Custom E-Motor Slot & Phase Insulation, End-Turn Support, Gearbox Seals & Powertrain NVH
H-O Products converts high-temperature electrical insulation (aramid paper, polyimide film and PEEK film) and fluorosilicone sponge into die-cut slot liners, phase and wedge insulation, end-turn support, gearbox and shaft-area seals, and powertrain thermal and acoustic parts for traction motors and e-axles. Made to your drawing, under an ISO 9001:2015 certified quality management system in Winsted, Connecticut.
Built for: stator slot and phase insulation, slot wedges and end-turn support, ground-wall and layer insulation, gearbox and shaft-area seals exposed to oil, and thermal and acoustic parts for the e-axle.
EV powertrain and e-axle insulation, sealing and thermal-acoustic parts are the materials that let a traction motor run hot, hold off high voltage, stay sealed and stay quiet. They do three different jobs. Electrical insulation uses a high-temperature dielectric barrier in the stator slots, between phases and at the ground wall. The remaining zones and duties are mapped in the When-to-spec list on this page. Values are per the TDS on file; see the material reference below for ordering details.
IEC 60034-18 (Rotating electrical machines – Functional evaluation of insulation systems; how a motor's insulation system is tested and classified) · UL 1446 (Systems of Insulating Materials – General; the recognized thermal-class framework for insulation systems and their component materials) · IEC 60317 (Specifications for particular types of winding wires; the magnet-wire context the slot and phase insulation works alongside).
Materials are evaluated against and support compliance with these methods; H-O does not independently certify materials to them unless explicitly stated on the quote.
- Stator slot liner & ground-wall insulation: aramid paper
- Thin, high-dielectric phase & layer barrier: polyimide film
- Stiff slot wedge & mechanical insulation: PEEK film
- Highest continuous-temperature film barrier: polyimide film
- Oil- and fuel-resistant gearbox / shaft seal: fluorosilicone sponge
- Laminated slot-liner / barrier build: aramid paper laminated with polyimide film
Where are you in the spec process?
This page serves engineers who already know the material family they need and engineers still working out whether the problem is insulation, sealing, or NVH. Pick the path that matches where you are; you don't have to read the rest.
Send a drawing, get a quote
An aramid paper slot liner, a polyimide film phase barrier, a PEEK film wedge, or a fluorosilicone sponge seal on your drawing – with thickness, dielectric and any adhesive or liner called out.
Skip to the quote form →Walk through the material decisions
The decisions that drive an insulation or sealing choice (insulation vs. sealing vs. NVH, thermal class, working voltage, mechanical and slot-fill duty, chemical exposure), the insulation-class framework, an interactive slot-liner selector, and a material-family reference with cited standards.
Start with the decisions →
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1Send drawingUpload a DXF, STEP, or PDF of the slot, liner, wedge, barrier or seal, or describe the motor and the duty. A sample part works too.
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2Material reviewEngineering reviews the call-out against the manufacturer's current TDS and checks the goal (insulation, sealing, or NVH), the thermal class, the working voltage and dielectric, the slot-fill and mechanical duty, and the chemical exposure.
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3PrototypeSamples typically 3–5 business days for common configurations. Standard production runs about 2 weeks; special orders run custom lead times.
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4ProductionTooling refined, ongoing converted parts to drawing with material traceability and lot-level TDS records.
This page is for EV powertrain and e-motor engineers, electrical-machine and winding designers, e-axle and gearbox engineers, and sourcing teams specifying slot and phase insulation, slot wedges, end-turn support, ground-wall and layer insulation, gearbox and shaft seals, or powertrain NVH parts for traction motors and e-axles. It introduces the theme and routes you up to the EV & Battery industry page and its application overview, and across to the related EV sub-applications.
Insulation, sealing or NVH requirement → material selection → converted part → prototype → production supply → ongoing program.
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1Define the problemName the job: insulate a stator slot or phase, support an end turn, seal a gearbox, or quiet an e-axle.
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2Select the material familyMatch the goal, the thermal class, the working voltage, the mechanical and slot-fill duty and the chemical exposure to a family direction (use the selector tool).
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3Converted partDefine the slot liner, wedge, phase barrier, end-turn part or seal geometry, fold and cuff details, adhesive side and liner.
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4PrototypeH-O die-cuts, kiss-cuts, laser- or waterjet-cuts a prototype to your drawing for fit in the slot and a first insertion check.
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5Production supplyTooling is refined and converted parts ship to drawing with material traceability and lot-level TDS records.
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6Ongoing programReleases against a blanket or kanban, with kitting and revision control as the design and volume evolve.
What are you solving?
Slot liners, phase barriers and class — what the terms actually mean.
The most common specification error in this field is confusing a thermal class with a material, or a single material with a qualified insulation system. A slot liner is not a phase barrier; a thermal class is a system property, not a sheet property; a seal is not an insulator. These six terms separate the jobs. Get the job right and the material family follows.
