Page 505 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 19.1 Introduction 48
TABLE I9.|

General Characteristics of Forming and Shaping Processes for Plastics and
Composite Materials

Process Characteristics
Extrusion Continuous, uniformly solid or hollow, and complex
cross sections; high production rates; relatively low
tooling costs; wide tolerances
Injection molding Complex shapes of various sizes; thin walls; very high
production rates; costly tooling; good dimensional
accuracy
Structural foam molding Large parts with high stiffness-to-weight ratio; less
expensive tooling than in injection molding; low
production rates
Blow molding Hollow, thin-walled parts and bottles of various sizes;
high production rates; relatively low tooling costs
Rotational molding Large, hollow items of relatively simple shape; relatively
low tooling costs; relatively low production rates
Thermoforming Shallow or relatively deep cavities; low tooling costs;
medium production rates
Compression molding Parts similar to impression-die forging; expensive
tooling; medium production rates
Transfer molding More complex parts than compression molding;
higher production rates; high tooling costs; some
scrap loss
Casting Simple or intricate shapes made with rigid or flexible
low-cost molds; low production rates
Processing of composite materials Long cycle times; expensive operation; tooling costs
depend on process

require much less force and energy to process. Plastics in general can be molded, cast,
formed, and machined into complex shapes in few operations, with relative ease, and
at high production rates (Table 19.1). They also can be joined by various means
(Section 32.6) and coated (generally for improved appearance) by various techniques
(described in Chapter 34). Plastics are shaped into discrete products or as sheets,
plates, rods, and tubing that may then be formed by secondary processes into a vari-
ety of discrete products. The types and properties of polymers and the shape and
complexity of components that can be produced are influenced greatly by their
method of manufacture and processing parameters.
Plastics usually are shipped to manufacturing plants as pellets, granules, or
powders and are melted (for thermoplastics) just before the shaping process. Liquid
plastics that cure into solid form are used especially in the making of thermosets
and reinforced-plastic parts. With increasing awareness of our environment, raw
materials also may consist of reground or chopped plastics obtained from recycling
centers. As expected, however, product quality is not as high for such materials.
In this chapter, we follow the outline shown in Fig. 19.1, which describes the
basic processes and economics of forming and shaping plastics and reinforced plas-
tics. We also describe processing techniques for metal-matrix and ceramic-matrix
composites, which have become increasingly important in various applications with
critical requirements. We begin with melt-processing techniques (starting with ex-
trusion) and continue on to molding processes-both categories involving the appli-
cation of external pressure during processing.