantities of cemented side-seam cans are

produced for dry and nonthermal processed type products.

Most three-piece tinplate cans currently made have

wire-welded side seams. The traditional soldered can is

now in the minority and only is used for irregular-type

meat cans and specialty cans. The solder used in canmak-

ing is now generally composed of tin and silver. In July

1995 the U.S. Food and Drug Administration (FDA) has

issued a ﬁnal rule prohibiting the use of lead solder in the

manufacturing of food cans, including imported products.

The tin/silver solder is categorized as a soft solder and

has a relatively low melting point, usually below 4501F,

(2321C). Tinplate cans are easily soldered because the tin

solder alloy readily fuses with the tin on the surface of the

steel. In addition to providing a hermetic seal, solder also

contributes to the mechanical strength of the seam by

forming a metallurgical bond with the tin plate. Solder

also has a certain amount of ductility and can be plasti-

cally deformed within certain limits. This characteristic

permits the soldered laps of the body to be ﬂanged and

then incorporated into the double-seaming process. The

speed of the can manufacturing line, the temperature of

the molten solder, and the length of cooling all play an

important role in good soldering operations. The soldered

side seam can has been replaced for many can structures

with an electrical resistance—welded seam. Beneﬁts in-

clude a clean compact process and a much narrower weld

area, which allows virtually wrap-around decoration and

greater integrity, particularly for seaming-on of ends,

because there is much less metal overlap thickness at

the weld compared with the soldered lock and lap seam

employed on soldered cans. The welding unit is easily

integrated and ancillary equipment used on solder body-

maker lines.

Two reliable welding systems were developed and used

commercially: The forge–wheel welding system developed

A device used to measure the surface hardness of canmaking

plate. A 1/16-in.-diameter ball penetrator is impressed into the

plate surface with a 30-kg load. The measurement expressed as a

Rockwell 30T reading, is inversely proportional to the penetra-

tion. An arbitrary scale is used to convert Rockwell readings into

plate temper values. The stiffness increases from T-1 to T-6.

CANS, STEEL 211

by Continental Can Company, and the wire weld system

was developed by Soudronic AG (15, 16). Both are resis-

tance-welding systems and variations of a forge weld

(see Figure 3). The materials to be welded together are

heated by an electric current passing through the materi-

als. The resistance of the overlapped side seam produces

the required welding heat. To obtain a weld, the heated

material must be pressed together or clamped. This

clamping capability is provided by wheels in the welding

process. Because welded seams are basically lap seams, a

raw edge exists, which generally requires a side-seam

stripe coating application to minimize metal exposure.

For economic reasons, the forge-wheel welding process

is no longer a viable commercial operation. Such forge-

welded cans used tin-free steel (TFS) plate and started

with the grinding of the body blank edges to remove oxide

and chromium to ensure acceptable welds. After blanks

are formed into cylinders, the edges are tack welded

together to produce an overlap, and the seam is made by

rolling electrodes that weld the interior and exterior of the

lap simultaneously.

In the wire welded operation that is used extensively

today, the welding process consists of passing the seam

with a small overlap between two electrode wheels over

which runs a copper wire. The use of copper wire as an

intermediate electrode is necessary to remove the small

amount of tin picked up from the tinplate during the

welding process, which would otherwise reduce the weld-

ing efﬁciency. A constantly renewable copper surface is

presented to the weld area. To obtain a weld, the heated

material is pressed together or clamped by wheels in the

welding process (see Figure 3).

Another innovation has been the introduction in the

early 1960s of the seamless two-piece can (integral

base and body, with one customer end). Metal forming

technology has been used to produce these cans from a ﬂat

sheet. Drawn and ironed cans employ tinplate and are

manufactured by ﬁrst drawing a shallow cup and then

extending the sidewalls by thinning the metal between

two concentric annular dies in an ‘‘ironing’’ process. This

results in a can with a normal base thickness and thinner

walls, and it is economical in materials usage. The capital

cost of D&I canmaking plants is high, but unit costs are

the lowest of all container types if the throughput is

sufﬁciently great. Because of the thin walls, body beading

is essential to maintain rigidity for vacuum-packed food

cans. The other two-piece can achieves ﬁnal can dimen-

sions through a series of consecutive drawing operations.

