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Nomex, Kapton, G10, or Mica: Choosing Aerospace Machine Insulation by Class
Aerospace machine insulation is organized by two questions, and a good spec answers both before naming a material: the thermal class the part must live at, set by the machine hot spot rather than its ambient, and the mechanical role the part plays, liner, separator, structural standoff, or compliant pad. The families on the H-O aerospace electrical insulation page sort cleanly along both axes, and this guide walks each one by the decision it wins, with methods named by designation.
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Two axes sort every part: thermal class and mechanical role
Aerospace machine insulation is organized by two questions, and a good spec answers both before naming a material. The first is thermal class: the IEC 60085 framing (the 130, 155, 180, and 220 rungs) names the temperature an insulation system is assigned to live at, and the number that matters is the machine's hot spot, not its ambient. The second is mechanical role: a slot liner, a phase separator, a busbar standoff, and a compliant dielectric pad are different jobs that the same dielectric strength cannot fill.
This guide walks the families on the H-O aerospace electrical insulation page along both axes, with methods named by designation: IEC 60085 for thermal class, ASTM D149 for dielectric breakdown, NEMA LI 1 and MIL-I-24768 for the laminate framing, MIL-STD-704 for the aircraft power classes including 270 VDC, and UL 94 for flammability. Per-grade values and class assignments are the maker-TDS figures cited on the parent page.
The slot: aramid paper and its laminations
In the stator slot, Nomex aramid paper is the working liner and separator family: Nomex 410 as the baseline, 411 calendered and 414 high-density variants per the slot, with class behavior per the IEC 60085 framing on the maker data and breakdown per ASTM D149. Where the slot runs hot or pressboard stiffness is wanted, Nomex 818 pressboard carries the structural separator duty.
Where space or temperature tightens past what aramid alone clears, the answer steps to a Nomex-Kapton lamination or to Kapton and Apical polyimide film, which buys dielectric strength and thermal class in less thickness.
The winding wrap and the barrier plies
Windings and coils wrap in Kapton HN polyimide film, with ManniGlas glass-fiber paper as a thermal-margin ply where the design calls for it. The arc and high-temperature barriers are mineral: muscovite mica and phlogopite mica sheet hold the locations where flame, arc, and radiant exposure exceed what any polymer should see, because mica still beats every polymer at the extreme. Mica is a barrier, not a bulk insulator, and the two jobs are not interchangeable.
The structure: laminates and tracking-resistant composites
Busbar supports, standoffs, and phase barriers are machined and die-cut from glass-epoxy laminate: Norplex FR4 (NP511) and G11 (NP512) from the structural laminate family, framed by NEMA LI 1 and MIL-I-24768 designations. Where the bay carries contamination, condensation, and the threat of surface tracking, the support steps to a Durostone UPM-203 composite from the tracking-resistant composite family, chosen on its comparative tracking data, with the creepage-path geometry engineered alongside the material rather than after it.
The higher-voltage and compliant layers
Higher-voltage DC power electronics, the 270 VDC class of the MIL-STD-704 framing, add thin film-tier barriers where stress and altitude derating compress the margins: APTIV XPI PEEK film (the APTIV 1000 grade) and the standard APTIV series carry documented per-thickness breakdown, and mica plies extend the creepage paths.
The compliant dielectric interfaces, terminal-board pads and standoff cushions, ride on BISCO HT-12xx solid silicone, and 6113-05 skived PTFE from the PTFE film and tape family serves the low-friction and chemical-resistant separation duty. Breakdown values stay per ASTM D149 on each TDS.
Machine Insulation Families by Class and Mechanical Role
| Family | Mechanical role | Thermal-class position | Dielectric note | Where it lives |
|---|---|---|---|---|
| Nomex aramid paper | Liner / separator | Per grade, IEC 60085 | ASTM D149 per TDS | Slot liners, phase and layer separators |
| Kapton / Apical | Thin wrap / lamination | High class per TDS | High breakdown per thickness | Winding wrap, tight-slot laminations |
| Muscovite / phlogopite mica | Arc / heat barrier | Mineral, extreme | Barrier, not bulk insulation | Arc plies, severe-temperature barriers |
| G10 / FR4 / G11 / Durostone | Structure / standoff | Per grade | Tracking data on Durostone | Busbar supports, standoffs, barriers |
| APTIV PEEK | Thin HV barrier | High class per TDS | Per-thickness breakdown | 270 VDC barriers, creepage extenders |
| BISCO HT-12xx | Compliant pad | Silicone window | Compliant dielectric | Terminal boards, standoff cushions |
Illustrative and relative only; confirm each grade on its current technical data sheet. Breakdown per ASTM D149; thermal class per IEC 60085; laminate grades per NEMA LI 1 / MIL-I-24768; system dielectric design and machine qualification belong to the OEM.
