How to Trade Off Strength, Chemical Resistance, and Cost in Plastic Part Design

You need a part that’s strong enough to carry a load, resistant to the chemicals it’ll contact, and cheap enough to hit your BOM target. Three requirements. No single plastic delivers all three at once. Engineering is about trade-offs.

I’ve spent twenty years watching engineers try to beat this reality. They spec a material that nails one requirement — strength, say — only to discover it stress-cracks in the presence of lubricant. Or they pick the cheapest resin and wonder why the part creeps under load. Here’s the thing: there is no perfect plastic. But there is a rational way to pick the best compromise.

If you visualize the trade-off as a radar chart with three axes — Strength, Chemical Resistance, Cost — every material occupies a different shape. Some stretch far in one direction and collapse in another. A few sit near the center with balanced but unexceptional scores. One or two push all three axes outward but at a price that changes your whole program economics.

Here’s how the six most common engineering candidates stack up.

PP — The Chemical Champ on a Budget

Polypropylene is cheap — reliably the lowest-cost engineering-grade thermoplastic on the market. Resin runs about $0.85–1.30/lb, and its processing window is wide and forgiving. Cycle times are short, drying is unnecessary, and tooling lasts forever because PP doesn’t contain abrasive fillers.

Chemically, PP is outstanding. It resists acids, bases, alcohols, and most aqueous solutions. That’s why it dominates battery cases, chemical storage containers, and laboratory equipment. On a chemical resistance scale, PP is a 9/10.

The weakness is mechanical. Tensile strength runs 30–35 MPa. That’s about one-third of PA6 and one-quarter of PPA. Creep resistance is poor — PP deforms under sustained load faster than any other material on this list. For structural parts carrying real weight, it’s the wrong answer.

Scorecard: Strength 3/10, Chemical 9/10, Cost 10/10.

PP works when chemical exposure is the primary threat and loads are minimal. Battery cases, fluid reservoirs, chemical tank liners. For a deeper look at chemical compatibility across plastics, see our chemical resistance guide.

ABS — The Default for a Reason

ABS sits at the low end of engineering plastics. Tensile strength around 40–50 MPa. Chemical resistance is mediocre — it survives dilute acids and bases but swells or stress-cracks in contact with hydrocarbons, ketones, and many solvents. Cost is excellent at roughly $1.80–2.50/kg.

Where ABS shines is the combination of decent surface finish, good impact at moderate wall thickness, and easy processing. It’s the most forgiving material in the mold — wide processing window, good flow, predictable shrinkage.

But ABS won’t solve a demanding trade-off problem. Its strength is adequate for light structural duty. Its chemical resistance is borderline for anything beyond mild exposure. It’s the safe middle when no axis is pushed to its limit.

Scorecard: Strength 4/10, Chemical 3/10, Cost 8/10.

ABS is your fallback for cosmetic housings, enclosures, and light-duty brackets where neither extreme chemical exposure nor high structural load is present.

PA6 — Strong, Chemically Capable, Moisture-Sensitive

Nylon 6 brings real structural performance. Tensile strength hits 75–90 MPa. Chemical resistance is solid — excellent against hydrocarbons, oils, greases, and most solvents. PA6 is the go-to for automotive under-hood components, industrial gears, and structural brackets that live in oily environments.

The catch, and it’s a big one, is moisture absorption. PA6 takes up 1.5–2.5% water at equilibrium. That water plasticizes the material and drops the tensile modulus by 30–50%. A PA6 bracket that passes FEA at dry-as-molded conditions can creep 1% or more in a humid environment.

Cost is moderate — about 1.5–2.0x the resin cost of PP. You’re paying for real strength and real chemical resistance, but you’re managing moisture as a design variable.

Scorecard: Strength 7/10, Chemical 7/10, Cost 6/10.

PA6 is the right call when your part sees oils and moderate loads in a reasonably controlled environment. If humidity is a concern, our comparison of PA6+GF vs PPA+30GF for outdoor use covers the moisture management strategy.

PC — Maximum Impact, Minimum Chemical Tolerance

Polycarbonate delivers 60–70 MPa tensile strength — solid but not class-leading. Its real claim is impact resistance: 600–900 J/m notched Izod, roughly ten times what PA6 offers. If your part gets dropped, hit, or abused, PC survives.

