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.
