FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

Подождите немного. Документ загружается.

have adequate humidity control for proper

performance of papers. Different grades of

paper have different points of equilibrium

where fibers neither give up, nor take on,

moisture. Dry substrates may be more sus-

ceptible to press web breaks because many

flexographic presses pull the web through

the press rather than drive it through the

many nip points. Excessive moisture will

cause welts and wavy edges making further

converting difficult (Figure

pH. The measure of whether a paper is alka-

line or acid is expressed as pH. The pH value

determines the paper’s permanence. Acid

papers with values lower than pH 7 have the

shortest life, while papers with pH values

greater than 7 have maximum longevity.

Sizing.Sizing refers to one or more chemicals

added during papermaking to improve the

substrate's end-use performance. There are

two types of sizing: internal and surface.

Internal sizing is mixed in the pulp at the wet

end of the paper machine. Surface sizing

involves applying a very light continuous film

of starch or other materials to one or both

sides of the web with a size press which is

usually located about two-thirds of the way

down the dryer section of the paper machine.

Benefits of sizing include an increase in the

ability of cellulose fibers to bond to each

other, which reduces the potential for internal

delamination and surface linting. Sizing also

increases the resistance of paper to absorbing

liquids such as water, ink, grease or blood. It

improves ink holdout by slowing the rate of

ink absorption into the fiber structure which

otherwise might contribute to unwanted

wicking, feathering, chalking and print densi-

ty loss. Surface smoothness, surface strength,

Mullen burst strength, tensile strength and

stiffness are enhanced by surface sizing.

Figure

shows a plate plugged with fibers

which were poorly bound to the paper.

Wet Strength. Wet strength is important for

any paper that will be exposed to outdoor

weather conditions. Wet strength is obtained

by the addition of resins in the papermaking

furnish that increases fiber bonding.

Graphics. Conversion to a paper substrate

offers the important advantage of superior

graphic capabilities. The finish of the paper

can be altered to meet the customer need. A

printer is not limited to the use of kraft but

can obtain semibleached or full-bleached

kraft for improved graphic appearance.

Printing on these papers requires following

recommendations similar to those for plain

or coated papers.

Alkaline/Acid Paper

Paper is classified as acid or alkaline

depending upon the papermaking process

used. The pH of the paper identifies this dif-

ference. The factor pH is measured on a scale

of 1 to 14 with values less than 7 indicating

greater acidity and above 7 greater alkalinity.

Paper is traditionally manufactured on the

acidic side (pH 4.0–5.5) because of the rosin

sizing and alum that are used as fixing agents

to waterproof the paper. Clay is the filler com-

monly used for acid paper, and while it

improves opacity it is not very bright.

Titanium dioxide (TiO

) is added to improve

brightness. The use of clay has to be carefully

controlled because excess clay can weaken

the sheet. Titanium dioxide is expensive and

abrasive. It also must be controlled because it

SUBSTRATES 133

A plate plugged with

fibers poorly bound to

the paper.

134 FLEXOGRAPHY: PRINCIPLES & PRACTICES

can dull slitter knives more quickly in con-

verting operations. Acid papers can eventual-

ly yellow and become brittle due to small

amounts of residual acid attacking the fibers.

Alkaline papers are manufactured in the pH

range of 7 to 8. This is considered essentially

neutral. The seemingly small difference

between the pH values are deceptive because

the scale is logarithmic, meaning each incre-

ment is 10 times greater than the last. Calcium

carbonate is the primary filler for alkaline

paper and this improves the appearance and

brightness without the addition of titanium

dioxide. Calcium carbonate can be abrasive in

its natural form, but paper makers use a chem-

ically manufactured precipitated form (PCC)

of the material. The advantages of alkaline

paper include higher natural brightness,

which gives greater printed ink contrast;

improved bulk, for better handling; greater

stiffness to enhance runability; and improved

archival qualities. The United States Govern-

ment has required that all its documents must

be printed on alkaline paper.

Acid and alkaline papers print equally well

by flexography.

Coated Papers

A large portion of papers printed by flexo,

such as label stock, some liner board and gift

wrap, are coated (Figure

). The primary

reasons to coat a paper are to enhance the

appearance and to improve the printing sur-

face. Coatings also provide functional quali-

ties such as water resistance, grease proof-

ing and heat sealability. The paper acts as a

base for the coating., which improves the

surface of the paper by filling in the inherent

micro spaces created by the fibers overlay-

ing each other. It also improves the print for-

mation by making the flocs (hills or dark

areas of densified fiber) and voids (valleys,

areas with less fiber) produced during the

manufacture of paper less apparent. A more

uniform surface gives more consistent print

density, gloss and print smoothness. Coat-

ings must allow the escape of water-vapor

during printing, especially with heavy ink

coverage. The higher a basis weight and

coating thickness the greater the risk of the

paper blistering when drying, especially at

web temperatures above 212° F.