Show all 6 terms tap to expand
The insulating sheet that lines the wall of a stator slot, between the laminated steel core and the copper winding. It is the ground-wall barrier that keeps the winding off the grounded core and protects the wire during insertion. Aramid paper is the classic slot-liner material, often laminated with a film for higher dielectric.
A barrier placed between coils of different electrical phases inside the slot or at the end turns, where the full phase-to-phase voltage appears. Because the space is tight and the voltage is high, phase insulation rewards a thin film with a high dielectric strength per unit thickness, such as polyimide.
The stiff strip that closes the open mouth of the slot over the winding, holding the conductors down against electromagnetic and vibration forces. A wedge is a mechanical part first and an insulator second, so it favors a stiff, strong, temperature-stable material such as a PEEK film or a rigid laminate.
The temperature rating of an insulation system, named by letter (for example Class H at 180 °C), that sets the allowable hot-spot temperature. It is a property of the qualified system under UL 1446 and IEC 60034-18, not of a single sheet, though each material has a thermal reach that supports a class.
The combination of materials, wire, varnish and processing that together make up a motor's insulation, functionally evaluated and classified as a system under IEC 60034-18. A converted slot liner or phase barrier is a component that supports the system; the class belongs to the tested system.
The voltage a barrier withstands before breakdown, expressed per unit thickness (for example kV/mm). It sets how thin a slot, phase or ground-wall barrier can be for a given working voltage and margin. It is grade- and thickness-specific and confirmed on the manufacturer's data sheet.
Decisions that drive your insulation or sealing choice
Material selection for e-motor and e-axle work returns to a handful of decisions, in order. Answer them and the family follows; the interactive selector below walks the same logic. None of these is a headline dielectric figure – the job and the duty decide the material, and grade-level values are always confirmed against the manufacturer's current technical data sheet.
Show all 6 selection factors tap to expand
What is the job: insulation, sealing, or NVH?
This is the first fork and it sorts the families. Electrical insulation wants a high-temperature dielectric barrier (aramid paper for the slot and ground wall, polyimide film for thin phase and layer barriers, PEEK film for stiff wedges). Sealing wants an oil- and temperature-resistant elastomer (fluorosilicone sponge for gearbox and shaft seals). Thermal and acoustic control wants a damping, decoupling or thermal-barrier part. Naming the job correctly prevents the classic mistake of using one where another belongs.
What is the thermal class and hot-spot temperature?
A traction motor's hot-spot temperature sets the thermal class, and the class narrows the material. Aramid paper covers the widely used Class H range; polyimide and PEEK films reach higher continuous temperatures where a hotter design or a higher class is required. The class is a property of the qualified insulation system under UL 1446 and IEC 60034-18, so send the target class and the hot-spot temperature, and the material is chosen to support it and confirmed on the data sheet.
What is the working voltage and required dielectric margin?
The voltage across the barrier – slot-to-ground, phase-to-phase, or layer-to-layer – sets the dielectric strength and thickness the part needs, with margin for the inverter's fast switching edges. A high-voltage traction motor with a tight slot rewards a thin film with a high dielectric strength per unit thickness, such as polyimide, sometimes laminated to aramid for handling. Send the working voltage, the partial-discharge concern if any, and the dielectric margin so the grade and gauge can be confirmed.
What mechanical and slot-fill duty must the part take?
Insulation in a motor is also a mechanical part. A slot liner has to survive winding insertion without tearing; a wedge has to hold the conductors against electromagnetic and vibration forces; an end-turn part has to support the windings. This favors tough, tear-resistant materials for liners (aramid paper) and stiff, strong materials for wedges (PEEK film, rigid laminate). Send the slot-fill, the insertion method and the mechanical loads so the thickness and material can be matched.
What is the chemical exposure: oil, coolant, or varnish?
The chemistry can override the mechanics. Insulation must be compatible with the impregnating varnish and any oil-cooling the motor uses; a gearbox or shaft seal sees lubricant continuously. Fluorosilicone resists oil and fuel where standard silicone would swell, which is why it leads for gearbox seals; the insulation films and papers are chosen partly for varnish and fluid compatibility. State the oils, coolants and varnishes the part will see so compatibility can be confirmed on the data sheet.
How is the part formed and assembled?
Slot liners are folded and cuffed, wedges are inserted, phase barriers are placed by hand or machine, and seals are compressed in a groove. The forming and assembly method drives the cut detail: fold lines and cuffs on a liner, chamfers on a wedge, the right adhesive side and liner on a barrier or seal. Tell us how the part is installed – hand-laid, machine-inserted, or automated – and the fold, cuff and adhesive details can be set on the drawing.
EV e-axle sealing & insulation failures you can prevent at spec
E-axle failures show up as coolant ingress, a winding hot-spot, or gear whine — all decided in the material callout.
The motor’s thermal class and the axle’s ingress rating belong to the qualified assembly. An under-rated seal or insulation puts them at risk.
Show all 5 failure modes tap to expand
1. An elastomer where the motor slot or winding runs hot
Fix — use mica insulation matched to the motor thermal class (IEC 60034 / IEC 60317).
