Flame-Retardant 3D Printing: What UL 94 Ratings Mean for Printed Enclosures

The Short Answer

UL 94 is a set of small-scale flame tests that classify a plastic material by how it burns: HB for horizontal burning, V-2, V-1 and V-0 for vertical burning, and 5VB and 5VA for a much harsher vertical flame. A classification belongs to one material grade, at a stated minimum thickness and color, as tested and listed by UL. A part printed from a classified material is not itself UL listed or UL certified, and whether a printed enclosure is acceptable is decided by the safety evaluation of the finished product. What you control is the material, its classification at the wall thickness you design to, and the documents that prove it.

What UL 94 Tests

UL describes UL 94 as small-scale flame tests run under controlled laboratory conditions, with 12 flame classifications that distinguish a material's burning characteristics. Six of them (HB, V-2, V-1, V-0, 5VB and 5VA) apply to plastics commonly used for enclosures, structural parts and insulators in consumer electronic products. The others cover very thin materials (VTM) and low-density foams (HBF, HF-1, HF-2). The three tests that matter for enclosures:

• Horizontal burning (HB): a specimen held horizontally is exposed to a 20 mm flame for 30 seconds, and its burning rate is measured.
• 20 mm vertical burning (V-0, V-1, V-2): a vertical specimen receives two 10-second applications of a 20 mm flame. The test records how long it keeps burning and glowing, and whether burning drips ignite cotton placed below.
• 500 W, 125 mm vertical (5VA, 5VB): a flame UL describes as about five times more severe, applied five times. The test also checks flat sheets for holes burned through.

What Each Rating Requires

A summary of UL's published criteria, from least to most demanding:

Reading the Classes Correctly

• In all three V classes, no specimen may burn up to the holding clamp, and the burning and glowing time after the second flame application is limited too (30 seconds for V-0, 60 seconds for V-1 and V-2).
• HB means the material may keep burning when horizontal, at a limited rate. It is not a self-extinguishing class.
• UL's guide notes that materials meeting V-0 or V-1 may be put through the 5V test, which is where 5VB and 5VA come from.
• A VTM class is not a V class. UL states that VTM-0 cannot be considered equivalent to V-0, because thin films are tested a different way.
• UL 94 methods are harmonized with IEC, ISO and ASTM standards, which is why a datasheet may list IEC 60695-11-10 beside UL 94.

A Rating Belongs to a Grade, a Thickness and a Color

UL issues a Yellow Card when a plastic earns a UL Recognized Component Mark, and certified materials are listed in UL's Product iQ database. The card lists each grade's flame class by color and minimum thickness. UL's guide to reading it explains three limits that matter for a printed enclosure:

• Thickness. Performance can vary with thickness. Where a card lists specific thicknesses, the material is recognized from that thickness up to the 13 mm the standard covers, so a grade can be V-0 at one thickness and only HB at a thinner one.
• Color. Color can change performance. An ALL color classification comes from testing natural material plus the heaviest white, heaviest black and heaviest other pigment loadings, it applies only to material as delivered by its manufacturer, and adding a color concentrate needs further evaluation.
• Grade. The grade designation together with the company name identifies what was recognized. A different grade, even of the same polymer, needs its own listing.

Why a Printed Part Is Not UL Listed

Three different things often get blurred together:

• The material. A plastic grade earns a UL 94 class at a thickness and color, and with UL recognition it appears on a Yellow Card. UL describes recognized components as parts of a larger product, permitted to claim compliance with the standards they were tested to within conditions of acceptability, such as a minimum thickness.
• The filament. A filament maker may build its product on a classified base resin and publish that resin's classes. That is a statement about the resin.
• The printed part. Printing creates a new structure, with layer seams, perimeters, infill and the wall thickness you chose. UL says UL 94 small-scale data is intended for judging plastic materials used in components of end-product devices and appliances, where the acceptability of the combination is determined by UL. The end product's evaluation decides whether your enclosure is acceptable.

So What Can a Printed Enclosure Carry?

Not a listing or a certification. We never describe printed parts as UL listed, UL certified or compliant with a standard. What a printed enclosure can carry is a documented material: a named grade with a known UL 94 classification at a known thickness and color, plus its datasheet. That is what an evaluator asks for first, and it is what you should collect before the first part prints. ABS-FR is printed by FDM, so the general FDM design rules apply as well.

Example: Reading One FR Filament's Data

The ABS-FR in our catalog is COEX CX16 flame-retardant ABS. COEX's technical data sheet (Rev 3.0, October 2026) presents its flame data as typical values for the base resin and lists UL 94 HB at 1.0 mm and V-0 at 1.5 mm, with relative thermal indices of 80–85 °C (UL 746B) at 1.8 to 3.0 mm. COEX's product page adds that these are the UL 94 ratings of the base resin used to make the filament, that they are not ratings of a finished printed part, and that printed performance depends on geometry, wall thickness and print settings, so parts should be tested for the application. Read that the way an evaluator would:

• 1.5 mm is the thinnest wall at which the base resin was classified V-0. It is where the manufacturer tested, not a guarantee that a 1.5 mm printed wall performs to V-0.
• At 1.0 mm the listed class is HB, which is not self-extinguishing.
• The values describe the resin. Whether the printed enclosure is acceptable is a question for the end-product evaluation.
• Check the current listing in UL Product iQ before relying on any class, and keep the datasheet with your design file.

What UL 94 Does Not Tell You

• It is not a building-code fire rating. UL notes that UL 94 does not cover materials for building construction, finishing or contents, that model codes do not recognize the small-scale ratings, and that most UL 94 results do not apply to material thicker than 13.0 mm.
• It does not test your geometry. Bars and plaques at a set thickness say nothing direct about a vent slot, a thin boss or a screw tower.
• It is not the whole electrical picture. Yellow Cards also list properties such as relative thermal index, hot-wire ignition and high-current arc ignition, and the end product's evaluation may look at those too.

Designing a Printed Enclosure Around the Rating

The general enclosure rules (board mounting, cutouts, ventilation) are in our electronics enclosures guide. For flame performance, add these:

• Design every flame-relevant wall at or above the listed minimum thickness for the class you need, and check thin features such as vents, bosses, ribs and snap tabs separately.
• Specify solid walls where flame performance matters. The classification was established on specimens of the material, not on sparse infill between two thin skins.
• Stay within the color the classification covers, and ask your evaluator before adding paint, dye or other finishes.
• Remember that inserts, gaskets, labels and adhesives are separate materials in the end product.
• Plan for the end-product evaluation to test samples of the printed enclosure itself.
• Use the free printability review when you upload the enclosure: an engineer checks wall thickness, orientation and material choice. It is a printability review, not a flammability assessment.

Documents to Ask For

Whoever supplies the material or the parts, collect these. Our material selection guide covers the other properties to weigh, and the full catalog is on the materials page.

• The filament grade name and its maker.
• The technical data sheet, with the UL 94 class at each listed thickness.
• Whether the class belongs to the filament or to its base resin, and the UL listing that backs it.
• The color the classification covers.
• The safety data sheet.
• The relative thermal index at your wall thickness, if the enclosure runs warm.