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3. PAPER MAKING
Once the wood has been
reduced to pulp it is often made on a machine similar to a Fourdiner into
a semi-paper form known as “wet-lap”, for shipping to, or transport
around, the paper mill. The paper mill receives the wet-lap as rolls,
bundles, or pallets of pulp sheets. The wet-lap usually requires
additional refinement and treatment before being used to create the
finished paper product.
3.1 FIBER SLUSHING,
BEATING AND REFINING
The wet lap and any
make-up materials, used in creating blends, must be mixed together,
repulped, beaten and refined, before it is suitable for making paper. In
rag paper, the wet-lap cottons, and linens are measured by weight and
placed in a hydrapulper or digester with water, and “slushed” back into a
pulp mixture. The digester disperses the fibers into the water with
minimal fiber damage, by gentle agitation and heated water. After the
wet-lap has been returned to pulp, it’s blown out of the digester and fed
into a breaking engine.
The breaking engine and
Hollander are the same as discussed in section 2. The main purpose of
beating engine (sometimes referred to as the “Bertram” after it’s
manufacturer) is to further breakdown the pulp into separate fibers and
filter out of any non-paper-making materials. The beater roll is kept
well off of the bed plate and is gradually lowered ensuring that there are
no sheaves (fiber bundles). After a short period of time in the breaking
engine, the pulp is transferred to the Holland beater, where the fiber
properties are further developed.
Bertram Beater refining the pulp
In the beater, the
outer fiber wall is removed and the fiber’s cells are allowed to swell.
At the same time the fiber length is adjusted by adjusting the distance
between the blades on the drum and the blades or surface of the bed
plate. As the drum revolves the blades collect fibers on the leading edge
of the blade. As the rotating blades approach the stationary blades the
fibers are either sheared or compressed between them, resulting in either
frayed or shorter fibers. While the pulp is in the beater additives can
be introduced to alter the properties of the finished paper. Some
additives, such as clay (aluminum-silicate), Calcium carbonate or Titanium
dioxide are introduced to increase opacity and increase brightness; dyes
and pigments are added to adjust color and brightness, while Aluminum
Sulfate activated rosin and Synthetic Alkaline resins are added for
internal sizing. Internal sizing helps the paper to resist water and ink
Pulp in the Bertram Beater
After the beater, the
pulp is further manipulated by refiners. Refiners increase the bonding
and surface areas of the fibers through a process known as fibrillation.
Fibrillation is the fraying of the very fine, threadlike structures,
called fibrils, that make up the fiber. The degree of fraying and the
fiber length are determined in the same manner as the Hollander, by the
type of blades used, the distance between them, and the time spent in the
refiner. Two main types of refiners are used for this purpose, the
conical refiner and the disk refiner.
The most common types
of conical refiners are the Jordan and the Claflin. Both refiners
consists of a stationary conical structure, lined with bars or blades
running in the same direction as its axis, and a rotating cone structure
called the “plug” which can be moved in or out of the stationary conical
structure. The pulp enters the narrow end of the refiner and is forced
out near the large end.
Disk Refiners can be
used to further refine stock from a beating engine or in place of it.
Disk refiners consist of at least two disk, that have bars or “ribs”
across their faces. They are positioned face to face, so that one disk
can rotate, while the other disk is stationary or rotated in the opposite
direction. The refiner can be configured so that four disk are used
instead of two, with two of the disk positioned back to back. The pulp
enters either from the center and is discharged out through the perimeter,
or enters through the perimeter and is discharged through the center.
Increase in refinement results in
decreasing fiber length, increase in intra-fiber bond breaking (internal
fibrillation), fiber end maceration (external fibrillation). These
processes increase surface, bonding area, and fiber length continuity,
which increase the fold, tensile and bursting strength, density, and
retention of filler. The increase in refinement, decreases fiber length,
which decreases the tearing strength and increases the paper’s sensitivity
to humidity, changes resulting in paper curling and dimension change.
