Abstract
Ink jet printing at elevated heights is known to initiate
unsteady laminar flows which lead to imaging defects often
referred to as wood-grain or fogging. An investigation of the
unsteady flow development was c
onducted using both experimental
and numerical simulation tec
hniques. High speed imaging
revealed that the interaction be
tween the air flow induced from
droplet drag and the couette flow
entrained from the substrate
motion develops large eddies which roll along the droplet stream
and lead to unsteady flows causi
ng wood-grain defects. ANSYS
CFX computational fluid dynamics (CFD) simulations are
presented to support the experim
ental results and to provide a
higher degree of understanding of
the flow dynamics. Several
techniques are revealed which illu
strate how wood-grain defects
can be improved. These techniques strive to enable ink jet
technologies to be used in a wider range of applications with large
height variations or which requi
re jetting at elevated heights to
prevent contact with print heads.
In addition to unsteady flow de
fects, two other phenomenons
related to elevated height printing
were included in this study: (1)
excessive nozzle plate wetting and (2) excessive drag on droplets
ejected into a still flow field.
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