Impact printers are the oldest printing technologies still in active
production. Some of the largest printer vendors continue to manufacture,
market, and support impact printers, parts, and supplies. Impact
printers are most functional in specialized environments where low-cost
printing is essential. The three most common forms of impact printers
are dot-matrix,
daisy-wheel, and line
printers.
The technology behind dot-matrix printing is quite simple. The
paper is pressed against a drum (a
rubber-coated cylinder) and is intermittently pulled forward as
printing progresses. The electromagnetically-driven
printhead moves across the paper and strikes
the printer ribbon situated between the paper and printhead pin. The
impact of the printhead against the printer ribbon imprints ink dots
on the paper which form human-readable characters.
Dot-matrix printers vary in print resolution and overall quality
with either 9 or 24-pin printheads. The more pins per inch, the higher
the print resolution. Most dot-matrix printers have a maximum
resolution of around 240 dpi (dots per
inch). While this resolution is not as high as those possible in laser
or inkjet printers, there is one distinct advantage to dot-matrix (or
any form of impact) printing. Because the printhead must strike the
surface of the paper with enough force to transfer ink from a ribbon
onto the page, it is ideal for environments that must produce
carbon copies through the use of special
multi-part documents. These documents have carbon (or other
pressure-sensitive material) on the underside and create a mark on the
sheet underneath when pressure is applied. Retailers and small
businesses often use carbon copies as receipts or bills of
sale.
If you have ever worked with a manual typewriter before, then you
understand the technological concept behind daisy-wheel
printers. These printers have printheads composed of metallic or
plastic wheels cut into petals. Each petal has
the form of a letter (in capital and lower-case), number, or
punctuation mark on it. When the petal is struck against the printer
ribbon, the resulting shape forces ink onto the paper. Daisy-wheel
printers are loud and slow. They cannot print graphics, and cannot
change fonts unless the print wheel is physically replaced. With the
advent of laser printers, daisy-wheel printers are generally not used
in modern computing environments.
Another type of impact printer somewhat similar to the daisy-wheel
is the line printer. However, instead of a
print wheel, line printers have a mechanism that allows multiple
characters to be simultaneously printed on the same line. The
mechanism may use a large spinning print drum
or a looped print chain. As the drum or chain
is rotated over the paper's surface, electromechanical hammers behind
the paper push the paper (along with a ribbon) onto the surface of the
drum or chain, marking the paper with the shape of the character on
the drum or chain.
Because of the nature of the print mechanism, line printers are
much faster than dot-matrix or daisy-wheel printers. However, they
tend to be quite loud, have limited multi-font capability, and often
produce lower print quality than more recent printing
technologies.
Because line printers are used for their speed, they use special
tractor-fed paper with pre-punched holes along
each side. This arrangement makes continuous unattended high-speed
printing possible, with stops only required when a box of paper runs
out.
Of all the printer types, impact printers have relatively low
consumable costs. Ink ribbons and paper are the primary recurring
costs for impact printers. Some Impact printers (usually line and
dot-matrix printers) require tractor-fed paper, which can increase the
costs of operation somewhat.
