When designing a product, the application of the right material is key to ensuring the best outcome and for many, the use of flame-retardant plastics is an essential design choice. When used in conjunction with other fire prevention design measures such as the structural design, it is an obvious material choice for many developer and designers.
The flame-retardant plastic chosen for a product is determined by a number of factors from the physical application requirements to the required fire resistance standard. However, when it comes to using flame retardant plastics, the type of polymer must be considered. There are different types of plastics and compositions that are used for the creation of flame-retardant materials to meet the required standards.
When choosing the right flame retardant plastic one has to consider the method that’s going to be used to create the housing and the plastic mouldings. When it comes to thermoplastics, many resin manufacturers create their own branded products, with specifically tested UL ratings that can be purchased in resin form or pellet form ready for injection moulding.
Thermosetting plastics, on the other hand, have flame retardant materials incorporated into their polymer structure through polycondensation or grafting which involves the condensation of organic materials in which small molecules are split out. These polymers can then be reclassified into two more important classes; amorphous polymers and crystalline polymers.
If we focus on injection moulding using a thermoplastic like ABS for now. The standard used to define thermoplastics flame retardancy is part of the UL body of materials testing, specifically the UL94 standard of flammability.
There are various ratings that can be attained according to the tests available, much like IP ratings for housings the materials are subjected to certain tests with levels of protection and achievement available.
There are 5 tests currently
|UL94 – HB||Horizontal Burning|
|UL94 – V||Vertical Burning (50W @ 20mm)|
|UL94 – 5V||Vertical Burning (500W @ 125mm)|
|UL94 – VTM||Vertical Thin Material||Testing on thin films|
|UL94 – HBF||Horizontal Burning Foamed Material||Testing for low density foam|
Within each of the ratings there are a variety of subsets, which will govern which version of the certification the material achieves, for example, UL94 – V has within it 3 levels of attainment based on the flame retardant nature, V0 – 1 – 3.
There’s a fantastic guide from an injection moulding company on picking the correct material and UL rated plastic for your components but also making sure that you consider any additives such as colours will alter the specifically tested flame retardancy of the plastic. As well as this it’s important to bear in mind that UL ratings are issued to very specific thickness of plastics and making something with an increase/decrease in thickness will drastically alter this.
It’s also important to remember the principle of occam’s razor, in the fact that simpler solutions are more likely to be the correct one. In this slightly warped way we’re saying, do not waste money and time on something with a higher flame retardancy than you actually need, such decisions can increase your total cost and influence your enclosure unnecessarily.