Page 92 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012K-587 July 26, 2001 10:35

462 Plastics Engineering

II. DESIGN CONSIDERATIONS is very important that these early mistakes not be repeated,
so it is essential that designers are aware of the advantages
Plastics offer the designer a very wide range, and interest- and limitations of plastics. This applies to their physi-
ing combinations, of attractive properties. Their ease of cal properties, their performance characteristics, and their
manufacture into complex shapes also presents exciting molding/fabrication methods. Most material suppliers
opportunities for the creative mind. As a result, plastics are now recognize that it is better not to choose plastics if the
serious contenders for most types of engineering compo- performance specification dictates use of another material.
nents, and they are only excluded from those in which the Of course, this puts a lot of responsibility on the de-
demands are extreme—for example, continuous-service signer because it means that he or she must be aware of
◦
temperatures above 400 C or situations in which high the broad spectrum of properties of a very wide range of
electrical or thermal conductivity are required. plastics and also be knowledgeable about their processing
However, it is important that the designer recognize that characteristics. Not all plastics can be injection-molded.
plastics are only part of the portfolio of materials—metals, (See Table II.) Many shapes cannot be produced by blow-
ceramics, etc.—that are available. In the early days of plas- molding. Processes such as rotational molding may put
tics it was very common to find them misused, so that the restrictionsontheproductionrate.Inmanycasesthemold-
public image of these materials was low. The problem was ing operation will alter the properties of the plastic. For
not with the plastic but with the designer, who would quite example, the molder may be obliged to use process condi-
often replace a metal article with a plastic one without any tions that improve the flowability of the melt, but this will
redesign to cater to the special characteristics of plastics. It in turn reduce the ductility and toughness of the product.

TABLE II Compatibility of Plastics with Processing Methods
Process
Blow- Compression Injection Rotational Thermo- Transfer
Material molding molding Extrusion Foaming molding molding forming molding
ABS — — —
Acetal — — — — —
Acrylic — — — —
Aminos — a a — — —
Epoxides — — — — — — —
Fluoroplastics — a — — — —
Ionomers — — — —
PEEK — — — —
Phenolics — — —
Polyamides a — — a — —
Polycarbonate — — — - —
Polyester — — — — — —
(unsaturated)
Polyester — — — — —
(thermoplastic)
Polyimides — — — — — — —
Polyphenylene — — — — —
oxide
Polyphenylene — — — — —
sulfide
Polypropylene — —
Polysulfone — — — —
Polyurethane — — — —
Thermoplastic — — — —
polyetheresters
Thermosets — — — — —
a
Special grades.