What is UL94 Flammability Testing

In product structural design, especially in scenarios involving high temperature, high voltage, circuit transmission, or human contact, the flame retardancy of a product's enclosure is directly related to usage safety, market compliance, and risk prevention. The ul94 flammability rating standard is a core compliance requirement for most product enclosures.

For example, for high-temperature and high-voltage products like electricity meters and connectors that we frequently develop for clients, UL94 certification for the enclosure is a strict mandatory requirement due to their long-term exposure to electrified and heat-generating environments.

ul 94 is currently the most universally applied plastic flammability rating standard globally. Its core focuses on four major indicators: the material's ability to self-extinguish after flame removal, burning time, whether it drips, and whether the drips ignite. If we can thoroughly understand it during our design work, we can avoid potential pitfalls when selecting materials, reading specifications, and conducting certifications.

Overview of UL 94 Ratings (3 Major Categories, 12 Levels)

Ratings are divided into 3 groups based on material types and testing methods. Within each group, a higher rating indicates better flame retardancy:

• General Rigid Plastics (Most commonly used): 5VA > 5VB > V-0 > V-1 > V-2 > HB

• Foam Plastics: HF-1 > HF-2 > HBF

• Films / Sheets: VTM-0 > VTM-1 > VTM-2

General Ratings (HB / V-2 / V-1 / V-0): Easy to Understand

1. HB: Horizontal Burning (Lowest Flame Retardancy)

• Test: The sample is placed horizontally and burned with a flame for 30 seconds.

• Criteria:

• Thickness 3–13mm: Burning rate ≤40 mm/min

• Thickness <3mm: Burning rate ≤70 mm/min

• Or the flame self-extinguishes before reaching the 100mm mark.

• Characteristics: Only flame retardant, not self-extinguishing, and can continue to burn.

• Application: Internal components with no specific fireproof requirements.

2. V-2 / V-1 / V-0: Vertical Burning (Mainstream for Electrical & Electronic Equipment)

• Test: The sample is held vertically and ignited twice for 10 seconds each time. It measures:

• Single afterflame time (t1, t2)

• Total afterflame time for a set of 5 samples

• Afterglow time

• Whether there are flaming drips and whether those drips ignite the surgical cotton placed below.

5VA / 5VB: Highest Ratings (Equipment Enclosures / Structural Components)

• Test: 5 ignitions of 5 seconds each + glow wire impact; the test is much more stringent.

• 5VB: Burn-through is allowed, but it must self-extinguish after flame removal, with no flaming drips igniting the cotton.

• 5VA: Burn-through is NOT allowed, it must self-extinguish, with no flaming drips igniting the cotton.

• Application: Cabinets, large equipment enclosures, and high-safety structural components.

Film Ratings VTM / Foam Ratings HF

VTM-0 / VTM-1 / VTM-2 (Films <0.76mm)

• The rules are similar to the V ratings, but the samples are thinner and tested by hanging vertically.

• VTM-0 ≈ V-0, VTM-2 ≈ V-2 (Flaming drips are allowed to ignite the cotton).

HF-1 / HF-2 / HBF (Foams)

• HF-1: No flaming drips, does not ignite the cotton, and extinguishes quickly after flame removal.

• HF-2: A small amount of dripping is allowed, but it does not ignite the cotton.

• HBF: Horizontal burning, only burns slowly, and is not self-extinguishing.

3 Key Points for Reading Specifications / Yellow Cards (Avoiding Pitfalls)

1. You Must Check the Thickness

For the same material, the thicker it is, the easier it is to pass a higher rating.

• Example: If labeled as V-0 (1.5mm) → It only meets the V-0 standard when it is at least 1.5mm thick. If it is thinner, it might only meet V-2.

2. Rating Order (From High to Low)

• 5VA > 5VB > V-0 > V-1 > V-2 > HB

• Commonly used in electronics: V-0 or V-1

• Low cost: V-2 / HB

3. How to Read a UL Yellow Card

• Material: The type of material.

• Flame Class: Flammability rating (including the specific thickness).

• Example: V-0 (2.0mm) → A thickness of 2.0mm meets the V-0 rating.