Brightness and opacity are also adversely affected by higher levels of
Final Screening and
Hand-made paper making
can be separated into two categories, professional manufacturing and
amateur or artistic papermaking. Professional hand-made production
represents the elite of the field. The finest paper available is produced
by the professional hand-made process and requires the highest expertise
and skill. The other category is composed of the amateur or professional
artist who make paper from improvised or small operations, creating sheets
for aesthetical purposes.
production, although relatively slow, labor intensive, and expensive,
produces the finest paper product available. Because of these reasons
only the finest grades of linen and cotton are used. The “rags” are
hand-sorted and pulped in small refining and breaking engines. After
breaking and refining, the pulp is screened and pumped into head-boxes or
stuff-chest, which in turn supply vats. Three craftsmen, two skilled and
one lay, are tasked with forming the sheets of paper. The skills of the
vatman and coucher determine the quality and consistency of the sheet. To
form the paper the vatman uses a wooden frame, covered by a wire screen of
either a “woven” or “laid” pattern, called the “mold,” and a deckle. The
deckle keeps the paper pulp from flowing over the edges of the mold.
The exact method of
forming paper varies with each vatman, but the basic operation is the
same. The vatman holds the edges of the mold about arms length with the
forming wire face up and the surface parallel to the surface of the pulp.
The vatman then dips the front of the mold/deckle into the vat submerging
approximately 3/4 of the mold/deckle. The vatman raises the mold/deckle
and tilts it back towards himself than away causing a small ripple and
evenly distributing the pulp as the water flows through the screen. He
then produces a side to side tilting or shaking motion causing the
settling fibers to orient themselves perpendicular to those already
settled. The settling fibers are allowed to “set” and the excess water
allowed to drain. The vatman is finished forming the sheet and places the
mold/deckle at an angle for further draining. After a couple of minutes
the coucher takes the drying mold/deckle flips and allows the other side
to drain. The coucher takes the mold/deckle and, while holding it at an
almost vertical angle, removes the deckle and places the mold face down
onto a felt. The coucher then positions and places another felt on top of
the drying paper sheet. The top felt must be placed with great care, any
surface disturbance will ruin the paper. After a completed post (stack of
sheets) has been formed, the “layman” or “layer” removes the post to the
pressing room, where the post is placed into a hydraulic press and pressed
with 100 to 150 tons of pressure, removing the excess water. The layman
then removes the sheets and creates another sandwich of paper and felt,
this time using felt board. This pile is placed in press, under slight
pressure, overnight. This is called “exchanging” or “parting” and can be
repeated depending on the type of finish desired.
Drying Sizing and
finishing can be done by hand or by machine depending on the size of the
operation and the size of the order. By hand the paper sheets are hung in
drying lofts. Sizing by hand involves quickly and skillfully skimming
sheets through a size vat without stretching the paper, or pooling the
size. In another method sizing is performed by fanning out the sheets in
a shallow tank, immersing, draining and pressing. There are also
continuous feeding machines that submerge the sheets through a size vat
and then presses and dries the sheets.
The majority of paper
is manufactured on the Fourdrinier paper machine. Nicolas-Louis Robert
developed his paper machine under the employment of paper maker Francois
Didot and applied for its patent in 1798. Didot purchased the patent from
Robert in payments, but fell behind following confrontations between the
two. Robert took back his patent, but not before Didot had given the
plans to his brother-in-law, John Gamble, who owned paper mill in
England. Looking for investors, Gamble interested Henry and Sealy
Fourdrinier in the development of the machine. Under the suggestion of
Gamble, the Fourdrinier brothers employed Bryan Donkin to build and
perfect the machine at the Frogmore mill, Two Waters, Hertfordshire,
England. Because of a loophole in their patent, the Fourdrinier brothers,
like Robert, never received royalties from their machine. But unlike
Robert, who was financially backed by Didot, the Fourdrinier brothers ran
into financial difficulty, due to lack of return on their investment.
Although they never received any monetary recognition of their efforts the
single forming wire machine carries their name.(hunter)
The Fourdrinier paper
making machine is composed of three main sections: the forming section,
the press section, and the dryer section. A Paper slurry consisting of
around 0.5-1.0% fiber, is pumped into a box where it flows out through a
slot onto a moving wire belt. Once on the belt the water is removed by
draining and suction, leaving the fibers to form a very wet, and weak
paper. The paper is then pressed, heated, dried, resulting in a
continuous roll or “web” which can be further finished as desired or
SIMPLE REPRESENTATION OF THE MODERN
FOURDRINIER PAPER MACHINE
Wet end of a Fourdrinier paper machine. Head
box is near the bottom. The drying section is towards the back .