But PC’s chemical resistance is poor. It stress-cracks in contact with hydrocarbons, greases, cleaning solvents — even some adhesives. That automotive bracket near an oil leak? PC fails within weeks. That power tool housing wiped with isopropyl alcohol? PC crazes within days.

Cost runs moderate at about $3.50–5.00/kg, similar to PA6 on a per-part basis when you factor in PC’s higher density (1.20 g/cm³ vs 1.13 g/cm³ for PA6).

Scorecard: Strength 6/10, Chemical 3/10, Cost 6/10.

PC is your pick when impact is the dominant risk and chemical exposure is controlled. See our PC vs ABS vs Nylon impact comparison for the full data on load-bearing applications.

POM — The Balanced Middle Child

POM (acetal) sits near the center of every axis. Tensile strength at 60–70 MPa matches PC. Chemical resistance is good — it handles hydrocarbons, alcohols, and most solvents, though it degrades in strong acids and bases. Cost is moderate, about 1.5–2.0x PP.

What makes POM special is dimensional stability and wear resistance. Water absorption is 0.2% — an order of magnitude better than PA6. That means a POM gear or housing holds its dimensions across seasons and climates. And POM’s wear factor (K = 1–3 × 10⁻⁶) makes it the standard for precision moving parts.

The trade-off: POM doesn’t excel on any single axis. It’s a 6/10 across the board. If you need peak performance in strength, chemical resistance, or cost, there’s a better material for each. But if you need reasonable performance in all three — without surprises — POM is hard to beat.

Scorecard: Strength 6/10, Chemical 6/10, Cost 5/10.

POM works when you need a part that performs adequately in every dimension. Our POM vs PA6 gear comparison covers the wear-specific data.

PPA — Everything Is Excellent. Except the Price.

Polyphthalamide is what you spec when requirements are non-negotiable. Tensile strength hits 90–110 MPa, best on this list. Chemical resistance is outstanding — PPA handles hydrocarbons, acids, bases, and most solvents at elevated temperatures. Heat deflection temperature at 1.8 MPa reaches 280°C, nearly 100°C higher than PA6.

Moisture absorption is 0.2–0.5%, which means PPA doesn’t lose stiffness in humid environments. Flexural modulus holds above 10,500 MPa wet — the gap over PA6 widens from 2x dry to nearly 3x conditioned.

The cost is the pain point. PPA runs roughly $8–12/kg, about 3–4x PP and 2x PA6. At volume, that delta adds up fast. A 50-gram PPA part at 200,000 units costs roughly $80,000–120,000 in material alone, versus $20,000–30,000 in PP.

Scorecard: Strength 9/10, Chemical 8/10, Cost 2/10.

PPA earns its place when failure isn’t an option — aerospace brackets, automotive transmission components, high-temperature electrical connectors, and outdoor structural parts that must perform for a decade without degradation.

Six-Way Comparison

The Decision Framework

Here’s how I work through the trade-off with clients:

Step 1 — Rank your requirements. Write down the minimum acceptable strength, the chemical environment, and the maximum cost per part. Only three numbers. Everything else is noise.

Step 2 — Eliminate non-starters. If you need 80+ MPa tensile, PP and ABS are out. If the part contacts hydrocarbons continuously, PC is out. If cost must stay under $0.15/part at 100K volume, PPA is out.

Step 3 — Test the top two candidates. CNC machine or prototype-mold your part in the two materials that survive elimination. Expose them to the actual chemical environment under load. Measure what happens, don’t guess from a data sheet.

Step 4 — Run the total cost, not the resin cost. A $4/kg material that lets you drop wall thickness by 20% can be cheaper per part than a $2/kg material. A $12/kg PPA part that eliminates a field failure replacement program saves money in the long run. Don’t optimize on resin price alone.

I’ve walked through this framework on hundreds of programs. Every time, the answer is different because every application has a different trade-off profile. That’s the point — there’s no universal winner.

If you’re stuck on a material selection with conflicting requirements, send us your design. Our engineering team will run the comparison, check the service conditions, and recommend the optimal material before we cut any steel.