Mylar (PET) Tubing for Bolts: Sleeves, Thread Protection and Flange Isolation
A few cents of PET film decides whether a flange isolation kit or a transformer clamping bolt passes inspection. The failures are nearly always fit and length, not material.
The short answer
- Mylar here means PET film tube. It is a slip-on or heat-formed sleeve, not a shrinking polyolefin. Working range is roughly -60 to +150 °C depending on grade, with 0.05 to 0.25 mm walls typical.
- Size the ID to the bolt shank and the length to the full stud. A sleeve quoted only by diameter will not cover head to nut, and one contact point defeats the isolation.
- PET is the cost-down route against PTFE. For standard flange and transformer duty it carries enough dielectric strength at a fraction of the material cost. Above the temperature ceiling, PTFE is the right answer.
- PVC is not the same thing. PVC plasticiser leaches out, the sleeve yellows and goes brittle, and the failure shows up in hot climates and long storage. PET is stable across the range and is halogen-free.
The job of a bolt sleeve is to break the metal-to-metal path. On a flange, a transformer core clamp or a structural joint, the bolt passes through two or more metal bodies. Without a dielectric sleeve between the shank and the hole, the fastener itself becomes a conductor bridging the joint, and the isolation washers at the faces do nothing. That path is what lets galvanic corrosion and stray current eat the assembly, and it is what turns a cathodic-protection scheme into a maintenance schedule.
PET film tube is the sensible default for that job. It is thin enough to fit inside the hole clearance, tough enough to resist puncture from a thread crest, and it cuts and heat-forms cleanly to length. The reason we push the sizing detail so hard is that most of the field failures we see are not material failures at all: the sleeve was too short, or the ID did not match the shank standard, so the bolt touched metal at one point and the whole isolation was lost.
Material choice: PET against the alternatives
| Property | PET (Mylar) film tube | PTFE bolt sleeve | PVC sleeve | Nomex / PET composite |
|---|---|---|---|---|
| Typical working range | -60 to +150 °C | -200 to +260 °C | -10 to +105 °C | Up to ~180 to 200 °C |
| Wall thickness range | 0.05 to 0.25 mm | 0.10 to 0.50 mm | ~0.3 to 1.0 mm | 0.05 to 0.80 mm |
| Bore range | ~1.8 mm to 64 mm | Custom | Limited | 1.8 to 64 mm |
| Dielectric strength | ≥ 15 kV/mm | Higher, ~25 kV/mm and up | Lower | High, plus thermal barrier |
| Puncture resistance | Good against thread crests | Very good | Ages and embrittles | Very good, tear resistant |
| Plasticiser leaching | None | None | Yes, yellows and hardens | None |
| Halogen-free | Yes | Yes | No, contains chlorine | Yes |
| Relative material cost | Low | High | Lowest | Mid to high |
Comparing costs is only meaningful on the whole kit. A PTFE sleeve can cost several times a PET one, and a flange kit with dozens of bolts multiplies that difference. PET is the default for standard duty; step up only where temperature or chemistry demands it.
Pick this, skip this
PET (Mylar) sleeves are the right call when
- Flange isolation kits for pipelines, tanks, valves and vessels
- Transformer clamping bolts and core or winding tie rods that must be dielectric from the live part
- Thread protection on studs and fasteners during shipping, storage and assembly
- Structural bolted joints that need a dielectric break at the shank
- Kits supplied to a bolt count, where sleeve cost per piece has to stay low
- Storage and transport through hot climates, where PVC would degrade
- Applications that must stay halogen-free for specification or ESG reasons
Move away from PET when
- Service temperature exceeds roughly 150 °C. Step up to PTFE or a Nomex composite
- The joint is exposed to aggressive chemistry or continuous solvent contact
- The bolt sees sustained vibration with sliding contact and the wall has to resist abrasion over years
- The kit spec names PTFE explicitly for a heat or chemical reason. Do not substitute silently; we will quote PTFE
- You need the sleeve to shrink and grip rather than slip on. Use a Mylar shrink grade, or move to a heat-shrink sleeve family
- The dielectric duty calls for a thick, high-voltage wall beyond what a thin PET film delivers
Specifications we hold
| Parameter | Value |
|---|---|
| Material | PET (Mylar / polyester) film tube, seamless |
| Bolt sizes covered as standard | M6, M8, M10, M12, M16, M20, M24, M30, M36, plus ANSI and DIN variants |
| Other bores | From ~1.8 mm up to ~64 mm ID |
| Wall thickness | 0.05 to 0.25 mm as standard, up to ~0.80 mm on request |
| Length | Cut to bolt length, roughly 8 mm to 6 m sections |
| Working temperature | -60 to +150 °C (grade dependent) |
| Dielectric strength | ≥ 15 kV/mm |
| Tensile strength | ≥ 50 MPa |
| Chemical resistance | Good against oils, greases, mild chemicals, fuels |
| Shrink behaviour | Non-shrink slip-on as standard; heat-formed grades available |
| Colour | Natural, clear, black, blue, green, red, custom |
| Standards cited | ASTM D880, IEC 60250, RoHS, REACH |
| Supply form | Loose sleeve, cut to length, or counted and packed as a kit component |
What goes wrong in the field
The sleeve is too short to cover head to nut
This is the failure that breaks isolation kits. If the sleeve stops short of the nut face, or does not reach past the flange faces, the fastener touches metal at one point and the dielectric path is broken. Always specify overall length and let us cut to the stud, not to a nominal size.
