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Even with all the advances in engineering polymers and processing methods, metal alloys still dominate the production of large parts used in power transmission and structural applications. And that dominance makes sense. Metals not only have the strength to withstand heavy loads over time but also have the advantage of familiarity. Most engineers know how to design with metals and how metals fail. Plastic properties can seem obscure in comparison.
It’s time, however, to shed some light on the capabilities of plastics in large part applications. Plastics can outperform metals in these some of these applications—but only under a very specific set of conditions:
Mechanical Properties Are Within Reach. To be sure, some large part applications have extremely high mechanical and thermal loads, which can rule out plastics of any kind. But don’t count out plastics prematurely. We have delivered larger polymer structural parts and power transmission components that withstand continuous forces of several metric tons. The key to these heavily loaded applications is picking the right material and manufacturing method. Our approach to large parts relies on a proprietary nylon material that we gravity cast. It has significantly different—and higher—strength properties than seemingly similar injection molded or cast polymers
The Application Requires Special Properties. No engineer in his right mind will replace a proven metal component with a plastic one—unless he has a good reason. Premature failure due to wear or corrosion represent two of the most common reasons to replace metal components with a high-performance plastic. The right plastic can eliminate wear altogether and resist corrosive environments in ways that metals cannot. Weight reduction is another good reason, since polymers like our Powercore (TM) weigh about 7x less than steel. The weight reduction comes in handy in many transportation applications where every ounce counts or in high-speed machines that have difficulties with inertia mismatches. Still other special properties revolve around the electrically and thermally insulatative properties of polymers.
Plastics Have To Cost Less. Plastics components often cost less than their metal counterparts, though their cost advantage is not always straightforward. Our Power-Core nylon material, for example, does not always offer an immediate cost reduction compared to metals. In fact, sometimes it costs more to produce the plastic component if it’s replacing a simple machined steel part. But the cost advantage will tip in favor of polymers over time whenever metal components fail prematurely due to wear or corrosion. Polymers also gain an advantage whenever they produce significant process advantage—such as higher speed or unit cost savings.The polymer component may also cost less because it eliminates the expensive lubrication needed to keep metal components running. When polymer components replace more complex metal parts, as opposed to a simple part, the cost reduction can be immediate. The reason boils down to a manufacturing edge. Polymer components can often be produced net-shape or near-net-shape, so they can cost less to produce then complex metal components that require multiple fabrication steps. A single casting can often replace multiple metal parts, producing more savings.
Polymer Properties Must Scale Up. A polymer’s physical properties won’t typically change with the size of the part. For example, a polymer that resists moisture will do regardless of part size. The inherent physical properties, however, paint only half the picture. Some polymer properties will change with size due to the influence of the processing method. Our gravity casting process, for example, can produce large parts without the internal stresses you would find in a high-pressure processing method such as injection molding. This low stress state is largely due to the resulting dense crystalline structure in the cast part as well as the gradual cooling in the mold. The low stress state allows the cast parts to remain dimensionally stable regardless of size. Indeed, we have produced parts that measure several meters across that have essentially the same dimensional stability parts measuring a few centimeters.