Mechanical Engineering Perspectives Part 1 Cost and Function of Materials

This is the first article in a six-part series entitled “Mechanical Engineering Perspectives for Efficient, Integrated Commercial Product Design”. When thinking about product design, it is common to immediately focus on developing what we see and experience in a product. While UX and UI design is critical to product success, less visible mechanical engineering functions can drive innovation to successful commercialization. Adam Smith, senior mechanical engineer at Product Creation Studio, has decades of experience designing and planning dozens of products to market. In this series, Smith shares insights and advice on how mechanical engineering supports turning product ideas into reality by working in sync with all disciplines throughout development.

Material selection is an important function in designing a product for sale. Early insight into the range of inputs and integrations that will be required throughout the product development lifecycle will pave the way for efficiency and potential cost savings in later development phases. Valuable information that influences material selection down the line can be missed in the early stages of development. This article shares examples and tips to help you think holistically about your material selection from start to finish to ensure the future success of your product development.

Consider future variables when selecting materials

Understanding important material properties, such as which stainless steel is the most corrosion resistant or which plastic has the greatest dielectric strength, provides valuable insight into how to approach difficult technical obstacles, but it does not always provide the most appropriate solution. The most popular materials are not always adequate. You may find that the most obvious electrical insulator is too brittle to survive the proposed physical environment, or the most corrosion-resistant metal does not provide the yield strength necessary to regain its nominal shape after being exposed to maximum prescribed loads.

Are exotic materials worth the cost?

This is where more exotic materials may be better suited to the final product. Most mechanical engineers are familiar with common plastics such as Polycarbonate (PC), Acetyl-Butyl-Styrene (ABS) and Polyamide (PA/Nylon), while engineers working on products intended for more extremes such as medical devices have experience with less common engineering plastics. such as polyoxymethylene (POM/Acetal/Delrin), polyether ether ketone (PEEK) and polyetherimide (Ultem). Those with experience in microelectromechanical assemblies may also have experience in the more exotic materials like glass-filled polyamide-imide (PAI/Torlon) and polyaryletherketone (PAEK/Arlon). Glass filled Torlon has proven to be a good solution for dielectrics with excellent mechanical strength and toughness while being injection moldable.

Although some exotic materials are becoming more common and cheaper, most exotic materials are still significantly more expensive than more common materials. Knowing when and how to use them is important to find the most appropriate solution while remaining profitable. An exotic or custom material may seem like the only option; however, combining lower cost materials that each provide some of the desired properties may be a better solution for this stage of development. A high-strength, spring-loaded stainless steel subframe with a thin layer of dielectric plastic can provide a more economical and sturdier solution than a single piece made from the most expensive glass-filled Torlon.

Exotic metals are also becoming more common and cheaper, but still cost significantly more than traditional materials. For example, 316 stainless steel and titanium are known to be two of the most corrosion resistant metals in most environments; however, others have higher mechanical strength and toughness. Finding a metal with the right mix of toughness and corrosion resistance is often 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 the 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 stainless steels commonly used in medical devices and consumer products that provide above average corrosion resistance with average mechanical properties. When a stronger material is needed and titanium is too expensive or not strong 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 stronger (less brittle) than 440C. It can also be heat treated to a number of yield points to achieve the ideal strength to toughness ratio.

Compatibility is an integral part of commercial product development

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 greater nobility of stainless steel compared to aluminum can actually cause parts to corrode. Electrons will easily travel through a conductive solution like salt water from aluminum to stainless steel and bring the aluminum along for the ride. This can increase the life of stainless steel, but can quickly reduce the life and strength of aluminum. A more suitable material for fastening aluminum in a corrosive environment might be a zinc-aluminum coated steel fastener. The underlying metal can be noticeably stronger than stainless steel and the aluminum blend coating virtually eliminates or minimizes the difference in nobility between the aluminum part and the fastener.

Another interesting consideration regarding material compatibility is their coefficient of thermal expansion (CTE). When very high CTE materials are rigidly connected or bonded to very low CTE materials, minor temperature changes can cause significant deformation as one part expands faster than the other. This isn’t always a bad thing, as this predictable behavior can be used to our advantage to trigger thermal events in circuit breakers, switches, and other thermal sensing devices. When warping is an issue, using materials with similar CTEs is a good idea. When this isn’t ideal, allowing parts to float freely from a single point can solve the problem. Thermally conductive pads and compounds can also help avoid overstressing the assembly.

Joining techniques can also be material specific. Ultrasonic welding, bonding and laser welding all require specific material properties to achieve an ideal connection. Some plastics can be easily painted or galvanized (ABS and PC-ABS) while others require exotic primers or base coatings. When bonding one material to another, three material compatibilities must be considered. Part A to Part B (usually not a problem), Part A to bonding material, and Part B to bonding material. Provided that the three materials are compatible, we still need to understand the interaction between them. For example, is the bonding material supposed to adhere to part A and part B, or does it dissolve part A into part B? Production assembly requirements may direct or minimize our material choices and may need to be considered early in the product development cycle.

Expertise in materials for the long term

Engaging mechanical engineers early in the process (taking a holistic approach to product development) can clarify the materials available to the design team and reduce cost and waste for manufacturing management across the board. line. Many factors drive material choices throughout product development. Having as much knowledge about the environment and compatibilities required for these materials will help you make appropriate selections early and minimize the need to change materials in later development phases.