The short answer
For a rigid part that must survive being dropped or hit, polycarbonate has the highest notched impact strength of the common rigid, clear plastics. It is also notch sensitive, so it needs generous radii, walls under about 5 mm and no solvent contact. PC/ABS and ABS are more forgiving and cheaper. Impact-copolymer polypropylene and TPO suit low-cost tough parts, impact-modified nylon adds heat and chemical resistance, and UHMW-PE, HDPE and TPU take repeated abuse by flexing instead of resisting.
Impact resistance is not one number. A part breaks because of how the resin, the geometry, the temperature and the chemicals around it combine. A tough resin in a part with a sharp corner can fail more easily than a modest resin in a well-designed part. This page compares the resins and then covers the things that undo them.
Impact-resistant plastics compared
| Resin | Notched Izod, 23°C (J/m) | Low-temperature toughness | Flex modulus (GPa) | Notes |
|---|---|---|---|---|
| Polycarbonate (PC) | 600–900 at 3.2 mm; often ~100–200 at 6.4 mm | Good unnotched to about -40°C (-40°F); notched toughness falls off between about 0 and -30°C (32 and -22°F) in standard grades | 2.2–2.5 | Clear. Notch, thickness and solvent sensitive |
| PC/ABS | 400–700 | Good; many grades hold useful toughness to about -30°C (-22°F) | 2.0–2.6 | Less notch and thickness sensitive than PC; easier to mold |
| ABS | 150–400 | Fair to good, depending on rubber content | 2.0–2.6 | Low cost, paints and plates well; poor in solvents |
| HIPS | 60–150 | Fair | 1.6–2.4 | Cheapest tough rigid option; low heat and solvent resistance |
| PP impact copolymer / TPO | 60–500; many TPO grades do not break | Good to about -20 to -40°C (-4 to -40°F), grade dependent | 0.9–1.4 (TPO lower) | Bumpers, totes, battery cases; low stiffness and creep |
| Nylon 66, impact-modified | ~800–1,100 dry as molded | Good; many grades stay ductile to about -30 to -40°C (-22 to -40°F) | ~1.5–2.0 dry, lower conditioned | Unmodified PA66 is only 35–60 J/m dry |
| UHMW-PE | No break in a standard notched bar | Excellent, well below -40°C (-40°F) | ~0.6–0.9 | Machined or compression molded; not injection molded |
| HDPE | 20–200 | Good | 0.8–1.5 | Rotomolded and blow molded tanks, drums, cases |
| TPU | No break | Good; polyether types best | Flexible (Shore A to D) | Absorbs impact by deforming; abrasion resistant |
| Acrylic / GPPS (for reference) | 15–25 | Poor | 2.9–3.3 | Brittle: crack or shatter on impact |
Typical values for general-purpose grades, ASTM D256 at 3.2 mm (1/8 in) unless noted; individual grades vary widely, especially impact-modified grades. Low-temperature limits are approximate and depend on grade, notch and wall thickness. Check the data sheet for the grade you run.
What notched Izod numbers mean
In the ASTM D256 test, a pendulum strikes a bar with a V-notch of 0.25 mm (0.010 in) tip radius, and the energy used to break it is divided by the bar width. Results are given in J/m or ft-lb/in (1 ft-lb/in is about 53.4 J/m). ISO 180 reports kJ/m², which does not convert directly. See Izod vs. Charpy impact.
The notch is there on purpose. It stands in for the sharp corners, holes, threads and weld lines in a real part. A resin with a high unnotched value and a low notched value, such as polycarbonate in thick sections or acetal, is tough only when the design avoids stress concentrators. Compare grades tested the same way, at the same thickness and temperature, and note whether nylon values are dry as molded or conditioned.
Polycarbonate: very tough, with conditions
Thin polycarbonate yields and bends before it breaks, which is why it is used for guards, face shields and housings. Three things change that.
- Notches. PC is among the most notch-sensitive tough plastics. In one published example, a 3.2 mm (0.125 in) right-angle part with a 0.25 mm (0.010 in) inside radius absorbed about 3.4 J (2.5 ft-lb); doubling the radius to 0.5 mm (0.020 in) raised it to about 27 J (20.2 ft-lb).
- Thickness. Above a critical thickness, notched PC breaks in a brittle way. A standard PC bar gives roughly 600–900 J/m at 3.2 mm but often only about 100–200 J/m at 6.4 mm (1/4 in). The critical thickness is near 5–6 mm for low-flow grades and lower for high-flow grades.
- Temperature. Unnotched parts stay tough to about -40°C (-40°F), but notched standard grades lose most of their toughness somewhere between about 0 and -30°C (32 and -22°F). Siloxane copolymer PC grades stay ductile in notched tests to -40°C (-40°F) or lower.
