Page 192 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management

FIGURE 5.16 Injection molding machine 171

the extrusion process, the injection molding process as shown in Figure 5.16
has a spiral screw which is fed through the hopper and presses the mix to a
strong, split steel mold (Lardinos and van de Klundert, 1995).
The rotating screw pushes the plastic while the heating element plas-
ticizes it. The melt is pushed into a closed steel mold which is kept cool by
a water jacket that accelerates the solidification process. Usually the process
is manually stopped, this is achieved by allowing the mold to have one small
hole allowing the exit of extra material, which indicates a full mold. The
worker then stops the process, opens the mold, removes the piece and leaves
it to cool. He then chops off all unwanted parts from the piece (Lardinos and
van de Klundert, 1995).
The process of blow molding is used to produce bottles. The operation
mainly takes place in two stages. First, the plastic is extruded in the form of a
tube, then it passes through a mold which closes around the tube. Compressed
air is then blown into a hollow mandrel which exists in the closed mold to
force the tube to take the shape of the mold. The product is left to cool, solid-
ify, and later removed from the mold: the process is then repeated (Lardinos and
van de Klundert, 1995). Plastic bags can be made using the blow-molding
machine as shown in Figure 5.17. The capacities of the blow-molding
machines (film) vary between 100 and 200 kg per day depending on the power
of the motor which ranges from 10 to 15 hp respectively.
In film blowing, the plastic is first extruded from a tubular die and
moves upward to a film tower with a collapsing frame, guide rolls, and pull
rolls driven by a motor. Air is compressed through the center of the die and
inflates the tube. Air rings are mounted above the die to cool the outside sur-
face. The tube is sealed and cut once it passes through the pull rolls.