2. An open-cell seal on the e-axle housing
Fix — use a closed-cell gasket rated for coolant and the ingress target.
3. NVH left untreated at the gearset
Fix — add isolation or damping sized to the gear-whine frequency.
4. Insulation under-rated for winding voltage
Fix — specify mica dielectric for the inverter output voltage.
5. Field-cut parts with gaps
Fix — die-cut seals and insulation to the drawing.
Interactive specification tools
Two interactive tools to take you from "I have an insulation, sealing or NVH requirement" to here is the material family to put on the drawing: a thermal-class map that shows how the material families reach across the insulation classes and where a hotter design needs a higher-temperature film, and an insulation and slot-liner selector that turns your goal, thermal class, voltage and duty into a cautiously framed family direction.
A third tool, an exploded 3D view of a stator slot-insulation stack, places the converted parts in context. Each renders with a static fallback when JavaScript is off.
Why this tool The thermal-class map is the relationship that decides which family can even be considered. Engineers sometimes pick a familiar paper or film and only later discover the design's hot-spot pushes past its continuous-temperature reach, or over-specify an expensive high-temperature film where a Class H aramid would do. Seeing each family's temperature reach against the insulation classes is the fastest way to land in the right family before the dielectric and mechanical details.
1. Insulation thermal-class map
The recognized insulation thermal classes, with the approximate continuous-temperature reach of each material family laid over them. The bars are illustrative of family reach against the class framework, not grade ratings; the qualified class belongs to the tested insulation system.
Interactive: Insulation Thermal-Class Map
The insulation thermal classes (B, F, H and higher) on one axis, with the approximate continuous-temperature reach of aramid paper, polyimide film and PEEK film shown as bars. Move the hot-spot slider to mark your design temperature and see which families clear it. With JavaScript off, the three family bars and the class bands render statically. Reach is illustrative of the family, not a grade rating; the qualified class belongs to the tested system under IEC 60034-18.
About this map. The class bands and the family reach bars are an illustrative orientation, not grade ratings. The recognized thermal classes come from UL 1446, and a motor's class is established by functional evaluation of the complete insulation system under IEC 60034-18, not by a single material. Each family's continuous-temperature reach is grade-specific and is confirmed on the manufacturer's data sheet; use this map to land in the right family, then confirm the grade, the dielectric and the system class.
Why this tool The insulation families overlap, and the right one depends on four inputs at once – the job, the thermal class, the working voltage and the mechanical duty. This selector encodes the same decision logic an H-O engineer applies, so you arrive at the material reference already pointed at the right family direction instead of reading all of it. It is a starting direction, deliberately cautious; the final grade is always confirmed against the manufacturer's data sheet.
2. Insulation / slot-liner selector
Pick your primary goal, thermal class, working voltage and mechanical duty. The selector returns a cautiously framed family direction and the reason. With JavaScript off, a static decision table covers the same ground.
Interactive: Insulation / Slot-Liner Selector
Four inputs in, one cautiously framed family direction out. This is a starting point that mirrors the decision logic on this page, not a substitute for an engineering review or the manufacturer's technical data sheet.
| If your goal is… | Lead family direction | Why |
|---|---|---|
| Slot liner / ground wall | Aramid paper (laminate with film for high V) | Tough, tear-resistant Class H paper survives insertion and lines the slot |
| Phase / layer barrier, tight slot | Polyimide film | High dielectric strength per unit thickness fits a thin, high-voltage barrier |
| Slot wedge / end-turn support | PEEK film or rigid laminate | Stiff, strong and temperature-stable to hold conductors against forces |
| Highest continuous temperature | Polyimide film | Reaches the highest continuous-temperature service of these films |
| Gearbox / shaft seal | Fluorosilicone sponge | Resists oil and fuel where standard silicone would swell |
All directions are cautious starting points; the final grade is confirmed against the manufacturer's current technical data sheet for your class, voltage and duty.
About this selector. The output is a family-level direction based on general engineering principles, not a grade recommendation and not a guarantee of fit. Several families can serve the same job; the selector points at the most common starting choice for the inputs given. Confirm the specific grade, thickness, thermal class, dielectric strength and chemical compatibility against the manufacturer's current technical data sheet, and send the part to H-O for an engineering review.
Why this tool A stator slot is a layered stack, not a single part, and where each converted insulation part sits relative to the core and the winding decides how it works. The exploded 3D view makes the stack legible – laminated core, slot liner, winding, phase barrier, slot wedge – so the parts H-O converts (the liner, barrier and wedge) are shown in their real context. It is a reference model, not customer CAD, and it degrades to a static caption when WebGL is unavailable.
3. Exploded stator slot-insulation stack (3D)
A representative stator slot insulation stack, exploded along the slot – laminated core, slot liner, copper winding, phase barrier, and slot wedge – to show where the converted parts live. Drag to rotate; click a layer to isolate it.