The forming section of
the Fourdrinier constitutes what is called the wet end of the machine and
is what the Fourdrinier brothers and their engineer, Donkin, perfected.
This section consists of the head box, the forming wire, foils, suction
boxes, couch roller, breast roller and dandy roll.
Pulp is pumped from the
machine box through the screens and cleaners to the head box. The purpose
of the head box is to deliver a uniform slurry to the forming wire. There
are several different designs, but all incorporate a method to induce
turbulence (deflocculation), while preventing cross currents, which would
inhibit the uniformity of the stock. The simplest design is the gravity
fed head box. It uses height/weight level difference to force the pulp
through several baffles and a through a perforated rotating cylinder,
before flowing through the apron and slice. A gravity-fed head box can
deliver an eight-inch stock depth at a rate of 400 feet/minute.
If faster production
speeds are required the stock must be fed under pressure. These machines
can operate at speeds greater than 4,000 feet per minute. The pressurized
head boxes are usually hydraulic and the stock is forced through conical
injectors, through a perforated plate and through a horizontally split
apron and the slice. The apron height and the slice height, which control
the jet of pulp can be independently adjusted by hydraulics.
The pulp flowing onto
the forming wire is approximately 0.5-1.0% fibers, with the make-up
consisting of water. As the water is removed from the slurry, the fibers
settle onto the surface of a traveling wire, forming a wet mat of paper.
Therefore, the main objective of the forming section is the controlled
removal of water. Originally gravity allowed the water to drain through a
brass forming wire 60-70 mesh per inch, 40-50 feet length and 70-90 inches
in width. But as production speeds increased, more efficient methods were
developed.(Sindall) The forming wire, now a fine polymer screen with
about 65 meshes per inch, carries the paper slurry over table rolls, foils
and suction boxes, providing precise control over drainage and agitation
control. As the slurry exits the slice onto the wire, the water starts
draining from the suspension. Water jets are positioned over the edges of
the forming wire to control the width of web, creating what’s called the
deckle edge. The first fibers forming the mat on the wire are oriented in
the direction of the machine; this is the wire side of the paper. If the
rest of the fibers in the slurry were allowed to orient themselves in the
same direction, the paper would have poor tear resistance and surface
properties. If gravity was the main method of dehydration, the machine
would have to be run at low speeds to overcome the orientation problem,
the alternative is to remove the water quickly while the fibers are still
agitated from the effects of the headbox.
The first set of
de-watering elements is a bank of table rolls. In earlier designs, table
rolls were a series of small solid rollers. If they are used today, they
much larger and are used as only the first water removal step. The
rotation of the roll in contact with the covered wire causes a vacuum to
form between the two, which pulls the water from the web.
Bank of table rolls removing the water
from the pulp
With increasing speeds
the table rolls cause problems with paper uniformity and aren’t able to
remove enough water before the presses. Foils have replaced most, if not
all of the table rolls. Foils remove water using a doctor blade on the
bottom of the forming wire. The blade causes a difference in pressures,
which draws water from the web behind the blade. This method allows for
more control over the removal process and is not significantly affected by
Water removal can be further enhanced by
placing a vacuum on the foil drainage system. The foil is essentially the
same as that diagrammed above with the addition of vacuum pumps. After the
foils, water is further removed using flat suction boxes. The suction
boxes remove the majority of the water, changing the stock consistency
from 2% to 20% fiber content. Above the first couple of suction boxes a
skeleton roll covered with wire may ride on the top of the paper mat.
This roll called a “dandy roll” compresses the paper, releasing any
trapped air and improving the surface. The dandy roll can be covered with
various wire patterns, which may simulate the forming wire and may have
recessed or raised elements-designs imparting a watermark onto the paper.