In these DRD cans, the thickness of the bottom end and

the side walls is largely the same and there is not the same

opportunity for saving on metal. Unlike the D&I can,

Figure 2. Double-seam three-piece can.

Figure 3. Welding process.

212 CANS, STEEL

which uses tinplate and inside spray coatings, the DRD

can uses TFS-precoated sheet or coil plate.

Organic sealing materials have been used as side-seam

sealants mostly before the advent of the welded can. They

have been used in situations where soldered structures

were not compatible with the product, when special wrap-

around lithography is desired, and when tin-free steel is

used. For the most part, polyamides and organosols are

used as the basic resin of organic sealing materials. They

are normally used for a variety of can sizes, shapes, and

styles that are used for dry and nonfood products. How-

ever, a polyamide hot-melt adhesive has been successfully

used in oblong meat cans that are subjected to a high-

temperature thermal process.

Much consideration has been given to cost reductions of

the metal can. In conjunction with thinner gauge and

higher temper plates, the reduction of the can diameter of

the two-piece beverage cans in the area of the end unit has

reduced the cost of metal cans signiﬁcantly. This body

diameter reduction is usually accomplished by a die neck-

ing-in process, which is performed prior to ﬂanging. The

necking-in process tends to alter the can height, so careful

body cut edge and/or can trim diameter must be calculated

to afford these accommodations. With three-piece cans,

this necked-in structure can be fabricated on both ends;

however, it is done on only one end of two-piece cans used

for liquid-type products.

The popularity of steel cans has been in large measure

caused by the double seam’s ease of fabrication and robust

resistance to physical abuse. This seam structure is the

portion of a can that is formed by rolling the curled edge of

the end and the body together to produce a strong leak-

proof structure. This seam is normally formed by a set of

ﬁrst and second operation seaming rolls while the end and

the can body are held together by a seaming chuck. The

ﬁrst operation seaming roll forms the seam by interlock-

ing the curled edge of the end with the ﬂange of the can

body. The second-operation seaming roll compresses the

formed seam to make a hermetic seal (Figures 4–6).

Because metal-to-metal contact does not produce good

sealing capability, a rubber-like material known as sealing

compound is applied to the loose end-unit seaming panel

that becomes engaged to the body ﬂange to form the

double seam. This material acts like a sealing gasket.

Compound placement and compatibility with the product

are important in providing for a hermetic seal. End-

sealing compounds are generally one of two types: (a)

water base—rubber/water dispersions and (b) solvent

base—rubber/solvent dispersions.

COATINGS

Organic protective coatings are used as can linings to

provide the additional protection above that provided by

the metal substrate. These organic materials are applied

to a ﬂat plate or a coil-coated plate and are cured by means

of continuous ovens. Spraying after complete or partial

fabrication is also done when low-metal-exposure cans are

required for aggressive-type products. It is desirable to use

can linings that possess a reasonable degree of latitude in

ﬁlm weight and bake. The roller coating machines and the

spraying machines used to apply linings provide means

for close control of ﬁlm weight. The ﬁlms, of course, are

relatively thin, about 0.0001–0.0005 in. thick or with a

weight of 2–6 mg/in.

Control limits generally allow for

variation of 70.5 mg/in.

Can linings must be nontoxic, meet government stan-

dards, be free from odors and ﬂavors, and be readily

applied and cured. They also must withstand the mechan-

ical operation of canmaking, must provide the required

barrier between canned product and metal, and ﬁnally

must be economical. Accelerated tests are used as screen-

ing or control tests, and evaluations under commercial

conditions are normally required before ﬁnal approval of a

can lining is standardized. When required, product test

packs will be made and monitored over a period of time to

conﬁrm that the coating material is compatible with the

intended product and/or container components.

Coatings accomplish several purposes. They make pos-

sible the use of less costly steels that permit the use of

Figure 4. Sequence of operations in seaming a can end onto a

can body.

