Halogen-Free Flame Retardant Heat Shrink 2:1
A single-wall 2:1 heat shrink tube in an irradiation cross-linked polyolefin compound whose flame retardance comes without halogen chemistry. The version of flame-retardant tubing specified where a fire must not add halogen acid gas to the smoke: rail and shipboard interiors, enclosed control cabinets, occupied equipment rooms. Service range -55°C to +125°C, rated ≤1 kV, UL224 and RoHS, in thin, medium and thick walls across a full size matrix. This page covers that one product; the multi-ratio DRS family sits on a separate page.
Specification
| Material | Polyolefin (irradiation cross-linked), halogen-free flame-retardant grade |
|---|---|
| Shrink ratio | 2:1 |
| Operating temperature | -55°C to +125°C |
| Voltage rating | ≤1 kV |
| Wall classes | Thin / medium / thick |
| Colours | Multiple |
| Standards | UL224, RoHS |
What "halogen-free" and "flame-retardant" mean here — and what they do not
The two words answer different questions. Flame-retardant is about whether the tube stops burning: this compound passes the flame test defined by its UL224 grade, and the specific flame class your batch carries is shown on the batch certificate. UL224 does not have one flame result (it grades several), so "built to UL224" is a construction statement, and the class is the number that belongs in your drawing. Halogen-free is about what the smoke contains: the flame-retardant package is not halogen-based, so the gases released in a fire do not include hydrogen halide acids that corrode electronics and endanger occupants. That is the property rail, marine and enclosed-equipment specifiers buy.
What halogen-free does not mean: it is not a general safety upgrade over standard flame-retardant polyolefin. A halogen-free compound is a chemistry choice driven by the installation environment and the specification you must meet; where no halogen restriction applies, a standard FR grade can be the more economical selection. Match the grade to the spec clause, not to a ranking.
Construction and recovery
The tube is extruded from the halogen-free polyolefin compound, cross-linked by electron-beam irradiation, then warmed, expanded to twice its recovered diameter and cooled in the expanded state. That frozen expansion is what sits on the shelf. Heating the installed tube above its recovery temperature relaxes the cross-linked network and the wall closes to the recovered size, roughly doubling wall thickness as the diameter halves. Recovery happens with a controlled heat source; the minimum and full-recovery temperatures for this grade are part of the verified data being published per the note above. After cooling, the recovered wall holds its shape and ≤1 kV insulation duty for the assembly's service life within -55°C to +125°C.
Selection inputs
- Largest pass-over feature. The biggest OD the tube must clear : connector, lug barrel or splice. At 2:1 this single number decides whether one tube works: the supplied ID must be above it, and half of it must still grip below it.
- Gripping diameter. The recovered ID should land 10–20% under the conductor insulation or jacket it must hold. If the pass-over feature is more than about twice the gripping diameter, 2:1 is the wrong ratio : take the higher-ratio DRS systems or use two sections.
- Wall class. Thin for dense harnesses; medium for general panel work; thick where abrasion stand-off and field-handled joints justify the extra heat and stiffness.
- Cut length. Cover the joint plus overlap onto sound insulation at both ends, plus allowance for axial change during recovery : the per-size figure is being published with the size table, so confirm the allowance with us on first orders.
- Documentation. UL224 flame class and RoHS declarations issue per batch; name the target market and the spec clause when you enquire and we align the paperwork.
Failure modes specific to 2:1 single-wall tube
Five ways this specific construction fails in the field, with the root cause and the counter-move for each.
Oversize selection: recovered but loose
Root cause: the tube was chosen from the pass-over diameter alone. At 2:1, a tube big enough to clear a bulky lug often recovers to an ID larger than the wire: the wall touches but never compresses, and the tube slides by hand after cooling. A single-wall tube has no adhesive to fall back on; fit is the only retention mechanism.
Fix: size from both diameters, supplied clears the lug, recovered sits 10–20% below the grip. When the numbers cannot both be met with one 2:1 size, the geometry needs a higher ratio or two tube sizes, not a bigger single tube.
Insufficient recovery: the wall never seats
Root cause: heat was withdrawn too early or applied too cold. The wall looks glossy and shrunk but sits proud of the connector, and the ends never rolled down. Thick walls are the usual victims: they need a longer heat soak than installers used to thin wall expect.
Fix: keep even heat on until the wall fully conforms and the end edges roll smooth; on thick wall, work the heat along the length twice. If the surface scorches before the wall seats, the source is too hot or too close: that is the next failure mode.
Overheating: scorch, thinning, bubble
Root cause: a concentrated flame or a gun parked in one spot drives the local wall past its limit. The polyolefin discolours, thins, or bubbles; the damaged patch keeps its recovered shape but has lost wall thickness and dielectric margin exactly where the conductor sits underneath.
Fix: a regulated heat gun kept moving, from the centre outward. No open flame on production work. A scorched section is a rejected section. Replace it, because the wall will not regain its rating.
Cut length comes up short
Root cause: the tube shortens along its axis during recovery. A cut sized to the finished length ends up exposing the joint ends.
