Home › Blog › EV Powertrain & E-Axle › Choosing slot, phase and wedge materials
Aramid Paper, Polyimide Film or PEEK? Choosing Traction-Motor Slot, Phase and Wedge Materials
A traction motor asks for insulation in four distinct places, and each place rewards a different converted material. The slot liner wants toughness to survive winding insertion. The phase barrier wants the most dielectric strength in the least thickness. The slot wedge wants stiffness and dimensional stability at the motor's hot-spot temperature. And the gearbox seal next door wants an elastomer that shrugs off hot lubricant.
This guide separates the four jobs, maps each to the material family H-O Products converts for it, and ends with a selector that turns your slot location into a cautiously framed material direction.
Request a quote for your project →
The application problem: one stator, four different material jobs
An engineer specifying "motor insulation" is really specifying several parts that share a slot but not a duty. The ground-wall liner sits between the laminated steel core and the winding and holds off the slot-to-ground voltage while protecting the wire during insertion. The phase barrier sits between coils of different electrical phases, where the full phase-to-phase voltage appears in a tight space.
The slot wedge closes the mouth of the slot and mechanically holds the conductors down against electromagnetic and vibration forces. Around the gearbox, seals and gaskets live in continuous hot-oil contact. Pick one material for all four and at least two of the jobs are compromised.
- Slot liner / ground wall: the success criterion is surviving insertion intact and holding the ground-wall voltage for the life of the motor. This favors a tough, tear-resistant insulation paper.
- Phase and layer barrier: the criterion is maximum dielectric strength per unit thickness, because every fraction of a millimeter in the slot competes with copper. This favors a thin, high-dielectric film.
- Slot wedge and end-turn support: the criterion is stiffness, strength and dimensional stability when hot. This favors a rigid, high-temperature film or laminate.
- Gearbox and shaft-area seals: the criterion is a resilient compression seal that does not swell in hot lubricant. This favors a fluorinated elastomer.
Why the choice is challenging: the limiting property changes with the location
The difficulty is that no single datasheet number ranks a material for all four jobs. The liner is limited by tear and insertion survival, so a paper that loses to a film on dielectric strength still wins the slot. The phase barrier is limited by dielectric strength per unit thickness and by the thermal class the system needs, so the film wins there.
The wedge is limited by mechanical stiffness at temperature, a property neither the paper nor the thin film leads on. And the seal is limited by fluid compatibility, where standard silicone, excellent everywhere else, swells in oil and loses its sealing geometry.
Temperature framing adds a trap of its own. A thermal class such as Class H, which corresponds to a 180 °C hot-spot allowance, is a property of the qualified insulation system as a whole, established by functional evaluation under IEC 60034-18 within the framework of UL 1446. A sheet material has a thermal reach that supports a class; the class itself is earned by the complete system of materials, wire enamel, varnish and processing tested together.
The honest way to select is to state the target class and hot-spot temperature, choose materials whose reach supports it, and confirm the class against the qualified system.
Recommended material families
Each location maps to a family H-O Products converts. Grade-level values such as dielectric strength, continuous-temperature rating and thickness are confirmed against the manufacturer's current technical data sheet before specifying.
For slot liners and the ground wall: aramid paper
Aramid insulation paper is the default ground-wall and slot-liner material because it pairs the thermal reach a traction motor needs with the toughness to be folded, cuffed and inserted without tearing. Calendered and uncalendered grades cover liners, layer insulation and wedge packing. Where the working voltage climbs, aramid is laminated with polyimide film so the part carries the film's dielectric strength with the paper's handling.
For phase and layer barriers: polyimide film
Polyimide film delivers the most dielectric strength per unit thickness of the families here and the highest continuous-temperature reach, which makes it the lead for phase and layer insulation in tight, high-voltage slots. It is a thin film, so for handling it is frequently laminated to aramid paper; aramid-polyimide-aramid builds are common in higher-voltage traction-motor slots. Where a design calls for an inorganic barrier, mica barrier sheet is the adjacent option for the hottest, most demanding zones.
For slot wedges and end-turn support: PEEK film and rigid laminates
PEEK film brings stiffness, strength, dimensional stability and a continuous-service temperature well above the Class H range, so it leads for slot wedges and end-turn supports that hold conductors against electromagnetic and vibration forces. Its chemical resistance also suits wet and oil-cooled motors. Where mechanical loads are highest, a rigid glass-composite laminate serves; denser calendered aramid grades cover lighter-duty wedges.
For gearbox and shaft-area seals: fluorosilicone sponge and FKM
Fluorosilicone sponge keeps silicone's broad temperature span 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 swell. For the most aggressive oil-and-heat combinations, solid fluoroelastomer (FKM) is the step beyond. Both are die-cut to the seal profile with adhesive sides and liners as drawn.
Slot Liner, Phase Barrier, Wedge or Seal: Material Direction Selector
Tough, tear-resistant Class H-supporting paper for the ground wall: it survives folding, cuffing and winding insertion, and it laminates with polyimide film when the voltage climbs.
Illustrative only; confirm against current material data and a drawing review. This selector returns a material family direction, not a grade or a guaranteed performance value. Dielectric strength, continuous-temperature reach, stiffness and fluid compatibility are confirmed on the manufacturer's current technical data sheet, and the insulation-system thermal class belongs to the qualified system per IEC 60034-18 / UL 1446.
Design and converting considerations
Once the location and family are settled, the converting decisions are where the part succeeds or fails:
- Thickness is a voltage decision, not a default. The minimum phase-barrier thickness follows from the phase-to-phase working voltage, the grade's data-sheet dielectric strength per unit thickness, and the margin you carry for the inverter's fast switching edges, with partial-discharge behavior considered for the highest-voltage designs.