CANS, STEEL 213

different types of tin-mill products in the can body and

ends. They protect the steel against corrosion and the

product from the steel and on the can exterior, minimize

rusting, and serve as a background to or improve adhesion

for lithography. Inorganic protective coatings that often

contain phosphates, chromates, or both are usually ap-

plied to tin-free steel at the tin mill to prevent rust and

provide a good surface for organic materials. If tin or iron

is detrimental to a product, then a suitable protective

coating can be applied. These coatings can prevent bleach-

ing discoloration by tin to dark pigments of foods such as

strawberries, blueberries, blackberries, and cherries. Also,

protective coatings can reduce the metal exposure for

metalsensitive products such as beer and certain soft

drinks. Other products would react with the unprotected

can interior to cause corrosion or cause discoloration of

both container and product. Finally, a protective coating

can mask unsightly discoloration of the can interior.

Currently, the major trend in coating technology is

toward waterborne coatings to replace conventional or-

ganic coatings. Typical organic coatings contain as much

as 70% solvents and are subject to stringent environmen-

tal restrictions. One answer is the aqueous coating, which

contains only a small amount of organic solvents and can

use the same coating and curing equipment. These aqu-

eous coatings, which were commercialized for food cans in

the early 1990s, exhibit the same good ﬂow and leveling

characteristics as found with solvent-base coatings.

Metal exposure in the side seam area is a common

occurrence with all three-piece can structures, and a

special protective coating material (a ‘‘side stripe’’) is

sometimes applied to the inside and outside side seam

area by spray or roller, online, followed by curing. Most

stripes used are organic liquid materials, which provide

only minimal coverage. Polyester powder (thermoplastic)

electrostatic spray stripes give superior performance.

These types of stripe coating provide 100% side-seam

coverage, but they are more costly than liquid stripes.

However, powder stripes have poor heat resistance and

are not recommended for use in cans that will be subjected

to aseptic process conditions that employ super-heated

steam.

Organic protective coatings were ﬁrst applied to can

interiors to protect the red fruit colors. Coatings used to

combat this problem were often called ‘‘regular’’ or ‘‘R-

enamels.’’ Now they are generally identiﬁed by a number,

letters, or a combination of numbers and letters.

If ordinary oleoresinous can linings are used for foods

containing appreciable quantities of protein such as corn,

peas, and ﬁsh, then a black tin sulﬁde stain will form

under the enamel; under certain conditions, black sulﬁde

discoloration will form at iron-exposed small scratches

and fractures. To minimize black sulﬁde discoloration,

oleoresinous enamels used for such products contain

about 6% of a ﬁne-particle-size zinc oxide, which is added

for its chemical reactivity and not as a pigment. The

sulﬁdes that form during the processing or heat steriliza-

tion of the protein-containing foods react with the zinc

oxide, which ‘‘de-zincs’’ and allows the formation of zinc

sulﬁde. Zinc sulﬁde is white and generally goes unnoticed.

Can linings containing zinc oxide were ﬁrst developed for

corn cans and were called ‘‘corn enamels’’; their use was

soon extended to other products, and they were then called

‘‘C-enamels.’’

Sulﬁde staining appears as a brown or brownish blue

stain on the surface of the tin coating and cannot be

removed by rubbing. Sulﬁde staining occurs not only on

exposed tin surfaces but also in tin surfaces protected by

many of the enamels because the volatile sulfur com-

pounds involved can permeate certain types of enamel

coating. To prevent underﬁlm sulﬁde staining, enamels

impervious to volatile sulfur compounds are used where

possible. However, such enamels are not always compa-

tible with the product being packed, in which case it is

necessary to use a pigmented enamel that will either

inhibit or mask the sulﬁde staining. These methods of

inhibiting or masking sulﬁde staining are limited to their

application, because not all enamels combine satisfacto-

rily with a given pigment or additive.

Many coatings are now in use and being developed for

steel cans used for a variety of food and nonfood products.

Some of those materials, which are supplied by coating

suppliers to the application facility and are now being

used, include acrylic, alkyd, epoxy amine, epoxyphenolic,

oleoresinous, oleoresinous w/zinc oxide, phenolic, polybu-

tadiene, vinyl, and vinyl organosol. Each material has its

own unique performance characteristics with regard to its

application, fabrication, product compatibility, ﬂavor and

odor, process resistance, and so on.

