All printing begins with an idea. A customer works with a
graphic artist to create the design on the package. Once the design is
complete, the art can be transferred to a package layout, scaling
items up or down to get a balanced appearance.
Before a sack is manufactured, its outer ply is usually
printed on rotary printing presses of various types. In general, the
process involves photographic film of the artwork being transferred to
plates which are mounted on a printing cylinder. The plate then
transfers the image to the paper surface.
Historically, (many years ago) the press most often used
for printing sacks has been the letterpress. The letterpress uses a
plate with raised type to carry the image into direct contact with the
paper being printed. Letterpress inks are viscous, tacky pastes which
usually are cured by oxidation.
|Printed image being transferred from printing cylinder.
Today, most paper shipping sacks are printed with a much
more efficient printing process called flexography. A flexographic
press uses photopolymer plates, either digital or conventional,
mounted on a printing cylinder.
This process uses very fluid, fast drying solvent or
water-based inks. It is especially adaptable to high speed, low cost,
in-line printing. Today's flexographic printing process is a worthy
competitor to other major printing processes.
Multiwall sack with four-color graphics.
The speed of a new, eight-color flexographic press can
range from 1200 feet per minute for a single color to 500-800 feet per
minute for process color and can be operated by two people: a
pressman and an assistant.
Newer photopolymer plates provide longer plate life, excellent print fidelity and added uniformity in durometer and thickness.
Also, commonly used are water-based printing inks that can
be printed at high speed in top-quality process printing while
greatly reducing pollution.
|Modern presses provide high-quality printing and can be operated by two people.
Methods of Printing
Here are the main printing methods used with paper shipping sacks:
Flexographic-Stack or Central Impression (CI) Press
A press commonly used for basic multiwall printing, from one to ten colors.
A. Image Carrier (Relief Plates)
The relief plate is made from photopolymer sheets or formed and etched from a liquid photopolymer resin.
B. Method of Printing
The flexible plate is mounted on a cylinder.
The plate picks up ink from the anilox roll which receives ink from
the ink chamber or a rubber roll. The plate cylinder turns and applies
ink directly to the sheet as it presses against the impression roll.
|A pressman strips negative film of graphics before “burning” or transferring it to plates.
Some companies use a small offset press to print labels, forms, letterheads, etc.
A. Image Carrier (Plano Plates)
Colors are separated onto film negatives. An exposure
light then penetrates the clear area of the negatives and burns the
image on a light-sensitive, flat aluminum plate for each color. Today,
plates are usually exposed with a digital image setter.
B. Method of Printing
The plate is attached to the cylinder with special
locking devices. As the plate cylinder turns, the plate is first
coated with water. Since water will not stick to the image on the
plate and ink will not adhere to water on the plate, ink is therefore
only accepted on the image area of the plate. As the plate cylinder
continues to turn, ink from the plate is transferred to a rubber
blanket cylinder and then to the paper.
Inks used for offset printing are of a pasty consistency,
oil-based and more expensive than other inks commonly used for multiwall sacks.
D. Advantages of Offset Printing
- Very high quality
- Inexpensive plates
- Quick and easy plate changes
- Fine line or process printing can be accomplished on
rougher substrates than with gravure or flexography due to the use of
the rubber blanket and the ink hold-out
E. Disadvantages of Offset Printing
- Higher ink costs
- Plates are not as durable as relief plates
- When water is used, presence of the fountain
solution can create problems in creasing, seaming, drying and
discoloration of non-printed areas
Rotogravure Printing (Gravure)
Companies often employ this type of press for
high-quality printing. Some regular multiwall presses also have one
or two gravure units for higher quality with certain colors.
A. Image Carrier (Engraved Cylinder)
The image carrier is normally a copper-coated,
metal cylinder which is chemically or electronically engraved, then
chrome-plated to resist doctor blade wear. The printing image consists
of many thousands of tiny recessed cells per square inch which vary
in depth and width so they can accurately meter the proper amount of
B. Method of Printing
The gravure cylinder picks up ink from the ink
fountain. A conventional angle doctor blade wipes off all excess ink
and allows it to return to the ink fountain. The ink remaining in the
cylinder's cells is transferred to the substrate when the substrate
passes between the engraved cylinder and the impression roll.
