What Do UL94 Ratings Mean for Your Plastic Part?

Last updated: 2026-09-01 · Reading time: about 4 min

UL94 is the flammability classification system for plastic materials: standardized burn tests sort each resin grade into ratings such as HB, V-2, V-1, V-0 and 5VA, describing how it burns and what extinguishes it.
The rating belongs to the material grade at a stated thickness — not to your part, not to the mold, and not to the molding factory — so specifying it correctly means naming the grade, the rating and the thickness together, and verifying the end product through your own compliance program. Here is what the letters mean and how to use them without buying flame retardancy you do not need.

Key facts

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  • UL94 ratings are earned by test specimens under defined conditions: horizontal burn (HB) or vertical burn (V-2, V-1, V-0), plus the more demanding 5VA/5VB panel tests.
  • The rating is thickness-specific — a grade listed V-0 at 1.5 mm may only V-1 or HB in a thinner wall, so the rating and the minimum wall must travel together on the drawing.
  • Flame-retardant packages change the melt: they can reduce impact strength, shift flow and complicate regrind, so the FR grade deserves the same datasheet scrutiny as the base resin.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

    Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. See also related guide.

    What does the test actually measure?

    The vertical-burn tests apply a standard flame to a bar of the material for a fixed time, then watch three things: how long the specimen keeps burning after the flame is removed, how long it glows, and whether burning drips ignite cotton beneath it. HB, the horizontal test, is the baseline — the material may burn, but slowly and without traveling past a mark.
    Up the vertical ladder, V-2 allows burning drips within limits, V-1 forbids the drips but allows longer afterflame, and V-0 extinguishes fastest with no flaming drips. The 5VA and 5VB tests scale up to panels and represent the most demanding end of the ladder.
    What the rating does not tell you: how your geometry, your wall thickness distribution, your assembly and your real ignition source behave — the test describes the material under the standard’s conditions, and everything else is your program’s verification.

    What do the rating letters mean?

    Rating Test What it says about the material
    HB Horizontal burn Baseline; burns slowly within rate limits — the minimum for many enclosures.
    V-2 Vertical burn Self-extinguishes; flaming drips allowed within limits.
    V-1 Vertical burn Self-extinguishes; no flaming drips; longer afterflame than V-0.
    V-0 Vertical burn The common target: fastest extinction, no flaming drips.
    5VA / 5VB Panel burn Most demanding; used where the spec calls for the top of the ladder.

    Two reading rules keep the table honest.
    First, compare ratings only at the same thickness — the listing sheets record the minimum thickness at which each grade earns each rating, and that line is the one your drawing must match.
    Second, treat the rating as material-level evidence: the grade’s listing comes from the resin supplier’s test records, and the datasheet or listing sheet for the exact grade is the reference you cite on the drawing — not the family name and not a habit from the last program.

    How do you specify flame retardancy without overpaying?

  • Let the application set the target, not fear — many enclosure specs require HB or V-2 at their wall thickness; demanding V-0 everywhere multiplies resin cost and property loss without adding safety the spec asked for.
  • Name grade, rating and thickness together — “FR-grade ABS” is not a specification; the grade, its rating and the minimum wall it applies to are.
  • Check the property trade-offs — pull the FR grade’s datasheet beside the base grade’s and read impact strength, melt flow and heat data before committing; the flame package is rarely free.
  • Decide regrind policy early — some FR packages tolerate limited regrind; verify the allowed percentage with the supplier before the process sheet promises one.
  • Plan the verification you will accept — agree at DFM which evidence proves conformity: the supplier’s listing for the grade, molded-and-tested samples, or both, and at whose laboratory.

    • Who certifies what in this conversation?

    The division of labor is worth stating plainly, because it prevents both loose claims and lost time.
    We do not certify the end product: the resin supplier’s records describe the material grade, and the compliance of your assembled product for your market is yours to run. What we deliver alongside is the engineering evidence chain — the grade as specified, processing that keeps the material’s properties intact, and molded samples you can build and test to your own program’s requirements.
    If your program requires a formal submission for the end product, tell us before the deposit and we will say plainly which parts of it we can support with material documentation and test samples, and which parts sit with you and your certification body.

    Frequently asked questions

    Does a V-0 grade make my product fire-safe?

    No — it tells you the material grade extinguishes quickly under the standard’s test conditions at the listed thickness. Product-level fire safety depends on your design, your assembly and your end-product testing, which is the program’s responsibility to verify.

    Why did my thin-wall part not meet the grade’s listed rating?

    Because the rating was earned at a thicker specimen. Check the listing sheet for the minimum thickness at that rating; if your wall is thinner, the grade may only support a lower rating there — and the drawing should say so explicitly.

    Can molding conditions change the rating?

    The rating belongs to the material, but poor processing can degrade it: overheated melt, wet resin or degraded regrind all consume the additives and polymer the rating depends on. The process sheet protects the property — which is why the drying and melt-temperature windows travel with FR programs.

    Key takeaways

    1. UL94 ratings describe material grades at stated thicknesses — HB through 5VA is a ladder of standardized burn behavior, not a product guarantee.
    2. Specify grade, rating and minimum wall together, cite the supplier’s listing, and respect the thickness line when walls slim down.
    3. Flame-retardant packages trade properties and regrind freedom for the rating — read the FR datasheet beside the base grade before committing.

    If your drawing carries a flammability requirement, send us the 3D model, the required rating with its thickness and the target grade before the deposit. You will get the grade-and-rating check in writing, the property trade-off summary beside the base resin, and the sample plan that lets your program verify what matters — from a named tooling engineer, before any steel is ordered.

    Sources
    Solutions — safety science organization, home of the 94 Standard for Safety of Flammability of Plastic Materials
    MatWeb — material property database with grade-level flammability listings
    Plastics Industry Association — industry resources on plastics processing practice

    Sources: Plastics Technology · Plastics Technology Online · Prospector — Resin Datasheets · MatWeb — Material Property Database