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Overview of UL94 Flame Testing

UL Standard 94 identifies test methods for flammability of plastic materials for parts in devices and appliances. This article is the first in a series intended to answer some common questions regarding this standard, and is based upon the UL Standard 94 text. It focuses on the vertical burning tests for sheet and molded solid plastics. Some of the differences between the various flame ratings and the tests used, are also discussed.

This article is the first in a series intended to clear up the myths and confusion surrounding the Tests for Flammability of Plastic Materials for Parts in Devices and Appliances known as UL Standard 94. This article will focus on the Vertical Burning Tests for sheet and molded plastics (solid). It will highlight some of the differences between the various flame ratings, as well as the differences between the tests. It should be noted that certain material types are customarily tested and certified by certain tests. This should not infer that a material is not capable of passing other test, but just means that it has the results of the tests which were conducted. These materials may be tested by one of the following methods:

20 MM Vertical Burning Test; 94V-0, 94V-1, 94V-2

Thin Material Burning Test; 94VTM-0, 94VTM-1, 94VTM-2

NOTE: This article is based upon the text of UL Standard 94.

The 20 MM Vertical Burning Test

This is the test which is conducted on the vast majority of solid plastics. A set of 5 plastic specimens are subjected to a 20mm flame in accordance with the prescribed test procedures. After the flame is removed from the specimen, afterflame and afterglow times are measured. If flaming particles or drops fall from the specimen to a piece of surgical cotton which is placed below, it is noted whether or not the cotton ignites. The material is classified into categories based upon characteristics such as after as follows:

Criteria Conditions 94V-0 94V-1
Afterflame time for each specimen t1 or t2 <10s <30s
Total afterflame time for any condition set (t1 + t2 for the 5 specimens) <50s <250s
Afterflame plus afterglow time for each individual specimen after the second flame application (t2 + t3) <30s <60s
Afterflame plus afterglow of any specimen up to the holding clamp No No
Cotton Indicator ignited by flame particles or drops No No

In summary, materials classified as 94V-0 exhibit afterglow and afterflame for less time than 94V-1 and 94V-2. Materials classified as 94V-1 do not have flaming particles which ignite surgical cotton, whereas materials classified as 94V-2 do ignite the cotton.

If the specimens tested exhibit afterflame plus afterglow up to the holding clamp, or if the specimens, die to their thinness, distort, shrink, or are consumed up to the holding clamp, then they may be tested in accordance with the Thin Material Burning Test.

The Thin Material Burning Test

Like the 20 mm flame test, the Thin Material Burning Test is conducted on solid plastics. The specimen is prepared in a different manner, by wrapping it around a mandrel. The same 20mm flame is used as the Vertical Burning Test, and specimens are treated and the procedures are followed in a similar manner. The times for the same criteria are recorded, and the material is classified into categories based upon characteristics such as after as follows:

Criteria Conditions 94VTM-0 94VTM-1
Afterflame time for each specimen t1 or t2 <10s <30s
Total afterflame time for any condition set (t1 + t2 for the 5 specimens) <50s <250s
Afterflame plus afterglow time for each individual specimen after the second flame application (t2 + t3) <30s <60s
Afterflame plus afterglow of any specimen up to the holding clamp No No
Cotton Indicator ignited by flame particles or drops No No

In general, the Thin Material Test is a very similar test, but it is used to classify thin gauge materials that can not be properly tested using the 20mm Vertical Burning Test. The criteria for classification listed in the chart above correlates directly to the chart for the 20mm Vertical Burning Test.

The Horizontal Burning Foamed Material Tests:

94HF-1 or 94HF-2

This is the test which is typically conducted on foamed plastics used in devices and appliances. This includes acoustical foams, sealing foams, and filter media. A set of 5 specimens, each measuring 150mm x 50mm, is prepared and subjected to a 38mm flame in accordance with the prescribed test procedures. After the flame is removed from the specimen, afterflame and afterglow times are measured. If flaming particles or drops fall from the specimen to a piece of surgical cotton placed below it, it is noted whether or not the cotton ignites. Also, the length of damaged material for each specimen is measured. The material is classified into categories based upon these characteristics as follows:

Criteria Conditions 94HF-1 94HF-2
Afterflame time 4/5 is <2s
1/5 is <10s
4/5 is <2s
1/5 is <10s
Afterglow time for each individual specimen <30s <30s
Cotton Indicator ignited by flaming particles or drops No Yes
Damaged Length for each indiviual specimen <60mm <60mm
Notes:
4/5 - fout out of a set of 5 specimens
1/5 - one out of a set of 5 specimens
s = seconds
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