Page 93 - 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

Plastics Engineering 463

A good designer must be aware of such factors. This is and chemical resistance is variable depending on the
particularly true in our modern society, which has come grades.
to expect high performance and regular design upgrades. 5. Fluoroplastics:Largefamilyoflow-strength,high-
The task of choosing the correct plastic may appear cost plastics that have a number of distinguishing features.
daunting, but to most designers plastics offer an exciting In particular, they offer excellent dielectric properties,
challenge. The design process is not difficult but it requires high-temperature stability, and chemical resistance. PTFE
a clear, logical, and open-minded approach. The first step has a coefficient of friction that is one of the lowest for
must always be a precise statement of the design speci- any material. Other plastics in this group are FEP, PFA,
fication. This must define (1) the structural demands on CTFE, ECTFE, ETFE, and PVDF.
the material, (2) the environmental factors relevant to the 6. Ionomers: Tough, tear-resistant plastics with ex-
product, (3) the special codes of approval that may apply cellent outdoor weathering resistance. Can be transparent
(e.g., food contact or aircraft authorities), (4) the appear- and have good chemical resistance.
ance required (transparent, colored, decorated, etc.), and 7. Polyamide: Engineering plastic with high tough-
(5) the number of articles required. ness and wear resistance. Coefficient of friction is low and
On the basis of this type of information, it is possible chemical resistance is excellent. Water absorption is high,
to draw up a short list of materials. Many materials sup- so dimensional stability tends to be low.
pliers and a number of commercial organizations provide 8. Polyamide-imide: A high-cost amorphous plastic
computerized databases that assist greatly at the material that offers high strength, high-temperature performance.
selection stage. The internet is also an excellent source of 9. Polyarylates: Family of tough, heat-resistant plas-
data and information on plastics. tics with excellent outdoor weathering resistance. Very
Using these data, it is necessary to finalize the shape low inherent flammability and high resistance to creep
and then identify the most appropriate molding method. and warpage.
These must be considered together because each molding 10. Polyaryletherketones: Strong, heat-resistant crys-
method places limitations on shape (e.g., wall-thickness talline plastics capable of continuous service at 250 C.
◦
distribution, undercuts, etc.). At the detailed mold design Excellent chemical and wear resistance. Family comprises
stage, when one is considering corner radii, draft angles, polyetheretherketone (PEEK), polyetherketone (PEK),
positioning of vents, shrinkage, etc., it is usual to seek and polyetherketoneketone (PEKK).
advice from both the molder and the moldmaker because 11. Polycarbonate: An amorphous plastic with out-
each will have a unique contribution to make. By the time standing toughness. Excellent outdoor weathering resis-
that metal for the mold is being cut it is extremely im- tance and good creep resistance. Susceptible to chemical
portant to have the selection of a plastic made because attack and stress cracking.
the shrinkage, dimensional tolerances, etc., that must be 12. Polyester: Polybutylene terephthalate (PBT) and
allowed for will be different for each plastic. Some of the polyethylene terephthalate (PET). Characterized by toug-
general characteristics of thermoplastics and thermosets hness with excellent dimensional stability. Good chemical
are indicated below. resistance (except to strong acids and bases) and low wa-
ter absorption. Generally notch-sensitive and not suitable
A. Thermoplastics for outdoor use. PET is capable of high optical clarity if
prevented from crystallizing.
1. ABS(acrylonitrile-butadiene-styrene):Easilymol- 13. Polyetherimide: A strong, tough amorphous plas-
ded, tough, hard, plastic with a relatively high modulus. tic suitable for use at high temperatures. Easily processed,
Good dimensional stability with low water absorption. with good dimensional stability and broad chemical resis-
Good wear resistance and some grades can be electro- tance.
plated. 14. Polyethylene: Basically a commodity plastic but
2. Acetal: Available as a homopolymer or copolymer. capable of performing in load-bearing applications if
Strong engineering plastic with a good modulus, low wa- properly designed. Easily processed, tough, inexpensive
ter absorption, and excellent dimensional stability. High plastic with excellent chemical resistance. Medium and
resistance to wear and chemicals and excellent property high density grades are stronger and harder.
retention in hot water. Low tendency to stress-crack. 15. Polyimide: A tough plastic with good wear re-
3. Acrylic: Hard, glossy plastic with high optical clar- sistance and low coefficient of thermal expansion. Out-
ity and excellent resistance to outdoor weathering. standing high-temperature properties but expensive and
4. Cellulosics: Family of tough, hard plastics com- not easy to process.
prising cellulose acetate, butyrate, propionate, and ethyl 16. Polyphenylene ether: The homopolymer is fre-
cellulose. Dimensional stability is generally fair. Moisture quently referred to as polyphenylene oxide (PPO). Rigid,