Hello and welcome to a new blog post. When it comes to creating a successful product, the material you choose can make or break your design—literally! Imagine you’re building a plate that needs to be stiff enough to withstand bending (Figure 1). You have a few plastics in mind: unreinforced Polyamide 6.6 (PA 6.6), Polypropylene (PP), and High-Density Polyethylene (HDPE). Which one should you pick? If you’re thinking, “Easy! Just go for the cheapest per kilogram,” think again.
Let’s dive into the fascinating world of material selection, where things aren’t always as simple as they seem.
The Price Tag Trap
At first glance, HDPE looks like a winner. It’s often the least expensive plastic by weight. But here’s the twist: not all plastics are created equal when it comes to stiffness. To achieve the same bending stiffness as PA 6.6 or PP, you’d need to make your HDPE plate much thicker. Why? Because HDPE has a lower modulus of elasticity—it’s just not as stiff.
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| Figure 1: Example material selection of rib plate - HDPE vs PP vs PA 6.6 [1]. |
The Domino Effect of Thickness
Making the plate thicker doesn’t just mean using more material (and thus, more cost). In injection molding, thicker parts take longer to cool. In fact, the cooling time increases with the square of the wall thickness! That means your production slows down, your output drops, and your costs go up. Suddenly, that “cheap” HDPE isn’t looking so cheap after all.
The Surprising Winner
In our example, the most expensive material per kilogram—Polyamide 6.6—turned out to be the most cost-effective overall. Its higher stiffness meant we could use less material and keep production fast and efficient. Sometimes, paying more upfront saves you money in the long run. Also, you can decrease the wall thickness even more, by using a PolyArylAmide (PARA; MXD6) instead a PA 6.6.
Beyond the Numbers
Of course, cost isn’t the only factor. When choosing a material, you also need to consider physical, chemical, and thermal properties, as well as how easy it is to process. The “right” material is the one that balances all these needs for your specific application.
The Takeaway
Next time you’re selecting a material, remember: the cheapest option per kilogram might not be the cheapest solution for your product. Think about requirements such as stiffness, processing time, and all the other requirements (chemical resistance) your product needs to meet. A little extra thought at the beginning can save a lot of headaches—and money—down the line.
Let’s Talk About Your Design!
Do you have a plastic part design or a material selection challenge? I’d love to hear about it! Share your project or questions with me here, and I’ll be happy to provide feedback and help you find the best solution for your needs. Let’s make your next product a success—together!
Thanks for reading and #findoutaboutplastics
Literature:
[1] Konstruieren mit Kunststoffen - IKKV, University Leoben
[2] https://www.findoutaboutplastics.com/2025/04/nature-is-built-on-5-polymers-modern.html
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