Understanding UL Burn Testing
Simplifying UL 94 Burn Ratings
UL 94 is a plastics flammability standard that contains the following tests: 94HB, 94V, 94VTM, 94-5V, 94HBF, 94HF, and Radiant Panel. Label and overlay manufacturers usually print on plastics and are frequently asked by engineers to use materials that pass the UL 94HB, 94V or 94VTM tests.
94HB HORIZONTAL BURNING TEST
UL 94HB is a horizontal burning test that is considered the easiest to pass and materials that pass any of the V (vertical) or VTM (vertical thin material) burn tests will usually be accepted by UL for UL label applications that require 94HB.
The test uses a 1/2″ x 5″ specimen held at one end in a horizontal position with marks at 25mm and 100mm from the free end. A flame is applied to the free end for 30 seconds or until the flame front reaches the 25mm mark.
94V VERTICAL BURNING TEST
This test includes three classifications – 94V-0, 94V-1, and 94V-2 – and would typically be acceptable for portable, unattended, intermittent-duty, household appliances. Material thickness is typically .010+ for this test.
This test uses a 1/2″x 5″ specimen which is held at one end in a vertical position. A flame is applied to the free end of the specimen for two 10-second intervals separated by the time it takes for flaming combustion to cease after the first application. UL 94V-0 is the highest burning rate, meaning the product label material is very unlikely to catch on fire.
94VTM MATERIAL VERTICAL BURNING TEST
Product label materials that are very thin (approximately 10 mil) may distort, shrink, or flex during the 94V test. These materials can be tested using 94VTM – the thin material version of the vertical burning test. For this product label test, the specimen size is 200mm x 50mm (8″ x 2″), and the two flame applications have a duration of three seconds instead of 10.
UL will generally accept a VTM rating of similar level where a 94V material is required i.e., 94VTM-0 = 94V-0, 94VTM = 94V-1, etc. UL does, however, reserve the right to make a determination on which rating a material should have based on the application.