Electrical insulating films are thin plastic films, usually between 5 µm and 0.5 mm thick, that electrically separate live parts in motors, transformers, batteries, power supplies and electronic equipment. They form the film group of electrical insulating materials. The two most common ones span, under IEC 60085, from class B at 130 °C (polyester film) to class R at 220 °C (polyimide film). How much voltage a film has to withstand is set by the IEC 60674 series for each material and thickness: a 23 µm PET film must hold a median of at least 174 V/µm, a 250 µm film only 60 V/µm. This article compares nine film materials using standard values, walks through selection in seven steps and shows how GOBA slits, stamps and forms them as part of its insulation materials range.
- Selecting an electrical insulating film starts with the continuous temperature at the hottest point, and only then with dielectric strength.
- The dielectric strength of a film falls with thickness, so design uses the standard value at the installed thickness and never the datasheet value of a 25 µm sample.
- Polyester film covers most motors and transformers up to 130 °C, and polyimide film only pays off when temperature or installation height demands it.
- An adhesive layer lowers the thermal class: polyimide film runs in class R as a material, but as a tape with acrylic adhesive only in class F.
Electrical insulating films from GOBA: supply forms, thicknesses and enquiry
GOBA supplies Electrical insulating films from a single piece, as material or as a finished part to your drawing.
- Supply forms
- Roll and slit roll, Sheet and board, Cut-to-size, Die-cut part to drawing, Formed part (folded, embossed or bent)
- Thicknesses
- Polyimide film from 7.5 µm, polyester film 12 to 350 µm, die-cut parts 0.023 to 3.0 mm
- Versions
- Polyester film (HOSTAPHAN, Mylar and others), polyimide film (Kapton), PP and PTFE film as well as laminates such as NMN, NKN and DMD. Die-cut parts also self-adhesive or kiss cut on a carrier liner.
- Slit rolls
- Widths from 5 to 1,600 mm, tolerance ±0.1 mm, cores of 25, 55, 76 or 152 mm
- Die-cut and formed parts
- Material thickness 0.023 to 3.0 mm, dimensions up to 1,000 × 2,000 mm
- Quantity
- From 1 piece, from sample to series
- Lead time
- Two weeks at most when the material is in stock
What we need for your quotation
- Drawing or dimensions
- Quantity
- Material and thickness
For rolls also width, length, core diameter, core material (for example plastic) and tolerances.
Nine plastics share the market for electrical insulating films
The table sets out the films that appear in specifications for motors, transformers and electronics. Where IEC 60674 defines a minimum value, that value is shown. It sits below the typical datasheet figure, but it is the only one every delivery has to meet. Where no standard sheet exists, the typical manufacturer value is given together with its thickness.
| Film (trade names) | Continuous temperature and class | Breakdown at thickness | Thicknesses | UL 94 |
|---|---|---|---|---|
| PET (Mylar, HOSTAPHAN, Melinex) | 130 °C, class B | min. 174 V/µm at 23 µm, min. 80 V/µm at 125 µm | 12 to 500 µm | VTM-2 |
| PEN (Teonex) | 155 °C, class F, electrical RTI 180 °C | 250 kV/mm typical | 12 to 250 µm | VTM-2 |
| Polyimide (Kapton, Upilex, Apical) | 220 °C, class R | min. 235 V/µm at 25 µm, min. 120 V/µm at 125 µm | 5 to 175 µm | V-0 |
| PP (BOPP, flame-retardant e.g. Formex) | 100 °C, flame-retardant 115 °C | BOPP 524 kV/mm at 25 µm, FR PP 13 kV at 0.127 mm | BOPP from 1.9 µm, FR PP 0.127 to 3.18 mm | HB, flame-retardant VTM-0 to V-0 |
| PC (Makrofol, Lexan FR) | 115 to 130 °C | 15 to 67 kV/mm depending on thickness | depending on grade | V-2 to V-0 depending on grade |
| PPS (Torelina) | material 200 to 240 °C | no public film value | depending on grade | flame-retardant without additives |
| PEEK (APTIV) | resin around 250 °C | 270 kV/mm at 25 µm, 70 kV/mm at 250 µm | 8 to 750 µm | resin V-0 |
| FEP (Teflon FEP) | 205 °C | 260 kV/mm at 25 µm, 70 kV/mm at 0.5 mm | 12.5 to 500 µm | VTM-0 |
| PTFE (Teflon) | 260 °C | 20 to 60 kV/mm depending on thickness and process | cast from about 25 µm, skived up into the millimetre range | per datasheet |
Sources: minimum values from IEC 60674-3-2 for PET and IEC 60674-3-4 for polyimide, PEN from the Teonex Q51 datasheet, PEEK from the APTIV 1000 datasheet, FEP from the DuPont FEP bulletin, flame-retardant PP from the Schupp catalogue, PPS from Toray's material data. For PEEK and PPS the values are those of the resin, because the film datasheets state no temperature or fire rating.
