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Since the earliest applications of synthetic polymers, plastics have steadily replaced traditional materials across a wide range of industries, offering numerous societal benefits. The first significant wave of metal-to-plastic conversions emerged in the early 20th century with the invention of Bakelite by Leo Baekeland. As the first fully synthetic plastic, Bakelite revolutionized the electrical and electronics industries by enabling the production of safer, more affordable and more complex components on a mass scale. Its success laid the groundwork for the broader plastics revolution that followed.

Today, more than a century later, it may seem that the most feasible metal-to-plastic conversions have already been achieved. However, advances in high-performance polymers, reinforcements and additives continue to push the boundaries of what plastics can do. These developments, coupled with an improved understanding of how polymers behave under high stress, at elevated temperatures and over time, are enabling the replacement of metal in increasingly demanding applications. With enhanced predictive tools and materials science knowledge, the risks traditionally associated with plastic substitution are being mitigated, opening new doors for innovation and efficiency across multiple industries.

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Considerations When Converting from Metal to Plastic

Converting a metal part to plastic involves a range of considerations, each specific to the part’s function, environment and performance requirements. A critical first step is to clearly define the purpose behind the conversion. Whether the goal is to reduce cost, lower weight, improve manufacturability or enhance sustainability, aligning objectives early on ensures a more effective outcome. Key benefits of converting a metal part to plastic include:

• Lower production costs: Plastic parts are often cheaper to produce due to lower material costs, simplified manufacturing and reduced assembly procedures.
• Corrosion and chemical resistance: Unlike metals, plastics resist corrosion and many chemicals, increasing longevity in harsh environments.
• Design flexibility: Plastics enable complex, multifunctional shapes and integrated features that reduce part counts and simplify assemblies.
• Cosmetic benefits: Plastics can be molded in virtually any color and finish, eliminating the need for painting or plating.  
• Improved performance: Plastics offer inherent advantages like electrical insulation, vibration damping and thermal insulation.
• Weight reduction: Plastics offer substantial weight savings that can improve energy, handling and shipping costs.
• Sustainability: Lighter parts reduce lifecycle energy use, and many plastics support recyclability.

Challenges When Converting from Metal to Plastic

Converting a part from metal to plastic introduces a unique set of challenges, many of which stem from the fundamental differences in material structure and behavior. Plastics offer many advantages, but their viscoelastic nature, which contributes to their versatility, can also create performance limitations. When designing with plastics, several critical factors must be considered to avoid premature failure or underperformance. Key considerations include:

• Strength: While certain plastics reinforced with fibers or fillers can achieve strengths comparable to some metals, most plastics fall short of matching the inherent strength of metals.
• Stiffness: Even more challenging than strength, plastics generally lack the stiffness of metals, regardless of the type or amount of reinforcement used.
• Temperature resistance: Plastics have a much narrower operating temperature range than metals, at both elevated and subambient levels.
• Effect of moisture: The mechanical properties of many plastics, including polyamide 6, are significantly influenced by moisture content. Figure 1 presents a stress-strain curve comparing conditioned polyamide 6 (50% relative humidity) to its dry counterpart. The dry material exhibits nearly double the tensile strength of the conditioned sample. Likewise, Figure 2 illustrates the modulus of both materials across a broad temperature range. At room temperature, the modulus of the dry polyamide 6 is approximately twice that of the conditioned version. 

FIG 1: Moisture affects the stress-strain response of polyamide 6. Source (all images): The Madison Group  

• Time-dependent behavior: The mechanical properties of plastics can change over time due to environmental exposure, loading and thermal cycling, which are often difficult to predict.
• Creep: Plastics are susceptible to creep, or slow deformation under sustained stress. Even low, continuous stress can lead to creep rupture over time.
• Chemical resistance: Although many plastics are highly resistant to certain chemicals, all plastics are vulnerable to attack or environmental stress cracking from specific chemicals.
• Oxidation and hydrolysis: Some polymers can degrade over time when exposed to oxygen or moisture, even under ambient conditions.
• Thermal expansion: Plastics typically exhibit a coefficient of thermal expansion (CTE) three to 10 times greater than that of metals, requiring special design attention for applications involving temperature fluctuations. Both expansion and contraction must be considered.
• Flammability: Most plastics are inherently flammable and can release smoke and toxic gases, requiring the use of flame-retardant grades in critical applications.
• Property variability: Metals typically exhibit narrow, well-defined property ranges, making their behavior more predictable. In contrast, plastics, especially those with fillers or reinforcements, often display a wider distribution of mechanical properties, which adds complexity to performance prediction and part validation. For example, Figure 3 illustrates the strength distribution of aluminum compared to polyamide 6 reinforced with 60% glass fiber by weight. While the reinforced plastic may achieve high strength values, aluminum not only exhibits higher overall strength but also a much tighter strength distribution.

