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590 Carraher’s Polymer Chemistry

(a) (b)
Plates with heating channels
Laminates and binding polymer

Heated metal plates

Heated metal plates
Sample

Applied pressure

FIGURE 18.8 Assembly employed for (a) the fabrication of laminates and (b) reinforced plastics.

shape, the various layers are added to a mold, and the molding is completed by heating the mold
(Figure 18.8 (b)). This process is favored over the high-pressure process because of the use of a
simpler, lower-cost mold, and production of strain-free products.

18.9 MOLDING

Molding is a general technique that can be used with plastics and thermosetting materials when
employing mobile prepolymer. Molding is used to produce sheet-like, foamed, hollow, or solid
materials from very small to very large objects. Here we will look at various molding processes.

18.9.1 INJECTION MOLDING

The most widely employed processing techniques for thermoplastics are extrusion and injec-
tion molding. Injection molding is also used to produce some thermoset products utilizing fl uid
prepolymer.
Injection molding involves forcing, injecting, a molten polymer into a mold where is cools becom-
ing solid. The mold separates allowing the molded material to be released. The mold parts are again
joined and the process begins again.
Injection molding allows the rapid, economical production of small to large parts. It provides close
tolerances and the same machine can be used to mold many different articles. Parts can be molded
combining the polymer with other polymers and with any number of additives. Further, it can be run
so that various parts can be easily married as part of an entire or combined-parts production assembly
of an article. The ability to easily modify the operating conditions of the injection molding machine
is important because of the variety of articles that may be needed, variety of material employed to
produce the same (general) article, variety of materials to produce different injection molded articles,
and the variability of supposedly the same polymer material from batch to batch.
Injection molding is not new. A patent was issued in 1872 for an injection molding machine for
camphor-plasticized cellulose nitrate, celluloid. Almost all of the machines used today are recipro-
cating or two-stage screw types. Both types employ a reciprocating Archimedean-like screw simi-
lar to that of a screw extruder. A few are of the plunger type.
A traditional injection apparatus consists of a hopper that feeds the molding powder to a heated
cylinder where the polymer is melted and forced forward by a reciprocating plunger or screw. The
cooled part is ejected when the mold opens and then the cycle is repeated. The molten material
passes from the nozzle through a tapered sprue, a channel or runner, and a small gate into the cooled
mold cavity. The polymer in the mold is easily broken off at the gate site and the materials in the

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