There’s nothing new about plastic as a substitute for other materials. From its very beginnings, plastic took the place of whalebone, tortoiseshell, wood, and stone. The reasons for substitution were simple: availability and cost. Mixing up a vat of combustible celluloid had its risks, but they were small compared to those of hunting whales in tiny boats. Plastics were cheap compared to the semiprecious stones they replaced in costume jewelry, and they were easier to work than wood or stone.
| Today, plastic still makes financial sense, but it often replaces other materials on the basis of performance as well. In the example shown here (Figures 1 & 2), a single plastic part replaced seven metal components: top and bottom brackets, a bearing assembly, and four screws. The part, a clip-in shaft bearing, carries the rotating paper-feed shaft in a printer. To be molded as a single part, it had to be totally redesigned, not just to ensure functionality, but for manufacturability as well. It had to be reinforced to support the stresses created by side loads on the shaft. To allow molding in a simple two-part mold, the new design had to eliminate undercuts. The part was designed with relatively constant wall thickness to avoid sink, voids and warp. And it had to be drafted to facilitate ejection from the mold. But the resulting part was lighter, less expensive, maintenance free and required no assembly. | 
Figure 1 | 
Figure 2 |
Of course every conversion is different. In some cases, a single metal part might have to be broken into two or more for production in plastic. These could then be joined by molded-in snaps or clips, by welding, or with connectors. Plastic parts may require ribs or other reinforcements in order to withstand stresses. And they might require specific resins in order to perform their functions. For example, a part requiring metal-like strength might be made of carbon, Kevlar, or glass-filled resins.
Of course metal isn’t the only material being replaced with plastic.
- Glass can be replaced with acrylic, which can be brittle but is low cost and has excellent optical properties or polycarbonate, which is light and impact resistant, making it ideal for applications from eyeglass lenses to bullet-proof windows
- Rubber has been widely replaced with thermoplastic elastomers (TPE) and thermoplastic olefins (TPO), both inexpensive, and thermoplastic polyurethanes, which are tear, abrasion, and chemical resistant
- Metal foils are replaced with staticdissipating plastics for packaging sensitive electrical components
- Thermosetting plastics can be replaced with a variety of high temperature/high stiffness thermoplastics to simplify manufacturing
- Enamels and paints are being replaced with thermoplastic powder-coating
- Ceramics are replaced with plastic; everyday examples include plastic tableware, both disposable and non-disposable
Actual food is being widely replaced with plastic display dishes in restaurants, and as many a disappointed fish can attest, living creatures, from minnows to frogs to earthworms, are now being replaced with live-looking and live-smelling plastic fishing baits.
In describing the plastic part pictured opposite in Figures 1 & 2, we said that the molded part had replaced seven metal parts. In fact, one of those metal parts was a bearing assembly consisting of races, a cage and individual balls. So with the ball bearings gone, what reduces friction? The plastic itself.
It’s a rare plastic that can take the place of an engineered metal bearing assembly, but RTP 200 AR 15 TFE 15 does the trick in this application. Offered by RTP Company and consisting of 15 percent aramid fiber for strength and abrasion resistance, 15 percent PTFE for lubricity, and 70 percent Nylon 6/6, it is a very specialized plastic. But it did its job of replacing metal admirably. Not only did it provide a low friction bearing surface for the printer’s metal shaft, but it proved so durable that, over time, the steel shaft wore before the plastic bearing did.
This is just one example of the extremes to which plastic resins have been engineered. Other examples include:
- 60% long glass fiber nylon with tensile strength of 33,000 psi and a heat deflection temperature (at 264psi stress) of 460°F, making it suitable in applications requiring very high strength and heat resistance.
- Ryton R4 PPS (polyphenylene sulfide), while not quite as high in tensile strength as the glass reinforced nylon has even higher heat temperature deflection (500°F) and is resistant to automotive/diesel fluids and combustion byproducts. This makes it suitable for long-term use in the grueling conditions produced under the hoods of diesel vehicles.
- RADEL R-5000 PSU provides hydrolytic stability, high deflection temperatures and outstanding resistance to environmental stress cracking. Its Izod impact resistance is a very high 13.0 ft-lb/sq. in. It is popular for medical applications and its heat stability and good electrical properties suit it for electrical and electronic applications in which high impact resistance is required.
- Zytel ST nylon has an extraordinarily high impact resistance of 44 .3 ftlb/sq. in.; no surprise considering that the “ST” stands for “super tough.” That, along with excellent flow characteristics, allows the production of parts with thinner walls, longer rails, and stronger knit lines. The resin is used in sporting goods, safety equipment, power tools, and appliances.
Other resins boast different sorts of unique characteristics.
- Polyetheretherketone* (aka PEEK) melts at around 350°C (662°F) and is highly resistant to thermal degradation. The material is also resistant to both organic and aqueous environments, and is used in bearings, piston parts, pumps, compressor plate valves, and cable insulation applications. It is one of the few plastics compatible with ultra-high vacuum applications.
- Ultem*, one of the strongest resins available, has excellent chemical resistance and low shrink so it can make low warp, accurate parts. It is used in many applications including medical, chemical resistant, and power supplies.
- Glow-in-the-dark blends of PP, PE, AS, ABS, PC, PS, PET, and other plastics are available for a variety of applications from toys to latches which allow people locked into car trunks to escape.
- Scent impregnated plastic is used for fish bait, chocolate scented mousetraps, air fresheners, and more.
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