3D Exploded View: Stator Slot-Insulation Stack
Representative stator slot insulation stack, exploded along the slot: laminated steel core → die-cut slot liner → copper winding → phase barrier → slot wedge. Drag to rotate, click a layer to isolate its role, toggle explode with the icon or the E key. The hero layers in amber are the converted parts — the liner, barrier and wedge H-O converts.
Open the interactive 3D exploded stack — drag to rotate, click a layer to isolate
3D viewer unavailable
The interactive 3D model could not load. This pilot needs WebGL; the stack it shows is, in order: laminated steel core, slot liner (an H-O part), the copper winding, a phase barrier (an H-O part), and the slot wedge (an H-O part). Please try a current desktop browser with hardware acceleration enabled.
Select to isolate
Representative stator slot insulation stack; not customer CAD.
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Skip ahead and request your engineering review now
If your drawing already calls out an aramid paper, polyimide film, PEEK film or fluorosilicone part – send it over for engineering review against the current data sheet.
The five jobs this theme covers
E-motor and e-axle work is not one application but five related ones, each with a different physics and a different material lead. Click a tab to see the environment, the standards commonly referenced, and the material families H-O converts for that job. Each maps to the traction-motor and gearbox assemblies these parts go into.
Stator slot liners and ground-wall insulation
This is the foundational insulation job: a barrier that lines the stator slot, keeps the copper winding off the grounded steel core, and survives the mechanical abuse of winding insertion.
The material has to combine a high thermal class, a strong dielectric to ground, and the toughness to be folded, cuffed and inserted without tearing. Aramid paper is the classic slot-liner and ground-wall material because it covers the widely used Class H range and is tough and tear-resistant; for higher working voltage it is laminated with polyimide film for added dielectric. H-O these to the slot profile with fold lines and cuffs.
Grade-level properties, standards and caveats for these families are in the material reference below — one card per family, with the TDS links.
Phase and layer insulation
Between coils of different phases, and between winding layers, the full phase-to-phase voltage appears in a very tight space.
The barrier has to deliver a high dielectric strength in the thinnest possible gauge so it fits the slot fill and stands off the voltage with margin for the inverter's fast switching edges. Polyimide film leads here for its dielectric strength per unit thickness and its high continuous-temperature reach; where a stiffer barrier is wanted, PEEK film serves. H-O these phase and layer barriers to shape with the fold and placement detail the winding needs.
Grade-level properties, standards and caveats for these families are in the material reference below — one card per family, with the TDS links.
Slot wedges and end-turn support
Once the slot is wound, the conductors have to be held down against the electromagnetic and vibration forces that try to lift them, and the end turns have to be supported. These are mechanical parts first: the slot wedge closes the slot mouth, and end-turn support pieces brace the winding overhang.
The material has to be stiff, strong and temperature-stable. PEEK film leads here for its stiffness, strength and high continuous-temperature reach; stiff laminates and high-temperature papers also serve. H-O wedges and end-turn parts to profile with the chamfers and fold detail insertion needs.
Grade-level properties, standards and caveats for these families are in the material reference below — one card per family, with the TDS links.
Gearbox and shaft-area seals
The reduction gearbox and the shaft area of an e-axle run in oil, so the seals and gaskets there see continuous lubricant contact across a broad temperature span.
Standard silicone swells in oil, so the material of choice is a fluorinated elastomer that keeps its properties against oil and fuel. Fluorosilicone sponge leads here, giving an oil- and fuel-resistant compression seal that holds across temperature; closed-cell elastomers serve where the chemistry is milder. H-O these seals and gaskets to the housing and cover profile, with the compression set point and any adhesive set on the drawing.
Grade-level properties, standards and caveats for these families are in the material reference below — one card per family, with the TDS links.
Powertrain thermal and acoustic control
An e-axle has its own NVH signature – the high-frequency whine of the motor and the gear noise of the reduction stage – alongside heat that has to be managed away from the windings and electronics. The fix is a set of converted parts: damping and decoupling layers that drain structure-borne noise, and thermal barriers and pads that move or block heat.
The families overlap with the NVH and thermal pages, so parts are chosen by the specific job; fluorosilicone and silicone parts cover the oily, hot regions, and damping foams handle the acoustic work. H-O converts and, where useful, kits these together for the e-axle assembly.
Grade-level properties, standards and caveats for these families are in the material reference below — one card per family, with the TDS links.