In areas where the watermark elements, usually a wire design, are above
the surface of the dandy roll, fewer fibers are allowed to settle, and the
paper appears light. If the watermark elements are below the dandy roll
surface, more fibers are allowed to settle than in the rest of the paper,
and the paper appears darker in these areas.
An alternative to
using a dandy roll to create watermarks is the Molette. The Molette is a
rubber stamp roll located before the wet press of the machine. This type
of watermark actually embosses the paper and squeezes the fibers to the
edges of the stamp.
3.4 TWIN WIRE
A variation on the
Fourdrinier was developed in the 1960’s and employed the use of two
forming wire, allowing the paper mat to be dried from both sides
The First Twin Wire
machines were constructed so that the headbox sprayed a vertical stream
between the forming wires at the nip of twin breast rolls. The paper web
was then further drawn vertically, while vacuum boxes operate from both
sides. Newer designs returned to a horizontal feed system with both
forming wires traveling horizontally and vacuum boxes drawing suction from
below and above the web. Another variation is the use of a de-watering
mat above the suction boxes on a Fourdrinier; this is referred to as a
Hybrid Twin Wire Machine.
3.5 CYLINDER MOLD
In 1809, in
Hertfordshire, England, John Dickinson invented another mechanical method
of manufacturing paper, the cylinder mold machine. Dickinson approached
the problem in a slightly different way then Nicholas-Louis Robert.
Instead of pouring fibers through the forming wire, his machine dipped the
forming wire into a vat, much in the same manner as hand made paper. This
allowed him to create water marks and four-sided deckled edges comparable
to hand couched paper.
The modern cylinder
mold machine, also known as “cylinder vat” or “mold made,” is used to make
fine bond paper with shadowed watermarks, currency and security papers,
art papers, extremely heavy stock, corrugated cardboards, and multi-ply
The key to the cylinder
mold machine is the use of a cylinder wire covered by the forming wire
(now called the cylinder blanket or cover), partially submerged in a vat
full of pulp. As the cylinder rotates into the paper stock, the slurry
flows onto the surface of the cylinder, and the water flows through the
wire cover to the inside of the cylinder where it is discharged. The
fiber mat that accumulates onto the cylinder surface is removed or
“couched” by a traveling felt belt. This traveling felt “the cylinder
felt” is sometimes referred to as the forming wire, even though the paper
is already formed by the cylinder. If multiple layer paper is desired,
several vats and cylinders can be placed in series with the paper web
acting as the cylinder felt for the additional paper mat. There are two
main cylinder vat designs, contraflow and direct flow; and the cylinder
felt can be above or below the drying stock.
3.6 PRESS SECTION
After the paper has
been partially dried while on the forming felts and wires, it still
contains vast amounts of water which must be removed to stabilize and
strengthen the paper. The first of these additional drying sections is
the press section. The press section or “wet-press section” contains
several drying felts and suction pick-up (vacuum) rolls, which remove
water from the paper mat until it reaches about 50%. Although the paper
web can support its own weight, carrier felts support the web, while at
the same time removing water from the surface and directing it through a
series of vacuum rollers. The transference of moisture from the web to
the felt is enhanced by the nips (the contact points between the rollers)
of the rollers, suction boxes, and rollers.
3.7 DRYER SECTION
After passing through
the press section, water is removed until the consistency of the web is
about 5-12 % water. The removal of water is accomplished by a series of
steam heated drums “dryer cans,” which evaporate most of the water from
the web. The web is forced against the dryer cans by the dryer felt (a
synthetic material that allows water vapor to pass through without
absorption). As the water vapor leaves the dryer felt it is pushed away
by forcing air across the felt. The dryer cans are usually around 4-5
foot in diameter and have highly polished shells.
Another type of drying
section utilizes a single large highly polished can instead of several
smaller cans. This system, known as a Yankee dryer, is used with light
grades of paper, like tissue.
After the drying
section, the web is subjected to several finishing steps prior to shipping
it as a final product. The web can be sized, giving the paper surface
resistance, or if other properties are needed, the web can be surface
coated. The web can also be supercalendered giving the surface a very
smooth the uniform surface. In the final stages the web is rewound and
slit into two or more rolls and if needed sheeted.
resistance to liquids on the paper surface, a property necessary for
paper used for writing or printing. Without external sizing, ink would
bleed and feather. External or Surface sizing can either be performed on
the paper machine or on a stand alone unit.
