Page 510 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

0 Chapter 19 Plastics and Composite Materials: Forming and Shaping

Rigid Plastic Tubing. Extruded by a process in which the die is rotated, rigid plas-
tic tubing causes the polymer to be sheared and biaxially oriented during extrusion.
As a result, the tube has a higher crushing strength and a higher strength-to-weight
ratio than conventionally extruded tubes.

Coextrusion. Shown in Fig. 19.4b, coextrusion involves simultaneous extrusion of
two or more polymers through a single die. The product cross section thus contains
different polymers-each with its own characteristics and function. Coextrusion
commonly is performed in shapes such as flat sheets, films, and tubes, and is used
especially in food packaging where different layers of polymers have different func-
tions, such as (a) providing inertness for food, (b) serving as barriers to fluids such as
water or oil, and (c) labeling of the product.

Plastic-coated Electrical Wire. Electrical wire, cable, and strips also are extruded
and coated with plastic by this process. The wire is fed into the die opening at a con-
trolled rate with the extruded plastic in order to produce a uniform coating. Plastic-
coated wire paper clips also are made by this process. To ensure proper insulation,
extruded electrical wires are checked continuously for their resistance as they exit
the die; they also are marked automatically with a roller to identify the specific type
of wire.

Polymer Sheets and Films. These can be produced by using a specially designed
flat-extrusion die, such as that shown in Fig. 19.3a. Also known as the coat-/ranger
die, it is designed to distribute the polymer melt evenly throughout the width. The
polymer is extruded by forcing it through the die, after which the extruded sheet is
taken up--first on water-cooled rolls and then by a pair of rubber-covered pull-off
rolls. Generally, polymer slieet is considered to be thicker than 0.5 mm, and #lm is
thinner than 0.5 mm.

Thin Polymer Films. Common plastic bags and other thin polymer film products
are made from blown film, which in turn is made from a thin-walled tube produced
by an extruder (Fig. 19.5 ). In this process, a tube is extruded continuously vertically
upward and then expanded into a balloon shape by blowing air through the center
of the extrusion die until the desired film thickness is reached. Because of the molec-
ular orientation of thermoplastics (Section 7.3), a frost line develops on the balloon
and its transparency is reduced.
The balloon usually is cooled by air from a cooling ring around it, which can
also act as a barrier to further expansion of the balloon, thus controlling its dimen-
sions. The cooled bubble is then slit lengthwise, becoming wrapping 191141, or it is
pinched and cut off, becoming a plastic bag. The width of the film produced after
slitting can be on the order of 6 m or more.
The ratio of the blown diameter to the extruded tube diameter is known as the
blow ratio, which is about 3:1 in Fig. 19.5. Note that, as described in Section 2.2.7,
the polymer has to have a high strain-rate sensitivity exponent, rn, to successfully be
blown by this process without tearing.

Plastic Films. Plastic films, especially polytetrafluoroethylene (PTFE, trade name:
Teflon), can be produced by shaving the circumference of a solid, round plastic bil-
let with specially designed knives in a manner similar to producing veneer from a
large piece of round wood. The process is called skiving (see also Section 24.4).

Pellets. Used as raw material for other plastic-processing methods described in
this chapter, pellets also are made by extrusion. A small-diameter, solid rod is
extruded continuously and then chopped into short lengths (pellets). With some