Two materials are deliberately left out of the table. As a semi-finished product, PVC film only holds 60 °C continuously and works as insulating tape, not for windings or barriers in equipment. Papers, pressboard and laminates are not films, and they are covered further down. For the plastic films a simple rule of thumb applies: in thin layers BOPP and polyimide withstand the most voltage, PTFE, PEEK and polyimide hold the highest temperatures, and PET and PP are the least expensive.
Which electrical insulating film belongs where
The material table answers what a film can do. Design asks the reverse question: what does the spot where the film sits demand? The following overview shows which films have become established at the most common installation points and which property decides there.
| Installation point | Usual films | What decides there |
|---|---|---|
| Slot insulation and phase separation in industrial motors | PET 50 to 250 µm, DMD and NMN laminates | thermal class B or F, stiffness when inserted into the slot |
| Traction motors and inverter duty | polyimide, NKN laminate, PEN | class H, partial discharge, height in the slot |
| Layer and cover insulation in transformers, chokes, relay coils | thin PET film | breakdown at low thickness, compatibility with impregnating resin |
| Power supplies, housing liners, PCBs at 230 V | flame-retardant PP film, PC film | UL 94 V-0, tracking resistance, scoring and folding |
| Battery modules and high-voltage housings | polyimide, flame-retardant PP film | fire behaviour, breakdown at high voltage, dimensional stability |
| Film capacitors | BOPP, ultra-thin PET, PPS | dissipation factor, thickness below 10 µm, temperature |
| High frequency, cables, chemicals | FEP, PTFE | permittivity around 2.0, media resistance |
| Masking during soldering and coating | polyimide tape with silicone adhesive | short-term heat, clean removal |
The PCB case deserves a second look, because it comes up often in searches and is rarely solved properly. Anyone insulating the 230 V side of a circuit board against a metal housing tends to reach for polyester film because it is electrically strong. Its fire rating, however, is only VTM-2. Flame-retardant PP film reaches UL 94 V-0 and a tracking resistance of CTI 600 V, and it can be scored and folded into a cap. The Schupp catalogue cites a minimum thickness of 0.4 mm between live parts and chassis for IT equipment under the former EN 60950, which is why these films come in 0.43 mm. Check the limit against the equipment standard that applies to your product today, such as IEC 62368-1.
Dielectric strength is a value per thickness, not a material constant
Datasheets give 303 kV/mm for polyimide and 280 kV/mm for PET, both measured at 25 µm. Scaling those figures up to a 125 µm film overestimates its voltage withstand by around a factor of two. A thicker film holds more defects in the stressed volume and sheds the heat of breakdown less easily, so the value per micrometre drops as thickness grows. IEC 60674 accounts for this and sets minimum values per thickness.
| Thickness | PET, standard minimum | Polyimide group A type 1, standard minimum | Kapton HN, typical | PEEK APTIV 1000, typical |
|---|---|---|---|---|
| 23 to 25 µm | 174 V/µm | 235 V/µm | 303 kV/mm | 270 kV/mm |
| 50 µm | 130 V/µm | 195 V/µm | 240 kV/mm | 190 kV/mm |
| 75 µm | 105 V/µm | 175 V/µm | 205 kV/mm | not stated |
| 125 µm | 80 V/µm | 120 V/µm | 154 kV/mm | 120 kV/mm |
| 250 µm | 60 V/µm | no standard step | not stated | 70 kV/mm |
1 V/µm equals 1 kV/mm. The unit is the same, the test is not: the PET value is a median that single specimens may fall below, the polyimide value is a minimum, and the typical values from DuPont and Victrex come from short-time tests to ASTM D149. For design this means a 125 µm PET film must withstand at least 10 kV by the standard, a polyimide film of the same thickness at least 15 kV. Work with the standard value and a safety margin to the test voltage. How the measurement works and why the value falls further with heat and humidity is explained in the article on electrical breakdown strength.