Powertrain insulation & sealing families compared by job
The families an e-motor and e-axle engineer weighs, compared on the properties that actually decide the spec. Values are cautious and qualitative; where a property is grade- and thickness-specific, the cell says so. This is a family-level orientation, not a grade datasheet – confirm exact values against the manufacturer's current TDS.
| Property | Aramid paper | Polyimide film | PEEK film | Fluorosilicone sponge |
|---|---|---|---|---|
| Primary job it leads | ||||
| Best-fit job | Slot liner & ground wall | Thin phase / layer barrier | Slot wedge & end-turn | Gearbox & shaft seal |
| Electrical & thermal behavior | ||||
| Dielectric strength | Good Strong as a paper; laminate film for high V | Very high per mil Highest dielectric per unit thickness | High Good dielectric in a stiff film | Not an insulator Chosen for sealing, not dielectric |
| Thermal class / reach | Class H Covers the widely used 180 C range | Highest Highest continuous-temperature film here | High Reaches well above Class H | Broad Wide span; verify on grade TDS |
| Mechanical & chemical | ||||
| Mechanical behavior | Tough, tear-resistant Survives winding insertion | Thin, flexible A barrier film, not a structural part | Stiff & strong Holds conductors against forces | Resilient seal Recovers; holds a controlled set |
| Oil / chemical resistance | Varnish-compatible Confirm fluid compatibility on TDS | Good Chemically stable film; verify on TDS | Excellent Very chemically resistant film | Oil & fuel Resists oil and fuel where silicone swells |
How to read this. Aramid paper leads for tough Class H slot liners and ground-wall insulation; polyimide film leads for thin, high-dielectric phase and layer barriers and the highest continuous temperature; PEEK film leads for stiff slot wedges and end-turn support; fluorosilicone sponge leads for oil- and fuel-resistant gearbox and shaft seals. Several families overlap and are often laminated together, so the job, the class and the duty decide the lead.
All values are family-level and qualitative; confirm grade-level thermal class, dielectric strength, mechanical properties and chemical compatibility against the manufacturer's current technical data sheet for your part.
What H-O converts these materials into.
H-O Products is a precision converter. We do not extrude or mold the raw material; we buy sheet, roll and film stock from the material manufacturers and convert it to your drawing. For e-motor and e-axle work, that converting capability turns the families above into finished parts in low and high volume.
Show all 6 part types tap to expand
Slot liners and ground-wall barriers cut to the slot profile from aramid paper and aramid-film laminates, with the fold lines, cuffs and creases the slot insertion needs.
Thin phase and layer insulation pieces from polyimide or PEEK film to fit between phases and winding layers, with placement and fold detail to the drawing.
Stiff slot wedges and end-turn support pieces cut from PEEK film and rigid laminate, with chamfers and profiles to close the slot and brace the winding overhang.
Oil- and fuel-resistant seals and gaskets from fluorosilicone sponge and fluoroelastomer to the gearbox and cover profile, with the compression set point set on the drawing.
Multi-layer laminations of aramid paper and polyimide film, built to a target thickness and dielectric, converted into a single slot or phase part for the line.
Sequenced, kitted sets of slot liners, phase barriers, wedges and seals delivered ready to install in winding order, with revision control across a production program.
Converting processes include rotary and flatbed die-cutting, kiss-cutting, scoring and creasing, laser and waterjet cutting, adhesive lamination, slitting, and kitting. Tolerances, fold and cuff detail, adhesive systems, liners and packaging are set on the drawing and confirmed at quote. See die-cutting, lamination and kitting under related capabilities.
Insulation and sealing materials H-O converts
Family-level notes on the materials referenced on this page, with where each one fits across the five jobs. These are the families H-O converts; they are commonly used for the duties described, and a given grade may be suitable depending on thermal class, voltage, mechanical duty and chemical exposure. Grade-level values are thickness- and grade-specific; confirm against the material manufacturer's current technical data sheet. H-O converts these to drawing in low and high volume.
Aramid PaperTough Class H insulation paper · slot liners & ground wall
- Nomex 410the baseline Class-H aramid paper
- Nomex 411uncalendered, conformable
- Nomex NMN laminatearamid-polyester triplex laminate
Polyimide FilmHighest continuous temperature · thin, high-dielectric phase barrier
PEEK FilmStiff, strong, high-temperature · slot wedges & end-turn support
- APTIV 1300general-purpose PEEK film
- APTIV 2000unfilled PEEK film
- APTIV XPI-A105polyimide-blend PEEK film
Fluorosilicone SpongeOil- & fuel-resistant elastomer · gearbox & shaft seals
- R10490fluorosilicone sponge rubber
- 550-70 FVMQcorrosion-resistant solid fluorosilicone
The governing specifications these materials are designed to meet.
The standards an e-motor and e-axle insulation spec returns to, grouped by what they govern. Materials are evaluated against and support compliance with these methods through the manufacturer's data sheet; H-O does not independently certify materials to them unless explicitly stated on the quote. Cite the designation, not a pass: "evaluated under IEC 60034-18," not "certified to."
Show all 4 standards groups tap to expand
- IEC 60034-18 – Rotating electrical machines, functional evaluation of insulation systems. The framework by which a motor's complete insulation system is tested and classified; a converted part supports the system.
- System, not material – the thermal class belongs to the qualified system; each material has a thermal reach that supports a class but is not itself "rated" to the system class.
- UL 1446 – Systems of Insulating Materials, General. The recognized framework for thermal classes (for example Class F, H) of insulation systems and their component materials.
- Class reference – the basis for the thermal-class map above; the family reach bars are illustrative, and the class is established by system evaluation.