The coating is generally applied on the

paper machine after the size press. Sizing

helps hold the coating on the surface of the

fibers. Coatings are comprised of pigments

(clay or calcium carbonate), a binder (starch

or latex), flow modifiers (carboxyl methyl

cellulose), brighteners and whiteners.

Coated paper is then super-calendered either

on- or off-machine to give the final degree of

surface finish. A super-calender is a stack of

rolls that are a combination of alternating

hard cotton rolls and steel rolls which polish

the surface of the sheet. The number of nips

through which the paper passes determines

the final gloss, smoothness and caliper.

Coatings can be produced in a number of fin-

ishes from matte to enamel. Paper finish will

determine the final ink absorbency, gloss and

drying rate. The color of the coating will

determine the amount of “snap” or contrast

between the ink and the paper surface.

Problems encountered with coated papers

can take a number of forms. A mottled

appearance in the unprinted coating may

produce a mottled print. Imperfections in

A magnified view of the

fiber structure in coated

and uncoated paper.

Uncoated

Coated

the coating surface such as pits, scratches

and streaks will negatively affect the fin-

ished print, especially in full ink-coverage

areas where voids will be noticeable but will

also negatively affect process work. Lower

gloss finishes can produce wet-ink rub

issues. The printing of high gloss papers

needs care in the selection of ink due to dry-

ing and ink- trapping issues resulting from

the higher ink holdout. Excess ink moisture

and some solvent-based inks can soften the

coating binders, so care should be taken to

match the paper and ink to the job and end-

use requirements.

Roll Quality

Most paper printed by the flexographic

method is in rolls, but some paperboard and

corrugated grades are printed as sheets.

While all of the previously discussed paper

properties are applicable to rolls, the quality

or construction of the roll is an additional

important issue. Due to advances in on-line

paper machine technology, variations in

basis weight and caliper are rare.

Paper immediately off the paper machine

is referred to as a log. The log is slit down to

the proper size for the end-use customer.

Rewinding and slitting of the rolls into the

ordered size can result in a number of prob-

lems, especially if the slitters are worn or the

rewind is not at the proper tension. Poor roll

quality can adversely affect converting pro-

ductivity.

Rolls should have uniform tension across

the grain, face or outer surface. Poorly

wound rolls will result in bounce and ten-

sion disturbances that can affect print regis-

ter. This bounce can also result in uneven

unwind and can cause the web to break.

Poor splicing may result in web breaks.

Cracked or damaged edges on a roll may

result in web breaks. Creases and wrinkles

may result from baggy rolls. Linting can

result from dull slitter knives and cause dirty

print due to fibers being liberated and accu-

mulating on the plate and in the ink.

If the roll is not smooth, or appears to be

ragged on the edges, it can be checked by

running a dark cloth over the edge. If an

accumulation of white debris is evident, it is

advisable not to use the roll. The printer/con-

verter should always save the roll identifica-

tion label to report roll condition to the man-

ufacturer.

Determining the amount of paper needed

for a specific job or number of labels has

become easier. Most manufacturers now list

the linear footage for each roll, but some-

times this information may be missing. Basis

weight affects the size of the roll and linear

footage. Lower basis weight paper will have

more linear footage than a heavier weight

paper.

The formula to determine the number of

feet in a roll is as follows:

WEIGHT

LINEAR



OF ROLL



BASIC SIZE



41.667

FEET (LBS) (LW)

WIDTH OF ROLL  BASIS WEIGHT

(IN)

Example: Book paper basis size is 25" x 38",

55#, 40" wide, 2,000 lb. roll

LINEAR



2,000  25  38  41.667  35,985 feet

FEET

40  55

Basis weight may vary up to 5% above or

below the advertised weight for that particular

grade. If the actual basis weight is higher, the

footage will be slightly under; if the actual

basis weight is lower, then more footage will

be available. Most rolls are not run to the very

end due to problems with roll-set curl, so a

minimal thickness of paper is usually left on

the butt roll when splicing to a full roll.

