Plastics and Polymer Properties Reference

Selecting the right plastic for an application requires balancing multiple competing properties: strength, stiffness, temperature resistance, chemical resistance, processability, and cost. No single plastic is best at everything.

Key Properties for Plastic Selection:

Density (g/cm³): Directly drives part weight and material cost. PP at 0.90 g/cm³ gives you more volume per kilogram than PVC at 1.40 g/cm³, a 36% saving on mass per liter of finished part, all else equal. For injection molders, lower density means more shots per kilogram of resin.

Tensile Strength (MPa): The force per area the material can withstand before breaking. PEEK (100 MPa) and Nylon 66 (82 MPa) are high-performance structural options. LDPE (12 MPa) is suitable only for flexible, non-structural applications.

Elongation at Break (%): How much the material stretches before fracturing. High elongation (HDPE 800%, PP 400%) means ductile, tough materials that bend and stretch before failing. Low elongation (Epoxy 2%, PS 3%) means brittle materials that crack suddenly without warning.

Service Temperature (°C): The maximum continuous use temperature before the material softens, deforms, or degrades. PTFE (260°C) and PEEK (250°C) are exceptional; LDPE (80°C) fails in a hot car.

Thermoset vs Thermoplastic:

• Thermoplastics (HDPE, PP, PC, ABS, Nylon, etc.) can be melted and reprocessed. They are generally recyclable. They soften when heated.
• Thermosets (Epoxy, silicone RTV, polyurethane foam) undergo irreversible chemical crosslinking during cure. They cannot be remelted. They offer superior heat and chemical resistance.

Resin Identification Codes (RIC):

Two things people get wrong about that little triangle. It is an identification code, not a recycling promise: codes 1 and 2 are widely accepted, 4 and 5 depend entirely on your local facility, and 3, 6 and 7 are refused almost everywhere. And thermosets carry no code at all, because they cannot be remelted, so there is nothing for a sorter to do with them.

Material Selection Example: Outdoor Electronics Enclosure: Options: ABS vs Polycarbonate (PC)

• ABS: cheaper, easier to machine/paint, good impact resistance indoors
• PC: superior impact resistance (nearly unbreakable), better UV stability outdoors, higher service temperature
• Winner for outdoor use: PC: despite higher cost, it resists UV yellowing and cracking that would ruin ABS after 2–3 years of sun exposure.

Part Weight Calculation: Weight (g) = Volume (cm³) × Density (g/cm³) This is the first calculation any injection mold designer performs to estimate material cost per shot.

Reading the strength-versus-temperature scatter. The chart plots every material on the list, tensile strength against maximum service temperature, with your selection highlighted. The commodity plastics cluster in the bottom left, cheap and neither strong nor hot. PEEK alone occupies the top right, which is most of the reason it costs what it costs. The interesting one is PTFE, sitting far right at 260°C but low at 31 MPa: it buys its heat tolerance by giving up strength, so choosing it for a hot application only works if the part is not carrying load. That trade is the whole point of looking at two properties at once instead of sorting by one.

Chemical Resistance Note: HDPE and PP resist most acids and bases. PVC resists many solvents. PTFE is nearly universally chemically inert. ABS is attacked by acetone (nail polish remover) and many organic solvents, which is exactly why acetone vapor smooths and welds ABS 3D prints. The same reaction is a defect in one context and a finishing technique in the other.