What impact strength means
Impact strength describes how much energy a material can absorb during a sudden blow before it cracks or breaks. It is different from tensile strength, which describes resistance to a slow, steady pull. A plastic can be fairly strong when pulled slowly and still shatter when dropped, which is why the two properties are listed separately on datasheets.
In everyday terms, a tough plastic dents, bends or deforms when struck, while a brittle plastic cracks. Toughness is the property people usually care about when they ask whether a plastic is “strong.”
How impact strength is measured
The two test methods you will see most often are Izod and Charpy. Both swing a pendulum into a small test bar and record how much energy the bar absorbs. The main difference is how the bar is held: Izod clamps it upright like a cantilever, while Charpy supports it at both ends like a beam.
A few details matter when reading results:
- Notched vs unnotched. Most published values are for notched bars, where a small V-shaped cut concentrates stress. Some plastics are very sensitive to notches and look much weaker with one, while others barely change.
- Units differ. Values may be given in J/m, kJ/m2 or ft-lb/in. These are not directly interchangeable, so compare numbers only within one test method and unit.
- Some tough plastics do not break. Certain grades are reported as “no break” in a test, which means the number is not a true ceiling.
Properties tables such as the one Curbell Plastics publishes let you sort materials by Izod impact, but they also note that values can represent a specific grade or formulation, so treat any single number as an approximation.
Common plastics compared
The table below gives a rough, qualitative ranking. Exact values depend heavily on grade, fillers, thickness and test conditions, so use it for orientation rather than design calculations.
| Plastic | General impact behavior | Notes |
|---|---|---|
| Polycarbonate (PC) | Very high | Often chosen for shatter resistance; can be notch sensitive |
| ABS | Good | A common balance of toughness and rigidity |
| Nylon (PA) | Good to high | Properties change as it absorbs moisture |
| PETG | Moderate to good | Generally tougher than PLA in typical use |
| Polypropylene (PP) | Moderate | Tough and flexible, but gets brittle in the cold |
| HIPS | Moderate | Rubber-modified polystyrene, tougher than standard PS |
| Acrylic (PMMA) | Low to moderate | Stiff and clear, but cracks rather than bends |
| PLA | Low | Stiff but tends to be brittle |
Acrylic is worth a special mention. Manufacturer material suggests it compares favorably with ordinary glass in impact tests, yet it is noticeably more brittle than polycarbonate. See our notes on acrylic heat resistance for other limits to keep in mind.
Why published numbers can mislead
Datasheet values are measured on small, molded, conditioned samples. Real parts behave differently for several reasons:
- Temperature. Many plastics lose impact strength as they get colder. One engineering forum discussion on parts needing impact resistance down to -40 C noted that polypropylene was a poor choice for that reason, and that other materials suited cold service better.
- Notches and sharp corners. Internal corners, screw holes and thin sections act like notches. Adding fillets often helps more than changing material.
- Thickness. A thick part can behave more brittlely than a thin one of the same plastic.
- Aging and UV. Sunlight and heat can embrittle some plastics over time. ABS, for example, is often cited as less suited to long outdoor exposure than ASA.
- Moisture and additives. Nylon changes with humidity, and impact modifiers, glass fibers and other fillers can raise or lower toughness.
Polycarbonate specifications and trade-offs
People often ask about the specifications of polycarbonate. It is an amorphous, transparent engineering thermoplastic known for high impact resistance and good heat tolerance relative to commodity plastics. Suppliers often position it as a glass alternative where shatter resistance matters, such as guards and glazing.
Exact figures vary by grade, so check the manufacturer’s datasheet. Common trade-offs include scratching more easily than acrylic, possible yellowing outdoors without UV protection, sensitivity to some solvents, and difficulty printing it on basic 3D printers because of high temperatures and warping. Our PETG vs polycarbonate comparison covers the printing side in detail.
Disadvantages of high impact polystyrene
High impact polystyrene (HIPS) adds rubber to polystyrene to improve toughness and is popular as an inexpensive, easy-to-machine plastic. The downsides people usually raise are:
- Lower stiffness and heat resistance than engineering plastics.
- Limited chemical resistance; some solvents and cleaners can attack it. One forum respondent suggested ABS instead when chemical resistance matters.
- Poor outdoor stability without protection.
- It is not the best choice for very cold or demanding structural loads.
In 3D printing, HIPS is mostly used as a soluble-in-limonene support material for ABS, rather than as a main structural filament.
Impact strength in 3D printed parts
Printed parts add a variable that molded parts do not have: layers. A printed part is usually weakest between layers, so a hit that tries to pull layers apart will fail at lower energy than the same blow applied along them.
Practical ways to improve toughness:
- Orient the part so that expected loads run along layers rather than across them.
- Increase wall count and perimeters, which usually helps more than extra infill (see best infill density for strength).
- Print slightly hotter within the filament’s range for better layer bonding.
- Add fillets to corners and avoid sharp internal angles.
- Keep nylon dry, since moisture-laden filament prints poorly.
Results vary between printers, filaments and brands, so test a few printed samples before committing to a design.
Choosing a plastic for an impact-loaded part
Work through a few questions:
- What temperature range will the part see? Cold favors tougher grades; high heat limits many common plastics.
- Is it outdoors? UV exposure may push you toward UV-stable options.
- Does it touch chemicals? Cleaners, fuels and solvents can crack some plastics.
- Does it also rub or slide? If so, see high wear resistance plastics.
- Could a different material be better? For adhesives and coatings, see epoxy durability.
FAQ
Is plastic a strong material?
It depends on the plastic and what kind of strength you mean. Some plastics are tough and absorb impacts well, others are stiff but brittle, and engineering plastics with fiber reinforcement can be very strong for their weight. Plastics generally have lower stiffness and temperature limits than metals, so they are not a universal substitute.
Can I rely on one number to pick a material?
No. Use impact numbers as one input, alongside temperature, load, chemicals and how the part is made.