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

470 Plastics Engineering

TABLE IV Typical Prices of Engineering Plastics (Relative (plastics, metals, ceramics, etc.) to obtain a ranking of the
to Polypropylene) cost effectiveness of each. If it is desired to make the com-
Density Price Price parison on the basis of maximum stiffness for minimum
3
Material (kg/m ) (vol. basis) (wt. basis) weight, the bottom line of the equation should be simply
ρ instead of ρC.
Polypropylene 905 1 1
A similar analysis can be carried out for a range of other
ABC 1040 2.6 2.3
structural shapes. The relevant equations are summarized
Polyacetal 1410 5.7 3.7
in Table V.
Polyamide(66) 1140 5.7 4.5
Polyamide(66)/30%g 1300 6.3 4.4
Polyamide-imide 1380 82.8 54.3
IV. SUCCESSFUL ENGINEERING
Polycarbonate 1240 5.5 4
APPLICATIONS FOR PLASTICS
Polyetherimide 1270 13.8 9.8
Polyethersulfone 1370 19.3 12.7
There are many success stories where the use of plastics
Polysulfone 1240 13.8 10.1
and composites has resulted in improved performance and
Polysulfone 30%g 1450 13.4 8.4
new markets for products that had previously been manu-
Polyimide 1400 160 103.4
factured from more traditional materials. Some examples
Mod. PPO 1100 3.4 2.8
of these applications are as follows.
Mod. PPO/30%g 1270 6.8 4.8
Polyester(PET) 1360 5.4 3.6
PET/30%glass 1680 6.2 3.3 A. Automotive Applications
Polyph. sulf(PPS) 1350 15.2 10.2
It has been evident for many years that the plastics industry
PPS/30%glass 1650 12.4 6.8
views the automotive sector as one of its most demanding
PEEK 1320 75.9 52
challenges, both in terms of materials performance, qual-
PEEK/30%C 1420 82.8 52.8
ity, and the production rates required. Initially the vast ma-
Liquid crystal 1600 69 39
jority of uses of plastics were in vehicle trim (that is, sec-
Fluoropol. (ECTFE) 1580 42.9 24.6
ondary, non-loadbearing applications). In 1960 a typical
Allyls 1820 9.7 4.8
family car contained about 1% by weight of plastic. Today
Allyls/glass 2000 11.7 5.3
the average weight of plastic in a car is about 175 kg, rep-
Aminos (urea) 1500 2.6 1.6
resenting between 9 and 18% of the weight of the vehicle.
Aminos (melamine) 1500 2.9 1.7
Interior trim still accounts for the biggest proportion of
Cyanates 1250 63.4 45.9
plastics in cars (about 40% of the total) but the use of plas-
Epoxies 1300 4.8 3.3
tics in demanding under-the-hood applications is increas-
Phenolics 1400 2.1 1.4
ing rapidly and is expected to reach 20% soon. At present
Unsat. polyesters 1300 2.3 1.6
more than 25% of the air inlet units made for cars world
wide are plastic. Cost savings of over 50% are quoted in
regard to this conversion from metal to plastic, accompa-
Stiffness = α 2 Ed 3 (2) nied by similar benefits in weight reduction. Exterior body
panels also represent an exciting challenge for plastics in
where α 2 is a constant. terms of design for manufacture and performance. Sheet
Now the cost of the beam will be given by: molding compound (SMC) is a composite that is popu-
lar in this sector, and at the current time approximately
Cost = α 3 ρdC (3)
13 million units are manufactured annually for automo-
where ρ is density, C is cost per unit weight, and α 3 is a tive use. Body panels from other materials such as PET or
constant. polypropylene blends reinforced with glass fibers and al-
So substituting from Eq. (2) into Eq. (3) for the depth loys of polycarbonate/polybutylene terephthalate are also
d, proving to be very successful. A primary target in this ap-
plication is to achieve a Class A, durable finish straight
1/3
Cost = α 4 (ρC/E ) (4)
from the mold so as to remove the need for the relatively
Hence, the desirability factor in the material, in or- expensive painting stage.
der to minimize cost for the given stiffness, is the ratio Polypropylene continues to be the popular choice for
(E 1/3 /ρC), which should be maximized. Using this re- automotive applications. U.S. consumption of this mate-
lationship it is possible to compare a range of materials rial for cars is currently about a billion pounds. Many