- IEC 60317 – Specifications for particular types of winding wires. The magnet-wire standard the slot, phase and ground-wall insulation works alongside in the slot.
- System fit – the sheet insulation and the wire enamel together make the slot insulation system; compatibility is confirmed at the system level.
- Dielectric strength – reported on the grade data sheet for each paper and film; sets the barrier thickness for a given working voltage and margin.
- Chemical compatibility – varnish, oil and coolant compatibility for the insulation, and oil and fuel resistance for the seals, confirmed on the manufacturer's data sheet.
Standard editions are current as of June 2026; verify against the publishing body before final spec. Thermal class is established by functional evaluation of the complete insulation system; individual material thermal reach is grade-specific and is confirmed on the manufacturer's data sheet.
Related EV & Battery sub-applications
This page sits inside the EV & Battery industry and its energy, power and high-voltage application family. These links route up to the industry and application overview, and across to the sibling EV sub-applications where related materials do their jobs. (Some pages are being published; links that are not live yet resolve gracefully.)
E-motor & e-axle insulation: engineer-grade FAQ
Ten of the questions we hear most from EV powertrain, e-motor and winding engineers. If your question isn't here, send a drawing or describe the motor and call, engineering picks up.
What is the difference between a slot liner and phase insulation?
They insulate against different voltages in different places. A slot liner is the ground-wall barrier that lines the stator slot, between the laminated steel core and the winding; its job is to hold off the slot-to-ground voltage and to protect the wire during insertion, so it favors a tough, tear-resistant Class H material like aramid paper.
Phase insulation is the barrier placed between coils of different electrical phases, where the full phase-to-phase voltage appears in a tight space; its job is maximum dielectric strength in minimum thickness, which favors a thin film like polyimide.
Many motors use both: an aramid slot liner for the ground wall and a polyimide phase barrier between phases, sometimes with the two laminated together for the highest-voltage builds.
How do I choose between aramid paper, polyimide film and PEEK film?
Start with the job, then the temperature and voltage. For the slot liner and ground wall, aramid paper is the workhorse: tough enough to survive insertion, Class H rated, and economical. For a thin phase or layer barrier in a tight, high-voltage slot, polyimide film wins because it has the highest dielectric strength per unit thickness and the highest continuous-temperature reach.
For a slot wedge or end-turn support that has to be stiff and hold conductors against electromagnetic and vibration forces, PEEK film wins on stiffness, strength and dimensional stability. Many builds combine them, an aramid liner, a polyimide phase barrier, a PEEK wedge, and aramid is often laminated to polyimide to get toughness and dielectric in one part. Send the class, the working voltage and the mechanical duty and the family follows.
What does a Class H or higher thermal rating actually mean for my material?
It is a property of the insulation system, not of a single sheet. The thermal class, Class H is 180 °C, is the allowable hot-spot temperature of the qualified insulation system as a whole, established by functional evaluation under IEC 60034-18 within the framework of UL 1446. A material like aramid paper has a thermal reach that supports a Class H system, and a film like polyimide reaches higher, but the class is earned by the complete system of materials, wire enamel, varnish and processing tested together.
So the honest way to specify is to state your target class and hot-spot temperature, choose materials whose reach supports it, and confirm the class against the qualified system, not to treat a single sheet as "Class H rated" in isolation.
How thin can a phase barrier be for a high-voltage traction motor?
As thin as the dielectric strength and your margin allow, which is exactly why polyimide film is the lead. The minimum thickness is set by the phase-to-phase working voltage, the dielectric strength of the material per unit thickness, and the safety margin you carry for the inverter's fast switching edges and any partial-discharge concern. A film with a high dielectric strength per mil lets you hit the voltage in a thinner barrier, which matters in a tight slot where every fraction of a millimeter competes with copper for space.
The right answer comes from the working voltage, the required margin, and the grade's data-sheet dielectric strength, with partial-discharge behavior considered for the highest-voltage designs. Send the voltage and margin and the gauge can be set against the TDS.
Why is aramid often laminated with polyimide film?
To get the best of both in a single converted part. Aramid paper is tough and tear-resistant and survives insertion, but its dielectric strength is that of a paper; polyimide film has a very high dielectric strength per unit thickness but is a thin film that benefits from a carrier for handling.
Laminating a polyimide film between or onto aramid paper produces a slot or phase barrier that carries the high dielectric of the film with the mechanical toughness of the paper, which is why aramid-polyimide-aramid and similar laminates are common in higher-voltage traction-motor slots.
H-O laminates and these builds to the slot profile, so you specify the dielectric and thickness you need and the laminate is converted into one part with the fold and cuff detail for insertion.
What material should a slot wedge be made from?
A wedge is a mechanical part first, so it wants a stiff, strong, temperature-stable material. The slot wedge closes the mouth of the slot over the winding and holds the conductors down against the electromagnetic and vibration forces that try to lift them, all at the motor's hot-spot temperature. PEEK film is the lead because it combines stiffness, strength, dimensional stability and a high continuous-temperature reach, and it resists the oils and fluids a wet or oil-cooled motor may see.