PAPER AND ROLL

STORAGE/HANDLING

Paper is shipped in a moisture-proof wrap-

ping to maintain its production target mois-

ture. Cold paper should be brought into the

SUBSTRATES 135

136 FLEXOGRAPHY: PRINCIPLES & PRACTICES

pressroom and sufficient time allowed for

the roll to come to the same temperature as

the environment. Pressroom humidity is

important when printing paper. Paper per-

forms best when the relative humidity is less

than 8% drier or wetter than that of the

paper. Paper is hygroscopic, meaning it will

absorb or lose moisture. Once the paper has

picked up or lost moisture it is impossible to

change back to the original manufactured

moisture content. Some performance char-

acteristics of the paper will be lost by the

change in moisture. To avoid problems with

dimensional stability, curl, register problems

and dot gain, it is advisable not to unwrap a

roll of paper until ready to put it on press. A

partially used roll should always be wrapped

again to avoid changes in the paper proper-

ties. Paperboard behaves in the same man-

ner. The degree of moisture change in paper-

board is dependent on fiber type, condition

of fiber (degree of recycle) and structure.

Rolls should be handled carefully. Dropping

a roll can crush the cores, making it difficult to

put on the press shaft. Damaged cores can

also result in tension variation that translates

into print misregister, repeat-length abnormal-

ities and slower press production. Bumping a

roll with a fork-lift can damage many layers of

paper in the roll, making it unusable. Rolls

should always be moved with a clamp truck.

PAPER FINISHES

There are basically two types of paper fin-

ishes, uncoated and coated. Uncoated papers

can vary greatly in appearance from a very

smooth and shiny surface to an antique finish

that has a rough, distinctive texture. In con-

trast, coated papers have a smooth finish and

are classified by their gloss or shininess.

Uncoated Paper Finishes

Antique, Eggshell and Vellum. Refers to

uncoated papers with a rough, distinct sur-

face texture. Antique is the roughest, fol-

lowed by eggshell with a pitted texture and

vellum as the smoothest.

Machine and English. Differs from the antique

finishes by having a smoother, more pol-

ished surface.

Felt. Made by having the felt mark trans-

ferred to the paper and is partly dried to imi-

tate handmade paper.

Laid. Paper which has a fine crisscross of

vertical and horizontal lines.

Embossed. This finish is applied after the

paper is off the machine. These papers are

made by passing the paper through two rolls

that stamp a three-dimensional pattern into

the paper.

Supercalendered. This paper is characterized

by a smooth and shiny finish.

Coated Paper Finishes

Enamel Coated. The highest or heaviest coat-

ed paper which is highly supercalendered,

generally to a 75° gloss of 60% or higher.

Dull Coated. This paper is lightly supercalen-

dered to have a low gloss (typically 30% to

40% at 75°) and is very good for readability.

Matte Coated. A fully coated, nonsupercalen-

dered finish that has virtually no gloss

(below 25% at 75°), but tends to be more

prone to poor ink-rub resistance, due to the

large rough-surfaced pigments used to scat-

ter light and reduce gloss.

Embossed Coated. This paper is coated, then

embossed with a three-dimensional pattern,

producing a textured surface having a slight-

ly lower gloss.

Cast Coated. Paper which has its pigmented

coating dried, while held against a highly

polished surface similar to a glossy photo-

graph. Cast-coated papers are usually mea-

sured with a 20° glossmeter rather than an

75° instrument normally used to character-

ize other pigmented-coated papers.

Coated One Side (C1S). Widely used for

labels, the uncoated side is compatible with

adhesives.

PAPERBOARD

There is no strict definition distinguishing

paperboard from paper. Paperboard is sim-

ply referred to as board and is a major raw

material in packaging. It is distinguished

from paper by physical properties like stiff-

ness and thickness. Since the structure con-

sists of cellulose fibers, it has the same

strength-to-weight ratio as paper. Board

includes boxboard, chipboard, container

board and solid fiber. It can be used for fold-

ing cartons, containers for liquids, or con-

verted into drums for transportation of bulk

chemicals. The structure of paperboard is

determined by its end use. Structure can be

single-ply or two to eight thinner plies.

Multiple plies allow for the use of different

types of fibers in each ply. Bonding of the

fibers between the plies is extremely impor-

tant because deterioration in strength during

processing is undesirable. The combination

of ingredients produces a paper product that

is strong enough to protect heavy or large

materials during shipping.