Two rules that sort the page
First, assign the thermal class to the hot spot, not the ambient: an insulation system specified to the housing temperature while a winding hot spot runs a class higher ages on the wrong curve, and life halves and halves again with no failure at acceptance. The class line on the drawing is the life of the machine.
Second, match the support material to the surface environment, not just the bulk voltage: a G10 standoff that is perfect in a dry panel can track in a bay with de-icing chemistry, condensation, and conductive dust, which is exactly where the tracking-resistant composites and engineered creepage geometry earn their place.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take aramid paper, polyimide film, mica sheet, glass-epoxy and tracking-resistant laminate, PEEK and PTFE film, glass-fiber paper, and silicone stock from the material manufacturers and die-cut, machine, laminate, crease, and kit it to your lamination drawing, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We recommend a family and grade direction from your hot-spot thermal class, dielectric duty, mechanical role, forming needs, environment, and system voltage class, and grade-level values and class assignments are confirmed against the manufacturer's current TDS. Insulation-system design and machine qualification stay with the OEM.
Frequently asked questions
What material is used for slot liners in aerospace motors and generators?
Nomex aramid paper is the working slot-liner and separator family: 410 as the baseline, with 411 calendered and 414 high-density variants chosen per the slot, and Nomex 818 pressboard where structural stiffness is wanted. Class behavior follows the IEC 60085 framing on the maker data and breakdown is per ASTM D149. Where space or temperature tightens past what aramid clears, the slot steps to a Nomex-Kapton lamination or to Kapton polyimide film, which buys dielectric strength and thermal class in less thickness.
What do the thermal classes 130, 155, 180, and 220 actually mean?
They are the IEC 60085 framing for insulation thermal endurance: each class names the temperature an insulation system is assigned to live at, derived from thermal-aging behavior. The number that matters is the machine's hot spot, not its ambient, and every grade's class assignment lives on its maker's TDS. Run an insulation system above its class and life shortens on an aging curve; specify the class to the real hot spot, derated honestly for altitude and duty, and the family list largely writes itself.
Where does mica still beat every polymer?
At the extreme: arc exposure and severe temperature. Muscovite and phlogopite mica sheet hold the locations where flame, arc, and radiant exposure exceed what any organic polymer should see, retaining dielectric and structural integrity at temperatures that would carbonize a polymer. The key is that mica is a barrier, not a bulk insulator, so it is layered into a system to carry the arc or heat duty while another family carries the bulk dielectric job. The two roles are not interchangeable.
When is Durostone specified instead of a standard G10 laminate?
When the surface environment, not just the bulk voltage, is the threat. A standard G10 or FR4 standoff serves perfectly in a dry panel but can track in a bay with de-icing chemistry, condensation, and conductive dust: surface leakage carbonizes a path and a phase-to-ground fault follows. Durostone-class composites are chosen on their comparative tracking data for exactly those contaminated, condensing locations, and the creepage-path geometry, chamfers, ribs, and distance, is engineered with the material rather than after it.
Does H-O design insulation systems or certify machines?
No. H-O converts the documented materials to your lamination drawing and kits the system in winding order, with material traceability and lot-code TDS records. Designing the insulation system, assigning the thermal class to the hot spot, sizing creepage and clearance, and qualifying the finished machine belong to the machine OEM and its test houses. Our role is bounded: die-cut, machine, laminate, and crease the liners, separators, wraps, barriers, and standoffs to your drawing, repeatably and traceably.
Get a machine insulation recommendation
Tell us the hot-spot thermal class, the dielectric duty (working, test, and altitude), the mechanical role of each part, the forming needs, the bay environment, and the system voltage class, and we will recommend a family and grade direction and quote die-cut and machined parts to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- IEC 60085 (thermal classification of electrical insulation, by designation); ASTM D149 (dielectric breakdown voltage and strength).
- NEMA LI 1 (industrial laminate grades G-10, FR-4, G-11, by designation); MIL-I-24768 (laminate military specification family, by designation).
- MIL-STD-704 (aircraft electric power characteristics, including the 270 VDC class, cited qualitatively); RTCA DO-160 (environmental framing, qualitatively).
- UL 94 (flammability listings, per grade and thickness on each TDS).
- Manufacturer technical data sheets for the aramid paper, polyimide film, mica, glass-epoxy and tracking-resistant laminate, PEEK and PTFE film, and silicone grades named on the parent application page.
This article summarizes electrical-insulation material-selection principles for aerospace machines and power systems using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values and thermal-class assignments quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your hot spot, voltage, and altitude. Standards (IEC, ASTM, NEMA, MIL-STD) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not design insulation systems, certify machines, or perform dielectric qualification; those belong to the machine OEM and its test houses.