Thermal class, RTI and short-term value: three temperatures on one datasheet
For polyimide, the trade quotes 400 °C, the thermal class says 220 °C and the UL file says 200 to 240 °C. All three numbers are correct, they measure different things. The thermal class under IEC 60085 gives the continuous temperature at which the insulation reaches its planned service life. The Relative Thermal Index under UL 746B is the temperature at which a property falls to 50 percent of its initial value in long-term testing, split into electrical and mechanical properties, as the RTI overview summarises. The short-term value only says what the film survives during soldering or in a fault.
PEN shows the difference most clearly: thermal class F at 155 °C, electrical RTI 180 °C, mechanical RTI 160 °C. For a phase separator that vibrates in service, the mechanical value counts. For a film that only blocks voltage and is firmly embedded, the electrical value may set the limit. Never design to the short-term value. Insulation that runs permanently above its class ages faster than maintenance is planned for.
Selecting electrical insulating films: seven steps
The order decides the result. Starting with thickness fixes the most expensive parameter first and leaves the temperature to be rescued afterwards. This sequence has proven itself in production:
- Measure or calculate the continuous temperature at the hottest point and fix the thermal class with a margin. This usually rules out several films in the table straight away.
- Determine test and operating voltage and derive the minimum thickness from the standard value at that thickness.
- Clarify fire protection. If the equipment standard requires UL 94 V-0 or VTM-0, standard PET and PEN drop out.
- Check the media: impregnating resin, transformer oil, coolant, damp heat. In damp heat PEN lasts about four times as long as PET, and one adhesives maker cites 200 against 50 hours at 120 °C and 100 percent humidity.
- Define mechanics and shape: laid in flat, scored and folded, formed under heat or pulled into a slot.
- Decide supply form and finish: slit roll, cut-to-size, stamped part or formed part, plain or self-adhesive, loose or kiss cut on the carrier liner.
- Specify the film on the drawing by standard rather than by brand, for example IEC 60674-3-2, PET, type 3, 125 µm. Only fix the brand when a UL listing or a customer release depends on it.
The seventh step saves purchasing the most time. IEC 60674-3-2 defines five PET types: type 1 as standard, type 2 ultra-thin for capacitors, type 3 with higher temperature resistance for windings, and types 4 and 5 additionally hydrolysis resistant for windings and solar backsheets. Once the type is on the drawing, any producer meeting it may supply, whether the roll is called HOSTAPHAN or Mylar.
"We will use Kapton, then we are safe": why that is rarely true
The sentence comes up regularly in project meetings, and it sounds reasonable. Polyimide has the highest thermal class among winding films, the highest standard breakdown value and reaches UL 94 V-0. The numbers tell a different story. Polyimide film costs a multiple of PET film of the same thickness. At 50 percent humidity it absorbs 1.8 percent water, PET 0.3 percent, and after six hours at 93 percent humidity the standard allows up to 3.0 percent for polyimide. Insulating a class B winding with polyimide means paying for 90 kelvin of headroom that the motor never uses, and buying a moisture problem in storage on top.
Our recommendation follows the temperature. Up to 130 °C, use polyester film. Up to 155 °C, PEN if the layer has to stay thin, otherwise an NMN laminate. Up to 180 °C, aramid paper or a laminate with a polyimide core such as NKN. Plain polyimide film pays off above that and in three special cases: when only half the height is available, when partial discharge threatens on an inverter drive, and when soldering briefly exceeds 260 °C. This ladder meets the required class and in series production costs far less than the blanket solution.
