Polyester film is biaxially oriented PET film, or BOPET: a film made from polyethylene terephthalate that is stretched lengthwise and crosswise during production and then heat-set under tension. In electrical insulation it runs as thermal class B, with a continuous rating of 130 °C per IEC 60085 and a dielectric strength of around 180 V/µm for thin grades. Mylar, HOSTAPHAN, Melinex and Lumirror are not separate materials but trade names for exactly this film.
Key points at a glance
- Polyester film, PET film and BOPET all describe the same class of material.
- The brands Mylar, HOSTAPHAN, Melinex and Lumirror differ in manufacturer and type series, not in the base material.
- Design work uses the 130 °C continuous rating, not the frequently quoted 150 °C short-term value.
- Dielectric strength falls as thickness rises, from around 174 V/µm at 23 µm to around 65 V/µm at 500 µm.
- The same PET layer only reaches higher thermal classes as a laminate, with polyester nonwoven as DMD or with aramid paper as NMN.
Polyester film is made by extrusion and biaxial stretching
PET granulate is melted and laid down as a thin film on a chilled roll. Two stretching steps follow: first in machine direction over heated rolls that turn progressively faster, then crosswise in a tenter frame. Stretching aligns the molecular chains in both directions, and that alignment is the source of the properties PET is bought for in electrical engineering: high tensile strength in both directions, dimensional stability under load, and water absorption of only 0.3 percent.
The final step determines behaviour in service. The stretched web is heat-set under tension, otherwise it pulls back toward its original dimensions when it sees heat. How much a film shrinks therefore depends on the producer's heat-setting and on the grade, not on PET as a material. In lamination, in winding hot assemblies and in potting, shrinkage is the value that creates scrap whenever nobody asks for it.
PET belongs to the thermoplastics and can therefore be formed, folded and shaped under heat. The upstream film extrusion process defines which thicknesses a producer can run at all, and therefore which thicknesses are available on the market.
Mylar, HOSTAPHAN, Melinex and Lumirror are trade names for the same film
The most common knowledge gap in this market concerns the brands. Datasheets, drawings and specifications carry four names that all mean the same class of material. They differ in manufacturer, in type series and in the surfaces available.
| Trade name | Manufacturer | Role in the market |
|---|---|---|
| Mylar | Mylar Specialty Films, formerly DuPont Teijin Films | the best-known name, often used as a stand-in for the generic material |
| HOSTAPHAN | Mitsubishi Polyester Film | widely used in electrical insulation, broad thick-film range |
| Melinex | DuPont Teijin Films | frequently specified in printing, graphics and label applications |
| Lumirror | Toray | less often called out by name in the European electrical market |
For procurement this distinction has a practical consequence. When a drawing names a brand, clarify whether the designer means that brand or the generic material. If the generic material is meant, sourcing opens up to several producers and lead times drop. If the brand is meant, because a UL listing or a customer release depends on it, the brand stays. GOBA works with all major European producers and is the largest European converter for Mitsubishi Hostaphan in thick films, while supplying the sister brands just as readily.
Properties of polyester film
| Property | Value | Note |
|---|---|---|
| Continuous operating temperature | 130 °C | thermal class B per IEC 60085 |
| Short-term peak | up to about 150 °C | not a design value |
| Lower service limit | about minus 70 °C | remains workable and dimensionally stable down to this point |
| Dielectric strength | around 180 V/µm for thin grades | drops markedly as thickness increases |
| Water absorption | 0.3 % | largely insensitive to ambient humidity |
| Density | 1.395 g/cm³ | basis for calculating area weight |
| UV resistance | none | unsuitable for outdoor use without stabilisation |
| Commercial thicknesses | 23 to 500 µm | GOBA converts 12 to 350 µm |
One value is routinely carried over incorrectly. The dielectric strength of PET is not a material constant, it falls with thickness. The catalogue values of an insulation distributor for Mylar A and HOSTAPHAN RN allow a direct calculation: a breakdown voltage of 4 kV at 23 µm works out at around 174 V/µm, while 32.5 kV at 500 µm is only around 65 V/µm. Anyone extrapolating the frequently quoted 180 kV/mm linearly onto a 350 µm film overestimates the voltage withstand by more than a factor of two. What counts for design is the breakdown voltage at the thickness actually installed.
Temperature deserves a second look as well. The literature carries 100, 130 and 150 °C side by side without anyone naming the difference. For electrical insulation, the continuous rating of thermal class B at 130 °C applies. The 150 °C figure is a short-term value, the 100 °C figure comes from applications without electrical stress. Designing to the peak produces insulation that ages in continuous service.
Selecting polyester film: the sequence that prevents mistakes
Selection rarely fails on the material and usually fails on the sequence. Starting with thickness means fixing the most expensive parameter first and then rescuing the temperature rating afterwards. This order has proven itself in production:
- Fix the thermal class. If the winding stays below 130 °C continuously, plain PET film is the right choice. Above that, a laminate or a different material is unavoidable.
- Determine the required breakdown voltage and check it against the thickness, not against the catalogue figure in kV/mm.
- Choose the thickness from voltage withstand and required stiffness. Thin films lay into tight slots, thick films stand as formed parts.
- Fix the surface: clear, matte, chemically pretreated for paint and adhesive bonding, or metallised.