The inks used are very thin and can be adjusted to operating viscosity at the press.
D. Advantages of Rotogravure Printing
- Relatively simple mechanical features
- Excellent process and line printing quality
- A very high degree of consistency throughout the run
- Prints a number of substrates such as paper, film, foil, etc.
- Excellent for long runs and repeated runs of medium quantities
E. Disadvantages of Rotogravure Printing
- Extremely high engraving costs
- Very high plate costs for short runs unless the job will be repeated many times
Flexographic printing is the most commonly used
for multiwall printing. It is used for regular line printing and
also for high-quality process printing.
A. Image Carrier (Relief Plates)
Flexible photopolymer plates are produced and then
mounted on a cylinder of the desired size and repeat length. The ink
is pumped into a fountain or directly on the anilox roll.
B. Methods of Printing
1. Three-roll system – The ink is transferred from the
fountain directly to the anilox roll by the fountain roll. The
fountain roll also serves as a metering device to allow all excess ink
to return to the fountain.
2. Two-roll system (reverse angle doctor blade) – The
ink is pumped directly to the anilox roll above the doctor blade
assembly. The doctor blade wipes off all excess ink and allows it to
return to the ink fountain. In both systems the anilox roll applies a
uniform film of ink to the printing plate which is mounted on the
plate cylinder. The plates transfer the ink to the substrate by
pressing against the impression roll.
The inks for flexographic printing are liquid and can be adjusted to the press for proper viscosity.
D. Advantages of Flexographic Printing
- Good quality process and line printing
- Fast press changeover with easy clean-up
- Less expensive plate costs than rotogravure
- Can print on different substrates
- No fire hazard with water-based inks
- Excellent registration
E. Disadvantages of Flexographic Printing
- Replacement of anilox rolls due to wear
- High drying costs
- Rotogravure cannot be matched for consistency throughout a run
Marketing Applications Of Multiwall Sacks
|Pet food sacks are an example of the industry's high-quality printing.
Paper shipping sack users have begun to realize the
value of shipping sacks as a marketing tool to help sell the product
inside the sack. Historically, shipping sacks were constructed of
plain brown kraft paper, with either minimal or no graphics. Now,
rather than just being a way to transport goods to market, shipping
sacks can also carry a product's image – right to the point of sale
At the Point of Sale
Marketers spend millions of dollars every year for
point-of-sale (POS) or point-of-purchase (POP) materials such as store
banners, posters, shelf-toppers and mobiles to influence consumers
who are about to make a purchase. But with improvements in four-color
offset and flexographic printing producing high-quality art and
graphics, the paper shipping sack itself can function as a POS item.
Because of the fierce competition among brands for
shelf space in retail outlets such as supermarkets and discount and
hardware stores, store managers often stock shipping sacks on their
faces due to stacking ease or, to save space, on their ends with one
side facing the store aisle. With this in mind, package designers try
to devise shipping sack graphics which include the side of the sack.
The Traveling Billboard
Image-conscious marketers use paper shipping sacks to
carry their advertising message as the product travels from the POS in
the store into the home. When consumers see related advertisements on
television or in magazines, they are constantly reminded of their
purchase, thus reinforcing buyer loyalty.
Paper Shipping Sacks as a Communications Medium
Many marketers also use paper shipping sacks to carry
coupons, rebate offers and direct response materials, either by
inclusion inside the sack or attached to the sack's face. For example,
a pet food manufacturer might insert a coupon good for a discount on
future purchases. Stapled or "tipped-on" coupons (coupons that are
affixed by an adhesive allowing easy removal) appearing on the outside
of shipping sacks visually attract consumers, thus serving as an
excellent POS persuader.
Marketing Plans Built with the Sack
The role of the shipping sack in purchasing decisions
can be an influential one. In the design and construction stages it is
important to consider the marketing usages of the package. For
example, a seasonal promotion such as a Christmas offer might be
particularly effective if the graphics had a holiday theme. This theme
used on shipping sacks in conjunction with television and print ads,
could serve as a successful marketing and communications tool.