Figure 5. First operation.

Figure 6. Second operation.

214 CANS, STEEL

It has been apparent since the early days of canning

green vegetables that the bright green darkens as a result

of the required thermal process, that is, the bright green

chlorophyll undergoes a chemical change to an olive green

pheophytin. There have been several studies conducted

to inhibit this chemical change. They all demonstrate an

inhibition capability by using additives containing mag-

nesium ions and careful control of pH by the addition of

hydroxyl ions. All of these procedures cause some tough-

ening of the product, which probably discourages any

commercial pursuits. The Crown Cork and Seal Company,

Inc. has a patented process called ‘‘Veri-green,’’ which

incorporates additives to the product as well as the

protective coating. The materials added are speciﬁc for

particular products, and pretesting is necessary (17–21).

DECORATION

Decorative lithographic designs afford an external protec-

tive coating. The process of application is known as offset

lithography. Lithography is a printing process based on

the fact that oil and water do not mix. The decorative

design to be printed on the tinplate is etched onto a plate

known as a master plate in such a manner that the image

area to be printed is ink receptive, whereas the portion to

be blank is water receptive. A lithographic design is a

system or a series of coatings and inks printed on plate in

a particular sequence. The order of laydown is determined

by the purpose of the coat, the kind of coating or ink, and

the baking schedule for each coat or print. The print

process is usually carried out with sheet stock prior to

slitting into can body blanks or scroll shearing into the end

stock. The sequence and type of equipment used depends

on the design and the ultimate use of the can (see the

Decorating article).

With the advent of two-piece cans, less elaborate de-

signs are used on beer and soft-drink cans because it was

necessary to use presses that print completely fabricated

cans. The ﬁnishing varnishes are usually the external

protective coatings. They not only protect against corro-

sion but also must be rugged enough to resist scufﬁng and

abrasion.

For drawn-and-iron beaded food cans, the outside is

coated with a water-based colorless wash coat material.

After packing, the customer glues a paper label to the body

wall, which identiﬁes the contents.

TECHNOLOGICAL DEVELOPMENTS

The steel can has grown despite competitive pressures.

During war years, tin and iron become controlled materi-

als, and other more available packaging materials usually

make inroads into the steel-can market. These same

constricting conditions often give rise to the development

of methods that reduce the consumption of the controlled

materials. During World War II, the electrolytic deposition

of tinplate was commercialized, which resulted in sub-

stantial reductions in tin consumption. Competition from

aluminum in the beverage-can business gave impetus to

pursue the use of tin-free steel to afford more favorable

economics. Successful cost reductions with no reduction in

performance, however, has been the cause for the contin-

ued growth of the steel can. The advent of cold-rolled,

double-reduced plate permitted signiﬁcant reductions in

base weight. To accommodate the fabricating character-

istics of this material, new approaches in end manufacture

and double seaming had to be developed. Another direc-

tion to cost reduction was to reduce materials usage. This

was accomplished by using smaller-diameter ends and

necking-in the can bodies to accommodate the reduced-

diameter ends. This saving can be directed to one or both

ends on three-piece cans.

The use of convenience features has been an important

stimulus to the growth of steel cans. Key-opening lids and

rip strips have been part of ﬁsh and meat cans for

generations. This style was also standard for coffee and

shortening cans, but economic pressures caused a change

to less expensive open-top food cans. Development of the

integral rivet and scored end solved many can opening

problems, particularly in the beverage and snack-food

businesses.

There have been dramatic developments in the overall

operation of can-making plants with increasing use of

automated procedures and computerized control systems

and techniques to optimize production processes. There

has been a reduction in the number of operatives, but the

working conditions of those remaining have been greatly

improved. These new developments are ensuring that the

can remains cost effective and reliable.

With thinner gauge plates and faster production

speeds, can-making equipment has become increasingly

precise in its operations. Reliability is all important, and

quality-control measures have been put in place at every

critical step to ensure container integrity. These include

gauge measurements, weld monitors, and leak testing.

Also there is more use of sophisticated statistical control

charts, statistical analysis, and design of experiments to

solve complex problems, particularly in manufacturing

and technical areas.

