Over 20,000 thermoplastic grades exist on the market. Selecting the right one requires filtering through this complexity to find the handful that actually fit your requirements. Most selection failures occur not from choosing the wrong material among valid options, but from starting with the wrong question. Engineers often ask “which material should I use?” before defining what problems that material must solve. A systematic approach that starts with application requirements and progressively narrows options produces better results than starting with familiar materials and hoping they work.
The Selection Hierarchy
Material selection follows a hierarchy: define requirements, eliminate materials that fail critical requirements, then optimize among survivors for cost and processability.
Start with application requirements, not materials. The question isn’t “is ABS good enough?” but rather “what must this part do, and what materials satisfy those requirements?” This reframing prevents the tunnel vision that comes from early material assumptions.
Define must-have versus nice-to-have properties. Must-have requirements are non-negotiable: the part fails if it doesn’t meet them. These might include temperature resistance above 120°C, FDA food contact compliance, or specific UL94 flame rating. Nice-to-have properties improve the application but aren’t essential. Conflating these categories leads to over-specified (expensive) or under-specified (failing) materials.
Environmental conditions shape material requirements more than most other factors. Temperature range (continuous and peak), chemical exposure (specific chemicals, not general categories), UV exposure, humidity, and outdoor weathering all eliminate materials from consideration.
Mechanical requirements include static loads (tensile, compression, flexural), dynamic loads (impact, fatigue), creep behavior under sustained load, and wear resistance in sliding applications. Quantify these where possible: “must withstand 50J impact at minus 20°C” eliminates more materials than “needs good impact strength.”
Regulatory requirements can be absolute constraints. Medical applications may require biocompatibility testing. Electrical applications may need specific UL or IEC ratings. Food contact requires FDA or other regulatory compliance. These requirements often override performance and cost considerations.
Commodity Resins
Commodity thermoplastics serve high-volume, cost-sensitive applications where their properties suffice.
Polypropylene (PP) offers the lowest density of common thermoplastics, excellent chemical resistance, fatigue resistance for living hinges, and the lowest cost per volume. It suits containers, caps, automotive interiors, and packaging. Limitations include poor UV resistance without stabilizers and brittleness at low temperatures (for homopolymer).
Polyethylene (HDPE, LDPE, LLDPE) provides exceptional chemical resistance, good impact even at cold temperatures, and low cost. Applications include bottles, containers, film, and pipe. HDPE offers stiffness; LDPE offers flexibility; LLDPE balances properties. All have high shrinkage and limited temperature resistance.
Polystyrene (PS) is rigid, dimensionally stable, and inexpensive. Standard PS (crystal or general purpose) provides clarity but is brittle. High-impact polystyrene (HIPS) adds rubber for toughness at the expense of clarity. Applications include packaging, disposable food service, and consumer electronics housings where impact isn’t critical.
PVC resists flames, chemicals, and weathering. Rigid PVC suits pipe, profiles, and window frames. Flexible PVC (with plasticizers) serves wire insulation, medical tubing, and flooring. Processing requires care due to thermal degradation and HCl release at high temperatures.
When commodity resins satisfy requirements, there’s rarely justification for engineering plastics. The cost difference (often 2x to 5x) and processing simplicity of commodities make them the default starting point.
Engineering Resins
Engineering thermoplastics provide higher performance at higher cost for applications that demand more than commodities offer.
ABS (acrylonitrile butadiene styrene) combines rigidity, impact resistance, and good surface appearance. It machines, paints, and plates well. Temperature resistance is moderate (about 80°C continuous). ABS suits automotive trim, appliance housings, consumer electronics, and any application requiring toughness with good aesthetics.
Polycarbonate (PC) offers exceptional impact strength (essentially unbreakable in typical thicknesses), transparency, and dimensional stability. Temperature resistance reaches 120°C continuous. PC serves safety glazing, protective equipment, automotive lighting, and medical devices. Limitations include chemical sensitivity (stress cracking in solvents) and surface scratching.
Nylon (PA6, PA66) provides high strength, wear resistance, and chemical resistance. PA66 offers higher heat resistance than PA6. Both absorb moisture, which affects dimensions and properties (typically requiring dry-as-molded specifications). Applications include gears, bearings, structural components, and automotive under-hood parts. Glass reinforcement improves stiffness and reduces moisture effects.
Acetal (POM) combines high stiffness, excellent fatigue resistance, low friction, and dimensional stability. It machines precisely and serves gears, bearings, fasteners, and precision mechanical parts. Homopolymer (Delrin) and copolymer grades offer slightly different property balances. Acetal bonds poorly without special preparation.
PBT (polybutylene terephthalate) offers chemical resistance, electrical properties, and fast crystallization for short cycles. It serves automotive electrical connectors, housings, and industrial components. Glass-filled grades provide structural capability with excellent dimensional stability.
Engineering resins cost 2x to 5x commodity prices. The selection decision compares this premium against the performance requirements that justify it.
High-Performance Resins
When temperature, chemicals, or mechanical demands exceed engineering resin capabilities, high-performance materials provide solutions at premium prices.
PPS (polyphenylene sulfide) withstands continuous temperatures above 200°C, resists nearly all chemicals, and provides inherent flame resistance. Glass-filled PPS serves automotive under-hood, industrial pumps, and electrical components in harsh environments. Cost runs 5x to 10x commodity resins.
PEEK (polyether ether ketone) represents the highest performance semi-crystalline thermoplastic, with temperature resistance above 250°C, exceptional mechanical properties, and broad chemical resistance. PEEK serves aerospace, medical implants, and extreme industrial applications. Cost runs 30x to 50x commodity resins.
