The Physics of PET Shrinkage
Polyester (PET) film is manufactured via biaxial orientation—stretching the film in both directions. This induces internal stress memory. When exposed to temperatures above its glass transition point (Tg ≈ 78°C) but below melting, the film attempts to revert to its unstretched state, causing dimensional shrinkage.
Consequences in Transformer Winding
In transformer applications, tape shrinkage during the curing cycle exerts compressive force on the copper windings. While some compression is desirable, uncontrolled shrinkage can deform the coil geometry, crack the enamel wire coating, or create gaps in the insulation layers (creating dielectric weaknesses).
Selecting Low-Shrinkage Alternatives
For critical applications, manufacturers must use heat-stabilized (pre-shrunk) polyester tapes or switch to dimensionally stable carriers like Polyimide or Glass Cloth, which exhibit near-zero shrinkage at standard varnish curing temperatures (130°C - 150°C).