There have been innovations in coatings technology

such as the introduction of waterborne coatings, coil coat-

ing, powder coating in electrostatic spray systems, and

organic resins that are deposited electrophoretically on

the can components from an aqueous solution. Increasing

use is being made of the new ultraviolet systems, which

dry-sheet coatings and inks in seconds at ambient tem-

peratures

. Also,

laminate polymer ﬁlms have been intro-

duced to compete with liquid organic coatings to metal

substrates that are used for can components.

Early in the 1970s, health authorities and the FDA

became concerned about the increment, if any, of lead and

other heavy metals that are picked up by foods packed in

soldered cans. The manufacturers of baby foods and baby-

formula foods or ingredients, such as evaporated milk,

were the ﬁrst to be asked to reduce the level of lead in

their canned products. Better care during soldering opera-

tions and ventilation resulted in major reductions but did

not eliminate all lead in the respective foods. Currently all

soldered cans being produced are using lead-free solders.

Also, the introduction of the three-piece welded and

CANS, STEEL 215

two-piece cans have virtually eliminated the concern of

lead contamination (22).

BIBLIOGRAPHY

1. Canned Foods, Principles of Thermal Process Control, Acid-

iﬁcation and Container Closure Evaluation, 3rd edition, Food

Processors Institute, Washington, DC, 1980, pp. 7, 141.

2. American Can Company, Canned Food Reference Manual,

McGraw-Hill, New York, 1947, pp. 25–29.

3. S. C. Prescott and B. E. Proctor, Food Technol. 4, 387 (1937).

4. E. J. Cameron and J. R. Esty, Canned Foods in Human

Nutrition, National Canners Association (NFPA), Washing-

ton, DC, 1950.

5. G. A. Hadaby, R. W. Lewis, and C. R. Ray, J. Food Sci. 47, 263–

266 (1982).

6. B. K. Watt and A. L. Merrill, ‘‘Composition of Foods—Raw,

Processed, Prepared’’ in U.S. Department of Agriculture

Handbook, 8th edition, Washington, DC, 1963.

7. Crown Cork and Seal Company, Inc., unpublished information.

8. Safety of Damaged Can Food Containers, Bulletin 38-L, Na-

tional Canners Association (NFPA), Washington, DC, 1975.

9. R. R. Hartwell, Adv. Food Rev. 3, 328 (1951).

10. R. P. Farrow, J. E. Charboneau, and N. T. Loe, Research

Program on Internal Can Corrosion, National Canners Asso-

ciation (NFPA), Washington, DC, 1969.

11. N. H. Strodtz and R. E. Henry, Food Technol. 8, 93 (1954).

12. J. S. Blair and W. N. Jensen, ‘‘Mechanism of the Formation of

Sulﬁde Black in Non Acid Canned Products,’’ paper presented

on June 12, 1962 at the 22nd Annual Meeting of Food

Technologists, Miami Beach, FL.

13. J. E. Chabonneau, ‘‘The Cause and Prevention of ‘Sulﬁde

Black,’’’ in Canned Foods, National Food Processors Research

Foundation, Washington, DC, 1978.

14. ‘‘Tin Mill Products’’ in Steel Products Manual, American Iron

and Steel Institute, New York, 1968.

15. U.S. Pat. 3,834,010 (Sept. 10, 1974), R. W. Wolfe and R. E.

Carlson (to Continental Can Co.).

16. Soudronic AG, CH-8962, Bergdietikon, Switzerland.

17. F. A. Lee, Basic Food Chemistry, Avi Publishing, Westport,

CT, 1975, pp. 163–165.

18. U.S. Pat. 2,189,774 (Feb. 13, 1940), J. S. Blair (to American

Can Co.).

19. U.S. Pat. 2,305,643 (Jan. 5, 1942), A. E. Stevenson and K. Y.

Swartz (to Continental Can Co.).

20. U.S. Pat. 2,875,071 (May 18, 1955), Malecki and co-workers

(to Patent Protection Corp.).

21. U.S. Pat. 4,473,591 (Sept. 25, 1984), W. P. Segner and co-

workers (to Continental Can Co.).

22. Bakker, ‘‘The Competitive Position of the Steel Can’’ in

Technology Forecast, Avi Publishing, Westport, CT, 1984.