PEI (polyetherimide, Ultem) provides high temperature resistance (180°C continuous), transparency in some grades, and excellent dimensional stability. It serves aircraft interiors, medical devices, and electrical insulation. Cost runs 10x to 20x commodity resins.
LCP (liquid crystal polymer) offers exceptionally thin-wall flow capability, high temperature resistance, and excellent dimensional stability. It serves miniature connectors, electronic packaging, and precision components. Cost varies but typically runs 10x to 15x commodity resins.
High-performance materials justify their cost only when lower-tier materials cannot meet requirements. Specifying PEEK when glass-filled nylon would work wastes substantial money.
Critical Property Considerations
Certain property requirements dominate selection for specific applications.
Temperature resistance requires understanding both continuous use temperature (what the part can withstand indefinitely) and heat deflection temperature (where the material softens under load). Applications near heat sources, under hoods, or in outdoor enclosures must quantify actual temperature exposure.
| Material Class | Typical HDT (0.45 MPa) | Notes |
|---|---|---|
| PP | 100-105°C | Higher with fillers |
| ABS | 90-100°C | PC/ABS blends higher |
| PC | 130-140°C | Glass filling increases |
| PA66 | 65-75°C (dry), higher filled | Moisture dependent |
| PPS | 260°C+ (glass filled) | Near-metal performance |
| PEEK | 160°C (unfilled) | Exceptional overall |
Chemical exposure requires identifying specific chemicals, not just categories. “Solvent resistant” means nothing useful. Specify contact with brake fluid, or gasoline, or sodium hypochlorite at concentration X. Chemical compatibility charts provide guidance, but testing in actual use conditions is essential for critical applications.
Mechanical requirements vary by loading type. Impact resistance differs from tensile strength differs from fatigue resistance. Quantify the actual loads and decide which properties matter most. Many applications require adequate properties in multiple categories rather than excellence in one.
Electrical properties matter for electrical applications. Surface resistivity, dielectric strength, and arc resistance ratings determine suitability. UL94 flame ratings (V-0, V-1, V-2, HB) are often specified for electrical housings.
Regulatory compliance is either met or not met. FDA food contact, USP Class VI for medical devices, UL recognition for electrical applications, and NSF certification for water contact are binary requirements that eliminate materials regardless of other properties.
Processing Considerations
Not all materials process with equal ease. Selection must account for manufacturing capabilities and constraints.
Drying requirements range from none (polyolefins) to stringent (nylons, PC, PET). Materials sensitive to hydrolysis degrade if moisture exceeds limits, typically 0.02 to 0.05 percent. Operations without proper drying equipment cannot process moisture-sensitive materials without quality problems.
Temperature sensitivity affects processing window. Some materials (PP, PE) tolerate wide temperature ranges. Others (PVC, POM) degrade if held too long at temperature. High-performance materials often require precise temperature control and may need specialized equipment.
Mold material compatibility matters for certain plastics. Some materials (especially flame-retardant grades) corrode standard tool steels. PVC releases corrosive gases. These materials require stainless or hardened tool steels that increase mold cost.
Cycle time impacts differ by material. Crystalline materials (PP, PE, nylon) release latent heat during solidification, extending cooling time. High-performance materials often require elevated mold temperatures that increase cycle time. Processing economics depend on these factors.
Cost vs. Performance Tradeoffs
Material cost typically represents 30 to 50 percent of molded part cost. Optimizing this tradeoff affects product economics significantly.
Over-specifying wastes money. Selecting glass-filled nylon for an application that unfilled polypropylene would satisfy adds cost without value. The temptation to choose “better” materials for safety margin creates unnecessary expense.
Under-specifying creates field failures. Saving material cost by selecting a marginally adequate material leads to warranty claims, recalls, and reputation damage that far exceed material savings. The cost of failure must inform material decisions.
Finding the minimum acceptable material is the optimization target. This doesn’t mean the cheapest material, but the least expensive material that reliably meets all requirements with appropriate margin.
Total cost perspective includes material cost, processing cost (cycle time, scrap rate, equipment needs), assembly cost (can this material be welded, bonded, or assembled as needed?), and quality cost (inspection requirements, failure risk).
The Decision Matrix Approach
A structured evaluation process narrows thousands of potential materials to a short list for detailed evaluation.
Step 1: Define absolute requirements. List must-have properties with quantified values. Example: continuous use temperature above 100°C, impact strength above 20 kJ/m² at minus 20°C, FDA food contact compliant.
Step 2: Eliminate failing materials. Any material that fails any absolute requirement is eliminated regardless of other properties or cost. This typically reduces candidates from thousands to dozens or fewer.
Step 3: Rank remaining requirements. Weight remaining properties by importance: dimensional stability might be weighted 3, surface appearance weighted 2, cost weighted 4 (on a 1-5 scale).
Step 4: Score candidates. Rate each surviving material on each weighted requirement. Multiply scores by weights. Sum for total score.
Step 5: Verify top candidates. Before final selection, verify that top-scoring materials actually process in your facility, are available from reliable suppliers at acceptable lead times, and perform in prototype testing.
Step 6: Select and document. Choose the material with the best total value (performance and cost). Document the selection rationale to support future reviews or variations.
Material selection is a filtering process. Start with all options, systematically eliminate those that fail critical requirements, then optimize among remaining candidates for cost and processability. This approach replaces intuition and habit with analysis, producing better results and defensible decisions.
Sources
- Modern Plastics Handbook, McGraw-Hill.
- Rosato, Donald V. “Plastics Engineered Product Design.” Elsevier, 2003.
- Material Supplier Technical Data Sheets (SABIC, Covestro, DuPont, Celanese, Solvay).
- Plastics Technology. “Material Selection Guide.” https://www.ptonline.com/
- UL Prospector Materials Database. https://www.ulprospector.com/
- IDES Plastics Database. https://www.ides.com/