Ucr; Advantages & Disadvantages To Ucr - Microtek ArtixScan 4500t Reference Manual

Adding black to CMY reduces ink coverage TAC, and thus improves the ability of
paper to firmly hold each layer of wet ink, known as ink trapping.
So, from the concerns about ink coverage and ink combinations have come
tried-and-true approaches to producing excellent films for excellent separations.
Two aspects of the offset printing process are undercolor removal (UCR) and gray
component replacement (GCR).

UCR

Undercolor removal is the practice of removing quantities of yellow, magenta, and
cyan ink from the dark neutral areas in a reproduction and replacing what was
removed with an appropriate amount of black. Kodak implements UCR in its Color
Profiles within a TAC constraint: CMY gets replaced by the maximum amount of K
up to the TAC limit—so you get the highest possible density.
The neutral center of both diagrams show different UCR/TAC settings. With UCR
applied, less process inks and more black increases the density in the shadows.
Advantages & Disadvantages to UCR
Undercolor removal within a TAC constraint reduces the problem of printing four
solid layers of ink, one on top of the other, while each previous layer is still wet.
Reducing the ink coverage, TAC, improves the ability of the paper to firmly hold
each layer of wet ink.
Less ink means better control, and faster drying times. Also, replacing cmy ink with
the less expensive black ink has proven more cost efficient on long runs that use
more ink.
Also, blacks and neutrals that print black are not influenced easily by shifts in the
chromatic inks which cause color casts in the shadow tones. Separations produced
with greater UCR produce darker blacks which result in better shadow detail.
However, some printers don't like high UCR because of on-pr
contrasty rosettes.
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Reference: Appendix 139