- Ask for shrinkage data as soon as the film sees heat in the process. Producers offer dedicated low transverse shrinkage grades, for example in 36, 50 and 75 µm.
- Consider lamination when thermal class and mechanical protection are required together.
For matching thickness to application, this rough scale is enough in most projects. It does not replace a design calculation, but it sorts the enquiry:
| Thickness range | Typical use |
|---|---|
| 12 to 36 µm | layer and turn insulation, winding film, dielectric in film capacitors |
| 50 to 125 µm | slot lining and phase insulation in small and medium motors |
| 150 to 250 µm | slot and phase insulation in larger machines, formed parts with fold lines |
| 300 to 350 µm | rigid formed parts, covers, spacers and separators |
Surface finish is almost always forgotten in enquiries and later decides adhesion. Clear grades are the standard, and from about 190 µm the film turns increasingly hazy anyway. Matte and roughened grades diffuse light and take printing ink better, chemically pretreated grades are made for paints, adhesives and laminating adhesives, and metallised grades act as a barrier or a shield. If a film is to be bonded or printed later, fix the pretreatment on the first run, otherwise the second production batch turns into fault finding.
Lamination: how class B becomes a higher-class composite
At 130 °C the story is far from over, provided the build-up is right. The same PET core moves into higher thermal classes through lamination, because the outer plies take the thermal load and protect the film mechanically. Two composites dominate motor and transformer construction: DMD with two plies of polyester nonwoven over a PET core, and NMN with two plies of aramid paper over the same PET core.
The step up is commercially attractive because it restricts the expensive material to the outer layers. An NMN composite costs a fraction of a solid aramid layer and still holds the required class. Finished sheet insulation materials such as Trivoltherm follow the same logic. For selection this means checking whether a composite delivers the class before switching materials altogether.
When polyester film is enough and when you have to move up
Our recommendation is clear: up to a continuous 130 °C, plain PET film is the right material, including where the specification calls for headroom. PET is inexpensive, tear resistant, dimensionally stable and available in every common thickness. Moving to aramid or polyimide out of caution means paying a multiple for a property that service never calls on.
Above 130 °C continuous, the sensible range ends. Between 155 and 180 °C, aramid paper and NMN composites are the right choice; from 180 °C, and for anything approaching 200 °C, the route leads to polyimide, usually specified in the market as Kapton. The price gap between PET and polyimide runs into multiples, which makes it worth measuring the real winding temperature rather than estimating it. Two further points bound PET: it is not UV-stabilised and therefore unsuitable for permanent outdoor use, and it hydrolyses under sustained high temperature in a humid environment.
Related PET grades and neighbouring films
Not every PET is BOPET. Amorphous A-PET and recycled R-PET have different priorities and different approvals, and they do not replace stretched electrical insulating film. Anyone looking for a crystal-clear rigid film for non-electrical uses often ends up with polystyrene film, which is cheaper but far more brittle. For an overview of the whole material group, in which PET is only one of several webs, see the article on electrical insulating films.
What matters when converting polyester film
What arrives is a roll, what gets installed is a part. Converting sits between the two, and that is where the problems arise that no datasheet mentions. GOBA slits PET film into slit rolls from 5 mm to 1,600 mm wide with a width tolerance of ±0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inner diameter. Master rolls up to 1,000 kg run through the line at up to 400 m per minute, depending on the material.
With thin films below 36 µm, the cut edge determines the voltage withstand of the finished part. A frayed edge is a field concentration, and that is where the insulation breaks down first. We therefore run the knife shaft up more slowly on thin webs and inspect the edge on the first roll before the series runs. Winding quality matters just as much: a roll with telescoping or uneven winding hardness will not feed cleanly into the customer's winding machine, even when width and thickness are correct.
As a formed part we process PET film by stamping, folding, embossing and flanging, in material thicknesses from 0.023 to 3.0 mm and part sizes up to 1,000 x 2,000 mm. For small batches and samples we use steel-rule dies, because the tool is cheap and quickly available. For series work with tight tolerances and high volumes we use solid steel tools, which last longer and cut more accurately. Complex geometries are formed under heat so that the fold line sits correctly and the surface stays undamaged.
Is a converter worth it, or is in-house slitting cheaper?
The objection comes up regularly in purchasing: standard rolls are available on the market, so slitting could be done internally. On material cost alone that holds up. In practice the cost appears elsewhere. A master roll ties up capital and floor space, every format change costs setup time, and the trim waste at the web edges falls entirely on the processor when slitting in house. Then there is inspection: without systematic checks on cut edge and winding hardness, the fault only shows up in the winding shop.
Our recommendation: from a stable annual demand in a few fixed widths, in-house slitting pays off. With changing widths, thin films and parts to drawing, the converter is cheaper, because it spreads trim waste across several customers and already has the line set up. If you are unsure, send us a drawing and the annual quantity and we will cost both routes.
GOBA Takeaway
Polyester film is the standard material for electrical insulation up to 130 °C. Most selection errors happen in three places: confusing brand with generic material, extrapolating dielectric strength linearly, and never asking about shrinkage. Clear those three and the design holds. GOBA carries the generic material as a polyester film programme covering all common brands and converts it into slit rolls and stamped parts. For an overview of the remaining materials, see our page on insulation materials.