Rigid glass-composite laminates serve where the mechanical loads are highest, and denser calendered aramid grades suit lighter-duty wedges. Send the slot geometry, the mechanical loads and the temperature and the wedge material and thickness can be matched against the data sheet.
Why use fluorosilicone instead of standard silicone for a gearbox seal?
Because the gearbox and shaft area run in oil, and standard silicone swells in oil. Silicone is prized for its broad temperature span and resilience, but conventional silicone absorbs hydrocarbon oils and fuels and loses its sealing geometry. Fluorosilicone keeps silicone's temperature range and recovery while adding resistance to oil, fuel and many solvents, so it holds a controlled compression seal in continuous lubricant contact where standard silicone would degrade.
For the most aggressive oil and high-temperature sealing, a solid fluoroelastomer is the step beyond fluorosilicone. Send the lubricant, the temperature and the seal geometry, and the right fluorinated elastomer and compression set point can be confirmed against the data sheet.
How do I keep slot insulation from tearing during winding insertion?
With a tough material and the right converted detail. Insertion is where slot liners fail, the wire or the insertion tool can nick or tear a barrier as it goes in, so the first defense is a tear-resistant material like aramid paper rather than a bare film. The second is the cut detail: properly scored and creased fold lines, cuffs at the slot ends that protect the wire entry, and a liner sized so it seats without bunching.
For higher-voltage builds, laminating the film between aramid layers keeps the dielectric film protected by the tougher paper. H-O converts liners with the fold, crease and cuff geometry your insertion method needs, so send the slot profile and how the motor is wound and the liner can be detailed to survive the process.
Can a converted insulation material be certified to IEC 60034-18 or UL 1446?
Those standards classify an insulation system, not a single converted part. IEC 60034-18 is a functional-evaluation method for a motor's complete insulation system, and UL 1446 is the framework for thermal classes of insulation systems and their recognized component materials; a slot liner or phase barrier is a component that supports a system, but the class is earned by the system tested as a whole.
So the honest framing is that H-O materials are evaluated against and support compliance with these standards through the manufacturer's data sheet and any component recognition the material carries, and H-O does not independently certify a converted part to a system class unless that is explicitly stated on the quote. If you need a system class, that comes from evaluating the complete insulation system, of which the converted part is one element.
Does H-O make the raw paper and film, and can I get custom parts with lead times and samples?
H-O is a precision converter, not a raw-material producer. We do not extrude or mold the paper, film and elastomer stock; we buy sheet, roll and film stock from the material manufacturers and convert it to your drawing, by die-cutting, kiss-cutting, scoring and creasing, laser and waterjet cutting, adhesive lamination, slitting and kitting, with material traceability and lot-level data-sheet records.
Every slot liner, phase barrier, wedge and seal is made-to-order; we do not carry finished parts in stock and we do not advertise a no-minimum policy, though prototype quantities through full production runs are equally welcome and the minimum varies by material and part. Prototype and production timing is summarized in the process strip near the top of the page and on the quote form.
Send your drawing or describe the motor through the form below for a specific quote.
Can I get material samples before committing to a design?
Yes — material swatches and cut samples are available on request, subject to material availability. For evaluation builds, the usual path is to send the part drawing so prototype parts are cut from the actual grade and thickness under consideration; that puts representative parts in your fixture instead of a generic swatch.
Are there minimum order quantities?
Minimums depend on the material and format rather than a single policy. Prototype quantities are quoted case-by-case, and production minimums are typically driven by the vendor’s sheet or roll purchase unit for the specific material. Stating your target annual volume on the RFQ lets H-O quote realistic break points up front.
Glossary: terms used on this page
The vocabulary of e-motor and e-axle insulation and sealing, defined as it is used on this page. Click a term to expand its definition.
Slot liner
The insulating sheet that lines a stator slot between the laminated steel core and the copper winding, providing the ground-wall barrier and protecting the wire during insertion. Classically aramid paper.
Phase insulation
A barrier between coils of different electrical phases, where the full phase-to-phase voltage appears. Favors a thin film with high dielectric strength per unit thickness, such as polyimide.
Slot wedge
The stiff strip that closes the mouth of a stator slot over the winding, holding the conductors down against electromagnetic and vibration forces. A mechanical part first; favors stiff PEEK film or rigid laminate.
Thermal class
The temperature rating of an insulation system, named by letter (for example Class H at 180 °C), set by functional evaluation of the system under UL 1446 and IEC 60034-18. A property of the system, not a single sheet.
Insulation system
The combination of materials, wire, varnish and processing that together make up a motor's insulation, functionally evaluated and classified as a system under IEC 60034-18. A converted part supports the system.
Dielectric strength
The voltage a barrier withstands before breakdown, expressed per unit thickness (kV/mm). Sets how thin a slot, phase or ground-wall barrier can be for a given working voltage and margin. Grade- and thickness-specific.
