Page 183 - Materials Chemistry, Second Edition

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154 Waste Management Practices: Municipal, Hazardous, and Industrial
6.11.4 INJECTION MOLDING
In injection molding (Figure 6.15), plastic is placed into a hopper which feeds a heated injection
unit. A reciprocating screw pushes the plastic through this long heating chamber, where the mate-
rial is softened to a fluid state. At the end of this chamber a nozzle abuts firmly against an opening
into a cool, closed mold. The fluid plastic is forced at high pressure through this nozzle into the
mold. A system of clamps holds the mold halves shut. As soon as the plastic cools to a solid state,
the mold opens and the finished plastic is ejected from the press (SPI, 1999). Food tubs used for
yogurt and cottage cheese are made by injection molding.

6.11.5 COMPRESSION MOLDING
Compression molding is simply the squeezing of a material into a desired shape by the application
of heat and pressure to the material in a mold. Compression molding is used for forming thermoset
materials but not for thermoplastics. Plastic molding powder, mixed with polymer feedstock and
fillers such as cellulose to strengthen or impart other qualities to the finished product, is placed
directly into the open mold cavity. The mold is then closed, pressing down on the plastic and caus-
ing it to flow throughout the mold. While the heated mold is closed, the thermosetting material
undergoes a chemical change that permanently hardens it into the shape of the mold (SPI, 1999).
Other methods of plastics formation include thermoforming, transfer molding, and reaction
injection molding (SPI, 1999).

6.11.6 PLASTICS RECYCLING
The American Plastics Council estimated that about one half of all U.S. communities, nearly
19,400, were collecting plastics for recycling (Testin and Vergano, 1992). PET and HDPE were the
primary recycled polymers. About 7400 communities collect plastics at the curb and another 12,000
collect plastics at drop-off centers. In addition, thousands of grocery stores in the United States
accept plastic bags for recycling into new trash can liners and other products.
A wide range of consumer products can be manufactured from recovered plastics. Some are
listed in Table 6.8. The U.S. Food and Drug Administration (FDA) regulates the use of recycled

FIGURE 6.15 Injection molding apparatus.