Metal-to-Plastic Conversion: When (and When Not) to Switch

Every year, we help customers convert metal components to plastic—and every year, we talk others out of it. Metal-to-plastic conversion can deliver dramatic cost and performance improvements, but only when the application genuinely benefits. Forced conversions often fail, wasting engineering time and damaging confidence in plastic materials.

This guide provides a framework for identifying real conversion opportunities and avoiding the pitfalls.

The Case for Conversion

When plastic replaces metal successfully, the benefits compound:

Weight Reduction Engineering plastics weigh 5-7x less than steel, 2-3x less than aluminum. In mobile equipment, vehicles, and handheld devices, weight reduction translates directly to:

• Reduced fuel/energy consumption
• Lower shipping costs
• Improved ergonomics
• Decreased wear on moving assemblies

Corrosion Elimination Plastics don’t rust, oxidize, or require protective coatings. In corrosive environments, this eliminates:

• Painting, plating, or coating operations
• Corrosion inspection and maintenance
• Premature replacement due to corrosion
• Contamination from corroded surfaces

Part Consolidation Complex plastic parts can integrate features—bosses, snap fits, living hinges, ribs—that would require multiple metal parts and fasteners. Consolidation reduces:

• Assembly labor
• Fastener inventory
• Potential failure points
• Tolerance stack-up

Machining Efficiency Most plastics machine faster than metals with less tool wear. For machined components:

• Cycle times drop 30-60%
• Tool life increases 3-5x
• Coolant requirements decrease
• Finishing operations often eliminated

Self-Lubrication Many engineering plastics are self-lubricating, eliminating:

• Lubrication systems and maintenance
• Contamination from lubricants
• Lubricant compatibility concerns
• Re-lubrication scheduling

Where Conversion Works Best

Certain applications consistently benefit from metal-to-plastic conversion:

Wear Components

Bushings, bearings, wear strips, and guides. Materials like UHMW, acetal, and nylon provide excellent wear properties at lower cost than bronze or other bearing metals. Self-lubrication eliminates maintenance.

Corrosive Environments

Chemical processing, marine, food processing, and outdoor applications. Plastics like PVDF, PTFE, and polypropylene resist chemicals that destroy metals.

Weight-Critical Applications

Aerospace, automotive, portable equipment, and robotic end-effectors. PEEK, glass-filled nylon, and carbon-fiber composites approach metal strength at a fraction of the weight.

FDA/Food Contact

Food processing and medical equipment. FDA-compliant plastics eliminate concerns about metal contamination, don’t require coatings that can chip, and resist cleaning chemicals.

Electrical Isolation

Components requiring electrical insulation. Plastics are inherently non-conductive; metals require additional insulating components.

High-Volume Production

When production volumes justify tooling, injection-molded plastics can cost pennies per part versus dollars for machined metal.

Where Conversion Fails

Some applications genuinely require metal. Forced conversion attempts in these areas waste resources:

High-Temperature Service

Most plastics are limited to 200-300°F continuous service. PEEK extends this to ~480°F, but metals routinely operate at 1000°F+. If operating temperature exceeds plastic capabilities, conversion isn’t viable.

High-Stress Structural Loads

Steel yields at 30,000-100,000+ psi; most plastics yield at 5,000-15,000 psi. For highly stressed structural members, the required plastic cross-section may be impractically large. Fatigue performance under cyclic loading also generally favors metals.

Precision Fits with Temperature Variation

Plastics expand 5-10x more than metals per degree of temperature change. In precision assemblies spanning significant temperature ranges, thermal expansion can cause fit and function problems.

Threaded Fastener Loads

Plastic threads strip more easily than metal threads. Applications with high fastener torque or frequent assembly/disassembly often require metal or threaded inserts.

Wear Against Abrasive Materials

While plastics excel at sliding wear, abrasive wear (sand, aggregate, particulate) erodes many plastics faster than hardened metals. UHMW is an exception, outperforming most metals in abrasion resistance.

Radiation Exposure

Most plastics degrade under UV, gamma, or other radiation. Metal is required for radiation-exposed components unless specialized radiation-resistant grades are available.

The Evaluation Framework

Before committing to conversion engineering, work through these questions:

1. Temperature Check

Operating temperature range?

• Below 180°F: Most engineering plastics viable
• 180-300°F: Limited selection (PEEK, PPS, PEI, high-temp grades)
• Above 300°F: PEEK maximum, or conversion likely not viable

2. Load Analysis

What loads does the part see?

• Calculate stress levels and compare to plastic yield strengths
• Consider safety factors (plastics typically need higher SF than metals)
• Evaluate creep under sustained loads
• Review fatigue behavior if cyclically loaded

3. Environmental Exposure

What chemicals, conditions, or environments?

• Chemical exposure: Check compatibility charts
• UV exposure: Requires stabilized grades
• Moisture: Hygroscopic plastics may swell
• Radiation: Usually excludes plastics

4. Dimensional Stability

How tight are tolerances and how much does temperature vary?

• Calculate thermal expansion across temperature range
• Compare to tolerance requirements
• Consider moisture absorption effects on hygroscopic materials

5. Economic Analysis

Do the numbers work?

• Material cost comparison (account for density differences)
• Machining time comparison
• Assembly cost changes (part consolidation, fastener elimination)
• Secondary operation elimination (coating, plating, deburring)
• Maintenance cost changes (lubrication, corrosion, replacement frequency)

Calculating the Real Cost

Material cost per pound is misleading because plastic weighs less. Compare cost per part volume:

Example: 4” x 4” x 1” Block

Material	Density	Block Weight	Material $/lb	Block Cost
Steel	0.28 lb/in³	4.48 lb	$1.50	$6.72
Aluminum	0.10 lb/in³	1.60 lb	$4.00	$6.40
Acetal	0.051 lb/in³	0.82 lb	$5.50	$4.51
UHMW	0.034 lb/in³	0.54 lb	$4.00	$2.16
Nylon	0.041 lb/in³	0.66 lb	$6.00	$3.96

Even at higher per-pound prices, plastics often cost less per part due to lower density.

Add machining cost differences:

Factor	Plastic Advantage
Cutting speed	2-4x faster
Tool wear	3-5x longer life
Coolant	Often dry machining
Deburring	Usually eliminated
Secondary ops	Coating/plating eliminated

A complete analysis often shows 40-70% total cost reduction on suitable conversions.

Conversion Process

For serious conversion candidates:

1. Application Analysis Document all requirements: loads, temperatures, chemicals, tolerances, environment, life expectancy, safety criticality.

2. Material Selection Match requirements to material properties. Often, multiple materials qualify—select based on cost, availability, and margin.

3. Design Modification Plastic parts shouldn’t replicate metal geometry. Redesign to:

• Add ribs for stiffness (plastics are less stiff)
• Include generous radii (stress concentration)
• Allow for draft where needed
• Integrate features (bosses, snap fits)
• Account for thermal expansion

4. Prototype and Test Build prototypes in the candidate material. Test under actual service conditions. Don’t shortcut this step—material properties on a datasheet don’t capture everything.

5. Production Transition Plan transition carefully. Maintain metal part supply until plastic version is validated.

When to Call Us

NextGen Components supports metal-to-plastic conversion at every stage:

• Material selection: Our engineers can recommend materials matching your requirements
• Design review: We identify conversion candidates and flag potential issues
• Prototyping: We supply prototype quantities for testing
• Production supply: We provide ongoing production material supply

Have a component you’re considering for conversion? Send us the details and we’ll provide an initial assessment.