The main difference between paper and

paperboard is caliper. Generally, substrates

with a thickness of 0.012" (12 points or 305 mi-

crons) or more are paperboards; thinner

structures are considered paper. Other differ-

ences are in the description of the compo-

nents that make up the board. An example is

the top side of the board. It is called the felt

side for paper, but is often referred to as the

liner side in board. The liner is usually a ply or

layer of good quality fiber applied to the top

side, which ultimately will be printed. The

underliner is the layer in between the two

external layers. Triplex boards consist of

three different types of fiber plies. Multiplex

boards have more than three layers.

Paperboard, due to its higher thickness, has

greater stiffness. When converting paper-

board, scoring is sometimes necessary to

avoid cracking or breaking the internal struc-

ture of the sheet. A flexographer must consid-

er the side of the board to be printed since the

finish, whether coated or uncoated, will affect

ink drying and printed color.

Paperboard is usually opaque due to its

greater thickness. To obtain good printing

characteristics, paperboard will have a

bleached chemical pulp for the liner. This also

improves the appearance of the final product.

Darker plies are usually used for the interior.

Printing and Handling

Like paper, paperboard is surface-sized or

pigment-coated for good printability. Like

paper, paperboard will absorb moisture and

change its dimensions. The degree of change

depends on the fibrous structure. Paper-

board will often be glued and so the surface

structure must be absorbent. Often the liner

side of the board will be printed and the back

side glued. Some grades of paperboard are

known as double-lined; in this case the two

faces of the web are both of high quality.

Types of Board

Can Board. A paperboard used for composite

fiber drums. The cans can be used for a vari-

ety of liquids and powders.

Carton Board. A paperboard of various com-

positions used for the manufacture of fold-

ing cartons.

Chip Board. A board made from waste paper,

and used for low-grade packaging and book

board.

Coated Board. Paperboards of various grades

that have a coating for high quality graphics.

Cup/Plate Stock. High stiffness paperboard

containing one or more plies of virgin

bleached pulp suitable for converting to

paper cups or plates. The stock may be poly-

ethylene coated before flexo printing or wax

coated after printing.

Double-lined Board. A board which is lined on

both sides; the outer surfaces are lined with

bleached pulp. Used for high quality packag-

ing of food and cosmetics.

Fluting Medium. A board made from semi-

chemical hardwood pulp or waste paper that

SUBSTRATES 137

138 FLEXOGRAPHY: PRINCIPLES & PRACTICES

is fluted and combined with liner board to

produce corrugated board.

Food Board. A hardsized board for water

resistance. Used for food packaging.

Frozen-food Board. Single- or multi-ply board

with resistance to high moisture and water

vapor. This usually is a coated board for high

quality graphics.

Kraft-lined Chipboard. A board with an

unbleached kraft liner on a wastepaper base.

Kraft Linerboard. A strong packaging paper-

board with two-ply construction made from

virgin kraft pulp. The top ply is added by a

second headbox. Used in combination with

a fluted medium for corrugated boxes.

Lined Board. A multi-ply board with a liner ply.

Liquid Packaging Board. Also called milk-car-

ton board, this strong board is usually plastic

coated.

Solid Bleached Sulfate. High quality board

usually made with 100% bleached kraft pulp

and coated on one or both sides to enhance

process color printing.

Solid Unbleached Sulfate. Unbleached chemi-

cal pulp provides an exceptionally strong

board with very good tear and burst resis-

tance; frequently coated white on one side

for printing.

White-lined Board. A board with bleached

pulp liner and the remainder composed of a

mixture of chemical and mechanical pulp.

Used for food packaging.

LABEL STOCK

Label stock is very diverse and can include

paper, laminates, metallic foils, plastic and

synthetic substrates. It can be divided into

groups like coated and uncoated, pressure

sensitive or heat sensitive, gummed or self-

adhesive. Plain paper labels are used exten-

sively but generally are coated on the printing

side so an adhesive can be applied to the back

side of the paper. This type of label is used for

large-volume items like soft drinks, wines and

canned foods. These papers have a tendency

to curl when printed. It is advisable to wet the

back or uncoated side during printing, if an

adhesive has not been applied, to help elimi-

nate the curl tendency.

MULTIWALL BAGS

The first U.S. patent for paper-sack making

machines was granted in the 1860s, when,

due to the Civil War, paper replaced the dif-

ficult-to-obtain cotton sacks. Multiwall bags

replaced the use of cotton sacks to ship flour

and grains to distant cities. Further expan-

sion of this type of shipping method was

fueled by the shortages created by World

War II. During this time the use of the multi-

wall bag came into wide usage. The develop-

ment of a wide range of special papers to

control moisture and insect infestation, plus

the ability to tailor-make the shipping con-

tainer to any required strength to fit difficult

handling, gave a big boost to this industry.