End-turn support
Parts that brace the winding overhang (the end turns) outside the slot against vibration and electromagnetic forces. Favors stiff, strong, temperature-stable materials such as PEEK film and rigid laminates.
Aramid paper
A meta-aramid insulation paper that is tough, tear-resistant and Class H rated, the workhorse for stator slot liners and ground-wall insulation. Often laminated with polyimide film for higher voltage.
Polyimide film
A high-temperature film with very high dielectric strength per unit thickness, the lead for thin phase and layer barriers and the highest continuous-temperature insulation. Often laminated to aramid paper.
Fluorosilicone
A fluorinated silicone elastomer that keeps silicone's temperature span and resilience while resisting oil and fuel, the lead for gearbox and shaft-area seals where standard silicone would swell.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
Standards, test methods & technical references
The standards referenced throughout this page, numbered for citation. Standard editions are current as of June 2026; verify against the publishing body before final spec. H-O materials are evaluated against and support compliance with these methods through the source manufacturer's technical data sheet, not independently certified by H-O unless explicitly stated on the quote. The references here are standards bodies and general engineering principles only.
IEC 60034-18
Rotating electrical machines – Functional evaluation of insulation systems (multi-part). Specifies how the complete insulation system of a rotating machine is tested and thermally classified. A converted insulation part supports the system; the class belongs to the evaluated system. International Electrotechnical Commission.
UL 1446
Systems of Insulating Materials – General. The recognized framework for thermal classes of electrical insulation systems and their component materials, including the Class F and Class H designations used for traction-motor insulation. UL Standards.
IEC 60317
Specifications for particular types of winding wires (multi-part). The magnet-wire standard whose enameled conductors work alongside the sheet slot, phase and ground-wall insulation to form the slot insulation system. International Electrotechnical Commission.
Dielectric & insulation-coordination principles
The standard treatment of dielectric strength per unit thickness, working-voltage margin and partial-discharge behavior used to size slot, phase and ground-wall barriers for an inverter-fed traction motor. Presented as general engineering principle. The basis for the thickness-versus-voltage reasoning on this page.
Updated . Standards editions current at publication; verify against the publishing body before final spec. H-O materials are “evaluated against” the standards cited through the source manufacturer's technical data sheet; H-O does not independently certify materials against these standards unless explicitly stated on the quote.
To review your insulation, wedge or seal part, send:
- Goal (slot liner, phase, wedge, or seal)
- Target thermal class / hot-spot temperature
- Working voltage and dielectric margin
- Slot or seal geometry / drawing
- Slot-fill and mechanical loads
- Fold, cuff and insertion method
- Varnish, oil or coolant exposure
- Adhesive / liner requirements
- Prototype and annual volume
Get an e-motor / e-axle insulation engineering quote
Send a drawing, BOM, or a description of the motor, the slot and the duty. We typically respond within one business day with a material-family recommendation, prototype lead time, and TDS verification against your thermal class, working voltage, mechanical duty, and chemical exposure.
Related H-O Products capabilities
The converting capabilities and adjacent application families that pair with e-motor and e-axle insulation work. Each page covers material selection and converter-side process detail for its area.
Capability
Precision die-cutting
Rotary and flatbed die-cutting, scoring, creasing, laser and waterjet cutting of the papers, films and elastomers that make up slot liners, wedges and seals, to your drawing and tolerance.
Read the page
Capability
Lamination & adhesive systems
Aramid-polyimide laminations, multi-layer insulation builds, and adhesive and liner systems that turn paper and film into a single converted slot or phase part.
Read the page
Capability
Kitting & assembly
Sequenced, kitted sets of slot liners, phase barriers, wedges and seals delivered ready to install in winding order, with revision control across a production program.
Read the page
Request a quote
Send a drawing for review
Upload a DXF, STEP, or PDF of the slot, wedge or seal with the thermal class, working voltage and duty, and engineering will confirm a material family, grade direction, and converting approach.
Open the RFQ form
Talk to an engineer
Contact H-O Products
Family-owned since 1971, ISO 9001:2015 certified, converting engineered materials in Winsted, Connecticut. Call or send a message and an engineer responds.
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Material data & standards. All material behavior described on this page – thermal class reach, dielectric strength, mechanical properties and chemical resistance – is taken from the source manufacturer's technical data sheets and the cited standards. Grade-level values are thickness- and grade-specific; verify against the source TDS for your part, gauge, voltage and environment before final spec.
H-O materials are “evaluated against” and “support compliance with” the cited standards through the source TDS; H-O does not independently certify materials against the standards unless explicitly stated on the quote.
System-level performance. The thermal-class map is an illustrative orientation of family temperature reach against the insulation-class framework, not a grade rating; a motor's thermal class is established by functional evaluation of the complete insulation system under IEC 60034-18 within the framework of UL 1446. Dielectric performance is a property of the as-built barrier at its working voltage. H-O is a precision converter and does not extrude or mold raw material; parts are made-to-order to your drawing.