UL 94 Test Standards for Flame Resistance

UL 94 Test Standards for Plastics

RAMPFS’ product line includes flame-retardant polyurethanes that are specially formulated for use in manufacturing medical devices, electrical/electronic housings and components for items such as appliances, and automotive parts.

To ensure a plastic’s ability to perform in these types of applications, material samples are typically tested according to the Underwriters’ Laboratories (UL) Standard UL 94 Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. UL has assigned 12 different flame classifications to materials based on the results of lab-scale flame tests. The classifications, shown below, are used to distinguish a material’s burning characteristics after test specimens have been exposed to a specified test flame under controlled laboratory conditions.

• Six of the classifications relate to materials commonly used in manufacturing enclosures, structural parts and insulators found in consumer electronic products (5VA, 5VB, V-0, V-1, V-2, HB).  RAMPF casting polyurethanes fit in this grouping.
• Three of the remaining six classifications relate to low-density foam materials commonly used in fabricating speaker grills and sound-deadening material (HF-1, HF-2, HBF).
• The last three classifications are assigned to very thin films, generally not capable of supporting themselves in a horizontal position (VTM-0, VTM-1, VTM-2). These are usually assigned to substrates on flexible printed circuit boards.

Horizontal versus vertical positioning

To test a plastic material, molded specimens are oriented in either a horizontal or vertical position and subjected to a defined flame ignition source for a specified time period. In the horizontal burn (HB) tests, the test flame is only applied to the plastic sample once.

The three vertical ratings, V-2, V-1 and V-0 indicate that the material was tested in a vertical position and self-extinguished within a specified time after the ignition source was removed. To comply with the 20-mm UL 94 V-O vertical burn test, each of five specimens must extinguish in less than 10 seconds.  A second flame is then applied, the total flaming and glowing time of the sample must be less than 30 seconds.  Total flaming time for all five samples must be less than 50 seconds.  In addition, to attain the UL 94 V-O rating, specimens can not burn through their entire link and cannot drip flaming particles onto a cotton indicator located below the sample.

For UL 94 5VA and 5VB testing, plastic samples are exposed to the flame ignition source as many as five times to determine the polymer’s suitability for use in severe conditions.  These small-scale tests measure the propensity of a material to extinguish or spread flames once it becomes ignited.