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5 Essential Engineering Guidelines for Designing Plastic Parts

When designing engineering plastic parts, treating plastics exactly like metals is a common pitfall. To ensure long-term performance, durability, and safety, engineers must apply specific “good engineering practice” guidelines tailored to the unique behaviors of polymer materials.

5 Essential Engineering Guidelines

Here are five critical rules to keep in mind when engineering plastic components.

Always Calculate a “Working Stress” Safety Factor

Unlike metals, plastic properties especially those involving ultimate strength values require a generous safety factor.

• To find a safe working stress, you should divide the material’s ultimate strength value by 4.
• For example, if a plastic has a compressive strength of 12,000 psi, you should design the part with a maximum working pressure of 3,000 psi.

Account for Temperature Fluctuations

Material data sheets generally report properties at a comfortable, ambient lab environment of 70°F-73°F. However, temperature affects plastics much more dramatically than other materials.

• High Temperatures: As heat increases, the material softens, properties tend to decrease, and chemicals become more aggressive against the plastic. Furthermore, the material can begin to oxidize from the surface inward.
• Low Temperatures: As the temperature drops, impact resistance decreases and the material becomes significantly more brittle.

Understanding “Creep” (Cold Flow)

Time is a massive factor in plastic design. The longer a plastic is exposed to external stresses whether mechanical, thermal, chemical, or electrical the greater the effect on the material.

• Under continuous mechanical forces, plastics can permanently deform even if the load is below the material’s “yield point”.
• This time-dependent deformation is called “creep” or “cold flow”.
• Creep becomes exponentially more severe as the operating temperature increases.

Scrutinize Test Criteria and “Word Games”

Not all spec sheets are created equal. It is critical to know exactly which test methods were used to generate the reported data.

• For example, ASTM D-648 tests Heat Deflection Temperature, but allows loads of either 66 psi or 264 psi; the lower load will obviously generate a much higher (and potentially misleading) temperature rating.
• Watch out for moisture absorption reporting. A supplier might report an “Equilibrium” content (2% to 4%), which relies entirely on ambient air and storage conditions. Reputable suppliers will usually report the 24-hour absorption rate (from a dry state) and the full saturation rate (the absolute maximum moisture it can absorb, often around 7%).

Remember That Fillers Change Everything

You must always review the properties for the specific filled material, not just the base resin. Adding a filler drastically alters a material’s capabilities:

• Compared to unfilled PEEK, a 30% carbon fiber filled PEEK will have completely different stiffness, heat transfer, and wear properties.
• Additionally, adding fillers like carbon fiber will cause the material to lose its FDA compliance. (Note: While colors usually do not affect physical properties, they can also affect food compliance ).

Expert Fabrication and Material Selection

Designing with plastics requires looking beyond the basic data sheet. At W.S. Hampshire, our experts understand the nuances of creep and cold flow, thermal expansion, and applying the proper safety factors to ultimate strength values to ensure your custom non-metallic parts perform exactly as intended.

Contact us today to discuss your next project.