How to balance Corrosion Resistance and Cost Efficiency for end users
When fastener engineers and injection molding engineers work together, projects often start with targeted goals for the component. Two goals that we often work toward with any partner at CFI are corrosion resistance and cost efficiency. With injection molding engineering partners, part integrity, surface damage avoidance and long-term performance are added to the equation.
The Corrosion Challenge in Plastic Assemblies
While plastics and composites are resistant to corrosion, metal fasteners can face significant challenges especially in humid or corrosive environments. Such environments can include HVAC interiors, outdoor enclosures, and even common automotive and marine components. These corroded fasteners can, in turn, stain carefully curated and matched interior cosmetic surfaces. Corrosion can even lead to joint failure.
In order to avoid such corrosion steps can be taken including choosing the type of fastener material or coating to ensure that your plastics are never affected. How can this be achieved? Stainless Steel or Coated Fasteners.
How Does Stainless Steel Work?
Stainless steel is an iron-based alloy with a minimum of 10.5% chromium. This chromium forms a passive, self-healing oxide layer on the surface, which protects the fastener from corrosion. For injection-molded components used in high-moisture environments, such as under-the-hood automotive parts or marine applications, this corrosion resistance is crucial. Engineers often choose between 304 and 316 grades, with the latter providing superior resistance due to its added molybdenum content.
Stainless Steel Fasteners: Strengths and Limitations
Stainless Steel, known for its exceptional corrosion resistance and bright polished look, is an excellent choice for exposed surfaces that can withstand demanding conditions. Type 304 stainless steel has a corrosion rate of 0.00215 mm/year1. Type 316 Grade stainless steel has the same corrosion rate2. These corrosion rates are twice that of Zinc-coated fasteners.
How do Coated Fasteners work?
On the other hand, coated fasteners have carbon steel as the base material and have sacrificial protective layers ranging from zinc to nickel platings. What these coatings bring to the table that stainless steel doesn’t offer is customization. These coatings are often specifically designed for the environment for which they are deployed. e.g. Zinc Coatings are often used for mild outdoor environments; nickel coatings are used for moderate to high-corrosive environments.
While they are cost-effective compared to stainless steel, these coatings can wear off with repeated installations or over-torquing, leaving the base metal exposed to corrosion. In addition, some coatings can also cause chemical reactions with plastics and composites, leading to discoloration.
Choosing the Right Fastener for Your Injection Molded Parts
When selecting fasteners for injection molded parts, consider factors such as the operating environment, expected load, and appearance. For outdoor or moisture-prone applications such as assemblies for an outdoor A/C unit, stainless steel offers superior longevity. For cost-sensitive projects where the environment is controlled, coated fasteners may provide sufficient protection without excessive expense.
Conclusion
Choosing between stainless steel and coated fasteners in plastic assemblies is about understanding the application environment and balancing performance with cost. In moisture-prone areas, stainless steel provides unmatched durability, while coated fasteners offer budget-friendly solutions for controlled conditions.
Need Help Specifying the Right Fasteners?
At CFI, we work directly with injection molding companies to optimize fastener selection. Whether you’re designing for corrosion resistance, appearance, or cost control, our team can help you choose the best solution for your molded components.
📩 Contact Us for fastener solutions designed to fit your needs.
