What Is a JIS K6251 Cutting Die Used For?

JIS K6251 is the Japanese Industrial Standard for determining the tensile properties of vulcanized and thermoplastic rubber, including tensile strength, elongation at break, and tensile stress at a given elongation. The current edition is based on the international standard ISO 37.

In Japan, and among suppliers to Japanese automotive, motorcycle, appliance, and electronics manufacturers across Asia, JIS K6251 is frequently specified as the acceptance standard.

Why does die quality matter? JIS K6251 requires dumbbell specimens to be cut with a die of the correct dimensions. Defective specimens must not be used, and specimens that break outside the gauge marks must be discarded and retested. A worn or deformed cutting edge leaves burrs or nicks that cause premature breaks, resulting in low readings, poor repeatability, and extra time and material spent on retests.

Kind Snail cutting dies are made of hardened alloy tool steel (HRC 55-60) and precision cut by electrical discharge machining (EDM). With a cutter wall of at least 5 mm, the die resists deformation and keeps every specimen consistent.

Suitable materials: NR, SBR, NBR, EPDM, CR, FKM, silicone, and other vulcanized rubbers, as well as thermoplastic rubbers such as TPE and TPV.
Typical applications: Automotive rubber parts, anti-vibration rubber, seals, O-rings, hoses, and belts.

JIS K6251 Dumbbell Numbers

The current JIS K6251:2017 defines seven dumbbell specimens: No.1, 2, 3, 5, 6, 7, and 8. According to the standard:

・No.3 and No.5: Recommended for general testing
・No.1: For materials with low elongation
・No.2: For materials with low tensile strength
・No.6: For narrow samples
・No.7 and No.8: Small specimens, used only when there is not enough material for larger specimens

Results from different dumbbell numbers are often not directly comparable. When comparing materials, use the same specimen number.

Note: No.4 was included in earlier editions of JIS K6251 but is no longer part of the current 2017 edition. If your specification or drawing still calls for No.4, Kind Snail can supply it. Contact us for details.

Always follow the specimen number required by your product specification or customer.

ISO 37 Equivalents

Because JIS K6251 is based on ISO 37, several dumbbell numbers correspond to ISO 37 types:

・JIS K6251 No.3 = ISO 37 Type 1A
・JIS K6251 No.5 = ISO 37 Type 1
・JIS K6251 No.6 = ISO 37 Type 2

JIS K6251 Specimen Dimensions

About Kind Snail Cutting Dies

・Material: Alloy tool steel, ensuring accurate cavity dimensions and a long service life
・Cutter wall: At least 5 mm thick to prevent deformation during cutting
・Hardness: HRC 55-60
・Machining: Precision cut by electrical discharge machining (EDM)
・Compatible machines: Kind Snail manual cutting press and pneumatic cutting press
・Custom service: Non-standard dies can be made from your drawing or sample (send us your drawing or requirements, and our specialist will assist you)

What’s in the Box

・Cutting die × 1 (as ordered)
・Dimension inspection report × 1
・Technical document × 1

Care and Storage

After each use, wipe the die clean, apply anti-rust oil, and wrap it securely. Store in a dry place to prevent rust and extend service life.

JIS K6251 Cutting Die FAQ

Q: Are JIS K6251 and ISO 37 dies interchangeable?
A: Several numbers correspond: JIS No.3 = ISO 37 Type 1A, No.5 = Type 1, and No.6 = Type 2. Please compare the dimension tables before purchasing, and state the standard and specimen number used in your test report.

Q: My specification calls for JIS K6251 No.4. Can I still buy it?
A: Yes. No.4 is no longer included in the current JIS K6251:2017, but some specifications still use it. Kind Snail can supply No.4 dies based on the original dimensions. Send us your specification or drawing and we will help you confirm.

Q: Not sure if your die can cut my material?
A: Send us a material sample. We will do a trial cut and report the results, so you can confirm performance before you buy.

Q: What is the lead time? Do you make custom dies?
A: Common types are usually in stock and ship quickly. Otherwise, standard dies take about 1 month and custom dies about 2 months, depending on complexity. Custom dies can be made from your drawing or sample. Contact us for details.