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 across multiple dimensions.

Weight Reduction. Engineering plastics weigh 5-7x less than steel and 2-3x less than aluminum. In mobile equipment, vehicles, and handheld devices, this weight reduction translates directly to reduced fuel and energy consumption, lower shipping costs, improved ergonomics for operators, and decreased wear on moving assemblies.

Corrosion Elimination. Plastics don’t rust, oxidize, or require protective coatings. In corrosive environments, switching to plastic eliminates painting, plating, and coating operations along with the ongoing corrosion inspection and maintenance they require. You avoid premature replacement due to corrosion failure and eliminate contamination concerns from corroded surfaces.

Part Consolidation. Complex plastic parts can integrate features that would require multiple metal parts and fasteners—bosses, snap fits, living hinges, and ribs can all be incorporated into a single molded or machined piece. This consolidation reduces assembly labor, fastener inventory, potential failure points, and tolerance stack-up issues.

Machining Efficiency. Most plastics machine faster than metals with less tool wear. For machined components, expect cycle times to drop 30-60%, tool life to increase 3-5x, coolant requirements to decrease (many plastics machine dry), and finishing operations to often be eliminated entirely.

Self-Lubrication. Many engineering plastics are inherently self-lubricating. This eliminates lubrication systems and their maintenance, removes contamination concerns from lubricants, avoids lubricant compatibility issues with other materials, and frees you from re-lubrication schedules.

Where Conversion Works Best

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

Wear Components. Bushings, bearings, wear strips, and guides represent prime conversion candidates. Materials like UHMW, acetal, and nylon provide excellent wear properties at lower cost than bronze or other bearing metals, with self-lubrication that eliminates maintenance.

Corrosive Environments. Chemical processing, marine applications, food processing, and outdoor installations all expose components to conditions that destroy metals. Plastics like PVDF, PTFE, and polypropylene resist chemicals that would quickly corrode metallic alternatives.

Weight-Critical Applications. Aerospace, automotive, portable equipment, and robotic end-effectors demand minimum mass. PEEK, glass-filled nylon, and carbon-fiber composites approach metal strength at a fraction of the weight, enabling performance improvements across the system.

FDA/Food Contact. Food processing and medical equipment benefit from FDA-compliant plastics that eliminate concerns about metal contamination, don’t require coatings that can chip into product, and resist the aggressive cleaning chemicals used in sanitary environments.

Electrical Isolation. Components requiring electrical insulation are natural conversion candidates. Plastics are inherently non-conductive, while metal alternatives require additional insulating components to achieve the same function.

High-Volume Production. When production volumes justify tooling investment, injection-molded plastics can cost pennies per part versus dollars for machined metal, fundamentally changing product economics.

Where Conversion Fails

Some applications genuinely require metal. Forced conversion attempts in these areas waste resources and create problems.

High-Temperature Service. Most plastics are limited to 200-300°F continuous service temperature. PEEK extends this range to approximately 480°F, but metals routinely operate at 1000°F and beyond. If your operating temperature exceeds plastic capabilities, conversion simply isn’t viable.

High-Stress Structural Loads. Steel yields at 30,000-100,000+ psi depending on grade; 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 that span significant temperature ranges, this thermal expansion causes fit and function problems that may be difficult or impossible to design around.

Threaded Fastener Loads. Plastic threads strip more easily than metal threads under torque. Applications with high fastener loads or frequent assembly/disassembly cycles often require metal construction or threaded metal inserts in plastic parts.

Wear Against Abrasive Materials. While plastics excel at sliding wear against smooth surfaces, abrasive wear from sand, aggregate, or other particulates erodes many plastics faster than hardened metals. UHMW is a notable exception, actually 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 and appropriate for your application.

The Evaluation Framework

Before committing engineering resources to conversion, work through these questions systematically.

1. Temperature Check

Start with the operating temperature range. Below 180°F, most engineering plastics are viable candidates. From 180-300°F, your selection narrows to high-temperature grades like PEEK, PPS, and PEI. Above 300°F, only PEEK remains viable, and even then you’re at the material’s limits—conversion is probably not the right answer.

2. Load Analysis

Understand what loads the part actually sees. Calculate stress levels and compare them to plastic yield strengths, remembering that plastics typically require higher safety factors than metals. Evaluate creep behavior if the part experiences sustained loads over time, and review fatigue characteristics if the loading is cyclic.

3. Environmental Exposure

Document the chemicals, conditions, and environments the part will encounter. Chemical exposure requires checking compatibility charts for candidate materials. UV exposure requires stabilized grades. Moisture can cause hygroscopic plastics to swell and change dimensions. Radiation generally excludes plastics from consideration.

4. Dimensional Stability

Assess how tight your tolerances are and how much temperature varies in service. Calculate thermal expansion across the expected temperature range and compare it to tolerance requirements. For hygroscopic materials, factor in dimensional changes from moisture absorption as well.

5. Economic Analysis

Run the numbers to see if conversion actually saves money. Compare material costs on a per-part basis, accounting for density differences—cost per pound is misleading. Compare machining times between metal and plastic. Account for assembly cost changes from part consolidation and fastener elimination. Include savings from eliminated secondary operations like coating, plating, and deburring. Don’t forget maintenance cost changes over the product lifecycle, including lubrication, corrosion, and replacement frequency.

Calculating the Real Cost

Material cost per pound is misleading because plastic weighs so much less than metal. The right comparison is 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 their lower density.

The machining cost advantages compound the material savings:

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, follow a structured process.

1. Application Analysis. Document all requirements thoroughly: loads, temperatures, chemicals, tolerances, environment, life expectancy, and safety criticality. Missing a requirement at this stage causes expensive problems later.

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

3. Design Modification. Plastic parts should not simply replicate metal geometry. Redesign to leverage plastic’s strengths and accommodate its weaknesses. Add ribs for stiffness since plastics are less rigid than metals. Include generous radii to reduce stress concentration. Allow for draft angles where needed for molding. Integrate features like bosses and snap fits that would require separate parts in metal. Account for thermal expansion in fits and clearances.

4. Prototype and Test. Build prototypes in the actual candidate material and test under real service conditions. Material properties on a datasheet don’t capture everything—thermal aging, chemical attack, and wear behavior in your specific application may differ from published values. Don’t shortcut this step.

5. Production Transition. Plan the transition carefully. Maintain metal part supply until the plastic version is fully validated in field service.

When to Call Us

NextGen Components supports metal-to-plastic conversion at every stage. Our engineers can recommend materials matching your specific requirements. We review designs to identify conversion candidates and flag potential issues before they become problems. We supply prototype quantities for testing, and we provide ongoing production material supply once conversion is validated.

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