Page 622 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Technology 585

Driven pinch rollers

Guides Wind up roll

Blown tube

Mandrel

Extruder Die

Air

FIGURE 18.5 Film formation employing extrusion.

extruded as relatively thick sheets. These sheets may also be produced by pressing a stack of fi lm at
elevated temperature (laminating) or by the calendering process.
Wire is coated by being passed through a plastic extruder, but most materials are coated from
solutions, emulsions, or hot powders. The classic brushing process has been replaced by roll coat-
ing, spraying, and hot powder coating. The application of polymers from water dispersions to large
objects, such as automobile frames, has been improved by electrodeposition of the polymer onto the
metal surface.
Printing inks are highly filled solutions of resins. The classic printing inks were drying oil-based

systems but the trend in this almost billion dollar business is toward solvent-free inks.

18.6.1 CALENDERING
Calendering is simply the squeezing or extruding of a material between pairs of corotating, paral-

lel rollers to form film and sheets. It can also be used to mix and impregnate such as in the case of
embedding fiber into slightly melted matrix material to form impregnated composite tapes. It can

also be used to combine sheets of material such as sheets of impregnated paper and fiber woven and
nonwoven mats to form laminar composite materials. It is also used in processing certain rubber
material and textiles. Calendering is also employed in conjunction with other processing techniques
such as extrusion in the formation of films from extruded material. It is also used to coat, seal, lam-

inate, sandwich, finish, and emboss.

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