FIG 2: The effect of moisture on the modulus of polyamide 6

Despite these challenges, plastics can still be excellent alternatives to metal when the right formulation is used. Additives, reinforcements and careful engineering design can significantly reduce the performance gap between plastics and metals, enabling successful conversions for a wide range of applications.

FIG 3: Material strength of aluminum and polyamide 6 with 60% glass-fiber reinforcement.

 

Understand Your Part Requirements

A thorough understanding of your part’s functional requirements, expected lifespan, environmental exposure and cosmetic needs is critical for proper engineering. Equally important is assessing the type and magnitude of stresses the part will face, along with the necessary material modulus and part stiffness. These considerations are vital to ensure reliable performance without excessive deformation or long-term failure such as cracking. Success also depends on the careful selection of resin, reinforcements, fillers and additives, in combination with thoughtful design, manufacturing and assembly planning. To accurately predict real-world performance, material testing and structural analysis are strongly recommended.

In parallel, optimizing processing conditions, often with the aid of molding simulation, is essential. Proper mold filling, controlled material solidification and fiber orientation help minimize molded-in stresses, shrinkage and warpage, leading to a higher-quality finished part. Even a well-designed part with the ideal material package can fail if molded under poor conditions — issues that often appear weeks or months after deployment in the field.

Engineering-Driven Design Enables Successful Metal-to-Plastic Fan Conversion

Air movement is fundamental to every occupied building, and HVAC systems, which heat, cool and circulate the air, are responsible for approximately 35% of a building’s total energy consumption, according to the U.S. Department of Energy. Improving the efficiency of these systems is a key objective for our client. Recognizing the unique advantages of plastic, particularly its ability to be molded into complex, aerodynamic shapes, the client saw an opportunity to enhance airflow performance, while reducing energy use and part cost.

The Madison Group assisted in converting this centrifugal squirrel cage PRV fan into a molded plastic part.

One specific target for improvement was an aluminum centrifugal fan used in rooftop HVAC units (shown first). The original fan consisted of 13 individually bent blades, assembled with 39 rivets. The client aimed to replace this multipart assembly with a single-shot molded plastic fan featuring a more advanced blade geometry for improved airflow.

To make this transition successful, the client, with the assistance of The Madison Group, applied a systematic, engineering-first approach. This included careful material selection, performance modeling, lifetime assessment and environmental analysis to ensure the plastic fan would meet operational demands and maintain durability over its expected service life.

By embracing the design freedom and efficiency of plastic, while fully addressing its engineering challenges, the client achieved a smart, high-performance, metal-to-plastic conversion (shown below). This highly successful 30% glass-filled polyamide 66 fan is made in one shot, with no rivets, and the metal hub is molded in. The benefits of this fan include, but are not limited to: decreased manufacturing time and part count, increased efficiency, corrosion resistance and noise reduction. This conversion has been highly successful for the client and has prompted the transition of additional legacy metal parts to plastic.

Fan redesigned as a single molded component made from 30% glass-filled polyamide 66.

True Convert

The replacement of metal with plastic has evolved significantly since the development of early synthetic polymers like Bakelite, and continues to progress with advances in high-performance plastics, reinforcements and predictive modeling tools. Converting metal parts to plastic offers benefits such as reduced cost, lower weight, corrosion resistance and improved design flexibility. However, it also presents challenges due to plastics’ lower strength and stiffness, limited temperature resistance, time-dependent behavior and greater variability in material properties. A successful transition requires a deep understanding of the part’s functional, environmental and structural demands, along with precise resin selection, thoughtful design and optimized processing.

ABOUT THE AUTHOR: Paul Gramann is one of the founders and current president of The Madison Group. Dr. Gramann received his Ph.D. from the University of Wisconsin – Madison, where he served as an adjunct professor within the Mechanical Engineering Department. He is dedicated to design verifications and failure analysis of thermoplastic, elastomeric and thermoset parts. His expert testimony has been invaluable in numerous depositions and trials for both defense and plaintiff counsels. Contact: 608- 231-1907; paul@madisongroup.com.