Mylar / PET Film for Capacitor Sleeves: How to Pick the Right Grade

PET (Mylar) film is the default dielectric for hundreds of millions of capacitors a year. The question is not which film, it is which grade of film, and where the cut-off between industrial and capacitor-grade actually sits.

The three things that matter

1. Thickness tolerance - capacitor-grade PET is 6-23 micron typical, with thickness tolerance of +/-5% or tighter. Commodity PET is wider.
2. Slitting edge quality - capacitor-grade is slit cleanly with no nicks or burrs. A rough edge creates partial discharge sites that age the cap.
3. End termination - the metal spray or foil termination is a separate engineering step, and the way the film ends are prepared controls the contact.

Voltage, thickness, and the practical chart

Voltage class	Typical PET thickness	Application
up to 100V DC	6-12 micron	Low-voltage electronics, signal
100-400V DC	12-23 micron	Industrial, lighting, drives
400-1000V DC	23-50 micron	Power electronics, EV
1kV+	Stacked or PP film	Specialty, traction

Where buyers go wrong

Data density point: the most common capacitor failure I see in field returns is not dielectric breakdown at full voltage. It is partial discharge at a void or rough edge, at 60-80% of rated voltage, after thermal cycling. The film is not the wrong choice - the slitting and winding are.

The same film, a different job: bolt and tube insulation

PET film is not only a capacitor dielectric. The same material, slit into a sleeve, insulates flange bolts and fasteners - breaking the metal-to-metal path so dissimilar flanges do not galvanically corrode, and taking the place of a PTFE sleeve at a fraction of the cost. The failure mode is different: a thin wall that splits on assembly, or a bore that is not consistent part to part, lets the bolt touch metal. So the grade question repeats - uniform wall and a clean edge matter as much on a bolt sleeve as on a capacitor winding. For cutout and flanged joints, the sleeve is part of a flange isolation kit, and the bolt sizes run from M6 to M36.

"PET is a commodity. Capacitor-grade PET is a controlled product. And a PET bolt sleeve is only as good as its wall consistency - specify the slitting edge and the bore, not just the thickness."

For PET film in capacitor and insulation sleeves our Mylar / PET film insulation tube, the flange bolt insulation sleeve, and the M6-M36 bolt sleeve size guide are slit to specification.