The major advantages of paper-shipping

sacks are low tare weight, flexibility, ease of

filling and handling, low cost, minimum stor-

age, biodegradability and good graphics.

ENVELOPE PAPER

Envelopes are made from many paper

grades, depending on end use and printing

requirements. The grades can vary from

bleached to unbleached kraft, to fine paper,

to some label stocks. In some cases, synthet-

ic paper is used for heavy shipping require-

ments. The ink must be specified for the type

of paper. Most envelopes are printed in-line

by the flexographic method, although a por-

tion are still printed by sheetfed offset and

later die cut into blanks. In some cases, the

blanks are cut, then flexo-printed and folded.

Papers used for envelopes must have good

dimensional stability, bulk and stiffness to

allow for manufacture at high speed with

good printability. Paper moisture is critical

when heavy ink coverage is used because

excess moisture will cause the die cutting to

become ragged and the sheet to lose stiff-

ness. Formation is important when printing

halftones because the higher the line screen

used for the art work, the more the nonuni-

formity of the paper formation will be

enhanced.

GLASSINE PAPER

The primary use of glassine paper is in the

flexible packaging and specialty applica-

tions. It is highly dense and resistant to the

passage of oil, grease and air. These proper-

ties are developed through a damp supercal-

endering process which produces a dense,

smooth, glossy paper. The glassine sheet is

an ideal substrate for heat-seal materials and

barrier coatings. Polyvinylidene chloride,

polyethylene, silicone, hot melts and waxes

all have been applied successfully to glas-

sine paper. Glassine paper can be modified

to accept almost any ink on virtually any

press. It has been used in envelopes, candy

wrappers, liners for cereal and cracker

boxes, potato chip bags, pie bags, medical

packaging, safety jar seals, pouches, soap

wrappers and photographics. Glassine can

be modified for virtually any printing or

strength requirements.

Physical Properties

Originally, glassine paper was highly trans-

parent (low opacity), but exhibited low

paper strength. Today glassine covers a wide

range of levels in strength, color and opacity.

Typically, softwood pulps of the kraft and sul-

fite process constitute the raw material.

Depending on color requirements, the fiber

may be bleached, unbleached or semi-

bleached. Bleached hardwood krafts have

been used, but only in small quantities

because of adverse strength and refining

properties.

Abrasion while in suspension during refin-

ing is used to increase the fibers' surface area.

Hydroxyl groups are exposed on the fiber sur-

face to provide sites for hydrogen bonding.

Natural grease and air resistance is indicative

of the amount of fiber contact developed in

refining. Damp supercalendering makes the

sheet denser to increase gloss and boost resis-

tance to grease and air passage.

Glassine paper contains no fillers and is

translucent (low opacity). These grades can

be made at varying levels of opacity, depend-

ing on weight and end use. Various pouches,

envelopes and bags are currently being

printed on this unfilled or bleached glassine.

In pigment-filled glassine paper, a filler (an

inert pigment such as titanium dioxide) is

added to block the flow of light through the

sheet. Opacity is highly desirable for candy

bar wrappers and potato chip bags to filter

out ultraviolet light. Opaque glassine paper

varies in weight, grease and air resistance,

opacity, gloss and application requirements.

Table 21 summarizes some of the physical

properties of glassine paper.

SUBSTRATES 139

Table 21

GLASSINE BASIS WEIGHT GLOSS OPACITY TEAR SMOOTHNESS

(#/3000 FT

) (%) (%) (G) (ML/MIN)

Lt. Choc. Glassine 25 35–45 70–80 18–22 under 125

Candy Bar Wrap 25 45–55 55–70 18–22 under 125

Bleached Glassine 25 35–55 20–30 16–20 under 125

PHYSICAL PROPERTIES OF GLASSINE

140 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Printing and Handling

Characteristics

Glassine paper is handled differently from

other substrates, mainly because the fibers of

the highly refined sheets are very reactive to

moisture. Many printers and converters use

water-based inks and coatings because of the

volatile organic compound (VOC) regula-

tions. However, improvements in wet-end

chemistry, refining and surface treatment has

produced glassine paper of greater dimen-

sional stability, so it is possible to obtain glas-

sine paper that will do just as well as other

substrates using solvent-based inks.

When converting glassine, the paper

should not be allowed to lose any more than

a minimum of its original moisture. Loss of

moisture can cause brittleness and weaken

web strength. As the paper loses moisture

below the supplied content, a small amount

of shrinkage should be expected.