Machine sizing can be
performed either by running the web through a size vat or by running the
web through a size press. In the case of the size vat, the web, after
exiting the dryer section, is directed down into a vat and through another
set of drying cans. Size presses are located after the between two dryer
sections and applies a coat of sizing by transference from rollers and the
metering is accomplished by the nip.
The most common types
of sizing consist of pigments and starches, although animal glue and
glycerin can also be used (art and banknote papers).
sizing (starch) applied to the paper
Coating paper may be
desirable or necessary to improve optical, printing/writing, and/or
functional properties. Functional properties can be for protection form
liquids, oils, gases, chemicals, improve adhesion characteristics, improve
wear, or some other property.
Coatings can be
classified as aqueous, solvent, high solids, or extrusion, coatings.
Aqueous coatings, used for commodity papers, contain water soluble binders
and are applied as a liquid. Common aqueous binders are Casein, Starch,
Protein, Acrylics and Polyvinyl Acetates. Solvent Coatings are used in
situations where the binders aren’t soluble in water and are used with
specialty papers. High solid and Extrusion coatings are used for
specialized papers, where chemical, gas or liquid resistance is
necessary. High solid coatings are applied as a coating of monomers and
are polymerized by UV or electron curing. Extrusion coatings are applied
as a molten film of wax or polymer.
CALENDERING, COCKLING, AND EMBOSSING
After the chemical
processes have been completed, physical processes, like super calendering,
cockling, and embossing, can be used to create the desired surface texture
to the paper.
Super calendering uses
friction and pressure to create a very smooth and glossy paper surface.
The super calender consists of a stack of rollers having surfaces
alternating between steel and cotton in construction. There is enough
pressure between the steel and cotton rollers to slightly compress the
cotton surface causing a drag. The difference in surface speed on either
side of the nip creates friction, which polishes the paper surface.
The cockle finish on
many bond writing papers is created by vat sizing the web then subjecting
it to high velocity air dryers under high tension, then under low
tension. The finished paper is usually heavily sized and has the
characteristic rattle associated with high quality bond paper.
Embossing is achieved
by running the web through an off-line press, where it is subjected to an
engraved cylinder. The concept is similar to the dandyroll, but since the
paper fibers cannot be redistributed the surface of the paper is raised or
SHEETING, AND SHIPPING
Once the paper roll
(machine log) is reeled from the paper machine it is removed and
transferred to a rereeler or a machine winder. A rereeler unreels the web
from the mandrels to create a full log. During this process any defects
can be removed and the web spliced. A machine winder is similar to the
rereeler, but is able to slit the web into multiple, narrower rolls.
These rolls can be further finished by supercalendering, embossing, etc.,
sheeted, or wrapped and shipped.
Paper roll at the winder
If the finished product
is sheeted paper, the rewound rolls are transferred to machines known as
cutters. The cutters can slit the web to form multiple narrower webs and
cut across the web creating sheets. The paper rolls are placed onto a
stand at one end of the machine. As the web unwinds it can be slit either
adjusting the web width or creating several parallel webs. After the
slitters, the web travels under a revolving knife, which cuts the web into
sheets. After being cut the sheets are jogged through an on-line
inspection system which checks caliper and dimensions. If the sheet does
not conform it drops down into a sheeter for recycling as broke. After
the cutters, the paper stacks are placed into guillotine trimmers, where
the edges receive their final trim.
paper rolls have inner headers (circular disks) applied to the ends, are
wrapped with a heavy moisture resistant paper or plastic and sealed with
outer headers. The sealed rolls are then placed flat, to prevent flat
spots from forming, and shipped. Sheeted paper can be prepared for
shipping in various ways depending on the size of the finished product.
If the finished sheets are small, such as 8 1/2” X 11,”
the sheets are stacked in junior cartons, cross
stacked on pallets, strapped and wrapped. Similarly larger sheets can
also be carton packaged, strapped and wrapped. Large orders, such as
those for printers, can be bulk packed on skids (slightly different
dimensions and design than a pallet), wrapped, and strapped.