1, HB: the lowest flame retardant grade in UL94 standard. It is required that the burning rate of samples with a thickness of 3 ~ 13mm should be less than 40mm/min；; For samples with a thickness of less than 3 mm, the burning speed is less than 70 mm per minute; Or go out before the 100 mm sign.
2.V-2: After the sample is tested twice 10 second, the flame goes out within 60 seconds. There may be combustible materials falling.
3.V- 1: After the sample is tested twice 10 seconds, the flame goes out within 60 seconds. No combustible materials can fall.
4.V-0: After the sample is tested twice 10 second, the flame goes out within 30 seconds. No combustible materials can fall.
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