If you’re specifying or designing with engineering plastics for the first time, the data sheets can feel overwhelming. This guide breaks down the most important material properties — what they measure, and why they matter for your application.
Engineering Notes: What to Keep in Mind
Before diving into individual properties, there are a few “good engineering practice” guidelines that apply across the board.
- Safety Factors. For properties that report ultimate strength values, divide the number by 4 to arrive at a safe working stress. For example, if a material has a compressive strength of 12,000 psi, design the part to a working maximum of 3,000 psi.
- Temperature. All properties are reported at 70°F–73°F and are typically tested in a controlled lab environment. Temperature affects all materials, but plastics more than most. As temperatures increase, properties tend to decrease (the material softens), and chemical resistance declines. At lower temperatures, properties also shift — especially impact resistance, as cold materials become more brittle.
- Time. The longer a material is exposed to any external stress — thermal, chemical, electrical, or mechanical — the greater the cumulative effect. Under sustained mechanical load, a material can permanently deform even below its yield point. This is called creep or cold flow (time-dependent deformation), and it becomes more severe as temperature increases.
- Test Criteria. Know which test method was used to generate a given data point. For instance, ASTM D-648 allows Heat Deflection Temperature (HDT) testing to be run at either 66 psi or the standard 264 psi. The lower load produces a higher — and therefore more favorable-looking — HDT number.
- How a Value Is Reported. There are “word games” in material data that can make one supplier’s product appear superior to another’s. Moisture absorption for cast nylon is a good example — a data sheet might report any of the following:
- 24-hour absorption (from a dry state): typically 0.3% to 0.7%
- Equilibrium content (average absorption from ambient air): typically 2% to 4% — highly dependent on geography, storage conditions, and time; often a “red herring”
- Saturation (maximum possible absorption): typically around 7%
For the same basic material, you can legitimately see reported values anywhere from 0.3% to 7%. Reputable suppliers report both the 24-hour and full saturation values.
- Fillers Change Properties. Always confirm the properties for the exact material formulation being specified. For example, 30% carbon fiber filled PEEK has different stiffness, heat transfer, and wear properties compared to unfilled PEEK — and it loses FDA compliance. Colors generally do not affect mechanical properties, but they can affect food-contact compliance and similar certifications.
Common Engineering Plastic Properties Defined
Mechanical Properties
- Specific Gravity (SG) / Density — ASTM D-792 The ratio of a material’s weight compared to the same volume of water. This determines part weight. Materials with an SG below 1.00 will float, as they are lighter than water.
- Tensile Strength — ASTM D-638 The pulling force (in psi) required to break a material at a given temperature. This determines how much tensile load a given cross-section can withstand without breaking.
- Tensile Modulus — ASTM D-638 A measure (in psi) of how stiff a material is when under tension. This allows calculation of how much a material will strain under a given tensile stress.
- Elongation — ASTM D-638 The percentage increase in a material’s length at the point of breaking. A higher value generally indicates a tougher, more ductile material.
- Flexural Strength — ASTM D-790 A measure of how much bending force a material can absorb before breaking. This determines the maximum bending load a given cross-section can withstand, whether fixed at one end or supported at both ends with a center load.
- Flexural Modulus — ASTM D-790 A measure (in psi) of how stiff a material is under flexural load. It represents a combination of tensile strain (one side stretching) plus compressive strain (the opposite side compressing).
- Compressive Strength — ASTM D-695 Determines how much compressive load a given cross-section can withstand without breaking.
- Compressive Modulus — ASTM D-695 A measure (in psi) of how stiff a material is when being compressed. Allows calculation of how much a material will strain under compressive stress.
- Shear Strength — ASTM D-695 Determines how much shearing force a given cross-section can withstand before failing.
- Hardness — ASTM D-785 Measures resistance to indentation. Various test methods and scales exist (Durometer, Rockwell, etc.) and there is no direct correlation between scales — only compare values reported on the same scale. Within a given scale, a higher number means harder. Most engineering plastics are reported using Rockwell scales.
- IZOD Impact Resistance — ASTM D-256, Method A (ft·lbs/in) A measure of impact resistance or “toughness.” It measures notch sensitivity and is typically used alongside other properties to identify the best candidate material for impact environments.
Thermal Properties
- Heat Deflection Temperature (HDT) — ASTM D-648 (°F) The temperature at which a ½” thick test bar deflects 0.010″. This is the key “working stress” number and a practical indicator of maximum operating temperature under load. Usually reported at 264 psi.
- Continuous Use Temperature (CUT) — UL 746 or resin data The maximum temperature at which a material can operate in air for 100,000 hours (~11 years) with no load and still retain at least 50% of its physical properties. Relevant mainly for lightly loaded parts in sustained high-temperature service.
