Page 41 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

INTRODUCTION TO POLYMERS AND PLASTICS 1.27

FIGURE 1.22 Orientation of polymer chains
during injection molding.

1.6.2.1 Injection/Compression Molding. Injection/compression molding refers to the
25
process wherein the cavity is not completely filled during injection of the resin. In this
process, the resin is injected while the mold is slightly open. The two halves of the mold
then close, distributing the resin and filling the cavity. This process is useful for products
that require high surface replication, such as compact discs or optical parts. Thin-walled
parts can also be molded by this process, as the pressure losses are reduced, and there is
less risk of premature resin solidification. Figure 1.23 illustrates this process.

1.6.2.2 Lost-Core Process. Products that are hollow or contain complex undercuts can
be fabricated using the lost-core process as illustrated in Fig. 1.24. Core materials are typ-
25
ically low-melting alloys (around 150°C), that are removed by heating the part. Before
each molding cycle, a core is inserted into the mold, and the part is injection molded. The
core is ejected with the part and then melted, resulting in the finished product. It is impor-
tant that the core material melt at temperatures low enough that the plastic material is not
affected by the heating cycle. Air manifolds for automotive and pump parts are often fabri-
cated using this method.

1.6.2.3 Gas-Assisted Injection Molding. In gas-assisted injection molding the mold is
partially filled with polymer, followed by a gas, which presses the polymer out to the sur-
face of the mold, resulting in a hollow part. This process can be used for producing lighter-
weight parts, often with reduced cycle times as a result of less material to cool. Thick-
walled parts can be produced with fewer surface imperfections, such as sink marks, but
equipment costs will be higher. Figure 1.25 shows the gas-assist injection molding
method.

1.6.2.4 Coinjection Molding. Coinjection molding refers to a process whereby two ma-
25
terials are injected into the same cavity. The first material is injected into the cavity and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.