Fix: cut long, joint length plus end overlap plus the axial-change allowance. We supply the per-size longitudinal change figure so the calculation is not a guess; ask for it with the size code.
Wrong grade for the fire specification
Root cause: two directions of the same mistake. Using a standard FR tube where the spec requires halogen-free fails the acid-gas clause; using this halogen-free grade where no halogen clause exists pays a compound premium for nothing. Either way the drawing and the installed product disagree.
Fix: read the fire clause of the specification, not just "flame retardant". If the clause names smoke or corrosive-gas limits, halogen-free is the requirement; if it only names a flame class, a standard FR grade usually satisfies it: tell us the clause and we quote the right grade.
Frequently asked questions
What does "halogen-free flame-retardant" mean for this exact grade?
Two separate facts, not one blanket claim. Flame-retardant means the compound passes the flame test defined by its UL224 grade. The flame class that applies to your batch is on the batch certificate, and UL224 covers several flame classes, so the class matters, not the word. Halogen-free means the formulation contains no halogen (chlorine, bromine) fire-retardant chemistry, so a burning section does not release hydrogen halide acid gases: the reason rail, shipboard and enclosed-occupancy specs call the property out. It does not mean the tube is automatically safer or better in every application: halogen-free compounds are one family of flame-retardant chemistry, and the fire-performance data behind this grade (smoke density, acid-gas emission, the specific test method) is being confirmed per the data note on this page. Ask for the batch test basis before you write the property into a specification.
When is a 2:1 ratio sufficient?
When the largest feature the tube must pass over is close to the diameter of the section it must grip. A 2:1 tube recovers to half its supplied ID; if a bare wire is 3 mm and the largest lug or splice over it is 5 mm, a 6 mm supplied tube clears the lug and recovers to 3 mm: one piece, tight fit. The ratio stops being sufficient when the step is steeper: if the lug needs 8 mm clearance and the wire still needs a firm grip at 3 mm, a 2:1 size either will not pass over the lug or recovers to a loose fit. Then use a higher ratio or two tube sections. As a working margin, treat a 2:1 selection as sized when the recovered ID sits roughly 10–20% below the gripping diameter. Below that you are forcing the fit; above it the tube has nothing to bite on.
What dimensions are needed before selecting a size?
Three diameters and one length. First, the largest object OD the tube must slide over, that is, the connector body, lug barrel or splice, whichever is biggest. Second, the smallest OD the recovered tube must grip, that is, usually the conductor insulation or cable jacket at each end of the covered span. Third, the largest OD that must pass through if the tube goes over an already-terminated end. Fourth, the cut length: cover the joint plus enough end overlap to seat on sound insulation, and allow for axial change during recovery; the per-size longitudinal change figure is on our data sheet and is being published per size as the size table is verified. Add the wall class you want (thin, medium or thick) and the colour, and we return the size code.
Which wall class should I specify — thin, medium or thick?
Follow the mechanical duty, not a habit. Thin wall suits dense harnesses and tight-clearance builds where the recovered wall mainly insulates and identifies. Medium wall is the general panel and machine-building class where leads get handled. Thick wall adds abrasion stand-off and stiffness for field-installable joints and heavier cables, at the cost of more heat and a larger bend radius during recovery. All three classes carry the same -55°C to +125°C service range and ≤1 kV rating on this page; the per-size recovered wall thickness figures are part of the size table being verified.
Is this tube suitable where low smoke and low corrosive gas are specified?
That is exactly the duty the halogen-free chemistry exists for. Enclosed spaces where people evacuate or equipment must survive a fire event, such as rail vehicles, ships and closed control cabinets. But a low-smoke specification is written against a measured value, and we do not publish smoke-density or acid-gas numbers we cannot trace to a test. The grade's fire-performance test basis is being confirmed; until it is on the data sheet, send us the spec clause your project cites (for example the applicable rail or marine standard) and we confirm against the batch certificate which requirements this grade meets and which need a different product.
How is this page different from the DRS series page?
This page is one product: a 2:1 single-wall halogen-free flame-retardant tube in thin, medium and thick walls. The DRS page is a family page covering several sub-series (including a dual-wall adhesive-lined system for sealing duty and an ultra-thin-wall system for tight spaces) across a wider ratio set. Choose this page when the duty is straightforward insulation with flame-retardant and halogen-free requirements at 2:1. Move to the DRS page when the position needs an adhesive liner, an ultra-thin wall, or a ratio above 2:1 to clear a connector in one piece.
Request a quote for Halogen-Free Flame Retardant Heat Shrink Tubing 2:1
Send the pass-over and gripping diameters, wall class, colour, cut length and quantity, plus the fire clause your spec cites. We return the size code, the batch UL224 flame class and lead time.
Certifications & Compliance
Built to UL224 and RoHS. The applicable UL224 flame class and the RoHS declaration issue per batch on request.
- RoHS
- UL224
Tell us your target market (EU / US / Asia) and we align the documentation to your filing.
View our verified UL / SGS / RoHS certificates →