UL 94 Flame Ratings Explained for Thermoformed Parts

If your part goes into an electrical enclosure, a vehicle, a battery pack or a transit interior, someone will eventually ask for its flame rating — and that almost always means UL 94. Here’s what the ratings mean and how to specify a flame-retardant thermoformed part correctly.

What UL 94 measures

UL 94 is a standard for the flammability of plastic materials in a device. It rates how a material behaves when exposed to a flame — whether it self-extinguishes, how fast, and whether it drips burning material. The common ratings, from least to most demanding:

• HB — slow horizontal burn. The baseline; many standard grades are HB.
• V-2 — vertical burn, self-extinguishes, but may drip flaming particles.
• V-1 — vertical burn, self-extinguishes, drips are not flaming.
• V-0 — vertical burn, self-extinguishes quickly, no flaming drips. The common target for enclosures.
• 5VB / 5VA — the most severe, for parts facing a larger flame source.

For most electrical, EV, battery and transit applications, V-0 is the rating specified.

Thickness changes the rating

This trips up a lot of specifications: a UL 94 rating is tied to a thickness. The same material may be V-0 at 3 mm but only V-2 at 1 mm. So “V-0” alone is incomplete — the rating must state the thickness at which it was achieved. When you specify a flame-retardant thermoformed part, specify the rating and the minimum wall thickness where it must hold, because thermoformed walls thin as they draw. This is exactly why wall-thickness control matters on FR parts.

Flame-retardant grades we thermoform

• Shield FR ABS — structural toughness with FR performance, V-0 verified from around 1.5 mm; good for rail interiors, electrical cabinets and enclosure shells.
• Shield FR PC (polycarbonate) — halogen-free, V-0 down to very thin walls, with high-voltage insulation and a high CTI (comparative tracking index) — the go-to for EV battery CCS trays, BESS modules and power-electronics parts.
• Rigid PVC — inherently flame-retardant (V-0) without additives, thanks to its chlorine content; suited to chemical and electrical environments.

Beyond UL 94

Depending on the sector, you may also encounter:

• Rail standards (e.g. EN 45545 / HL ratings) for transit interiors — a different, smoke-and-toxicity-aware regime.
• Glow-wire and CTI requirements for electrical parts — CTI matters where tracking across a surface under voltage is a risk (PC-FR grades reach CTI 600).

Tell us the standard and the environment, not just “make it FR,” and we’ll match the grade.

Specifying it right

1. State the rating (usually V-0) and the wall thickness it must hold at.
2. Note any sector standard (rail, automotive, electrical) beyond UL 94.
3. Flag electrical requirements (CTI, dielectric, halogen-free) if relevant.
4. Let us control wall thickness in the draw so the rating holds where it’s thinnest.

Because we extrude our own FR sheet and control forming, we can hold the rating through the part. Send your FR requirement, or see our PC-FR V0 battery insulation work.