How UL94 Grades Are Determined ？

The core UL94 testing method involves igniting a specimen with a specified standard flame and observing the degree of combustion, speed, presence of dripping, and whether the dripping ignites flammable materials (such as cotton wool) below. Different flame retardancy grades are assigned based on the specimen's thickness, burning behavior, and self-extinguishing ability.

It is important to note that material thickness significantly affects test results. For the same material, the thicker it is, the easier it is to achieve a higher flame retardancy grade. Therefore, the UL94 grade must be indicated along with the specimen thickness (for example, UL94 V-0 @ 1.5mm).

Detailed Explanation 

UL94 is divided into two main test series: vertical burning and horizontal burning. The vertical burning test has more stringent requirements and a higher grade.

1. Vertical Burning Grade (V-0. V-1. V-2)

This is the most common and most stringent grade. During the test, the specimen is held vertically and the flame is ignited from below.

Evaluation Criteria: After each flame is removed, the specimen's afterflame duration, whether it burns into the fixture, whether it drips, and whether any dripping ignites the cotton wool below.

Grade Differences:

UL94 V-0: The highest flame retardancy rating.

After each flame is removed, the afterflame duration is ≤ 10 seconds.

The total afterflame duration for a group of 5 specimens is ≤ 50 seconds.

No dripping of flammable material ignites the cotton wool below.

The specimen must not completely burn (burn out).

The flame must not reach the upper edge of the fixture.

UL94 V-1:

After each flame is removed, the afterflame duration is ≤ 30 seconds.

The total afterflame duration for a group of specimens is ≤ 250 seconds.

No dripping of flammable material ignites the cotton wool below.

Other requirements are the same as for V-0.

UL94 V-2:

The burning time requirements are identical to those for V-1.

Flaming material is permitted to drip, and these drips can ignite the cotton wool below.

The key difference between V-2 and V-1 is whether ignitable dripping is permitted.

2. Horizontal Burning Grade (HB)

This is the lowest-demand grade. During testing, the specimen is placed horizontally, and the flame is ignited from one end.

Evaluation Criteria: The rate at which the flame burns on the specimen is measured.

UL94 HB Requirements:

For specimens with a thickness of < 3 mm: Burning rate ≤ 40 mm/min.

For specimens with a thickness of ≥ 3 mm: Burning rate ≤ 75 mm/min.

The flame must extinguish itself before burning the 100 mm mark.

HB-rated materials are often referred to as "slow-burning materials." They burn, but slowly and are not self-extinguishing.

3. Higher Flame Retardancy Levels

For flame-retardant materials with superior performance, more stringent testing methods are available:

5VA / 5VB:

A 500W flame is used, which is much more intense than the 50W flame used in the V test.

The specimen is tested both vertically and horizontally.

5VB: Burn-through of the specimen is permitted, but the flame must meet specified burning duration requirements (afterflame duration ≤ 60 seconds), and dripping must not ignite cotton wool.

5VA: The highest level. This exceeds the requirements of 5VB, but with stricter requirements. Burn-through of the specimen is not permitted. This is the highest flame retardancy level currently specified in the UL94 standard.

VTM-0. VTM-1. VTM-2:

Applicable to extremely thin plastic film materials (typically less than 0.25mm thick).

The test method and evaluation criteria are similar to those for V-0. V-1. and V-2. but because the specimen is too thin to stand upright, a special metal frame is used for vertical testing.

UL94 Grade Comparison Summary

How to choose the right UL94 rating?

When selecting materials, the higher the rating, the better. Comprehensive considerations are needed:

Final product safety standards: Different products (such as mobile phones, TVs, and chargers) have their own safety standards that must be met. These standards may mandate that specific components meet a certain UL94 rating (most commonly, V-0).

Material thickness: Be sure to confirm the thickness at which the rating was tested. When selecting materials, ensure that your product's wall thickness is at least as thick as the tested thickness.

Cost-performance balance: Materials with higher flame retardancy ratings often come at a higher cost or compromise certain mechanical properties. Finding the optimal balance between safety, cost, and functionality is crucial.

Other performance requirements include strength, thermal conductivity (CTI), color, and FDA certification requirements.

UL94 provides a systematic, scientific framework for evaluating and comparing the flame retardancy of plastic materials. From the lowest HB rating to the highest 5VA rating, designers can clearly select the appropriate material based on the product's safety requirements, providing reliable safety for end users. Identifying the required UL94 rating at the early stages of product development is a key step in ensuring that the product successfully passes safety certification.