Environmental Engineering Reference
In-Depth Information
Figure 4.7
Upper: schematic diagram of a single-screw extruder. Lower: a
sheet extrusion die for plastics.
Source: Reproduced with permission from Styrolution, Styrolux, BASF.
Theengineering designofthescrewcanbecomplex.Differentregionsofthe
screw perform slightly different functions and are therefore designed with
appropriate pitch and depth. At least three basic zones can be identified:
a feed zone, a compression or compaction zone, and a metering zone. In
the feed zone, the plastic pellets begin to melt, and the material is a mix
of partially melted pellets and liquid melt. By the time the melt is in the
compression zone, it is homogeneous and has compacted during its travel
through the other zones. The metering zone of the screw ensures delivery of
homogenized melt to the die at a constant rate and pressure.
An extruder can also be used with a cylindrical annular die to extrude a
tube of thin-walled plastic. The tube is not allowed to collapse by blowing
air through the mandrel of die to balloon it out until it cools. The collapsed,
continuous tube of film can then be heat sealed and cut into plastic bags.
This process is commonly referred to as “film blowing” and used extensively
to produce plastic bag stock and other plastic films. Several extruders
working in concert can be used to deliver several layers of melted resin
to a single complex die to fabricate multilayered plastic laminates. Each
layer performs a different function such as enhancing barrier properties
(excluding oxygen and moisture), providing good mechanical strength, or
even acting as a “tie layer” that holds two incompatible layers of plastics
together. Multilayered plastic films can be produced in a film blowing
operation as well as in cast film mode.
4.3.3 Blow Molding
This process borrowed from glass technology is used extensively to fabricate
plastic bottles. Initially, a short thick length of polymer tubing (called a
parison) is extruded through an annular die into an open two-piece mold.
Once the mold closes, air is injected through the parison into the mold
to press the molten polymer against the walls of the cavity. The polymer
in mold is cooled, generally using chilled water.
Figure 4.8
illustrates this
process for molding a bottle. While the mechanics of the process are simple,
the pressure, temperature, and parison design have to be carefully
controlled to obtain a consistent quality of product.
