Biomedical Engineering Reference
In-Depth Information
direction. Non-uniformity in the machine direction is caused by eccentricity
of the roller with respect to its axis, roller vibration or a non-uniform feed.
The gap size can also change during operation because of hydrodynamic
forces that take place in the nip which deflect the rollers. In this situation,
the final product is thick in the middle and thin at the borders. In order to
avoid this problem, three methods have been developed:
∑ roll-crown,
∑ roll-crossing, and
∑ roll-bending.
In the roll-crown method, the roll diameter in the center is slightly greater
than at the edges. This allows for compensation of roller deviation. Roll-
crossing and roll-bending involve a continuous adjustment of gap size
distribution. In the roll-bending method, a bending moment is applied on
roll ends by additional bearings in order to increase or decrease bending
caused by hydrodynamic forces. roll bowing is another problem caused by
pressure that occurs in the nip region. This problem can be overcome by
placing rolls in a slightly crossed pattern rather than completely parallel. The
knowledge of pressure distribution in the nip is necessary to evaluate gap
thickness distribution and the load on the bearings. a calender line requires
a long time to reach the steady state, or, in other words, to stabilize. As a
result calender lines are used for long runs (Han, 2007, Tadmor and Gogos,
2006, Osswald and Hernández-Ortiz, 2006).
3.7 Coating
In the coating process, a liquid film is deposited on a substrate. Wire coating
is a good example of this type of process. it is used for primary insulation
of conducting wires, as well as increasing wire mechanical strength and
providing protection. The unwound bare wire is preheated to a temperature
above the glass transition temperature or the melting temperature of the
polymer that is to become the coating layer. This promotes adherence of
the layer to the bare wire and removes moisture or oils from the conductor
surface. The wire is fed into a 'guider tube' of a die. at the exit it meets
the molten plastic that covers it circumferentially. The coated wire is then
exposed to an air or gas flame for surface annealing and melt relaxation
which improves coating gloss. The coated wire then enters a cooling trough
with water usually employed as cooling fluid. Trough length depends on wire
speed, wire diameter and insulation thickness. it increases as these parameters
increase. It is longer for crystalline polymers, because crystallization is an
exothermic phenomenon.
after cooling, the wire passes over a capstan where its tension is controlled
and further cooling takes place. It then passes through a capacitance-measuring
Search WWH ::
Custom Search