Page 541 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Summary

TABLE |9.2
Comparative Production Characteristics of Various Molding Methods
Molding Equipment and Production Economical
method tooling cost rate production quantity
Extrusion M-L VH-H VH
Injection molding VH VH VH
Rotational molding M M-L M
Blow molding M H-M H
Compression molding H-M M H-M
Transfer molding H M VH
Thermoforming M-L M-L H-M
Casting M-L M-L L
Centrifugal casting H-M M-L M-L
Pultrusion H-M H H
Filament winding H-M L L
Spray layup and hand layup L-VL L-VL L
VH I very high; H = high; M 1 medium; L I low; VL = very low.

molding, these costs are relatively low. The most expensive are injection-molding
machines, followed by compressing-molding and transfer-molding machines. Tool
and die costs are also high. Thus, in an operation like injection molding, the size of
the die and the optimum number of cavities in the die for producing more and more
parts in one cycle are important considerations, as they are in die casting.
Larger dies may be considered in order to accommodate a larger number of
cavities (with runners to each cavity), but at the expense of increasing die cost even
further. On the other hand, more parts will be produced per machine cycle; thus, the
production rate will increase. Therefore, a detailed analysis must be made to deter-
mine the overall die size, the number of cavities in the die, and the machine capacity
required to optimize the total operation and to produce parts at minimum cost.
Similar considerations apply to the other plastic-processing methods described
throughout this chapter.
For the production of composite materials, equipment and tooling costs for
most molding operations are generally high, production rates are low, and econom-
ical production quantities vary widely. High equipment and tooling costs can be
acceptable if the production run is large.

SUMMARY

° Thermoplastics can be shaped by a variety of processes, including extrusion,
molding, casting, and thermoforming, as well as by some of the processes used in
metalworking. The raw material usually is in the form of pellets, granules, and
powders. The high strain-rate sensitivity of thermoplastics allows extensive
stretching in forming operations; thus, complex and deep parts can be produced
easily. Thermosetting plastics generally are molded or cast, and they have better
dimensional accuracy than forming thermoplastics.
° Fiber-reinforced plastics are processed into structural components using liquid plas-
tics, prepregs, and bulk- and sheet-molding compounds. Fabricating techniques in-
clude various molding methods, filament winding, pultrusion, and pulforming. The
type and orientation of the fibers and the strength of the bond between fibers and
matrix and between layers of materials are important considerations.