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July 25, 2022

Mechanical Engineering Perspectives Part 1 Material Cost & Function - Design News

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| Jun 27, 2022
This is the first article in a six-part series titled “Mechanical Engineering Perspectives for Efficient, Integrated Commercial Product Design.” When thinking about product design, it’s common to immediately focus on developing what we see and experience in a product. While UX and UI design are critical to product success, the less-visible mechanical engineering functions can be the workhorse driving innovation through to successful commercialization. Adam Smith, senior mechanical engineer at Product Creation Studio, has decades of experience designing and planning dozens of products for commercialization. In this series, Smith shares insights and tips on how mechanical engineering supports the transformation of product ideas into reality by working in sync with all disciplines throughout development.
Material selection is an important function in designing a product for commercialization. Early insight into the spectrum of inputs and integrations that will be needed throughout the product development life cycle will pave the way for efficiency and possible cost savings in later development phases. Valuable information that influences material selection down the line may be missed in early phases of development. This article shares examples and tips to help you think holistically about your materials selection from start-to-finish to ensure future success in developing your product.
Understanding important material properties—like which stainless steel is the most corrosion resistant, or which plastic has the highest dielectric strength—yields valuable insight on how to approach difficult engineering hurdles, but this doesn’t always provide the most appropriate solution. The most popular materials are not always adequate. You may find that the most obvious electrical isolator is too brittle to survive the proposed physical environment, or the most-corrosion-resistant metal doesn’t provide the yield strength to return to its nominal shape after being exposed to the maximum prescribed loads.
This is where more-exotic materials may be a better fit for the final product. Most mechanical engineers are familiar with common plastics like polycarbonate (PC), acetyl-butyl-styrene (ABS), and polyamide (PA/Nylon) while engineers working on products for more-extreme environments like medical devices have experience with less common engineering plastics like polyoxymethylene (POM/Acetal/Delrin), polyether ether ketone (PEEK), and polyetherimide (Ultem). Those with experience in microelectromechanical assemblies may also have experience in the most-exotic materials like glass-filled polyamide-imide (PAI/Torlon) and polyaryletherketone (PAEK/Arlon). Glass-filled Torlon has proven to be a nice solution for dielectrics with excellent mechanical strength and toughness while still being injection moldable.
Although some exotic materials are becoming more common and less expensive, most exotic materials are still substantially more expensive than more-common materials. Knowing when and how to use them is important when working toward the most-appropriate solution while still being cost effective. An exotic or custom material may seem like the only option; however, combining lower-cost materials that each provide a portion of the desired properties may be a better solution for this stage in development. A high-strength spring stainless-steel subframe with a thin layer of dielectric plastic may provide a lower cost, more robust solution, than a single part made from the most-expensive glass-filled Torlon.
Exotic metals are also becoming more common and less expensive but still cost significantly more than mainstream materials. For instance, 316 stainless steel and titanium are commonly known to be a couple of the most-corrosion-resistant metals for most environments; however, others have greater mechanical strength and toughness. Finding a metal with a good mix of toughness and corrosion resistance is frequently a challenge. Aluminum is a common lightweight metal that has decent corrosion resistance when properly treated but has medium to low strength and is prone to fatigue. Titanium is an attractive choice for extreme environments but is largely avoided given cost and manufacturing process requirements to maintain corrosion resistance. This leaves stainless steel as a very popular mid-cost material. For example, 316SS and 304SS are common stainless steels used in medical devices and consumer products that provide better-than-average corrosion resistance with average mechanical properties. When a tougher material is needed and titanium is too expensive or not tough enough, 17-4 stainless steel is an attractive choice. Also called grade 630, 17-4SS has a significantly higher yield strength than most other stainless steels and is tougher (less brittle) than 440C. It can also be heat treated to a number of yield strengths to achieve the ideal strength to toughness ratio.
Material compatibility is also a common concern among mechanical engineers. When fastening aluminum parts in a corrosive environment, the seemingly obvious choice would be to use 316 stainless steel to avoid corroding the fasteners; however, the higher nobility of stainless steel versus aluminum can actually cause the parts to corrode. Electrons will readily travel through a conductive solution like salt water from the aluminum to the stainless steel and bring aluminum along for the ride. This may increase the life of the stainless steel but can rapidly reduce the life and strength of the aluminum. A more-appropriate material for fastening aluminum in a corrosive environment might be a zinc-aluminum-coated steel fastener. The underlying metal can be substantially stronger than stainless steel and the aluminum-blend coating nearly eliminates or minimizes the difference in nobility between the aluminum part and the fastener.
Another interesting material compatibility consideration is their coefficient of thermal expansion (CTE). When materials with very high CTE are rigidly connected or bonded to materials with very low CTE, minor temperature changes can cause significant deformation as one part expands faster than the other. This is not always a bad thing, as this predictable behavior can be used to our advantage to trigger thermal events in breakers, switches, and other thermal-sensing devices. When deformation is a concern, using materials with similar CTEs is a good idea. When this isn’t ideal, allowing the parts to float free from a single point can resolve the issue. Thermally conductive pads and compounds can also help avoid over-constraining the assembly.
Assembly techniques may also be material specific. Ultrasonic welding, bonding, and laser welding all require specific material properties to achieve an ideal connection. Some plastics can be readily painted or electroplated (ABS and PC-ABS) while others require exotic primers or base coatings. When bonding one material to another, there are three material compatibilities to consider. Part A to Part B (usually not an issue), Part A to the bonding material, and Part B to the bonding material. Providing that all three materials are compatible, we still need to understand the interaction between them. For instance, is the bonding material expected to adhere to part A and part B, or is it dissolving part A into Part B? The production assembly requirements can drive or minimize our material choices and may need to be considered early on in the product development cycle.
Engaging mechanical engineers early on in the process (taking a holistic approach to product development) can provide clarity in materials available for the design team and reduce cost and waste for manufacturing management down the line. Many factors drive material choices throughout product development. Having as much knowledge about the environment and the compatibilities required for those materials will help you make appropriate selections early and minimize the need to switch materials in later phases of development.
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