“A kitchen appliance manufacturer decided to replace the metal shaft in their blender with a plastic one. Designers thought plastic would be lighter and cheaper, but just three months later, users started reporting broken shafts—the small plastic part simply couldn’t handle the everyday torque.”
This case is a clear reminder that switching materials isn’t just a “swap”—it involves structural design, stress analysis, and manufacturing considerations. In this article, I’ll walk you through the five most common pitfalls engineers encounter when replacing metal parts with plastic, along with practical tips that actually work.
Pit 1: Copying Metal Dimensions
One of the most common mistakes is directly using the metal part dimensions for plastic components. At first glance, it seems convenient, but it’s risky. Standard PA6 has a tensile strength of around 70 MPa, while aluminum alloys are about 250 MPa. Without adjusting cross-sections, plastic shafts are prone to breakage.
Practical tips:
- Thicken critical load-bearing sections or add ribs.
- Consider glass-fiber reinforced plastics (like GF30-PA6) to boost rigidity.
- Run finite element analysis (FEA) to verify the shaft can handle the torque.
Pit 2: Ignoring Creep and Thermal Expansion
One of the biggest differences between plastic and metal is time-dependent deformation. PA6 can creep 0.5%–1% over three months at 50°C, while aluminum barely changes. If ignored, this can lead to fit failure or jamming.
Practical tips:
- Factor in thermal expansion and creep when setting tolerances.
- Add ribs at key areas to spread stress.
- Allow slightly larger clearances in fits to accommodate long-term changes.
Pit 3: Overlooking Manufacturing Constraints
Metal parts can be machined into thin, complex geometries, but plastic parts have injection molding limitations such as wall thickness uniformity, draft angles, and shrinkage. Copying metal designs often causes warping, short shots, or internal stress concentration.
Practical tips:
- Keep wall thickness uniform and include draft angles and fillets.
- Add strategically placed ribs to improve structural stability.
- Conduct small-batch mold trials to see if the part can actually be produced as designed.
Pit 4: Underestimating Assembly Differences
Metal parts can handle threaded fasteners and press fits easily, but plastic is prone to thread stripping, clip failure, or insert loosening. For example, an M4 thread in PA6 can handle roughly 30–40 N·m, whereas metal can exceed 100 N·m.
Practical tips:
- Use metal inserts or nuts for threaded connections.
- Optimize clip designs and increase locking area or elasticity where needed.
- Add ribs or fillets at stress points to prevent cracking.
Pit 5: Overlooking Environmental Factors
Plastics have limited resistance to chemicals, oils, and cleaning agents, and they wear faster under friction. PA6 is water- and oil-resistant but not suitable for strong acids; high-friction areas wear out quickly.
Practical tips:
- Choose materials with proper chemical, wear, or temperature resistance for your environment.
- Add lubrication or surface treatment in friction zones.
- Conduct accelerated aging and durability tests for critical parts to ensure long-term reliability.
Practical Experience Summary
- Early assessment is crucial: Analyze torque, bending, and thermal expansion at the design stage.
- Collaborate closely with suppliers: Confirm manufacturing feasibility and material selection before finalizing designs.
- Mold trials and testing are non-negotiable: Produce small batches first, simulate real use conditions, and adjust before full-scale production.
Replacing metal parts with plastic isn’t a simple swap—it’s a systematic design iteration. Paying attention to dimensions, creep, manufacturing processes, assembly, and environmental factors, combined with simulation and prototype validation, ensures parts are lightweight yet reliable.
If you’re considering material substitution, starting early with design reviews and prototyping can prevent the dreaded “three-month breakage” scenario. RJC Mold offers comprehensive plastic part design review, rapid prototyping, and production support to help you safely implement substitutions while improving part reliability and service life.
Continue Reading:
Which Plastics Are Best for Replacing Metal Parts?
Can Plastics Really Replace Metal Parts?
CNC Machining vs. Injection Molding for Plastic Parts: How to Choose