Processing matters as well. PC molded above about 0.02% moisture hydrolyzes in the barrel, loses molecular weight and molds brittle parts that look normal. Dry it at about 120°C (248°F) for 2–4 hours (see the drying guide) and limit regrind. See polycarbonate.
Chemicals undo polycarbonate toughness
Polycarbonate under stress cracks when it meets many common chemicals: gasoline, some oils and greases, thread-lockers, many cleaners and alcohols, sunscreen, and plasticizer moving out of flexible PVC. The crack starts at a stressed spot (a screw boss, press fit or sharp corner) and the part then breaks at a light knock. PC/ABS shares the weakness to a lesser degree. If the part will see chemicals, test it under stress with the real chemical, or consider a semi-crystalline resin. See environmental stress cracking.
The other tough resins
PC/ABS trades some heat resistance for less notch and thickness sensitivity and easier molding. It is the usual choice for electronics housings that get dropped. See PC vs. PC/ABS.
ABS gives fair to good impact at low cost and keeps it in thick walls better than PC. Toughness rises with rubber content and stiffness falls. See ABS.
Impact-copolymer PP and TPO carry a rubber phase that keeps them tough below freezing. They are soft and creep under load, so they suit totes, bins, bumpers and battery cases rather than precise parts. See PP homopolymer vs. impact copolymer.
Impact-modified nylon (often called super-tough) reaches notched Izod values near or above PC while keeping nylon’s resistance to oils, fuels and heat. Plain nylon is notch sensitive when dry, about 35–60 J/m for PA66, and gets tougher as it absorbs moisture. Ask whether a data sheet value is dry as molded or conditioned. See nylon.
UHMW-PE, HDPE, LDPE and TPU absorb impact by deforming. UHMW-PE and TPU also resist abrasion, so they suit liners, bumpers, wheels and wear pads that are hit over and over. Standard UHMW-PE is not injection molded.
Design and molding for impact
- Radius every inside corner. Many design guides call for at least 0.5 mm (0.020 in) and preferably about half the wall thickness.
- Keep walls uniform and, for PC, under about 5 mm. Use ribs for stiffness rather than thick walls.
- Keep weld lines, gates and molded-in lettering away from the impact zone.
- Dry the resin to the producer’s target and control regrind percentage.
- Test finished parts at the lowest service temperature, not just at room temperature.
What to send us for a quote
- The grade you run now, or the part and how it gets hit (drop height, impact, repeated abuse)
- Wall thickness and the lowest service temperature
- Chemicals the part touches, including cleaners and lubricants
- Clear or opaque, color, and any flame or food-contact requirement
- Quantity for this order and per year, and ship-to city
Common questions
What is the most impact-resistant plastic?
Among rigid, clear-capable plastics, polycarbonate has the highest notched Izod, typically 600–900 J/m in a 3.2 mm (1/8 in) bar. Impact-modified nylon can match or beat it, and UHMW-PE, TPU and LDPE usually do not break at all in a standard notched Izod test, but they are much less stiff.
Is polycarbonate really unbreakable?
No. Polycarbonate is very tough in thin walls with generous radii, but it is notch sensitive: sharp corners, thick sections, cold, wet molding and chemical stress cracking can each make it fail in a brittle way at a fraction of its rated impact strength.
What does notched Izod mean?
It is the energy a pendulum uses to break a notched bar, reported per unit width (J/m or ft-lb/in under ASTM D256). The notch acts like a sharp corner in a real part, so the number shows how the material behaves when a stress concentrator is present.
Is ABS or polycarbonate tougher?
Polycarbonate is tougher in a standard notched test (about 600–900 J/m against 150–400 J/m for ABS), and unnotched it stays tough to about -40°C (-40°F). ABS is less notch sensitive in thick sections, cheaper and easier to mold, and PC/ABS sits between them.
Why did my polycarbonate part crack when it should be tough?
The usual causes are a sharp inside corner, a wall thicker than about 5–6 mm, resin molded without proper drying, or contact with a chemical such as a cleaner, thread-locker, oil or plasticized PVC while the part was under stress.
Which plastic stays tough in the cold?
Polyethylene, UHMW-PE, impact-copolymer polypropylene, TPU, impact-modified nylon and PC/ABS keep useful toughness well below freezing. PP homopolymer, polystyrene and acrylic turn brittle. Our cold temperature guide covers limits by resin.
Does glass fiber make plastic more impact resistant?
Usually not in the sense buyers mean. Glass fiber raises the notched Izod of brittle resins such as nylon and PBT, but it cuts elongation and makes failure sudden. For parts that must absorb a blow and bend, unfilled or impact-modified grades are better.
Tell us how the part gets hit
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province