Moving Away From Metals

Metal-to-plastic conversion is a growing trend, and while engineered thermoplastics have existed since the introduction of acrylonitrile butadiene styrene (ABS) and nylon in the 1950s, industries and applications that can benefit from today’s thermoplastics are still being realized.

Engineered thermoplastics provide a lightweight, high-strength alternative to metals. In the automotive industry, this has become increasingly transparent as the early adapters that were quick to exchange metallic non-critical components with plastic counterparts have moved forward, finding additional weight reductions and increased fuel economy with high performance thermoplastics.

The widening scope of today's engineered thermoplastics now addresses needs across a broad range of industries and applications, including aerospace components, agricultural equipment, building and construction supplies, consumer products and a wealth of other applications in industrial manufacturing.

Engineered Thermoplastics

Advanced engineered thermoplastics incorporate modified polymer systems and fillers. The ever-increasing range of material options now encompasses more than 25,000 engineered plastic materials, including advanced nylons, polyphenylene sulfides and polypropylenes.

Each polymer system begins with either an amorphous or semi-crystalline resin that is modified to address specific performance requirements by incorporating additives, fillers and modifiers. In some applications, an impact modifier may help achieve the desired toughness, while more advanced options include a laundry list of glass fiber additives, microspheres and other additives.

Benefits of Metal-to-Plastic Conversions

The cost benefits of replacing a metal component with thermoplastic counterparts was the initial driver that spawned metal-to-plastic conversions. Injection molding is faster, more efficient and capable of producing parts with higher tolerances when compared to die-cast metals. A single molded thermoplastic can replace multiple metal components, reducing assembly time, reducing parts count and even eliminating the need for secondary machining operations.

Additional benefits achieved through use of engineered thermoplastics include weight reduction, improved structural strength and increased design options. The specific gravity of engineered thermoplastics typically ranges from 0.9 to 2.0, drastically lower than any ferrous metal and even offering a weight reduction over aluminum. The thermoplastic component can also achieve greater strength when that polymer system has been optimized for the given application and design considerations are implemented, which may include strengthening members such as ribs, bosses and gussets.

Design Challenges

Converting metal components offers numerous benefits that can only be achieved with due diligence and a complex engineering process. If you're redesigning a nylon gear and you only consider the tensile strength of the teeth, you may find your design fails due to thermal degradation. To achieve a desired outcome, there needs to be a feasibility analysis, proper material selection, extensive testing, process design and a deeper understanding of new challenges that may arise.

Feasibility analysis looks to define a full suite of requirements. It needs to address functional requirements, environmental conditions, manufacturability and economic feasibility. A design can fail for a number of reasons, and if you choose to only address core requirements, you may find that the chosen solution does not address the full suite of challenges in its intended application.

Material selection is a second core competency required. Manufacturing a thermoplastic component isn’t as simple as determining the proper polymer system from a two-dimension plot. An amorphous resin may be needed to achieve tight tolerances, or for excessive chemical or wear resistance your only option may be higher performance semi crystalline resin systems backed with carefully designed additives and fillers.

Testing is a key requirement and even if you have a detailed understanding of part requirements and the proper polymer system, plastic replacement parts may require design modifications to address new challenges that arise. Prototyping allows design engineers to continually modify the part design to address these design challenges before committing to tool and mold designs.

One final point of contention is process design. Not all injection molds are created equal. Operators looking to achieve exceptional quality need to address how the mold cavity is filled, the cooling rate and ensure that design flaws are not inherent to the process design rather than the part design or material selection.

Resources

An Engineers Guide to Specify the Right Thermoplastic [PDF]

Converting Metal Automotive Components to Plastic: A Manufacturer’s Guide