Page 34 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.20 CHAPTER 1

for film, sheet, and extrusion coating. In die design, it is critical to avoid “dead spots”
where the polymer melt can become stagnant and risk thermal degradation. It is also im-
portant that the polymer molecules be allowed to return to an equilibrium position to the
greatest extent possible to minimize the orientation as a result of flow. Laminar flow is de-
sired, and finite element analysis is used to design dies that enable laminar flow to the
greatest extent. Multimanifold dies, such as plate dies, and feedblocks (along with film,
sheet, and extrusion dies) combine melt streams from multiple single-screw extruders to
produce co-extruded multilayer products. This common technique is used for producing
multilayer packaging films, where each layer provides a particular feature. For example,
garbage bags are often multilaminate constructions, as are packaging films where a PVDC
layer may be incorporated for moisture or oxygen barrier properties, and HDPE may be
used as a less-expensive, relatively strong, layer. EVA is a common “bonding layer” be-
tween different plastic layers. As many as eight or more extruders may be used to form
highly specialized, multilayer films.
Common defects encountered with extrusion include effects associated with the vis-
coelastic nature of plastic melts. As the melt is extruded from the die for example, it may
exhibit sharkskin melt fracture and extrudate (die) swell. Diagrams of these defects are
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shown in Fig. 1.16. Sharkskin melt fracture occurs when the stresses being applied to the
plastic melt exceed its tensile strength. Extrudate swell occurs due to the elastic compo-
nent of the polymer melt’s response to stress and is the result of the elastic rebound of the
polymer as it leaves the constraints of the die channel prior to cooling.

(a) (b)

(c)

FIGURE 1.16 Common defects described from rod dies: (a) shark-skin-
ning, (b) die swell, and (c) melt fracture. 23

Pressure generated in the extruder forces the melt through the breaker plate, die adap-
tor, and die. The die forms the melt into the desired shape. Downstream equipment, such
as a water bath cools the melt, and a puller draws the extrudate away from the die and
through the water bath away from the die. Figure 1.17 illustrates the downstream equip-
ment for tube extrusion. The annular tube exiting the die is pulled though a calibration
unit, which maintains the outside diameter of the tube, while being cooled by a water bath.
The puller stretches the molten tube, and a cutter slices the tube into preset lengths. In
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blown film extrusion (Fig. 1.18), the melt forced though an annular die is expanded into
a bubble using air blown through a hole in the die mandrel, stretched axially by take-up
rolls, and cooled by forced convention. This biaxial orientation, thinning of the tube of
film through the internal pressurization of the bubble, combined with the thinning of the
film as it is stretched upwards, results in a strong, biaxially oriented film. Stretching con-
tinues until the freezing line is reached, at which point the film has cooled off to such an
extent as to provide a high enough modulus to resist further deformation. Crystallization

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