UL94

UL 94 classifies the flammability of polymeric materials. For seals, moulded articles and flat seals in equipment and installations, the UL rating is often an important factor in obtaining release for series production.

What does UL 94 stand for?

UL 94 is a test and classification procedure of Underwriters Laboratories (UL). It assesses the flammability, afterflame and afterglow behaviour and the dripping behaviour of plastics and elastomers. The rating refers to a material- and thickness-specific specimen, not to the complete device.

Which products require a UL 94 rating?

Wherever flame-retardant or self-extinguishing seals are required:

• Electronics and electrical appliances: device feet, enclosure seals, cable bushings
• HVAC and white goods: sealing points close to combustion chambers, motor and fan installations
• Machinery and plant engineering: control cabinet seals, unit covers
• Mobility: battery housing seals, interior components

Commonly used materials are CR/Neoprene (halogen-containing, self-extinguishing) and many FKM/FPM and silicone types (depending on the compound) with UL ratings such as V-0, V-1 or V-2.

UL 94 classifications

Group	Classes	Test set-up (brief)	Key point
Vertical	V-0 · V-1 · V-2	Vertically suspended specimen, two flame applications, afterglow and dripping assessment	V-0 is the strictest of the three; V-2 permits flaming drips (cotton as indicator)
5V	5VA · 5VB	Stronger flame (approx. 500 W), five flame applications	5VA is stricter than 5VB; with 5VA, for example, no hole may form
Horizontal	HB	Horizontal specimen, burning rate or self-extinguishing	Basic class, minimum requirement for many housings
Thin films	VTM-0 · VTM-1 · VTM-2	Vertical test for very thin materials and films	Analogous to the V classes, designed for films
Foams	HF-1 · HF-2 · HBF	Horizontal, with assessment of afterflame and afterglow	For cellular materials (foam)

Which requirements must be met?

• Specimen: 125 × 13 × T (mm); T (thickness) typically 0.4 / 0.75 / 1.5 / 3.0 mm (up to 13 mm possible)
• Conditioning: standardised climate pre-treatment (e.g. 23 °C / 50 % RH), heat ageing to UL specifications if required
• Assessment: individual and total afterflame time, afterglow time, dripping (cotton indicator), burn-through and hole formation (5V)
• Documentation: the rating is thickness-specific and is published in the UL database (Yellow Card)

The UL rating is material-specific. Components made from the material may only carry the UL designation if design, thickness and colour are within the permitted range.

How do products achieve UL 94 conformity?

1. Material selection: check formulations and already listed types (Yellow Cards) against the required class and thickness
2. Design check: take into account minimum thicknesses per sealing point, dripping risk, and colour and pigments
3. Test strategy: material testing at the manufacturer or via an accredited laboratory, optionally component and assembly tests
4. Evidence: test report, UL listing and date code, internal release documents (CoC/DoC) for audits
5. Series maintenance: reassess changes to formulation, colour and thickness

Practical tips and common mistakes

• Thickness is decisive: a V-0 rating at 3.0 mm does not automatically apply to 0.75 mm.
• Colour and additive changes: pigments and flame retardants affect the class, so document and reassess any changes.
• Dripping behaviour: for vertical installation positions, V-0 or V-1 is preferred; V-2 permits flaming drips.
• Prototyping: 3D printing materials only have a UL class if exactly that filament or resin is listed.
• CR is not automatically sufficient: halogen-containing does not automatically mean V-0, always check the listed compound.