Self-adhesive electrical insulating films: the adhesive sets the class
A film with an adhesive layer is a composite, and the tape is rated as a whole. The adhesive sets the limit: with acrylic adhesive a polyimide tape ends in class F, with silicone adhesive in class H. The tape table of the insulation distributor SynFlex shows this for three carrier films, next to the class of the plain film from the first table:
| Carrier film | plain film | tape with acrylic adhesive | tape with silicone adhesive |
|---|---|---|---|
| PET 23 to 50 µm | class B, 130 °C | class B, 130 °C | class B, 130 °C |
| PEN 25 µm | class F, 155 °C | class F, 155 °C | class H, 180 °C |
| Polyimide 25 to 50 µm | class R, 220 °C | class F, 155 °C | class H, 180 °C |
For polyimide the adhesive therefore costs two to three classes, for PET none. SynFlex even lists the PEN tape with silicone adhesive one class above the plain film, because the class applies to the tested tape and not to the material. Always work with the class on the tape datasheet, never with the class of the carrier film. The standard confirms the picture. IEC 60454-3-7 for polyimide adhesive tapes sets thermal indices of 155 to 180, that is F to H. Fitting a self-adhesive polyimide film into a class R zone gives headroom on paper and, in service, an adhesive that gives up first. Silicone adhesive withstands more heat, acrylic adhesive is more compatible with impregnating resins. Fix the adhesive together with the film, not afterwards. We supply self-adhesive stamped parts loose or kiss cut on the liner so that assembly can peel them straight off.
Paper, fish paper and laminates are not films
Many material lists put films, papers and composites under one heading. For selection it pays to separate them, because each is strong in a different place. Films block the most voltage at the smallest thickness. Papers absorb impregnating resin and take more mechanical load. Laminates combine both.
- Fish paper is the common name for thin vulcanised fibre, pure cellulose in class Y at 90 °C with 7.7 to 15.7 kV/mm, robust and inexpensive but sensitive to moisture.
- Aramid paper such as Nomex 410 reaches class H and a UL listing at 220 °C, but at 18 to 34 kV/mm it blocks only around a tenth of what polyimide film blocks per millimetre.
- Laminates such as DMD (class B to F), NMN (class F) and NKN (class H) place a film in the middle as the voltage barrier and nonwoven or aramid paper outside as protection.
For an overview of all sheet materials including pressboard, see the article on sheet insulation materials, and for how the composites are built, the article on laminating film.
Standards for electrical insulating films
A handful of standards covers specification and incoming inspection. The key ones:
- IEC 60674, plastic films for electrical purposes. Part 1 covers definitions and general requirements, part 2 test methods, part 3 individual materials, including sheet 1 BOPP for capacitors, sheet 2 PET, sheet 3 PC, sheet 4 polyimide, sheet 7 FEP and sheet 8 PEN.
- IEC 60454, pressure-sensitive adhesive tapes for electrical purposes, with separate sheets for polyester and polyimide tapes.
- IEC 60626 for combined flexible insulating materials, that is, the laminates.
- The thermal classes from Y at 90 °C up to 250 °C are set by IEC 60085.
- Fire behaviour is tested to UL 94, using the VTM classes for thin films, with UL 746B adding the RTI.
- IEC 60664-1 for insulation coordination with clearances and creepage distances, that is, whether a film has to replace a missing clearance.
Converting: from master roll to finished insulating part
A film arrives as a master roll, and what gets installed is a tape, a cut piece or a formed part. GOBA slits electrical insulating films by roll slitting with shear cutting to widths of 5 to 1,600 mm with a width tolerance of plus or minus 0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inner diameter. Master rolls up to 1,000 kg are lifted in hydraulically. The tolerance is tighter than the standard requires: IEC 60674-3-4 allows plus or minus 0.5 mm for polyimide up to 50 mm wide and plus or minus 1.0 mm up to 300 mm. For narrow tapes in winding machines, that is the difference between clean running and edge offset.
Film becomes die-cut parts: in stamping and forming we process material thicknesses from 0.023 to 3.0 mm and parts up to 1,000 x 2,000 mm. Polyimide below 23 µm therefore stays roll goods or is laminated onto a carrier first. For samples and small batches we use steel-rule dies, for tight tolerances and long tool life solid steel tools. Fold lines are pre-embossed in the same stroke so the part can later be bent into its final position without cracking. PET with complex geometries is formed under heat, while flame-retardant PP film can be scored and folded cold. Finished parts go to assembly stacked or packaged, on request with marker lines that a sensor uses to detect and position the part.
GOBA Takeaway
The right electrical insulating film follows from temperature, voltage at the real thickness, fire class and adhesive, in that order. For most motors and transformers the search ends with polyester film, which we carry as a polyester film programme with HOSTAPHAN, Mylar and other brands. Above that the route runs through PEN and laminates to polyimide. Send us drawing, quantity, material and thickness, and for rolls also width, running length and core. We supply from one piece, within two weeks at most when the material is in stock, and make parts to drawing as custom products.