- Melt Point — ASTM D-348 (°F) The temperature at which a crystalline or semi-crystalline material becomes fluid. Most relevant for processing and extrusion.
- Glass Transition Temperature — ASTM D-348 (°F) The “softening” temperature for amorphous materials. Important for companies doing thermoforming — it’s the minimum temperature needed to thermoform materials like PC, PMMA, or PET-G.
- Coefficient of Linear Thermal Expansion (CLTE) — ASTM D-831 Measures how much a material shrinks or grows with changes in temperature. Important for designing clearances and fits over a broad temperature range. Values are reported as the linear average across 30°F to 300°F.
- Thermal Conductivity — ASTM D-5930 The rate at which heat is conducted through a material. Determines a material’s effectiveness as a thermal insulator — the lower the value, the better the insulation.
Electrical Properties
- Dielectric Constant — ASTM D-150 Describes a material’s ability to store electrical energy (act as a capacitor). Allows designers to compare materials for their ability to inhibit or pass electrical current.
- Dissipation Factor — ASTM D-150 Measures dielectric loss in an AC current. Loss is expressed as heat, and since heat is generally unwanted, materials with a low dissipation factor are preferred for electrical applications.
- Dielectric Strength — ASTM D-149 The voltage at which a 1mm sample fails as an electrical insulator. This is primarily a comparative test between materials and should not be used alone as a design criteria.
- Volume Resistivity — ASTM D-257 Provides an estimate of current flow through a material for a given applied voltage. Important when evaluating static dissipative material performance.
Other Properties
- Flammability — UL 94 A standardized measure of how a material burns under specific controlled conditions. An important safety consideration. Ratings are reported by material thickness, and actual UL testing generates a “Yellow Card” for the specific resin.
- Coefficient of Friction (COF) Measures the “slipperiness” of a material against another surface — typically steel for engineering plastics. Distinguishes between static COF (force to start sliding) and dynamic COF (force to keep sliding). Critical for designing slide bearings and wear pads.
- Moisture (Water) Absorption — ASTM D-570 The percentage increase in a material’s weight based on water absorbed, measured at 24 hours and at full saturation. Addresses two concerns: dimensional stability (more water absorption means more dimensional growth) and property changes (absorbed moisture generally makes materials softer and less wear resistant).
Common Engineering Thermoplastics at a Glance
WS Hampshire fabricates components from a range of high-performance thermoplastics. Here is a quick summary of four of the most widely used.
- Acetal (Copolymer & Homopolymer) Acetal offers excellent dimensional stability and is ideal for wear applications in wet environments. Its primary limitation is excessive wear in dry applications. Common uses include bearings, wear parts, solenoid blocks, hinge pins, gears, sprockets, nozzles, pistons, pump parts, guides, and fittings. It serves transportation, food processing, medical, and paper processing industries. Available in plate, rod, and tube; natural, black, and blue.
- PET (Polyethylene Terephthalate) PET combines the wear resistance of nylon with the dimensional stability of acetal, and performs well in both wet and dry environments. Its main limitation is that it is inherently a brittle material. Applications include bearings, wear parts, scrapers, nozzles, pistons, dispensing heads, guides, plungers, cams, tension wheels, pump parts, and valves. Industries include transportation, food processing, bottling equipment, and medical. Available in plate and rod (tube available on special order); internally lubricated bearing grade also stocked. Natural and black.
- Nylon Nylon outwears acetal by a 3:1 ratio in dry applications, making it a strong performer where lubrication is unavailable. Its limitation is moisture absorption, which can cause excessive wear and dimensional tolerance issues in wet environments. Applications include bearings, bushings, rollers, guides, slide pads, sprockets, gears, star wheels, chain guides, and wear strips. Used in construction, conveying, transportation, food processing, medical, and paper processing. Available in plate and rod; natural and black.
- PEEK (Polyether Ether Ketone) PEEK delivers excellent wear resistance, a continuous use temperature up to 480°F, strong chemical resistance, and long-term stability. Its primary limitation is cost — it is significantly more expensive than other mechanical plastics. Applications include valves, insulators, valve seats, pump wear rings, pump parts, washers, and bearings. Used in construction, semiconductor, transportation, chemical processing, food processing, and medical industries. Available in plate and rod; natural brown.
Need Help Selecting the Right Material?
Choosing the right engineering plastic involves balancing mechanical, thermal, chemical, and cost factors specific to your application. WS Hampshire’s team can help you match the right material to your design requirements — contact us at info@wshampshire.com or call 800.541.0251.