- Insertion detail decides liner survival. Scored and creased fold lines, cuffs at the slot ends, and a liner sized to seat without bunching are as important as the material choice. H-O converts liners with the fold, crease and cuff geometry your insertion method needs.
- Laminate when one material cannot do both. Aramid-polyimide laminates carry the film's dielectric with the paper's toughness in a single die-cut part, which is how the highest-voltage slots resolve the toughness-versus-dielectric trade.
- Wedges are mechanical parts first. Specify the slot geometry, the holding loads and the hot-spot temperature so the wedge material and gauge can be matched on stiffness and stability, not dielectric alone.
- Seals are sized by fluid and set. Send the lubricant, the temperature and the seal geometry; the fluorinated elastomer and its compression set point are confirmed against the data sheet.
- Process fit. These parts are die-cut, kiss-cut, scored and creased, laser or waterjet cut, laminated and slit to width, then kitted for the winding line.
Common failure modes
| Failure mode | Root cause | Consequence |
|---|---|---|
| Liner tears at insertion | A bare film or a paper grade without the toughness for the winding method, or missing fold and cuff detail | A nicked ground wall that fails later in service; the defense is a tear-resistant aramid liner with proper scoring and cuffs |
| Phase barrier too thick for the slot | A paper specified where a film was needed, spending slot space the copper needed | Lower slot fill; a thin polyimide barrier reaches the voltage in less thickness |
| Barrier undersized for switching stress | Thickness set on nameplate voltage without margin for fast inverter edges or partial discharge | Insulation distress at the highest-voltage zones; size against working voltage plus margin per the data sheet |
| Wedge relaxes or deforms hot | A material chosen on dielectric rather than hot stiffness and dimensional stability | Conductors lift under electromagnetic and vibration forces; PEEK or a rigid laminate was the call |
| "Class H" claimed from a sheet alone | Treating a material rating as a system rating | An unsupported compliance claim; the class belongs to the system qualified per IEC 60034-18 / UL 1446 |
| Gearbox seal swells | Standard silicone specified into continuous hot-oil contact | Lost sealing geometry and leakage; fluorosilicone or FKM was the right family |
How H-O Products helps
H-O Products is a precision die-cut converter. We convert aramid paper, polyimide film, PEEK film, mica barrier sheet, glass-composite laminates and fluorinated elastomers into slot liners, phase barriers, wedges and seals, cut to your drawing, as part of our broader EV and battery materials work.
As an ISO 9001:2015 certified organization, we help you settle the location-by-location material map, recommend a family and grade direction, laminate where one material cannot do both jobs, and convert the stock with the fold, cuff and corner detail the winding line needs.
We do not produce the raw paper, film or elastomer; grade properties are confirmed against the manufacturer's current technical data sheet, and the insulation-system class is earned by your qualified system.
Mapping materials onto a new motor design?
Tell us the location, the target thermal class and hot-spot temperature, the working voltage and margin, and the slot or seal geometry. We will recommend a family and grade direction and convert samples on your profile.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
IndustryEV & Battery materials overviewRead →
Frequently asked questions
What is the difference between a slot liner and phase insulation?
A slot liner is the ground-wall barrier lining the stator slot between the laminated core and the winding; it holds off the slot-to-ground voltage and protects the wire during insertion, which favors a tough, tear-resistant aramid paper. Phase insulation is the barrier between coils of different electrical phases, where the full phase-to-phase voltage appears in a tight space; it favors a thin, high-dielectric polyimide film. Many traction motors use both, sometimes laminated together for the highest-voltage builds.
Does a Class H material make a Class H motor?
No. A thermal class such as Class H, corresponding to a 180 degree C hot-spot allowance, is a property of the complete insulation system qualified by functional evaluation under IEC 60034-18 within the framework of UL 1446. A sheet material has a thermal reach that supports a class; the class itself is earned by the system of materials, wire enamel, varnish and processing tested together.
Why is aramid paper often laminated with polyimide film?
To combine the two properties no single sheet leads on. Aramid paper is tough and tear-resistant and survives winding insertion, but its dielectric strength is that of a paper. Polyimide film has very high dielectric strength per unit thickness but benefits from a carrier for handling. Laminating them produces a slot or phase barrier with the film's dielectric and the paper's toughness, converted as one die-cut part with the fold and cuff detail for insertion.
Request a quote for your EV powertrain & e-axle part
Send H-O your drawing or describe the part—engineering reviews every RFQ and converts samples on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- IEC 60034-18, Rotating electrical machines: functional evaluation of insulation systems. International Electrotechnical Commission.
- UL 1446, Systems of Insulating Materials, General. The framework for thermal classes of insulation systems and their component materials. UL Standards.
- IEC 60317, Specifications for particular types of winding wires. International Electrotechnical Commission.
- Manufacturer technical data sheets for the aramid paper, polyimide film, PEEK film and fluorosilicone grades discussed; values are grade-specific.
This article cites general engineering principles and published standards (IEC 60034-18 and UL 1446 for insulation-system thermal classification, IEC 60317 for winding wire). Material names refer to material families H-O Products converts.
Dielectric strength, continuous-temperature reach, stiffness and fluid-compatibility values are grade-specific; confirm all values against the manufacturer's current technical data sheet and an engineering drawing review. The insulation-system thermal class belongs to the qualified system, not to a single converted part.
All parts are custom manufactured to order and minimum production run quantities apply. H-O is a die-cut converter, not a raw-material producer, and does not certify the finished assembly.