CAPPING MACHINERY

In categorizing capping machinery that applies closures to

bottles and jars, the best place to start is with the closure

itself (see Closures). The different types of machinery for

applying these closures have features in common (e.g.,

straight-line vs. rotary). This article provides a basic

description of machinery used for continuous-thread

(CT) closures, vacuum closures, roll-on closures, and

presson closures.

CAPPERS FOR CT CLOSURES

There are four basic types of capper for CT closures, which

are as follows: hand cappers and cap tighteners; single-

spindle (single-head), intermittent-motion cappers;

straight-line, continuous-motion cappers; and rotary con-

tinuous-motion cappers.

Torque Control. All automatic cappers apply the clo-

sures mechanically, but they differ in their approaches to

torque control. In general, torque control is achieved with

chucks or spinning wheels (rollers). Straight-line, contin-

uous-motion cappers generally control torque mechani-

cally, but all other types use either pneumatic or

mechanical means, or combinations thereof.

A pneumatic chuck contains a round ﬂexible ring with

a hole in the center (1). The cap enters the hole when the

ring is in its relaxed state. When air pressure is applied,

the ring compresses and grips and holds the cap while it is

moved to the bottles and screwed on. The air pressure may

be applied by the downward movement of an air piston, or

it may be directed into the space between the ring and the

wall of the chuck (see Figure 1).

The amount of torque is controlled by a pneumatic

clutch operated by pressure from low-level pneumatic

lines (see Figure 2). It contains two or more sets of disks

that are pressed against each other when air is applied,

which connects the chuck and drive shaft. When the cap is

screwed on to the point that the torque being applied

equals the force being applied to the disks by air pressure,

the disks start to slip and the drive shaft is disconnected.

A typical mechanical chuck has jaws that close around

the skirt of the cap to maintain a grip until the closure

application is complete (see Figure 3). A mechanical chuck

can be controlled by a pneumatic clutch, but it also can be

controlled by a spring-loaded clutch or a barrel cam. The

pneumatic clutch is similar to the clutch used for a

pneumatic chuck. A spring-loaded clutch (see Figure 4)

uses a spring to disconnect the chuck from the power

source when the preset amount of torsion has been applied

to the cap (1). Torque can be increased by compressing the

spring by screwing down the collar on top of the spring; it

can be decreased by moving the collar to loosen the spring.

The chuck opens to release the bottle when the torsion on

the cap matches the torsion on the spring.

On some capping machines, the heads are raised and

lowered by a follower riding on a barrel cam (see Figure 5).

Manufacturers of capping machinery have different ap-

proaches to torque control based on the principles or

combinations thereof, described above.

Hand Cappers/Cap Tighteners. With operating speeds of

up to 20 caps/min, hand cappers are used for low

216 CAPPING MACHINERY

production volumes or unusual caps. Hand cappers work

with some kind of handheld chuck. Cap tighteners, a step

up in automation, are useful for pump and trigger-cap

applications, retorqueing after induction sealing, and

short production runs in general.

Single-Spindle, Intermittent-Motion Cappers. Using either

pneumatic or mechanical torque control, these cappers can

theoretically apply up to 60 caps/min, and they are useful

for relatively short production runs. Their versatility is

limited by the intermittent motion that would tend to cause

spillage from widemouth containers.

Straight-Line, Continuous-Motion Cappers. On these ma-

chines, torque-control clutches are built into multiple

spindles that turn the rollers (disks) that apply the caps

(see Figure 6). In contrast to the single-spindle cappers,

the bottles never stop and the speed of capping is limited

only by the speed of the conveyor. The capability of the

machines has traditionally been quoted as 60–300 caps/

min (in contrast to rotary machines that can be more than

twice as fast), but by using multiple spindles, speeds can

be increased greatly; for example, with eight spindles at

four stations, production can be as high as 600/min or

more, depending on the size of the cap and the container.

Figure 1. Pneumatic chuck (1).

Figure 2. Pneumatic clutch (1).

Figure 3. Mechanical chuck with jaws (1).

Figure 4. Spring-loaded clutch (1).

CAPPING MACHINERY 217