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Week #68

Plastics Testing – Flame Tests & Flame Ratings


General :Plastics Testing for Product Safety can be divided into two categories:

  1. Flammability Tests: Flame tests to evaluate the response of a plastic to a source of ignition = Does the plastic ignite? Does the plastic self-extinguish? Does the flame spread? Are flaming particles emitted? These tests are used to assign “flammability ratings” to specific plastics. This whitepaper will expand on flammability testing and ratings.
  2. Tests for Other Properties: Electrical properties, temperature limits, resistance to tracking, UV resistance, ability to withstand cold, etc. = these tests/properties can be required depending on the material’s application. These plastics tests will be discussed in a future whitepaper.

Flammability Test Objectives: There are four primary objectives for flame testing & flame ratings of plastics for Product Safety compliance (UL, CSA, EN-CE, IEC standards).

  1. Fire Containment: Preventing the spread of fire to the building structure is the #1 goal with preventing a Risk of Fire to the user. Fire containment from the enclosure is intended to provide protection when all else fails.
    a) Plastics used for a product enclosure are expected to contain any fire that occurs internal to the product.
    b) Plastics used for a product enclosure are expected to resist surface ignition from an external fire source.
    c) Enclosure plastics are therefore required to have a higher flame rating than plastics used for other product applications (decorative, internal). In addition, plastic enclosure materials used in stationary and fixed equipment are required to have the highest flame rating since they must be extinguished in place (vs. portable equipment that can be removed from the premises to be extinguished).
  2. Minimizing Fuel to a Fire: The standards assume that an internal fault can occur that results in a fire within the product. A secondary goal within Risk of Fire protection involves limiting the potential for fire to grow internally to the point that the enclosure cannot contain the fire.
    a) Internal plastics must have a suitable flame rating to limit the likelihood of ignition and ensure that they will not serve as fuel to a fire should one occur.
  3. Minimizing Flame Spread: Large external plastic surfaces are expected to be made of a material that minimizes the spread of fire across the surface. Such applications require an additional “flame spread rating” for large, external plastic surfaces.
  4. Decorative Parts: External decorative parts of sufficient size that they could become a fuel to a fire are required to have a minimum flame rating to protect against ignition from an external source.

Flammability Tests & Flame Ratings:

  • Flammability Testing can involve either applying a flame to a plastic or applying an electrically heated ignition source (i.e. glow wire, hot wire ignition).
  • Flame Ratings: To help limit the chance for products to catch fire, the safety standards require that all combustible materials have suitable flame ratings to insure that they will not serve as fuel to a fire. There are 12 common “flame rating classifications”:
    – 6 of the ratings apply to materials used for enclosures, major parts, and insulators. These ratings are 5VA, 5VB, V-0, V-1, V-2, & HB,
    – 3 of the ratings apply to foam materials. These ratings are HF-1, HF-2, & HBF,
    – 3 of the ratings apply to very thin films = materials that will not support themselves in a horizontal position = VTM rating suffix
    – In general, flame ratings are given to raw plastics based on testing performed on bar or plaque samples (UL94). Similar flame testing can also be performed on the finished product (UL746C).
  • Test Sample Position: Flame tests are performed with the material in either a horizontal or vertical position.
    – The HB rating reflects a horizontal burn test.
    – Materials are tested vertically for the 5V, V-0, V-1, and V-2 ratings = these materials must self-extinguish within a specified time limit after the test flame is removed.

Flame Rating System: Flame tests done with a Bunsen burner result in these ratings

Conclusions: Of the 6 Hazards of Product Safety (whitepapers #30 – 35), the most potentially severe is “Risk of Fire”, in a worst case scenario where fire starts in a product, then spreads to the supporting surface, and eventually the building structure. This is absolutely the greatest concern of all. In this manner, a large number of people can be put into a life threatening situation. Plastics are flammable by nature, so the use of plastics in product design increases the risk of such an event. Consequently, it is very important that product designers use properly flame rated plastic materials to insure that they will not add fuel to a fire or worse, allow a fire to escape the product enclosure.

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