PlasticsHow is plastic hardness measured? Shore D, ball indentation and Rockwell
In steels, hardness is measured in Rockwell C or Brinell, but those scales do not work for plastics: polymers deform differently and recover part of the indentation. That is why plastics data sheets talk about Shore D, ball indentation hardness, or Rockwell R and M. In this guide we explain what each scale measures, how the test is done, and how hard the twenty plastics we carry are.
Why don't plastics use the steel scales?
Hardness is a material's resistance to another body penetrating its surface. In metals, the indentation left by the indenter is permanent and is measured after the load is removed. In plastics, the material is viscoelastic: it deforms under load and recovers part of the indentation when the load is removed, and the reading also changes with the time of application. That is why plastics tests measure the depth of penetration with the load applied and fix a reading time.
Shore D: the most common scale for engineering plastics
The Shore test (ISO 868 and ASTM D2240) uses a hardness tester with a spring-loaded indenter. The scale runs from 0 (2.5 mm penetration) to 100 (zero penetration). The Shore A scale, with a truncated cone and 9.81 N of force, is used for rubbers and soft elastomers; the Shore D scale, with a 30° cone and 49 N, is the one that applies to rigid plastics and normally gives readings between 30 and 90. The specimen must be at least 6 mm thick and the reading is taken at 1 second (ASTM) or 15 seconds (ISO), which explains small differences between data sheets.
A value such as "83 Shore D" means 83 points on the D scale. The higher the number, the harder. A Teflon at 50 to 55 Shore D is soft to the touch; a nylon or a PET at 83 to 84 Shore D feels rigid; a Bakelite at 90 Shore D can no longer be marked with a fingernail.
Ball indentation hardness (ISO 2039-1)
This test presses a steel ball against the surface: first with a 9.8 N preload to set the reference plane, and then with a load of 49, 132, 358 or 961 N, chosen so that the penetration falls between 0.15 and 0.35 mm after 30 seconds. The result is expressed in N/mm² (equivalent to MPa) and is identified as HB or "ball hardness." Because the indentation is large, it averages the material well and works for both filled and unfilled plastics. It is the scale used in European data sheets (DIN EN ISO) for nylon, acetal, PEEK, PET and Bakelite.
Rockwell R and M (ISO 2039-2 and ASTM D785)
The Rockwell test for plastics measures the permanent depth of the indentation after the load is removed, using balls of different diameters. The most commonly used scales are R (12.7 mm ball and 588 N) for softer plastics and M (6.35 mm ball and 981 N) for harder plastics; L and E also exist. It is the usual scale on North American (ASTM) data sheets, such as that of polycarbonate (R125) or PETG (M/R 115).
How hard are our plastics?
Reference values taken from our data sheets. The three columns are not interchangeable: each scale uses a different indenter and load, so only values from the same scale can be compared.
| Material | Shore D (ISO 868) | Ball (ISO 2039-1) | Rockwell (ISO 2039-2 / ASTM D785) |
|---|---|---|---|
| Nylon M natural | 83 Shore D | — | — |
| Nylon SL black | 83 Shore D | 185 MPa | — |
| Nylon HS blue | 82 Shore D | 170 MPa | — |
| Nylon AL red | 82 Shore D | 170 MPa | — |
| Nylon XL green | 82 Shore D | 165 MPa | — |
| HD Polyethylene PE 300 | 64 Shore D | — | — |
| HMW Polyethylene PE 500 | 62 Shore D | 150 MPa | — |
| UHMW Polyethylene PE 1000 | 60 Shore D | 33 MPa | — |
| Ceramic UHMW Polyethylene | 68 Shore D | — | — |
| Acetal (POM) | 81 Shore D | 150 MPa | — |
| Teflon (PTFE) | 50 – 55 Shore D | — | — |
| Polycarbonate (PC) | — | — | 125 Rockwell R |
| PEEK | 88 Shore D | 230 MPa | — |
| PETG Copolyester | — | — | 115 |
| PET Polyester | 84 Shore D | 170 MPa | — |
| Polypropylene (PP) | 72 Shore D | — | — |
| Bakelite | 90 Shore D | 320 MPa | — |
| Celoron | — | — | 105 Shore D |
| G10 / FR4 | — | — | — |
| G11 | — | — | — |
What does hardness tell us when choosing a plastic?
- Scratch and mar resistance: the harder the material, the better the part resists scratches and contact marks. That is why Bakelite (90 Shore D) and PEEK (88 Shore D) are used in parts that take point loads.
- Don't confuse hardness with wear resistance: UHMW PE 1000 polyethylene is among the softest (60 Shore D) and yet it is the best at resisting abrasion in the whole family, thanks to its molecular weight. Hardness doesn't explain everything.
- Low hardness = better sliding and damping: Teflon (50 to 55 Shore D) and polyethylenes conform to the mating surface and absorb impact; they are ideal for seals, guides and wear plates.
- High hardness = better stability and finish: acetal, PET and nylon hold tight tolerances and give a good machined finish.
In summary
- Plastics are not measured in Rockwell C or Brinell: Shore D, ball indentation (N/mm²) and Rockwell R or M are used
- Shore D (ISO 868 / ASTM D2240) is the most common scale: 30° cone and 49 N; the higher the number, the harder
- Ball hardness (ISO 2039-1) is expressed in MPa and is the usual one on DIN EN ISO data sheets
- Only values from the same scale can be compared; hardness is not the same as wear resistance
- Our plastics range from 50 to 55 Shore D (Teflon) up to 90 Shore D (Bakelite)
Why buy your engineering plastics at NAT MAC?
We have more than 54 years of experience in the industry and ISO 9001 certified quality. We carry all ten families of engineering plastics, each with its own data sheet, and we help you choose the right material for your part, your load and your working environment.
- Material cut to the size you need
- Data sheets with verified properties for each grade
- Advice on choosing the right plastic for your load, temperature and environment
- Stock in bars, plates and bushings with delivery throughout Mexico
- Backed by ISO 9001 certified quality
Tell us what part you'll make, the conditions it operates in and the sizes you need, and we'll recommend the right material. Request your free quote.
Sources: NAT MAC Engineering Plastics data sheets and catalog; TecnoQuim (material data sheets); ISO 868:2003, Plastics — Determination of indentation hardness by means of a durometer (Shore hardness); ZwickRoell, Shore hardness test (ASTM D2240 / ISO 868); ZwickRoell, ASTM D785 and ISO 2039-1/-2, Plastics hardness test; ASTM D785, Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials.
