Hello and welcome to a new Rule of Thumb post discussing why mixing maximum with optimum in polymer material selection and plastics processing is not the best thing to do.
Understanding “Maximum” vs. “Optimum” in Plastic Material Selection
When selecting materials, it’s important to distinguish between “maximum” and “optimum.”
Maximum refers to the highest possible value of a single property (e.g., the highest tensile strength, the greatest heat resistance, or the lowest density).
Optimum means the best possible balance among several relevant properties for a specific application. The optimum is not always the maximum of any single property, but rather the material that meets all key requirements most effectively.
Why is this distinction important?
Choosing a material based solely on a maximum value (such as the strongest or most heat-resistant polymer) can lead to unnecessary costs, processing difficulties, or other trade-offs. The optimal choice is the one that delivers the best overall performance for your specific needs—even if it doesn’t have the highest value in every category.
Example: “Optimal vs Maximum” in Plastic Material Selection
When selecting a plastic material for a specific application, engineers often seek the “optimal vs maximum”—that is, the best possible balance between competing requirements, such as mechanical strength, cost, processability, and chemical resistance.
Case Study: Gear Wheel for Automotive Application
Requirements:
- High mechanical strength and stiffness
- Good wear resistance
- Dimensional stability at elevated temperatures
- Cost-effectiveness for mass production
Material Candidates:
- Polyamide 6 (PA6)
- Polyoxymethylene (POM)
- Polyetheretherketone (PEEK)
Selection Process:
-
Define Key Properties:
The gear must withstand high loads (tensile strength), resist wear, and maintain shape at temperatures up to 120°C. -
Score Materials:
Each candidate is evaluated for tensile strength, wear resistance, heat deflection temperature, and cost.
Find the “Optimal vs Maximum”:
- PA6: Good strength and cost, but absorbs moisture (affecting dimensions).
- POM: Excellent wear resistance and dimensional stability, moderate strength, good cost.
- PEEK: Outstanding properties, but very high cost.
After scoring, POM emerges as the “optimal vs maximum”—it offers the best compromise between performance and cost for this application, even though PEEK has higher absolute properties.
The “optimal vs maximum” is not always the material with the highest individual property, but the one that best meets all critical requirements for the application. In this case, POM is the optimal choice, delivering reliable performance at a reasonable cost.
Takeaway:
When selecting plastics, always look for the “optimal vs maximum”—the material that provides the best overall fit for your application, not just the highest value in a single property.
Optimum vs Maximum in injection molding
For example, in injection molding, the optimum flow rate is not necessarily the fastest possible, but rather the rate that minimizes shear stress, ensures proper mold filling, and avoids defects like air traps or weld lines. Similarly, the optimum temperature profile for a molding process might balance melt temperature for flow with cooling time for cycle time, while minimizing residual stress.
Figure 1 presents the concept of "maximum vs optimum" for plastics processing. In essence, while maximum settings define the boundaries, optimum settings represent the sweet spot within those boundaries for achieving the best possible results in plastics processing.
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| Figure 1: Difference between optimum and maximum in plastics processing. |
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Literature:
[1] https://youtube.com/shorts/q_KfChgTwdM
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