TISSUE

Tissues are a special category of sub-

strates which run the gamut from semitrans-

parent to totally opaque. They may be made

very weak and soft or surprisingly strong

and hard. Tissue is almost always character-

istically fairly thin, unless combined in mul-

tiple plies by embossing or adhesives.

Tissue-making paper machines are among

the fastest in the industry with production

speeds up to 5,000 ft/min. Tissue formed on

a Fourdrinier wire is further dried in a press

section and against a huge (typically about

18' in diameter) heated Yankee drum. Paper

towels and napkins are among tissue com-

monly printed by flexography using water-

based ink. Producing tissue with fiber

strength to resist linting during printing and

maintain fluffy softness desired in the end

product is a technical balancing act for the

papermaker.

SUBSTRATES 141

Corrugated Board

The different types of

corrugated board and its

construction.

orrugated packaging is the most

popular and cost-effective pack-

aging method for the transporta-

tion of manufactured goods

today. Corrugated board’s main

purpose is to protect its con-

tents by cushioning. It’s very high strength-to-

weight ratio allows this material to be strong

despite its light weight.

BOARD CONSTRUCTION

Corrugated material is a composite made

from paperboard components of the liner,

which is the outside face, and the medium,

which is the internal fluted structure that

gives the board its strength There are differ-

ent types of board construction as shown in

Figure

The Medium

The medium is generally made up of short-

er fibers and is more flexible than the liner. It

is usually made of hardwood fibers from

deciduous trees and is made by the sulfite

process. Resin is added to the pulp to

improve wet strength. It also has a lot of

stretch to it, allowing the paperboard to be

fluted without cracking. Flutes are the char-

acteristic that give the combined board its

rigidity and strength with low weight and den-

sity. The fluting provides the stiffness and the

resistance to crushing in the corrugated box.

Looking at the magnified view of the medi-

um in Figure

, it can be seen that there are

a lot of spaces or voids in the fiber construc-

tion of the medium. These void areas receive

the adhesive and serve as the anchor point for

the bond between the liner and the medium.

Because the medium is normally sandwiched

between two layers of liner, there is no real

concern for sheet appearance

The Liner

The outside face is made of natural kraft.

This kraft paper is made primarily of soft-

wood from coniferous (evergreen or pine)

trees. The pulp is made by the sulfate

process from wood chips. The kraft fiber

can be bleached to lighten dark components.

Triple

Wall

Double

Wall

Double

Face

Single

Face

Inside Liner

Outside Liner

Medium

Center Liner

Medium

142 FLEXOGRAPHY: PRINCIPLES & PRACTICES

After washing and refining, the paper is

made on a specialized Fourdrinier machine.

The difference between this machine and

normal paper machines is the use of two

headboxes. About 80% of the fiber first

deposited on the wire is less refined and

stronger. The remaining fiber is more refined

and smoother and is applied to the felt (top

or finished) side of the sheet.

Two types of finishes are possible, wet fin-

ish or dry finish. The wet finish has a coating

of starch applied before the calender and is

smoother than the dry finish, which is uncal-

endered.

Mottled white liner board is made by the

same method but has both kraft and

bleached kraft in its composition. The top

layer of bleached kraft is not enough to

make a perfectly white layer but is sufficient

to give an improved imaging surface for

packaging applications, thus the mottled

appearance and name.

Combined Board Construction

At the corrugator, the roll stocks of both

medium and liner are converted into com-

bined corrugated board and cut into sheets,

which are also known as blanks. Corrugated

board is manufactured in a number of con-

structions (see Figure

). The simplest form,

known as single-face, is used for wrapping

fragile items, but not for making shipping con-

tainers. Double-face is the most common con-

struction and uses a fluted medium sand-

wiched between two layers of liner. The dou-

ble-wall and triple-wall configurations provide

A magnified view of the

medium shows voids in

the fiber construction.

These void areas receive

the adhesive and serve

as the anchor point for

the bond between the

liner and the medium.

Blown-up View

of Medium

Blown-up View

of Liner

Roll of

Medium

Roll of

Liner

Roll of

Liner

Corrugated

Medium

Medium:

Liner:

Paperboard used to line or face corrugated

core board (on both sides) to form shipping boxes.

• Additional refining

• Smooth surface for printing

Bottom Liner

Top Liner

Blank

Fluted Medium

Light-weight board used for the fluted

inner ply of corrugated box stock.

• Stiffness

• Resistance to crushing

• High stretch

• Gaps provide holding volume for adhesive