The ID is matched to the hole, not the shank
Bolt hole diameter and bolt shank diameter are different dimensions. A sleeve sized to the hole will not grip the bolt, will slide during assembly, and may tear on the thread. Size to the shank and let the hole clearance stay an air gap.
PVC substituted to save cost
PVC is cheaper per metre and looks identical in a warehouse photo. It also contains plasticisers that migrate, yellowing and hardening the sleeve, and it is not halogen-free. In hot or long-storage conditions the substitution shows up as cracked sleeves in the kit, not as a clean cost saving.
A burr on the bolt or hole destroys the sleeve
The sleeve goes in last, over a thread that may have a burr at the start. A hand chamfer on the stud entry and a quick deburr of the hole save more rework than any material upgrade.
Not stating the bolt standard
M-series, ANSI and DIN bolts at the same nominal size do not share a shank diameter across all grades. Send the callout, for example M20 x 90 DIN 931, and we will size from the standard rather than from the nominal number.
The kit arrives with missing pieces
Project teams assemble at a remote site. If the sleeve count does not match the bolt count, or lengths vary per joint, the kit stalls. We cut to length and pack to count so a kit can be issued whole.
Related Engineering Guides
Components for this solution
Mylar Tube / PET Film Insulation Tube
Seamless PET tube in custom ID, OD and wall for sleeve and insulation duty.
Mylar Sleeve for Flange Bolt Insulation
PET bolt sleeves for flange isolation kits, cut to bolt length and packed to count.
Mylar Sleeves for Fastener and Bolt Thread Protection
Slip-on sleeves that protect threads in transit, storage and assembly.
Mylar Bolt Sleeve Sizes M6 to M36
The size reference page: bore, wall and length guidance across the bolt range.
Mylar Shrink Tubing
Heat-formed PET grade where the sleeve has to grip rather than slip on.
Flange Isolation Kit Sleeves
Complete sleeve range for gasket and flange-kit OEM supply.
PTFE Tubes
Step up here for high-temperature or chemically aggressive bolt duty.
Vulcanized Fiber Washers
Dielectric washers and spacers that pair with bolt sleeves in an isolation kit.
Related solutions
Common questions
What bolt sizes do Mylar sleeves cover?
M6, M8, M10, M12, M16, M20, M24, M30 and M36 as standard, plus ANSI and DIN variants, and other bores from around 1.8 mm to 64 mm on request. Sizes outside the standard list are a tooling question, so tell us the callout and volume.
How do I specify the length?
By the full bolt length from head to nut, plus whatever the isolation washers and flange stack add. Send the bolt callout, for example M20 x 90 DIN 931. A sleeve specified only by inner diameter will not cover the stud correctly, and an uncovered section is a lost isolation.
Should I use PET or PTFE for flange bolts?
PET for standard flange duty at lower cost. PTFE where the temperature is high, the chemistry is aggressive, or the specification names it. On a kit with dozens of bolts the material cost difference is significant, so the decision is usually commercial unless the duty rules it out.
Is Mylar the same as PET?
Yes in trade usage. Mylar is a DuPont trade name for biaxially oriented PET film, and the sleeve industry uses Mylar, PET and polyester interchangeably. If your drawing names a specific film grade, send it and we will confirm equivalency.
Why is Mylar better than PVC for bolt sleeves?
Three reasons. PET has no plasticiser to migrate, so it does not yellow or embrittle in hot climates and long storage. It has higher tensile strength, so it resists puncture from thread crests. And it is halogen-free, which matters when a specification or a sustainability requirement rules PVC out.
Can Mylar sleeves be supplied as a kit?
Yes. We cut to length and pack to the bolt count, so a flange isolation kit can be issued whole without missing or mismatched sleeves. Send the bolt schedule and the kit bill of materials.
Does Mylar tubing shrink?
The standard bolt sleeve is a non-shrink slip-on. Mylar can be heat-formed and shrink grades exist, with shrinkage up to roughly 50 percent depending on grade, but that is a different product and a different process. If you need a sleeve that grips, tell us and we will quote the shrink grade.
What is the temperature limit?
Roughly -60 to +150 °C depending on the grade, with 105 °C being a common conservative continuous figure for standard film. Above that, PET is the wrong material and PTFE or a Nomex composite takes over.
Are the sleeves used in transformers as well as flanges?
Yes. In power and distribution transformers the clamping bolts and tie rods that hold the core and windings need to be insulated from live parts. A PET or PET-and-Nomex composite sleeve provides the dielectric break and, in the composite version, a thermal barrier as well.
What wall thickness should I choose?
Thin enough to clear the hole and the nut, thick enough for the dielectric duty and to resist thread puncture. For most flange duty that lands in the 0.05 to 0.25 mm band. We select the wall against your dielectric requirement rather than maximising it, because a thicker wall can make the sleeve impossible to fit.
Will the sleeve survive shipping and storage?
PET is stable across the -60 to +150 °C range and resists moisture, oils and greases, so it holds up in container storage and hot climates. That is the specific weakness of PVC, which is why the substitution is not a like-for-like saving.
What tolerances can you hold?
ID and OD within roughly ±0.1 mm on heat-formed PET, tighter with custom die forming. Length is cut to your specification. Confirm the fit at both ends of the assembly and the gage method, and we will state the achievable tolerance for that bore.
Do you ship RoHS and REACH documentation?
Yes. PET grades are RoHS and REACH compliant, with batch traceability to the incoming film certificate. Certificates of conformance are supplied per shipment on request.
What do you need to quote a bolt sleeve order?
The bolt callout including size, standard and overall length; the required ID and wall; service temperature and any chemical exposure; whether the sleeve is loose or a counted kit component; colour; annual volume and first-order quantity.